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January 4, 2008: Sunday Evening Prospectus

A pattern reconfiguration is set to transpire on the planetary scale as the position of the troughs and ridges shifts to bring ridging back to the Eastern Pacific and Western North America, and troughing rebuilds to the Eastern half of the United States.  All the while, the atmosphere will retain below normal heights across most of Central and Southern Canada, with the deepest negative anomalies developing over the Eastern United States.  The result will be frequent visits from anomalously cold air, and the potential for winter weather when strong shortwaves can intensify quickly enough to develop closed circulations and tap Atlantic moisture. That's in the medium to long range - in the more immediate future, a quick-moving low brings some interest early Monday, and a more meaningful storm appears on tap for Tuesday night and Wednesday.

A quick warm front/cold front progression associated with an open surface wave is moving east, with current radar imagery showing the convective clusters moving east ahead of a prefrontal trough.  These clusters are fueled by weak cyclonic vorticity advection in the mid-levels, and the best dynamic lift pushes through Western and Northern New England Monday morning.  This keeps the best chance of early morning precipitation from CT (esp. Western) through Western MA, then east along and/or north of the Route 2 corridor of Northern MA.  Temperatures throughout the column are below freezing in the North Country, where light snow will fall.  850 temperatures are above zero south of this, however, while surface temperatures will be below freezing owing to the combination of an antecedent cold airmass and a light gradient wind limiting the potential for surface warm advection.  This ends up meaning light freezing rain for Central/Western New England on Monday morning, then the precip will end as dynamic lift diminishes, and more importantly, cold advection begins.

The shot of cold that follows is the result of a strengthening gradient flow between the large low northeast of the St. Lawrence, and the high building northeast across the Ohio Valley.  This high moves over New England on Tuesday, and though it will initially hold the cold by increasing the barometric pressure and subsidence while cold air is in place following the antecedent cold advection, the overall motion of this high doesn't really favor deep cold holding on, considering it's approaching from the southwest, then weakening as it moves over and northeast of New England. 

This leaves the next southern stream charged shortwave a bit short-handed when it comes to deep cold air to crank out snow later Tuesday night into Wednesday.  There's also plenty of indication that the northern and southern streams have trouble phasing until later in the game, meaning the moisture feed of deepest moisture and therefore highest precipitation amounts stay a bit east of New England.  Still, even if this doesn't change in the days to come, there will be a surge of warm and moist advection into the cold dome for a swath of accumulating snow from south to north Tuesday night.  There's concern for a change to freezing rain and sleet for the interior, and this will depend heavily on the strength and track of the low - the ECMWF is quicker to intensify the system and takes it very close to the coastline, which would favor developing a north and northeast wind through the interior to hold the cold for icing, but at this point is largely alone in such a scenario.  Nonetheless, winter weather will greet many on Wednesday morning, and the North Country will likely see significant snow as the low winds up and the mid-level front slows over the North Country.

Another shot of cold comes Thursday and Friday with the gradient between departing low and approaching high.  At the same time, a strong shortwave rotates through the base of the trough, acting as a fast-moving clipper that will intensify dramatically south of New England late Thursday.  At this point, it would appear development occurs too far south to affect New England, but this is subject to the evolution of the longwave pattern, especially in relation to the Wednesday storm's intensity and track.

A similar strong shortwave will be tracked for next weekend, but even greater uncertainty exists as to the longwave pattern, and therefore any potential track of this system.

Matt

December 17, 2008: An in-depth look at the normally unfavorable pattern for storms that suddenly is so favorable for snow

Thanks for checking back in - a late post today...as explained yesterday, pressed for time lately but wanted to pound something meaningful out before retiring for an early arrival and plenty of forecasting in the morning.  The current weather pattern continues to astound me, as it's virtually the opposite signals one would normally expect for a storm.  The NAO (North Atlantic Oscillation) is strongly positive, while the PNA (Pacific North America Pattern) is strongly negative.  I've seen such a pattern evolve a few times in the past, though, when unfavorable planetary longwave patterns actually become so extreme, that they can bring storms, nonetheless.  Consider that a positive NAO pattern features a large trough from Eastern Canada to Greenland, with strong negative height anomalies, and a large, anomalously strong ridge over the Atlantic.  If the trough gets deep enough, and the ridge gets strong enough, this means fast southwest to northeast flow heading into the Eastern Seaboard, and excellent baroclinicity.  Similarly, a negative PNA implies a Pacific Northwest or West Coast trough, and a warm anomaly in the Southeastern United States.  Combined, these two parameters, if strongly "unfavorable" enough (like they are now), bring a pattern in which a replenishing West Coast trough launches shortwaves northeast in a fast southwest/northeast jet stream flow that moves just south of New England.  Sound familiar?  This is the reason that in my discussions and the "White Christmas Probability" forecasts I wasn't really excited about long term potential, but didn't give up the ship and in fact kept the chance for a White Christmas at better than 50% for even the big cities of Southern New England.  I have to admit that while I saw potential in this pattern, I was ready to give up on it by this past Sunday.  Without power since Thursday night, I hadn't looked at a single guidance product all weekend and heard the talk that there was no chance of snow in sight.  I came into work on Monday morning ready to concede and slash probabilities...until I looked at the guidance!  What a delight I saw in the longwave pattern!

A small slice of that delight was delivered to my driveway, which I just finished snowblowing, today.  Three and a half inches of delight.  I let the freezing drizzle work its crusty magic, then blew it all off, in hopes of getting some melting with sunshine and above freezing temps on Thursday.  I also made sure to push the snow extra far off of the driveway, to leave plenty of room for the NEW snow that's on the way.

This entire setup is exciting and dynamic from a meteorological perspective.  The thought of strong Pacific disturbances responsible for mudslides and record cold rain in Southern Cali making their way virtually untouched across the nation and heading toward New England is intriguing enough.  To combine that with record cold in the Rockies and Plains, and bubbling warmth up the Eastern Seaboard, is the icing on the cake, and provides the baroclinicity needed to turn an otherwise fast and flat flow into something far more exciting.  For the timebeing, it's important to remember the core of this pattern though - fast...flat...flow.  At least for now.  So as we gear up for a substantial snow on Friday, it's not going to be from a powerhouse storm that rages along the coastline.  Rather, this storm should be impressive for its snowfall intensity, delivering a swift and decisive snowy blow to New England, driven by isentropic lift caused by a tightening baroclinic zone that is capitalized upon by the Pacific energy.  Today's event was an excellent precursor: was there a storm today?  Yes, but it was quite weak.  Nonetheless, that defined storm center is what produced a cold air drainage north and northeast wind to keep icing conditions in Eastern New England.  Will there be a storm center with Friday's event?  Yes, again, but stronger than its predecessor.  Still, it's the airmass collision that will drive the snow as an arctic front losing some of its arctic characteristics settles south across New England on Thursday, losing definition after crossing the North Country, perhaps but still representing a flow of new cold that will seep southward.  As the Pacific energy approaches, warmth will be drawn northward, and the thermal gradient will be far more pronounced than this last time around, with New England on the cold side of this storm.  The result should not only be heavy snow cranking out on Friday, but also very cold temperatures in the 20s during this snow, and cold enough air for snow even at the coastline, including Boston, but perhaps excepting the South Coast and Cape Cod where a change to rain is likely given the onshore flow that persists and the east-northeast trajectory.  Though there are some differences in the handling of the system that do have me at least keeping my eyes wide open - namely, the NMM delayed and less intense with the precipitation - the best agreement is with the very similar solutions of the GFS, GGEM, and ECMWF.  All of these models carry plowable snow way into the North Country, and this seems wise given the mid-level warm advection that is forecasted by these products, and the fact that such mid-level warmth has evidenced itself as changes to sleet and freezing rain in the last few events, and while it may not change precipitation type this time, it will manifest itself in the form of a broad snow shield.  The NMM raises an eyebrow because it's essentially saying that the low level cold is so dry it will evaporate the precip before reaching the ground on the northern end, and reduce amounts elsewhere...also delaying arrival.  I am wary given the model's good handling of low level cold airmasses, but prefer the superior handling of the mid-level dynamics for now.

One interesting point here is that, while the vort crosses Southern New England and this supports intense upward vertical motion, especially when coupled with intense warm and moist advection, there is also some evidence of a center jump on Friday.  That should be enough to scare any meteorologist at least a little, because some of the most notorious over-forecasted snow events come in center-jump scenarios, where the vorticity maximum is moving so fast the surface low can't keep up, and a new one develops farther east.  This happens on Friday with one surface low left behind in Western PA, and the new one developing south of Long Island.  While that's all well and good because it brings an intensifying cyclonic circulation south of New England for intensifying warm and moist conveyor belt, plus an intensifying Atlantic easterly inflow, it also means that if the center jump occurs just a touch later, somebody (or lots of somebodies) aren't getting the snow they're supposed to.  Right now, this concern certainly is warranted given the speed of the vort max, but also has very little foundation given excellent agreement among the guidance on developing the new center quickly enough to do the job.  In fact, 12Z guidance suite of the Canadian Ensemble continues to come on board with the leading GFS forecast, expanding the precip field and increasing amounts.  Rather impressive that even the GGEM at 12Z had nearly an inch of melted QPF for Southern New Hampshire, with greater amounts through Northern and Central MA!

This is a big reason why I've had no qualms about mentioning a foot or more for some of us in New England - looks like Central and Southern New England - because the QPF is there even without the trend for increasing liquid.  With such cold antecedent conditions, this is unlikely to be a straight 10:1 or 13:1 ratio, and likely will verify at least in the deformation band as a 16 or 18:1.  I haven't analyzed the thermal profile yet (will tomorrow, of course) but it just makes sense to have a decent ratio given the cold in place to start, and the good warm advection that also may give us our ideal 850 mb conditions for sticky snowflakes that can grow quickly upon descent.  So, right off the bat we're talking a 16-18" snowfall if the 1" QPF verifies.  Now, I haven't thrown those specific numbers out there for the taking in my broadcasts yet, for the obvious reason that a center jump or southward shift like the NMM tries to paint means a big decrease in amounts.  Regardless, I don't think either of those will happen, but figure saying a foot plus is bold enough from two and three days out.

As for placement of the deformation band/heavy snow band, there's likely to be two different mechanisms for banding: 1) mid-level frontogenesis, and 2) surface coastal frontogenesis.  The former appears in recent runs to set up near the MA Turnpike, but given the propensity for a farther north and heavier handed distribution of precipitation, I am suspect it may verify in Northern MA to Southern NH.  Nonetheless, this intense isentropic lift will mean a band of forced ascent that will create efficient snowflake production, and increased QPF.  The coastal frontogenesis is unlikely to spell a rain/snow line from Boston northward, though may in Southeastern MA where more ocean fetch takes place.  Still, it will define the difference in ratios, and also will help to increase surface and low level convergence.  At some point, the mid-level deformation band and low level coastal front will intersect.  This is likely to be your highest snow total.  It's worth noting that there's good agreement on some .50-.70" amounts in only 6 hours from 18Z to 00Z Fri/Sat, and this implies snowfall rates of 1-2" per hour for an extended duration Friday afternoon and eve.  Plows will not be able to keep up with this, especially at the time of evening commute, and if this still looks as menacing in tomorrow's runs, it will be essential to encourage no afternoon travel plans in my casts on Friday morning.  Perhaps sooner, but that's a lot to ask of people and you want to have excellent confidence (utmost) before you suggest or request it.

Of course, this is only the first storm of the series, right?  The NAO remains abnormally positive while the PNA remains abnormally negative, and this means the same strangely active pattern persists with another storm threat Sunday, then again Tuesday.  One would think at least one of these storms will fail to verify - how often can you really pull the hat trick?  But then again, such a strong backwards New England storm planetary pattern just might be the one scenario you could actually do it in.

See you back here before too long.
Matt

Freezing rain for some south, sure...but one to two feet of snow in the North Country? Is this guy kidding?

No joke...a major winter storm is on the way.  By the time you're done reading this, you'll know why I'm forecasting what I am, and we'll all sit back and see if nature can deliver.  This is a rather encyclopedic post, so sit back, relax, and continue reading...

Very active and wavy front likely to bring widespread winter storm of varying precipitation types to New England

In the short range, we're locked, loaded and ready to fire.  In the medium range, the moving target is becoming more clear, and thankfully in a direction that makes some sense.

Let's start at the beginning - a nice burst of warm advection snow especially across Northern New England has continued to push northeast.  Now, the impressive Gulf of Mexico tap and its associated thunderstorms and heavy rain are pushing east, along with the strengthening low.  Not surprising to see that the NMM has slowed its frontal progression, though I think the initial instinct of the model to slide the front through very quickly is a testament to the strengthening of the anticyclone upstream that will be so important by the medium range.  Though the NMM was, as it appeared to be, too hasty in sliding this anticyclone east and southeast, it does do the best job with handling low level cold airmasses thanks to its higher resolution, and this dense cold air still looks as though it will slide in quickly behind the front, which is getting a push from the shortwave that races across Quebec later Wednesday.  Confluent flow follows the shortwave, and this favors sliding the high east, then holding it and strengthening it given confluence aloft.  Prefrontal, most of New England will find the strongest wind late Tuesday night into Wednesday morning.  Of course, there is an inversion present at about 960 mb most of Tuesday night, which rises to about 925 mb Wednesday morning, but even given this limited mixing, gusts to 45 and 50 mph remain possible through the period owing to a strong low level jet in advance of the front produced by a pronounced pressure gradient.  As the front nears, the low level jet slides east with the pressure gradient, meaning that the actual frontal passage Wednesday afternoon should bring slackening though shifting wind to blow from the northwest.  I've been carrying a chance for thunder in the forecast with the frontal passage, though am tempted to drop it.  Total totals climb to near 50, which is impressive, but lift doesn't look sufficiently deep to reach cold enough air for multiple active ice nuclei for lightning generation, except perhaps near the immediate South Coast.  Also worth noting is the time of heaviest precipitation - likely to occur either side of 18Z in most of New England as the front settles through, though early in the morning Northern New England and late in the day South Coast/Cape Cod.  It's at this point that the front slows substantially as shortwave ridging in advance of the digging trough over the Southeastern U.S. combines with the longwave ridge holding steady across the Western Atlantic to stop the front dead in its tracks.  Of course, this sets up the pathway for the intense and moisture-loaded system pulling northward.

This system has been the cause of great forecast uncertainty in the medium range as the guidance has struggled with how to handle the shortwave energy when it rounds the base of the longwave trough.  Slowest and most pronounced in the 00Z runs from last evening was the GFS, and quickest was the GGEM (Canadian).  I looked at all available guidance before the early morning shows, and had 00Z Friday low centers ranging from Louisiana to Nova Scotia!  In these moments of great uncertainty, a forecaster must rely on what they know at the simplest level, and that's the upper level pattern.  The Westerlies seemed too fast to allow the upper low to both close off AND drift south of the jet stream so substantially, not to mention this is climatologically quite an anomaly to forecast.  Therefore, the GFS solution was discounted and weighted the ECMWF forecast slightly toward the GGEM and the bulk of its Ensemble members (ie: slightly farther east/north and slightly more progressive).  This seems to be the path worth taking, at least for now, as the 12Z guidance came around nicely, though the GFS is still too laggard in its northeast acceleration of the vorticity maximum.  It not only is not consistent with its own mid-level wind forecast and the confluent flow creating that increased wind, but lags behind its counterparts.  Therefore, still really don't trust the GFS solution in the 12Z run.  Instead, the ECMWF, GGEM, GGEM Ensemble members and to a large extent the NMM have come around nicely, and fall in decent agreement at the 500 mb level.

At first, this seems to have implications only in the "timing" of the storm.  A more in-depth critique of what this really means, though, is that there's more than just timing at stake.  Rather, a quicker ejection of the shortwave trough implies a quicker return to warm and moist advection across the stalled front and into Southern New England.  One has to ask: Does the precipitation ever really stop?  If the warm advection and isentropic lift begins sooner, this doesn't give the high time to build in like the GFS is trying to do, thereby keeping ascent near the front over Southern New England and meaning that, especially the farther south one is, precipitation doesn't really stop.  It does, however, begin to morph.  The cold air streaming southward from the high over the St. Lawrence Valley that strengthens to either side of 1030 mb will make inroads in the lower levels of the atmosphere.  With a failure to bring in colder air aloft, however, we're going back to the seemingly unimportant point of interest discussed at the start of this discussion - the NMM's desire to charge the front through in its earlier runs, and the meaning of that with relation to dense, cold air.  This dense, cold air should seep southward and hold its own interior of a coastal frontal boundary.  This would appear to be a rain/snow line, but the failure of deeper cold air to establish into Southern and even South-Central New England implies it will be freezing and not frozen precipitation for these interior locales - freezing rain.  Farther north, there's great agreement among the NMM and ECMWF that the 850 mb 0 C line runs from Casco Bay to Bennington, Vermont.  This, however, is not the rain/snow line, because of an above freezing layer indicated in the NMM thermal profiles between 700 and 800 mb that penetrates even farther northwest, which makes sense with the lack of a good mid-level cooling signature.  Having said that, I think the NMM is still slightly too far north and west with the 850 and 700 mb front as a result of pulling the 500 mb vort center just a touch too far inland, but it is in good agreement with both the ECMWF and the GGEM, and is close enough to use as a decent guide.  Even a slight adjustment southeast keeps most of these Central New England areas as sleet and holds the snow line from Rutland, VT, to the Northern Lakes/Mount Washington Valley of NH, to Rumford, ME.

If I'm right on this setup, we have an almost all rain scenario for most of coastal Southern New England (surprise, surprise), a substantial freezing rain scenario for interior Southern and South-Central New England, a substantial sleet event for much of Central New England including Concord, NH, and Portland, ME, and a one to two foot snow event for some of Northern New England, from Central/North Central VT to the Mount Washington Valley to the Maine Mountains.

Remember that those of you who read this blog get my raw thoughts - I wouldn't necessarily go on the air with a map of hard and fast predictions for exact locations of ptype and amounts, because there is still lingering uncertainty with the storm track, and the efficiency of precipitation production, though I expect the latter to be high with differential temperature advection underway - warm advection aloft and cold advection below - and the cold side of the mid-level front should form the necessary deformation band on the cold side of its location to verify the heavy snow band in the North.

So, meteorologists and weather-enthusiasts alike have our work cut out for us.  Watch the GFS to come in line and just a touch farther southeast than the current NMM solution, and both should be fairly close in guidance prior to the event.  The biggest difference will probably come with regard to surface temp and the location for the freezing line, but the north-northeast component of the surface wind thanks to ageostrophic flow will favor colder farther south, especially given what is a good cold air damming and cold air drainage surface pattern with the strengthening then maintaining anticyclone to our northeast.

I invite your thoughts in the comments section of this post!
Matt

December 3, 2008: Will anafrontal precipitation bring a white surprise later Thursday?

This is a quick post before bed - hey, I gotta get up early (!) - but here's something to consider that I thought of this morning, but didn't have a chance to sit down and post until this evening.  Looking back to our west, the cold front moving toward us finally has some prefrontal convection with it, but most of yesterday, overnight last night, and the first half of today (Wednesday) the precipitation was all falling behind the front.  Even as some rain showers develop ahead of the front as I write this, there's still snow falling anafrontal, or behind the front.  This raises an interesting thought that I ran by Danielle and Tim today, and the more I looked into it, the more I'm getting sold on it...and that idea is similar to the old Red Sox saying from the 2004 World Series: "Why not us?"  That is, why shouldn't New England also see snow behind the front tomorrow?  There are a few answers to that question, like the vorticity splits, the surface convergence splits and there is low ambient relative humidity.  But on the flip side, the frontal characteristics change little, and a close inspection of the model guidance shows that the models - and especially the NMM - are actually doing a good job of swinging the front through FIRST, then bringing the precip in.  So, if a meteorologist is surprised when more snow than rain falls later Thursday, shame on the meteorologist!  I actually fit into that shameful category until I really started to take a step back this morning and look at the radar, and then the 12Z run as it came in.  Something didn't line up - the precip was waaaayyyy too slow.  That's when it hit me.  So, a look at the 12Z NMM for someplace like Worcester, MA, shows only .03" in the first hour as rain, then .09" as snow, post-frontal.  Of course, then the 18Z run came in with only 1/1000th of an inch of rain, occurring exactly at the time of frontal passage.  So, how seriously do we really want to take this?  Well, there's no question the vorticity field is weak and split aloft, and the low level wind field is strongly downsloping behind the front as it blows out of the west.  Hence, the guidance is really keeping this a very light event.  The WRF is the most hefty with about .10" liquid equivalent, most falling as snow after 11 PM Thursday night.  Probably not a bad idea to put a chance of light snow/snow showers in the forecast for Thursday night with a dusting possible especially Worcester Hills north and west (falling earlier in Northern and Western New England - Thursday afternoon into eve, with some prefrontal possible even earlier), but this is something I will review in the 00Z and 06Z runs, prior to my 5 AM Thursday broadcast.

Farther out, the weekend scenario may be gaining clarity.  Good signals of a Norlun trough developing on Sunday, and perhaps the Southern coast of Maine will be targeted, which is a climatologically favored zone for such events.  Nonetheless, while that's the direction of the guidance right now - and the guidance is good enough at convective parameterization that it is picking up nicely on this potential right now (though one also must show concern for convective feedback overdoing the event) - let's still keep a close eye on the situation.  It won't take much to get the vort to amplify just a little quicker south of New England as it zips along, and make for a more interesting scenario, though the confluent middle and upper level flow over and just east of us really suggests it's Norlun or bust.  Let's keep an eye on it.

Noyesie

Tuesday, December 2: All's quiet on the home front, but for how long?

Wow...it's been quite awhile since I've been able to post here.  I appreciate you coming by and I apologize for the lack of posts lately.  I'm excited to sink our collective teeth into the pattern in this post, and...

Sunday night prospectus - November 16, 2008 - Winter will not wait

Last week's prospectus looked at the transition to a wintry pattern that was slated for this week, and the potential for a coastal storm development around the middle of this upcoming week.  The reality a week later is that we're ready for a trough that will continue to dig and amplify over the Eastern United States this week, dumping cold air across the Northeast.  The intense shortwave energy riding the fast westerlies will continue to drop over New England, and the aforementioned strong storm will indeed develop, but it appears just far enough offshore to preclude major storm development for New England.  This doesn't eliminate the chance for accumulating snow in the next seven days, however - rather, I think it will be very difficult for many areas NOT to accumulate snow between now and next Sunday's prospectus.

But let's start with the here and now, which features one installment of cold air to New England, with many more yet to come.  Westerly low level winds have instigated lake effect snow squalls, and these will carry east, enhanced by upslope flow against the western slopes of the Green Mountains tonight and Monday.  By later Monday, mean layer flow in the lowest several thousand feet of the atmosphere favors carrying some of the snow showers into the Berkshires, as well, though accumulation there should be limited, while the favored locales of the Southern Greens, on the other hand, could pick up a few inches by Monday night.

Also of interest is a pronounced wind shift dropping across the North Country Monday afternoon and evening, and the remainder of New England Monday night.  This shift in surface wind represents a cold front, providing a formidable shot of Canadian cold for Tuesday and the midweek.  The passage of this front in Northern New England Monday afternoon should bring some snow squalls - and a few of these will probably be intense in the Green Mountains.  Lifted index values are positive, but rise about 3 or 4 with the passage of the front, and the pressure rises 3 to 4 mb in the North Country after its passage, while there's plenty of mid-level instability.  These parameters appear to favor a weak windex event, though relative humidity parameters in the lowest levels are a bit dry for a decent event.  Nonetheless, orographic lift should make up for this in favored locales.

Tricky forecasting on Tuesday, though not for all parameters.  For example, temperature and wind seem pretty straight-forward with good cold advection underway.  What's trickier, though, is the precipitation forecast.  I hate looking at a situation like this, because I realize from a couple of days out that a big bust is possible right up to the event on Tuesday.  And the event is this:  The major storm we looked at as a potential last week will be coming together southeast of New England.  Though that's not a big deal for us in and of itself, there are problems here, because there's a ton of vorticity moving overhead, and how that vorticity interacts, combined with diffluence aloft and a mid-level speed max at 500 mb, as well as the approach of the trough axis at the jet stream level, creates a bulls-eye of upward vertical motion in all of the guidance products.  The tricky part is that there's not really a well defined surface feature to cling to here - perhaps a weak Norlun-type troughing that takes place, but it's really driven by a complex pattern aloft, and nailing down where that lift will take place is going to be very difficult.  Nailing down just how ripe the conditions will be for precipitation will be equally if not even more tricky, but so important given the temperatures through the column will support snow.  The NMM and GFS both do very little with this setup, but the higher resolution WRF and SUNYSB MM5 go to town with accumulating snow for Eastern MA - with the WRF starting in Southern ME then sweeping south down the coast, and the MM5 focusing on the South Shore and Cape Cod, where some ocean enhancement aids development of a snowburst.  At this point, I can see good evidence aloft to support such a snow burst, and following the guidance shows Eastern New England will be the closest to this burst, but further detail may remain difficult even right up to the event!

Wednesday looks to be the peak of the cold as the storm really deepens to our east, dragging down a shot of dense, cold air.  NMM temp forecasts of max temps in the teens high terrain north to lower or middle 30s south look reasonable and with the wind chill we can knock about another 10 degrees off.  Though the cold may relent a bit toward the end of the week, the longwave trough holds tough, and this raises eyebrows by next weekend.  The GFS and many of its Ensemble members are rather insistent upon a strong shortwave digging out of Central Canada, dropping south in the fast jet stream flow and amplifying over the Midwest, then shooting east across the Ohio Valley and south of New England.  This prompts coastal storm development in an airmass that would be cold enough for a mostly snow event except for areas warmed by a coastal front.  The timing would be later Saturday through Saturday night as a moderate to strong nor'easter.  Of course, we know the GFS is not the be all and end all of medium range forecasting, though good agreement among the Operational and Ensemble members certainly is of interest and warrants respect.  Those of you who've read my posts for a long time know that I always go with what the upper level pattern supports when there is uncertainty, especially in the medium range, and while the precipitation pattern on the GFS Ensembles may say a moderate to strong event, the 500 mb mean prog is not so supportive.  In fact, the GFS Ensemble mean 500 mb height fields look very similar to the ECMWF and especially the GGEM and GGEM Ensembles, which indicate some digging of the upstream shortwave, but also a lingering trough axis east of New England.  This keeps a faster and less amplified flow from the Ohio Valley through New England, meaning less deepening of the shortwave and resultant surface storm.  This is the solution I think should be followed for now, which actually can mean a colder scenario for more of New England, because the chance of a coastal front is much less with what essentially becomes a clipper going near or just south of the South Coast of New England, dropping a swath of accumulating snow on its northern side, which could drop anywhere between two and six inches on the way by.  Of course, at nearly a week out and with a changing and fast flow pattern, there's lots of room for change here, but I would favor the faster and less amplified solution for now.

That disturbance does reinforce the upper level trough over the Northeast, which means the cold continues coming on strong.  The Friday after Thanksgiving brings another strong shortwave worth watching, then by the first week of December, another strong shortwave coming off the Pacific will try to dig over the Plains and Midwest, and may bring another scenario that can be watched for coastal development.

Please do offer your thoughts in the comments section.

See you back here throughout the week.
Matt

Sunday evening prospectus, November 9, 2008 - A colder pattern ahead

A mild reprieve of the past few days will be a distant memory in short time as a well-defined Eastern United States trough will take hold.  In time, this may result in an accumulating snow event for much of New England by the middle of next week, but that's a long way out, and remember what this Sunday prospectus is - just my view of the weather that lay ahead from a relaxed, big-picture, Sunday spin-up perspective.  The week will bring the need to get down and dirty with the details, some of which will clarify and some of which will muddy the view, meaning it will be necessary to rely on this big-picture perspective for clarity.

The water vapor loop shows a very active continent this evening, with a couple of well defined shortwaves spinning east toward New England - one set to cross the region overnight Sunday night into Monday morning, and another likely to pass on Monday night.  Each pinwheels beneath a strong upper low spinning across extreme Southern Canada, and each will deliver slightly cooler air to New England.  Farther west, a strong upper low is evident moving across the Four Corners region of the Southwestern United States, while a series of strong upper level disturbances are racing into Western Canada from the Northern Pacific.  The Southwest U.S. low will dissipate but move over New England on Thursday as a vorticity lobe with warm and moist advection, and a substantial chunk of Aleutian Low energy will move through New England next weekend.

The short-term forecasting challenges will come from the most immediate shortwaves, and the cold air that follows them.  With cold air streaming east, lake effect snow will begin off of Lake Ontario, and though this often means only limited consequences for New England, indications are we should get a decent spray into Northern VT on Monday.  The challenge will be boundary layer temperatures, which appear to stay too warm for snow - at least for appreciable snow - in the valleys, including and especially the Champlain Valley.  The mountains, however, and the higher terrain from Montpelier north to Jay Peak, should find upslope flow with a west wind enhancing the lift, and cold enough air above about 2000 feet during the day Monday, then lowering to about 1000 feet Monday night, to produce accumulating snow.  Total amounts won't be blockbuster, for sure, but a few inches are likely in places like Jay Peak between Monday and Monday night.  Elsewhere, a westerly, downsloping flow will limit precipitation chances, keeping scattered instability produced showers the most likely precipitation form both Monday morning with the first vorticity axis, then again Monday night with the second.

Confluent mid and upper level flow takes over for midweek, ensuring subsidence and therefore surface high pressure will dominate.  Cool temperatures but dry conditions are expected.  Meanwhile, another impressive storm will take shape from the Northern Plains to the Upper Midwest, though it won't be anything like the last blockbuster blizzard of this past week, largely because an abundance of Gulf Moisture will remain untapped, with the southern stream shortwave remaining just a bit too far south and a bit too progressive to interact.  Instead, that southern energy rides into New England later Thursday, with quickly increasing clouds from Wednesday night into Thursday and a good chance of at least some rain developing in the southwestern half of New England by the time the day is out Thursday.  At first I was somewhat skeptical of this - and to be completely honest, I think this timing is one that still could change as we near it and the details become more clear - but the 850 mb warm advection is pronounced at the same time the vort max moves through, and this shortwave does have a history of Pacific moisture with some weak Gulf interaction, so some showers seem like the wise way to go for now.

Next weekend the trough associated with the Midwest energy really digs, and this ejects Gulf of Mexico moisture and energy northward so that after a break early Friday, heavy rain is likely Friday night into Saturday morning.  Rain is likely to exceed an inch for many spots, with some two or three inch amounts possible, at least given this cursory glance.  One thing that may alter this is if the system progresses a bit quicker, as the energy aloft will start piling east, and will shift the moisture axis east, too, by late morning or midday Saturday.  Thereafter, cold air surface and aloft means some drying but also some instability for scattered rain and snow showers as another shot of cold comes on.

What I find the most intriguing of this week's cursory glance, however, is the setup for midweek next week.  A series of strong northern shortwaves dig across the Upper Midwest and Great Lakes, and this not only keeps shots of cold air coming at the Northeastern corridor, but also gradually shifts the mean trough axis on the longwave pattern just a little farther west with time.  By next Wednesday, this takes a similar baroclinic setup to what we have coming up next weekend, but moves it slightly farther east and thereby introduces the possibility for at least weak coastal storm development with much of New England (interior) on the cold side of the system.  We're talking a week and a half out here, so we'll see what verifies, but the pattern does look favorable.  Additionally, the Ensembles are showing signatures I usually like to see to support such a scenario, including cold 850 mb temperatures (-4 C anomaly), baroclinicity along the coast and just east of  New England, and, though not nearly as important but still an important signal, a moderate swath of precipitation in the means, on the cold side of the storm.

So, plenty to watch - and what does seem likely is that the Northeast remains colder than normal at least until around Thanksgiving, while warmth will build in the west with time.

Matt

Quite a storm developing off the Mid-Atlantic coast...core expected to pass south of New England

The storm off the Mid-Atlantic coast has become very well defined this afternoon and continues to ramp up.  There are a few impressive features on this cyclone and I wanted to take a moment to contemplate them and work them out in a post this afternoon.

First, have you checked out the buoy observations on the western side of the storm circulation?  Wow!  58 mph gust just east of Hatteras, and 54 mph gust at the buoy east of Virginia Beach!  Seas correspond, registering at 14-18 feet with an 11 second period.  Meanwhile, winds near the center of the storm are much lighter, which corresponds well to an extratropical storm with a wind field removed from the center and driven primarily by pressure gradient forces, in this case the product of the bridged surface ridge from Nova Scotia over New England then down the Appalachian Mountains.  For now, the storm looks like it will remain extratropical in the next 12-18 hours, which is certainly supported by the dry air wrapping around the southern and eastern side of the storm and directly into its core.  This has suppressed any deep convective activity near the center, and instead an impressive swath of mostly stratiform rain has been thrust north and west of the circulation.  This is the band of rain that will affect New England in the coming 36 hours.

There appears to be decent agreement among the guidance on the storm reaching peak intensity tonight, well south of New England, and expanding its wind field far enough north to get Cape Cod and the Islands into an easterly wind with gusts to 40 mph - perhaps a few 45 mph gusts in the most exposed east-facing locales - then the wind field diminishes rather quickly on Thursday.  Meanwhile, aloft, the slug of warm and moist advection responsible for the rain outruns the surface/mid-level center (the storm becomes stacked) and therefore weakens as it drifts north and northeast in the steering flow on Thursday.  From Northern VT to far Nrn NH, little precip is expected, with heavier amounts farther south and east.  The ample tropical and Atlantic moisture feed, coupled with a mid-level baroclinic zone, will provide sufficient lift for as much as 2" of rain Wed Ngt and Thu in some of Eastern MA - esp on Cape Cod - with 1"-2" more widespread in Eastern New England.  Even along the Mid-Coast of Maine, though the rain may be weakening as it drifts up the coast and the Maine Turnpike on Thursday, a secondary max of precipitation may occur with a re-infusion of tropical moisture later Thursday and Thursday night.

This isn't quite the whole story on this storm, though, as there may be a few surprises with it Thursday.  Though the agreement among the guidance is to weaken the wind field, there is also remarkable agreement on transitioning the core from its present state of asymmetric warm core to a symmetric warm core storm later Thursday.  This is a big deal!  One of the traits we look for in a tropical transitioning storm is this symmetry and warm core.  Though the storm will be passing over water that's still 75 to 80 degrees over the Gulf Stream, it's unlikely to remain in such favorable conditions for long.  Nonetheless, later Thursday we may see a flare up of thunderstorms around the center with a renewal of wind, this time contracted near to the center.  Definitely something interesting to watch from a scientific perspective, and important for mariners and surfers, as well, as this may enhance and prolong waves on the southern waters/South Coast into Friday.

As for the sensible weather, the most successful viewpoint on the next few days is to view the airmasses as fluids - something we should always do in the field of meteorology, but an outlook that will really help to grasp the forecast for the coming days and latch onto what I think will be the right solution.  The bottom line is that we have one fluid from the Mississippi Valley to the Southeast US (warm air), another fluid from Eastern Canada and New England through Ontario (cool, but not cold air), and another over the Rockies (chilly).  It's interesting to note that the dewpoint of the air north of New England today is in the middle and upper 40s, so while it may be cooler than the stuff producing highs in the 70s and 80s over the Ohio Valley and Midwest, it's not a really cool or dry airmass, and this is part of the reason that, even with a northeast wind, I'm not going exceptionally cool for most of New England - not below the middle 50s - on Thursday, and by Friday the wind abates as a weak trough between the Upper Midwest low and our coastal low very sluggishly moves to and through New England from the southwest.  Of course, with very light winds either side of this trough as New England finds ourselves in a col between systems, clouds will be stubborn on Friday and low level differential advection will probably bring drizzle and areas of light rain, especially east ahead of the trough, which comes with weak warm/moist advection on the leading edge of the Southeast US air.  Saturday may start gray even though Central/Southern/Western areas are transitioning to the warmer side of the trough, but an increasing southwest wind ahead of the approaching occlusion/cold front will break the clouds up, and with warm air in place, at least Central and Southern areas will see enough breaks to warm considerably into the 60s.  By Sunday, cold advection is underway in installments, with the first installment not all that chilly, and the combination of drying for sunshine, and downsloping flow, resulting in a fair day with bubbling instability Cu.  Cool air continues building in for the start of next week.

Matt

Sunday night prospectus, October 19, 2008 - Doing a double take

Another Sunday night prospectus - the time when I sit down and look at the upcoming "big picture" in the forecast to have a foundation to build from as the week brings the need for attention to detail.

For those of you who've looked at the prognostics for this week, it's enough to do a double take for sure!  Aloft, we have a series of strong upper level lows near and over the Northeast, and this is going to keep things interesting.  In the short term, a formidable ocean storm develops Sunday night through Monday, stacked beneath a well defined upper low, but remains progressive and stays offshore as the upper low starts out positively tilted until it nears Nova Scotia Monday night, at which point it interacts with the fast Westerlies and gets a push out.

By that point, a new, strong shortwave is digging through the Central Great Lakes, and this one means business.  It's the combination of a piece of energy from the Aleutian Low by Alaska, and the polar vortex near the North Pole.  This shortwave amplifies as it nears the mean trough position over the Northeastern United States.  For those of you who've been keeping up with my day-to-day discussions on WeatherNewEngland.com, you've seen my thoughts on a colder-than-normal pattern with a chance for a significant precipitation event sneaking back in.  The first of this combination comes this week, as the amplifying shortwave becomes a closed upper low, transitioning from positive to neutral tilt as it moves from New England to Nova Scotia.  We should get a surface low developing in the cyclonic vorticity advection and upper level diffluence ahead of the vorticity maximum, and this will continue shooting east on Tuesday.  But the air aloft is so cold with the upper level low - to the tune of -27 to -28 C at 500 mb - that this provides strong mid-level instability, and a resultant inverted/Norlun trough, perhaps even with a surface low circulation along it!  This is important because, with a northeast wind behind the departing initial surface low, this means a moist several thousand feet in the lower levels of the atmosphere, and cold advection that brings widespread 850 mb temperatures below zero late Tuesday through Tuesday night and into Wednesday.  500 to 1000 mb thickness values are also below the critical 5400 dm values that would indicate snow for all of New England.

Of course, this time of the year it isn't all so simple.  Critical will be the lowest few thousand feet of the atmosphere, which may be moist and chilly on a northeast flow, but will have significant trouble cooling for anything but a cold, cold rain in most spots.  In the mountains of Maine and the Crown of Maine, however, a northeast flow means a surface wind directly from the center of the cold anticyclone east of James Bay, Canada.  At least from the big, Sunday night picture, that tells me it should be cold enough for snow in these northern and elevated areas of Maine Tuesday night into Wednesday morning, with the potential for the cold to make it into the higher terrain of the White Mountains of New Hampshire!!!  The hardest part of making this happen will be the surface/near ground cold advection, but in higher terrain this is always much easier to do.

Wednesday it looks like lots of junk lingers - low level moisture, chilly air, leftover convective bursts near a lingering inverted front as the upper low only slowly moves east.  Thereafter, the next huge high builds in with very chilly but dry conditions thru Friday.  There are divergent solutions for what happens with the deep upper level low from the nation's midsection by next weekend, but this all clearly points to a strengthening and retrograding longwave pattern.

You guys have any thoughts on this week?  Add 'em in the comments!

Matt