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

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