MATTNOYES.NET - Weather Website & Blog

SEVERE WEATHER THREAT ACROSS NEW ENGLAND FOR THURSDAY AFTERNOON

This will be a relatively quick update with lots going on in soon-to-develop severe weather setup.  Bottom line is that New England has successfully transitioned into the warm sector, though the wind shift/warm front is, as of this writing, just north of Boston and located near Essex County.  I expect this boundary to lift north into Southeastern NH or extreme Southern ME, as per NMM forecast.  The result of dry air in the midlevels has been for plenty of sunshine and temps have spiked as a result.  This insolation, combined with dewpoints running in the middle to upper 60s, will push CAPE values to in excess of 2000 J/kg and perhaps even as high as 2500-3000 J/kg in some spots of Northeast MA/Southern NH where the most sun has been found, and surface based lifted indices may drop as low as a highly unstable -8.  This is, of course, a dangerous combination of a nearby warm front in a CAPE loaded location, and the result is a high EHI (energy helicity index) value.  The EHI represents the combination of available energy for convective (thunderstorm) development, and the helicity (tendency for air to form a helix as it rises - to spin) of the atmosphere.  Of course, a high number represents a combination of energy and spin that can result in sustained vigorous updrafts resulting in possible rotating supercells.  The low level helicity enhancement comes with strongly veering winds in the lowest 9000 feet of atmosphere, and this was a major part of my reasoning for mentioning possible isolated tornadoes in my morning forecasts both on NECN and on the Forecast Page here on my website.  The greatest threat for supercells and an isolated tornado will be in Northeast MA, Southeast NH and far Southern ME, closest to but just south of the warm front.  A wave of low pressure crossing New England provides additional backing of the surface flow.  The cold front marching east across New York State will slow as it moves through New England this evening and tonight, carrying thunderstorms with it from west to east and overturning a warm and muggy airmass to keep scattered thunder of diminishing intensity through the overnight.

Drier air for Friday but upper low nearby will spawn renewed convection in the North Country and the stalling cold frontal boundary over extreme southeast MA may return northwest just a bit before becoming stationary, and may spark an afternoon storm over the South Shore of MA.

That's all for now.
Matt

QUIETER WEATHER SETTLES IN FOR THE TIME BEING...PATTERN OF REPETITIVE SHOWERS AND THUNDER TO PERSIST FOR WEEKS

A weather pattern with a bit more sanity has settled into New England for the time-being.  Pulse convection (thunderstorms that grow strong in pulses, then weaken) have been ongoing in Central and Northern Maine this Tuesday afternoon, but the remainder of the area finds dry enough air and a weak enough trigger to prevent storms from developing in most spots.  The action in Maine is the result of a spoke of vorticity pivoting around the Eastern Canada upper low, and farther removed from the cyclonic vorticity advection there's only a weak surface trough in Central and Southern New England.  That weak trough is evidenced by a shift of surface wind from southwest over the coastal waters to west over land.  Though this trough provides enhanced surface convergence today, it will not be enough to initiate convection outside of the terrain-assisted locales.  Speaking of terrain assistance, the Greens, Berkshires and Whites may spark their own isolated convection, but that should be about the extent of any action farther south/west than ME.

Before we go much farther, it's worthwhile to note that this break in the action expected for the midweek comes as a strong upper level low digs into the Pacific Northwest, digging south as an extension of the mammoth upper low near the North Pole, north of Siberia, which has been a persistent feature.  Beneath that large upper low has not been phenomenally cold air for where it's placed, at such a high latitude, but the pools of cold air have been migrating southward, contributing to both the Eastern Pacific and East Coast troughs.  When an especially vigorous upper low digs deep enough across the Eastern Pacific, it can dig into the Pacific Northwest, which is what will bring the rain and wind to Eastern Washington and Oregon.  At the same time, this allows a brief eastward expansion of the large Western/Central U.S. ridge - or perhaps more accurately stated, a relaxing of the Eastern U.S. trough, because the jet stream shift comes more from the quick ejection of energy eastward from the Pacific NW upper low, rather than any act of expansion by the ridge itself.  In fact, it's the combination of the stubborn refusal by the Western ridge to break down and the lingering Eastern Canada/East Coast trough that encourages the upper low to migrate east across Hudson Bay by the first days of August, and back to what seems to be a seasonal semi-permanent home for upper level lows in Eastern Canada.  This also means the East Coast trough will dig again for the beginning and middle of August.

Lots of concern has also come from viewers with regard to the recent pattern of frequent thunderstorms and why such severe weather has been recurring.  Keep in mind what an active tornado season we had in the nation's midsection, then recall why that was.  It was the confluence of many types of air coming together - in fact, during that outbreak, it was almost every type of airmass possible.  Cold Canadian, cool Pacific, warm tropical Pacific, dry continental tropical out of Mexico, warm and moist tropical out of the Gulf of Mexico, all meeting up.  The driving force was frequent Canadian cold pools and associated upper level lows, digging deep into the nation as winter and spring cool air refused to abate.  It wasn't the warmth coming in that was so unusual - that's simply called summer - but rather, it was the clash with a lingering cool air that would not abate.  A few months ago in this discussion, I conjectured that an active severe weather pattern with occasional tornadoes would head toward New England for much of the summer as that Canadian cold would still refuse to abate, but the summer warmth absolutely HAS to move north with the increasing sun angle and warming of the earth's surface with the summer months.  Suffice to say, that not only has come to fruition but will continue into and through much of August, meaning the threat for recurring severe weather in New England and the entire Northeast remains high.  I'm quite certain the F2 tornado in New Hampshire, and F1 tornado at the South Coast, will not be the last vortices we see this season.  We've also looked in this discussion at the role the recent "sink" in New England may play with regard to the tropics.  That is, the persistent below normal heights along the Eastern Seaboard, sandwiched behind an Atlantic subtropical high that remains anomalously far north and quite expansive, puts New England in not only a favorable position for fronts to die overhead, leaving lots of boundaries for thunderstorms, but also puts New England in a veritable weather sink for tropical systems, too, as they will recurve around the western side of the subtropical Atlantic high, and that means timing of northern stream shortwaves will be the key to tropical strike potential this season.

Of course, there's still the day to day weather to contend with, as well, and while the next couple of days will be quiet, there are some challenges in the foreseeable future.  I expect cirrus clouds to start moving into New England from west to east Tuesday night into Wednesday morning, then thicken on Wednesday in advance of a northern stream vorticity maximum that will be dropping southeast across the Upper Great Lakes.  That shortwave will be responsible for Wednesday night/Thursday morning rain with a weak wave of low pressure moving east across New York State at a triple point.  The attendant warm front will lift across New England Wednesday night, with the trailing cold front Thursday afternoon/evening.  This does put New England in a precarious spot Thursday afternoon as morning rain will taper, sun probably will break out, and the atmosphere will destabilize, especially in Southern New England, ahead of the vort and the cold front.  The result may be a very quick rise in temperatures where the sun comes out, but strong thunderstorms Thursday afternoon.

The GFS has been somewhat behind the 8-ball for Friday, but is catching on in its 12Z run.  The ECMWF, GGEM, and both American and Canadian ensemble members have featured fairly good agreement on shifting the surface low east of New England on Friday, leaving a northwest flow.  It's not common for Southern New England to see thunderstorms on a northwest flow, even with a cold pool aloft.  There's also dry air forecasted to wrap into the system on Friday, so for now it seemed wise to keep Southern New England dry with scattered orographically assisted convection the farther north one goes.  That said, the cold pool aloft is about -10 to -15 C, which isn't frigid but is enough to warrant the scattered thunderstorm potential for the northern half of New England.

Broadscale warm advection doesn't really take place over New England until Sunday, which should leave a beauty on Saturday before thunder returns to the forecast later Sunday under the warm advection regime, though the best chance would come late Sunday when diurnal destabilization peaks, and warm advection peaks, as well.  Of course, this warm advection must lead to a warm day somewhere along the way, and it does appeaer as though deeper warmth will run at New England by Monday.  The question about just how far north the warmth makes it before being beaten back down by an active northwest flow under the belly of the upper low north of the St. Lawrence River mouth is an important one in the early week forecast next week, and the uncertainty surrounding this issue is what would keep me from biting hook, line and sinker into the upper 90s that the GFS advertises.  While that may be a possibility in Southern New England, it's far less of a likelihood in the North.

Matt

EXCLUSIVE IMAGES OF TORNADO TOUCHDOWN OVER WATER OFF BARRINGTON, RI - SEVERE WEATHER/ISOLATED TORNADO THREAT, FLOODING THREAT, STRONG WIND THREAT CONTINUES FOR THURSDAY

Yesterday's forecast and discussion is verifying well, from the isolated tornado threat to the areas of flooding and overnight severe weather threat, to the continuing flooding/tornado/severe weather threat today, not to mention the active wind that is still expected.  Therefore, no extensive discussion is needed here today (see yesterday's if you need an overview of the setup, including a map of predicted rainfall totals - that discussion appears below this one, or you can use the archives on the right side of the page).  In fact, if anything, the winds may be even stronger than the 50 mph gusts I was originally expecting, as the atmosphere will mix to about 900 mb (a couple thousand feet above ground level) where the wind is cranking at 50-55 knots today, so I'd expect at least a few 55+ mph gusts ahead of the main rain band.  This also means the tropical downpours and thunderstorms that continue to develop will have no problem mixing damaging gusts to the surface.

The discrete threat for today continues to be the threat for isolated tornadoes with a deep southeasterly flow.  More on this can be found in yesterday's discussion, but the big problem in a tropically saturated airmass is that you get low cloud bases, as the "lifted condensation level" - the altitude where air saturates and a cloud forms - is quite low in such a moist airmass.  This can be problematic because you have a strongly favorable environment for rotation, but with such a low cloud base, the only place rotation can occur is within the cloud.  As a result, you end up with brief, usually weak tornadoes that develop embedded in a rainband, or even in the base of a cloud, that can evade radar by riding beneath the beam.  Therefore, I'd encourage folks to keep checking the radar links on the right side of this page through the day to follow the rain and storms, but also be aware that stronger storms may contain some unwanted features.

No question yesterday was an active day, and the isolated tornado threat not only verified but was captured on camera, thanks to Christopher Legro.  I can't tell you how much it means that folks not only think of me after snapping photos like this, but take the time to pass the pictures along.  These are compelling images of a tornado over water off of Barrington Beach, RI, that touched down on the late afternoon of July 23.  This was the same storm that prompted a tornado warning deep into Southeastern Massachusetts, and for good reason!  A classic "waterspout" develops with cold air aloft, above much warmer water.  In this case, that wasn't the setup, and this is a bona fide tornado over water.  Awesome shots Chris - who reports the tornado was moving northeast toward Rumstick Point, where Chris reports it lifted just prior to moving over land.  The sequence is shown below.  Enjoy...and keep your cameras at the ready today.

The tornado developing:

Barrington, RI, tornado reaches maturity:

Tornado begins to "rope out," or dissipate, as approaching land:

TROPICAL RAINS TO BRING MODERATE AND PERHAPS MAJOR FLOODING FOR SOME, SEVERE WEATHER FOR OTHERS, POSSIBLE ISOLATED TORNADO THROUGH THURSDAY

An overview of the weather pattern driving this remarkable mid-summer storm was laid out in this discussion yesterday, featured below or via the archives link at right.  The period through Thursday evening will be quite active, and I encourage one and all to make frequent use of the radar links at the right side of this page.  It may also be worthwhile to sign up with NationalWeatherOnline.com before the event to post images/observations/analysis on your own during the event.

The bottom line is that the expected confluence of ingredients for a flooding rain and potential severe weather event are coming together, and will be meeting over New England.  Water vapor satellite imagery depicts a well-developed moisture feed from the Tropical Atlantic into New England, and a steady feed of moisture from the Gulf of Mexico into New York State.  This is moisture-loading the atmosphere in advance of the strong vort moving east across New York - quite impressive on water vapor and infrared satellite imagery, and responsible for vigorous dynamic lift ahead of it.  This vorticity maximum is in the process of closing off as it interacts with the primary upper low that has begun to fill over the Quebec/Ontario Provincial line, and this southern energy center will become the new upper low of primary focus.  The dynamically unstable ridge discussed here yesterday that is upstream from the low has already begun to break down, but has done its job in amplifying the shortwave, and the subsidence in the wake of this shortwave is rather remarkable, evidence of the recent strengthening that's taken place with the vorticity maximum.  With the slowing of the upper level low has come a slowing of the surface front, of course, and the area of greatest lift has slowed dramatically over PA and NY, with multiple lines of convection, but the tropical feed and instability over Long Island and Southern New England have helped to feed new bands of convection, and the initial surge of warm and moist advection continues to produce mostly stratiform rains farther north.

The most pressing concern with this event is undoubtedly the threat for significant flooding in Western New England, and particularly in Vermont.  The NMM guidance computes a remarkable 9"-10" of precipitation in Western MA and VT ending Thursday night.  The 8"+ area is rather large, and implies even higher amounts, which would be epic for these areas.  Substantial amounts like this are not unprecedented - the Great May floods of 2006 were one example, when an amazing 17" of rain was recorded on the North Shore of Massachusetts.  Many of you will remember the catastrophic flooding that followed.  There are some similarities to such epic New England flood events in the evolution of this most current situation, namely the presence of both a Bahamas/Tropics Atlantic feed and a Gulf of Mexico feed of moisture.  The differences are also apparent, however, including weaker surface convergence, and a somewhat more progressive system.  Therefore, I have trouble believing in a widespread 8"+ amount for all of Western New England, though I do think such amounts will be realized in orographically favored regions and communities.  Where this happens, flash flooding will be significant, with multiple road washouts and smaller rivers rising rapidly above bankful.  Depending on where such amounts are recorded, some evacuations are certainly possible in Vermont and Western Massachusetts along smaller, quicker responding rivers, though I expect that to be the exception rather than the rule, as it normally is.  6"+ would be a reasonable assessment for average rainfall amounts from Litchfield County, CT, through the Berkshires, then again in favored terrain of the Green Mountains, including Rutland and Addison Counties.  A widespread 3"-6" is expected in Western New England, decreasing the farther east one is, and I expect 1"-3" across Eastern New England, with less rainfall possible on Cape Cod (where most of Thursday may stay dry) and Eastern Maine (same).  The reason for decreasing rainfall amounts is the increasing speed of the front and the upper low later in the period as the band of tropical rainfall moves east.  All of this having been said, there will be substantial deviations from the average amounts.  A town like Concord, MA - not far northwest of Boston - is a great example of recent variation, with observer Dave Cook picking up nearly 4" of rain in the last 72 hours!  Spots that see heavier tropical downpours that record one or two inches in only an hour or so will surely exceed expected amounts by a couple of inches.  The ground across New England is wet, but nowhere is it as wet as Northern Vermont, where greater than 3" of rain in a 6 hour period will cause flash flooding, and with tropical convective elements this is a certainty in many locales in the Green Mountain State.  It's not just Vermont that's seen wet conditions lately, however.  Almost the entire area forecasted to see 3"+ of rain in the Western half of New England, and across the North Country, has flash flood guidance values at both 6 and 12 hours of 3", so essentially I am forecasting flooding for most of Northern and Western New England.  Considering the precipitation forecast is based on sound scientific reasoning, there seems little reason to question the results of the rainfall, meaning a moderate flood event in most of VT, and minor to moderate for susceptible locales in Central, Western and Northern NH, and Western ME.  Smaller rivers will be the quickest to rise in most instances, and while NH rivers like the Swift and Pemigewasset may see minor or moderate flooding, even greater concern comes along the Otter Creek in Rutland, or the Mad River and Winooski River, where major flooding may be achieved.  Larger rivers like the Connecticut and the Housatonic, also are vulnerable to minor or moderate flooding, but take longer to rise.

It should go without saying that street and urban flooding will occur wherever prolonged tropical downpours are observed.

The next threat is severe weather, which is a more vague threat, but a real one, nonetheless.  Instability is limited owing to limited insolation with plenty of cloud cover, but the situation is one that lends itself to substantial situational instability - that is, a deep flow of tropical warmth and moisture in the lower levels of the atmosphere, while the slowly migrating upper low to our west brings falling heights and therefore steepening mid and upper level lapse rates.  The northward progression of the surface low ensures the low level flow will remain backed out of the south and southeast, and the increasing pressure gradient between a cool eastern Canada high and the low to our west also will serve to increase the wind.  The backed flow is essential to increasing low level helicity, and I think it's quite telling that as I write this, almost every cell is depicted as mesoscale rotation on the radar algorithm on our SkyTracker radar, and I actually just paused to investigate a TVS (tornadic vortex signature) that was showing up as a possible developing tornado on the south coast of Connecticut.  An examination of the radial velocity plot showed weak enough winds inside the cell that I'm not immediately concerned, but there is rotation showing up in many showers and thunderstorms.  This is not atypical for a tropical moisture influx, and brief, shallow tornadoes often are recorded in such situations, especially with the rainband associated with frontal passage (forecasted to occur from west to east on Thursday) as the rotation occurs within the cloud, and the tropical nature of the air creates a low lifted condensation level (cloud base) that means brief tornadoes can skip across especially hilly terrain, often not detected by radar and known only by the damage they cause.  Considering the low level jet will only increase Wednesday night and throughout the day Thursday, this means low level helicity will also only be on the rise, and I think the mention of an isolated tornado for New England is quite warranted given the setup, and I mentioned that potential several times in today's broadcasts.  The biggest motivator for severe weather potential Wednesday late afternoon and evening is the northward nudging warm front from the South Coast of New England that will reach for Southern NH by Wednesday late evening and night, and this influx of warmth and moisture will continue to produce intense isentropic lift in the backed wind environment through the overnight, so some severe storms will continue to be possible, especially the farther west one is, closer to the dynamic lift.  Thursday will dislodge the heavy rain band from the west and slowly mark time eastward, carrying the severe weather threat for straight-line winds and for aforementioned potential isolated tornadoes.

Of concern on Thursday will also be the strengthening low level jet.  A tropical airmass is well mixed, and even with a southeast flow off the ocean, such an airmass will allow for the mixed layer reaching to 880 mb to carry wind to the surface, with wind gusts to 50 mph possible during the midday and afternoon.  Strong winds are likely to begin even earlier across Vermont, where overnight Wednesday night may bring an onset to the strong wind, gusting 60 or 65+ mph for a time Thursday morning/midday just prior to the onset of the front, and downed trees/lines are possible as a result.  This steady wind will churn the seas, as well, and mariners will face dangerous conditions for pleasure craft boaters throughout the day on Thursday.

After frontal passage Thursday night, drier air will move across New England and my thinking into the weekend has not changed from yesterday's discussion, except that there's now more evidence to back it up, as the previously wet GFS for Saturday has jumped on board with a drier solution.  This means Friday should be a mostly dry day with the follow up vorticity maximum and associated cold pool sliding across the far North Country with a late shower possible in far Northern New England, but less likely farther south owing to ambient low relative humidity air.  The atmosphere still looks like it will try to reload in the lower levels on Saturday, but also still looks like it will hold onto dry air aloft, fighting the showers and thunder that move out of New York State in association with the next vort, and leaving New England with an increasingly breezy and warm day prefrontal, but then bringing greatest lift into only a marginally favorable New England sky for thunder with the diurnal max for heating passed by the time the greatest forcing arrives, meaning scattered late day and evening showers and thunder still looks like the way to go for mostly Western and Northern New england, with an evening scattered shower/storm in eastern New England, and probably nothing in Southeast MA.  By Sunday, one stronger vort moves across Northern New England early, and another drops into the lakes late, and this should result in more widespread convective activity.  Behind the vort, cooler and drier air is likely on Monday.

Should be quite a storm in the next 36 hours.  Noticing flood watches being hoisted by the NWS for more of New England (eastern) as I put this finishing sentence on - fasten your seat belts.

Matt

FLOODING RAIN TO FALL ACROSS THE NORTHEAST - A MATTER OF MILES WILL MAKE THE DIFFERENCE OF WHO GETS HIT HARDEST

The longwave pattern across North America features a strong upper low digging across the Northeast Pacific Ocean, another digging into the Great Lakes out of Canada, and a narrow but amplified ridge between these two features that will be important to the forecast.  To the east of New England, the subtropical high remains quite strong, and anomalously north and westward displaced, which also may play a significant role in the forecast.  At the surface, the series of complicated wind shifts and boundaries - many of them fronts that have died over New England in the past three weeks - have been shuffled around a bit by the passage of Tropical Storm Cristobal, who is now approaching the Scotian Slopes.  Aside from periodic heavy rain on Nantucket, the only sensible effects of Cristobal have been increasing ocean swell in the southeast waters, and along south and east facing coastlines.

In the long term, the subtropical Atlantic ridge remains anomalously far north and stays strong, and there's fantastic agreement among nearly all guidance members of the Western U.S. ridge not only holding its own, but also strengthening through the beginning of August.  This locks in a hot and dry pattern for much of the Western United States, while ensuring the maintenance of the East Coast, and East Pacific troughs in a two-wave longwave pattern stretched from Pacific to Atlantic.

In the short-term, this seemingly simple longwave structure will result in a complicated and perhaps even dangerous weather scenario...especially in Western New England or Eastern New York, dependent upon where the battle zone of two very different airmasses sets up.

The passage of Tropical Storm Cristobal allowed a northeast wind to penetrate far across Southern New England, thereby ushering in a low-altitude marine layer, responsible for a gray Tuesday morning in many spots.  In Southeast and extreme Eastern MA, most under the influence of a northeast marine flow, it will exceptionally hard to burn off the low altitude stratus cloud deck that's in place, though mixing with somewhat drier air aloft should allow for breaks of sun by later in the day.  Elsewhere, the influence of the ocean is much more limited on a northeast flow, and mixing has been able to do a better job of poking holes in the clouds.  With the stationary front analyzed over the South Coast of New England early this morning, we've seen waves along the front, allowing it to move northward, and perhaps even take on some warm frontal characteristics.  That will result in varying amounts of cloud cover for New England, though the "warm sector" south of the front clears out pretty nicely, while the entire Northern half of New England is holding onto low level moisture, and thereby popping lots of cumulus.

The first of the big changes in the atmosphere comes overnight Tuesday night.  The aforementioned narrow, amplified ridge over Central Canada, extending from the Northern Plains to Hudson Bay, is a big player in driving these changes.  Referred to as a "dynamically unstable ridge," these southwest to northeast oriented ridges result in warm advection not only on the backside of the ridge, as would normally be expected, but also around the top and front side of the ridge.  This sets up a scenario of increased isentropic lift and favorability for upward vertical motion in the downstream storm - in this case, the upper low located near James Bay.  But with a strong vorticity maximum rotating under the belly of that upper level low, and careening into the Upper Great Lakes, it's likely to be this vort that becomes the biggest beneficiary of amplification, and therefore becomes the dominant upper level feature.  Of course, with an anomalously strong, and anomalously northward displaced subtropical Atlantic high, this sets up a stronger gradient than would normally be observed in such a situation.  The result is not only to increase the baroclinicity of the situation, but also to bring a new, cooler, drier Canadian airmass beneath the upper low at the same time the subtropical high pumps deep tropical warmth and moisture northward from the Bahamas.  This is evident not only aloft, but even more so in the lower levels of the atmosphere, with the persistent and strengthening southerly wind flow.  Of course, between the two, the pressure gradient also strengthens the southwest flow out of the Gulf of Mexico, providing yet another moisture source.  This situation of moisture streaming in from multiple tropical sources...plus the deep heat and humidity already in place across the Northeast and the Eastern Seaboard, primes the pump for heavy rainfall and establishes a situation in which all that's really necessary for heavy rain is an appropriate trigger mechanism.  In this case, lobes of vorticity rotating around the upper low will be the impetus aloft, and a slow moving cool front will be the trigger at the surface.  With the upper level winds flowing nearly parallel to the surface winds, downpours and thunderstorms that develop in New York State Tuesday night into Wednesday will slow, then stall, forming a band of persistent tropical rain and thunder.  Earlier this week, the indication was that Eastern New York State would bear the brunt of this heavy rain, though the trend slowly had been farther east, and as of this morning it seemed wise to raise an alarm for potential flooding in Western New England.  By the 12Z model cycle, the unquestionable trend among the NMM, GFS and Canadian Global has been back over Eastern New York State.  Forgetting about the QPF model guidance for a moment, I am a bit conflicted because there's the question of just how far ahead of the upper low the frontal boundary will stall.  To stall over Eastern New York State would be awfully close to the upper low, and it seems more natural to place the surface front over Western New England when looking at the 500 mb height field that's forecasted.  On the flip side, the pulse of the Atlantic ridge doesn't favor quick motion eastward, and the slowing trend for the upper low and the surface front makes sense.  At this point, it makes sense to bring scattered tropical downpours into Western New England tonight, keep them blossoming here on Wednesday but hold the steady and heavy rain band west, then carry it into Western New England Wednesday night, and swing it gradually east on Thursday.  This more sluggish scenario may preclude 6"+ rainfall amounts from falling in the Green Mountains as it seemed would happen earlier today, but a 3"-4" rainfall is still a real possibility in Vermont, with already saturated ground ready to produce flash flooding, and possibly river flooding by the time this event is over.  Farther east, a shot of 1"-3" seems likely with the lowest amounts farthest east as the band swings through later Thursday/Thursday Night/Friday.  Again, timing is still a bit up for grabs, but the trend is to be a bit on the slower side of things.

The passage of the cold front Friday will be followed by drier, less humid air, though upper level energy will remain aloft.  This means that once the subsidence behind the band of forced ascent passes, instability cumulus with showers/scattered thunder would be possible beneath the cold pool, but if the front is sluggish enough getting through and lingers into Friday morning, the atmosphere may not recover enough for new showers Friday afternoon, and the mid-level steepening of lapse rates may take place overnight Friday night, without much fanfare owing to limited surface heating.

By Saturday, drier air is in place and sunshine will start the day.  The fly in the weekend ointment is a moderate strength vorticity maximum passing overhead later in the day, which is actually riding ahead of another strong, energetic lobe diving across the Great Lakes, all part of the same gigantic Southern Canada pinwheel of energy.  The Saturday forecast hinges entirely upon timing the incoming disturbance, and accurately predicting the amount of moisture return through the atmosphere ahead of its arrival.  At this point, it seems a 30-36 hour window is in store between Thursday Night/Friday vorticity lobe, and the next in line.  With a developing southerly flow on Saturday, the atmosphere should be able to reload, but this timing also keeps the shortwave slow enough to keep most of Saturday a warm and increasingly humid start to the weekend with thunderstorms moving across Upstate NY during the day, then moving into Western New England Saturday late afternoon and evening, spreading farther east Saturday late evening or night but likely weakening in the process as the showers/storms will be encouraged by diffluent air aloft, but retarded by the dry ambient air aloft, even if the surface and boundary layer had a chance to reload a bit with the strengthening southerly flow.  Behind the shortwave, another shot of drier air for Sunday.

So, in the short term the biggest challenge by far is the placement of heavy rain axis, and the ramifications, which will be significant for Vermont.  At this point, it seems the already soggy Green Mountain State is looking at least at a minor flood event, with the potential for high end moderate if the axis of heaviest precipitation moves in earlier, and stalls longer.

That's all for today.  Please see previous discussion (accessed via archives on the sidebar of the page) to see my thoughts on New England's location as a place where fronts have been coming to die, as well as what this may mean for the rest of the summer and tropical season.

Matt

POSSIBLE TORNADO IN SOUTHEAST NEW HAMPSHIRE AT MAINE LINE

Radar images are posted on the Northeast Page NationalWeatherOnline.com...if you have images or reports, join the spotter network there and post your observations.

Matt

NSIDC: A DIFFERENT PATTERN OF SEA ICE RETREAT

The latest assessment is out from the National Snow and Ice Data Center.  Highlights include:

• Arctic sea ice extent on July 16 fell roughly between the extent for the same day in 2007 and the long-term average. The spatial pattern of summer ice loss has evolved differently from last year; this reflects the prevailing pattern of atmospheric circulation. Areas of low-concentration ice are also developing at unusually high latitudes.
• Arctic sea ice extent on July 16 stood at 8.91 million square kilometers (3.44 square miles). While extent was below the 1979 to 2000 average of 9.91 square kilometers (3.83 million square miles), it was 1.05 million square kilometers (0.41 million square miles) above the value for July 16, 2007 (see Figures 1 and 2).
• How is this different from what we saw in the record-breaking year 2007? In early July 2007, an atmospheric pattern developed that featured high pressure over the Beaufort Sea. This pattern promoted especially strong sea ice loss. The pattern that has dominated the summer of 2008, so far, seems less favorable for ice loss. However, the melt season has a long way to go. Furthermore, as discussed above, large areas of the pack ice with fairly low concentrations are likely to melt out soon.

The complete report can be found by clicking here.

NOAA: EIGHTH WARMEST JUNE ON RECORD FOR GLOBE...D'ALEO: NOT SO FAST, NOAA

The National Oceanic and Atmospheric Administration released the following:

The combined average global land and ocean surface temperatures for June 2008 ranked eighth warmest for June since worldwide records began in 1880, according to an analysis by NOAA’s National  Climatic Data  Center in Asheville, N.C.  Also, globally it was the ninth warmest January – June period on record.

Climatologist Joe D'Aleo, responds:  DON’T BELIEVE A WORD OF IT. Just a few days ago, the University of Alabama, Huntsville came out with their global assessment and they reported the 22nd warmest in the 30 years of records in their data base (in other words the 9th coldest).

Who is right?  Continue reading to get both perspectives...

MUGGY AIR GRADUALLY RETURNS INTO FRIDAY AND THE WEEKEND...NEW ENGLAND'S WEATHER SINK MAY PROVE PRECARIOUS IN AN ACTIVE TROPICAL SEASON

The weather map across New England is becoming increasingly complicated this week, even if we're seeing a stretch of quiet and fantastic conditions.  First, we've seen the slow moving Tropical Storm Bertha, now nudging away from Bermuda, generating swell for our coastal waters.  This swell is diminishing today, but may increase again on Friday as four to five footers may return on an easterly/southeasterly trajectory.  Second, the front that moved into New England this past weekend and only slowly nudged through us on Monday is still sitting offshore.  Though there's been a decreasing airmass difference (frontolysis - the death of a front) either side of this boundary, a wind shift and slight airmass difference still exists, so I still analyze this as a stationary front running all the way from the west coast of Florida, over the Outer Banks, just east of Cape Cod, and then into Eastern Maine.  Third, a shortwave riding just north of the Canadian border yesterday and last evening pushed a cold front south into New England, and this front has also been slowing over our region.  Becoming a west to east stationary front, the slight enhancement of surface convergence along the front raises the potential for an isolated shower or thunderstorm this afternoon.  Additionally, sea breeze convergence will team with a weak trough extending from Southern New England to New Jersey...another weak but present feature that will persist into Friday.

What all of this complexity means is yet to be determined by nature, but it would seem we have a series of boundaries to contend with that could serve as convective initiators.  Of course, to make anything of that, we need both instability and lift.  With the jet stream lifting north, dynamic lift is somewhat limited, but a series of shortwaves will still pass close enough to, and occasionally over, New England to result in sufficient lift for some convection in the coming days. Thursday's shortwave moves across Northern New England, but in these Northern communities the dewpoint has lowered thanks to the slightly cooler and drier air behind the stalling cold front.  Close to the front as it becomes stationary, however, enough surface moisture lingers for CAPE to climb near or just over 1000 J/kg (lower than progged by the overly moist NMM), and with LIs running below zero in many spots, the enhanced surface convergence associated with the stationary front may touch off an isolated shower or thunderstorm in Central New England - somewhere between Central VT/NH and Northern MA into Southern ME.  Overall coverage will be quite limited given the isolated nature of any convection that manages to develop.  Most of this action will diminish after sundown with the departure of the vorticity maximum and waning daytime heating.  There's a decent chance that showers and perhaps some embedded thunder will fire up again Thursday overnight, though - especially after midnight - as another shortwave crosses New England.  This shortwave moves through in a warm air advection regime, which means isentropic lift will team with dynamic lift, and the most likely region for sufficient lift to generate precipitation will be in Northern New England, where clouds and a few showers may linger into early Friday.  Drier air moves in along with anticyclonic vorticity advection Friday morning, however, and this brings sun back out in almost all areas with a new, high theta-e airmass in place.  Dewpoints will run near 70 while temperatures soar into the 90s, creating heat index values in the middle to upper 90s Friday afternoon!  Of course, with such heat and humidity will come increased instability, with CAPE values climbing to between 1000 and as high as 2500 J/kg, very little convective inhibition, and lifted indices dropping to between -3 and -6 Friday afternoon.  There will be at least one shortwave embedded in the zonal westerlies along the Canadian border, and though the vorticity maximum straddles the border, so much instability will be present that it won't take much of a trigger to generate storms farther south.  This is actually something I've given more thought to while writing this discussion than I did when making my morning on-air forecasts, so I may end up hitting the convective potential a bit harder in the midday shows on NECN for Friday afternoon.  At issue here is that, while there is a lack of trigger in Southern New England on Friday, there are all the boundaries I laid out at the start of this discussion.  That's an awful lot of surface convergence to simply turn a blind eye to and dispel the threat for thunder from the forecast outside of the area of cyclonic vorticity advection, so I think it's wisest to keep the best chance for scattered thunder in Northern New England, but maintain a chance for scattered convection in Central and Southern New England during the afternoon, too.

The weekend:  The overall weather pattern this weekend doesn't change much, keeping a fast and relatively zonal jet stream flow over the Northeastern U.S. and Southern Canada.  Timing of shortwaves will be essential to the forecast, but I've been trying to convey the fact that the weekend, overall, should be decent for most spots.  Saturday in particular looks good, though I think it would be foolish to keep showers and thunder out of the forecast.  Fast flow aloft will carry at least week vorticity maximums overhead, including one during the afternoon, and while it is weak, the dynamic lift it provides may be just enough to take the 1000+ J/kg CAPE and turn it into scattered thunder along leftover boundaries both mentioned at the start of this discussion, and also resulting from any Friday convection.  The next essential question on shortwave timing comes as to how quickly a follow-up shortwave ripples across New England Saturday night/Sunday. With such fast and active flow, the truth is that there are likely to be a number of vorticity maximums and it seems silly to try and nail down exactly when each one will move through and what it will bring, except to say that moisture continues to load into the atmosphere throughout Saturday, Saturday night and Sunday across Central and Southern New England, and with a slowing/stalling front in Northern New England this weekend, this will allow for additional pooling of moisture and increased dewpoints, meaning that by Sunday we should be able to trigger convection very easily, so more numerous thunderstorms seem likely.  Still, as is often the case in a tropically enhanced atmosphere, the day will not be a loss, and most of it should be salvageable.

Monday and beyond:  We have an interesting conundrum here because we're working on a number of assumptions that I've made, and it's important to keep that in mind.  One or more incorrect assumptions makes a lot of the forecast wrong.  But, consider one of the assumptions made is that we still have a ghost of a stationary front off the Eastern Seaboard right now that I contend still IS a stationary front, and even has been focusing convection farther south along the front the last couple of days.  Does that front ever really disappear?  It will continue to experience frontolysis, but the convergence band very well may linger.  This is important because we're looking at a surface low sitting off the Georgia coastline that continues to show excellent closed circulation, but has thus far failed to develop deep convection, especially after its trek across the Florida peninsula yesterday.  That system is over warm waters off the Southeast Coast, now, and will be approaching the Gulf Stream as it drifts northeast.  If the surface convergent zone still exists through the weekend, there is the potential for these two to link up.  Bottom line here, though, from a forecasting perspective is that even if this thing doesn't bodily come north - which is still a distinct possibility - the moisture associated with it seems destined to.  With conditions aloft only marginally favorable for development presently, it may never really gets deep convection that can persist, it'll really just be a low-level circulation and tropical moisture infusion that will touch off relatively low-topped tropical convection as it meanders north, but this still can serve as input for New England tropical convective clusters by Monday, so I'd expect numerous downpours and thunder amidst breaks of sun on Monday as an early estimate in an uncertain pattern.

The pattern thereafter is really somewhat monotonous, but exciting nonetheless.  That is, with a lingering trough for the Northeast, but rising heights overall through the nation (and here in the northeast - the trough just remains as relatively low heights compared to the rest of the Lower 48), there will be more warmth and moisture to work with. Meanwhile, the central Atlantic seasonal subtropical high will be flexing, and doing so a bit farther north than normal.  In fact, the northward displacement of the subtropical high may prove very important to New England for a long time to come.  In the one to two week range - through the start of August - this means a southerly low level flow continues while numerous northern stream shortwaves continue to maintain a trough in the Northeast.  The result should be numerous severe convective events in the coming weeks for New England - especially Northern New England - with plenty of heat and humidity between them, even if beaten down for a day or two at a time behind the stronger of the shortwaves. 

In the longer term, though, there are real implications for a pattern like this and it should give one great pause to think with regard for the hurricane threat along the Eastern Seaboard this season.  Consider what we saw in the last week on a big picture scale - a storm named Bertha that had developing just off the African coastline made a relatively early-season jog WNW and then recurved over the ocean, passing near Bermuda.  That all made sense and fit the atmospheric pattern, right?  When Bertha emerged off the African coast, here in New England we were immersed in a trough and a fast flow aloft that showed no sign of weakening considerably, and a run to New England would have required some tremendous timing that was relatively impossible with the fast flow and numerous shortwaves digging across the East Coast.  So, Bertha reaches a weakness in the subtropical high that was induced by the strong shortwaves and recurves toward Bermuda.  What happened next posed tremendous challenges for hurricane forecasters and drove up hurricane forecast error significantly as Bertha meandered near Bermuda.  But what was going on with Bertha, I contend, is that she was feeling the first effects of an important atmospheric shift.  The jet stream retreated northward, the subtropical ridge flexed, and Bertha was stuck in a no-man's land.  What started as a clear path to recurvature rapidly changed with the de-amplifying trough and the expanding ridge.  Of course, the upper level low east of Bertha also was wedged in this pattern, but served to keep Bertha in check and prohibited an eastward drift, keeping her sitting off the coast of Bermuda for days.  So now we apply that to the big picture and to a forecast that pulls the westerlies farther north over the Atlantic, weakens them considerably, but keeps a trough defined in the Northeast.  Additionally, we see the expansion of a ridge across the Central and Western United States, leaving a weakness in the height field across the East Coast, with a relative sink over the Northeast. Finally, with the subtropical high setting up anomalously farther north that we'd expect, both surface and aloft, this has to give one pause to stop and ask if the implications here are that New England will serve as a sink for numerous Atlantic systems this season.  Not only does the described scenario favor Bertha look-alikes running WNW before recurving - but farther west as the subtropical ridge expands - but also favors a continuation of the very type of situation we're seeing played out in real time as I write this - not model forecasted, but actually observed - and that's a situation of frontolysis in the Northeast and along the eastern seaboard.  When you stop to think about the pattern, an area of frontolysis is caused by a number of boundaries and airmasses converging to one place where they enter a virtual no-man's-land of week steering flow and/or shifting wind patterns with height.  This sink so far has caught a front last weekend that still sits off of our coastline, a new front that came in last night and will die over us, a trough from New England to New Jersey that is the remnant of yet another long-lost frontal boundary, yet another cold front that will drop in this weekend before stalling and washing out, and perhaps a low level tropical origin circulation traveling up the first front out of the Southeast US waters.  If this is the pattern for the season to come, New England may be looking at not one, but a number of tropical interests.  It's a pattern that I will remain quite alert and to and aware of in the weeks to come.

Matt

NOAA: NOAA AND LOUISIANA SCIENTISTS PREDICT LARGEST GULF OF MEXICO "DEAD ZONE" ON RECORD THIS SUMMER

Direct from the National Oceanic and Atmospheric Administration:

NOAA-supported  scientists from the Louisiana Universities Marine Consortium and Louisiana State University are forecasting that the “dead zone” off the coast of Louisiana and Texas in the Gulf of Mexico this summer could be the largest on record.