WINDY WEDNESDAY, SNOW THURSDAY, REPRIEVE FRIDAY AND SATURDAY, STRONG EAST COAST STORM SATURDAY NIGHT INTO SUNDAY - NOW TAKE A BREATH!

Note:  I've updated the discussion as of 9 AM Wednesday.  I'll be out at a charity appearance in the Lakes Region of New Hampshire through most of the day, and if needed, will update the discussion again late afternoon or evening.

Chatter around the weather world has taken off here in New England with the active weather pattern of late, and that means more of you checking in here, and more frequently.  For those of you who are first time visitors to the site, I invite you to peruse the archives, linked on the lower right side of the page, and those of you really into the technical and scientific side of weather should enjoy the technical discussions that appear beneath this summary.

For now, we're looking at a Wednesday that started with a lingering northern wintry mix that's been pressing south as dying rain showers through Southern New England, where they should clear the South Coast by midday.  These showers are representative of the same band of moisture that's been tapped out of the Pacific Ocean over the past several days, responsible for the major ice event of the Plains and Midwest, and responsible for anywhere for a couple of fresh inches of snow in most of Northern New England to as much as six inches of new snow in Northern Maine last evening and night.  Now, drier air is muscling its way southward out of Canada and forcing the moisture plume south of New England.  Winds will gust to as high as 40 or 45 mph across New England Wednesday afternoon as this new air moves in behind the showers, and as a strengthening high pressure center builds into Southeastern Canada and into New England.  Though one may assume colder air would come streaming in with the fast wind, that cold air actually will come in high in the sky first, meaning mountaintops and high terrain of Northern New England holds in the 30s Wednesday afternoon while the lower elevations and remainder of New England see afternoon temperatures into the 40s with a few 50+ readings at the South Coast.  Wednesday evening, deeper cold air succeeds at moving into New England as the barometer rises in response to the building high pressure cell heading our way, and temperatures will drop into the single digits north and teens elsewhere with an active evening wind diminishing as the night wears on.

Of course, there's been very little change to the overall pattern in the past several days.  A steady flow of Pacific and Gulf moisture has set up beneath the jet stream corridor of wind, dipping down around an upper level storm and associated trough in the Western United States, and rising back up above a ridge, or bump, in the jet stream over the Southeastern U.S.  This has set up a conflict of airmasses - cold from the north and warm from the south, with an ample supply of Pacific moisture, enhanced by Gulf of Mexico moisture...all components for storm development beneath the fast, storm-steering jet stream winds.

The jet stream also acts as an atmospheric thermostat with cold air to the north and warm to the south, and by Wednesday night, as the jet stream settles south over New England for a time, cold air will come rushing in and become firmly entrenched by Thursday morning.  At the same time, a major chunk of the intense disturbance responsible for carving the Western U.S. jet stream trough and setting up this active pattern will break loose, and become caught in the jet stream winds racing across the Central United States.  The result for New England, after a cold start Thursday morning, will be for increasing clouds quickly late morning to midday.  With such an intense slug of moisture on the way, the potential exists for an intense burst of several hours of snow later Thursday into Thursday night.  The biggest question as the slug of tropical moisture and more importantly warmth drives northward, is just how far north it will penetrate.  This surge of warmth and moisture seems destined to throw snow into Southern and Central New England on Thursday afternoon and evening from southwest to northeast, but the fast winds aloft will more favor an easterly track of the snow rather than a northward expansion.  By Thursday evening, it truly ends up a combination of both factors - the thrust of warmth and moisture seems likely to produce snow all the way into Northern New England, but the heaviest snow should fall from Connecticut to Rhode Island and Southeast Massachusetts, including the South Coast.  In these locales, five inch amounts should be common with a few six inch readings.  Farther north, where less moisture penetrates northward, less snow will fall with perhaps three or four inches several miles either side of the Massachusetts Turnpike and a couple of inches into Southern New Hampshire.

Behind Thursday evening's storm will come a moderated airmass for Friday - deep chill hanging back to our west but warmth ushered away to our east - and in this "no man's land" temperatures will climb to near 40 with sunshine breaking through.  This entire process of unloading Pacific energy and moisture will helps to break down some of the Southeastern U.S. ridge of warmth, and will allow the western jet stream trough to move eastward gradually, toward the Central United States.  This sets the Eastern Seaboard close but just east of the storm path, which means coastal storms will become the next features to watch carefully for.  An arctic front will drive through New England Friday night, and may be accompanied by a few squalls, bringing a wind shift that will usher in a northerly wind to open the floodgates to deeper Canadian cold that will be well established on Saturday.  At the same time, the aforementioned unloading of energy and moisture across the Eastern United States will result in the development of a storm center that will move across the Southeastern United States on Saturday, tapping the Gulf of Mexico and adding yet more moisture to an already moisture-laden storm, bogged down with Pacific moisture from the persistent jet stream flow of wind over the course of this week. In addition, what is now an uncommon but not unheard of December Tropical Storm "Olga," which has been moving west across the Dominican Republic, will continue moving west as a bundle of moisture and energy, and as the Gulf of Mexico opens on a southerly flow feeding into the developing storm, this may provide extra tropical moisture, warmth and energy.  This warmth and moisture all converges near the coastline at the same time cold air builds across New England - an excellent combination for storm development along the coastline.  The biggest question has been how close this storm will come to New England, and the trend in guidance unquestionably has been closer to the coast with a powerful northeaster that would lash New England with heavy snow Saturday night, perhaps changing to rain in Southeastern Massachusetts Sunday morning but continuing as heavy snow farther inland and north. From this far out, all of you reading this know there can be a lot that changes, but the fact that the jet stream winds aloft are favorable for a coastal storm, and that we have a confluence of moisture, energy and airmasses over the Eastern United States really leaves very little question as to whether or not a coastal storm will form, and even little question as to whether it will strengthen (it's bound to strengthen quickly with so many factors working in its favor), but rather the exact track the storm will take.  This is something I'll continue to focus on both in my on-air segments, in this discussion, and in the technical discussion below this weather summary in the coming several days, but, as mentioned here, there's good reason to believe in a strong shot of snow as long as the moisture makes it in here Saturday night into Sunday, colliding with the cold air in place.

Farther down the line, the pattern continues with a somewhat broad and flexible, changing trough over the Central and Western United States, capable of shifting and flexing toward the East Coast when strong disturbances ride through it, and such disturbances will continue into the last week of December, keeping the prospects for coastal storms high, and meaning I'll stay on my toes in the short, medium and long range forecasts for the remainder of the month, though a period of above normal temperatures is likely for most of next week. There's certainly plenty to watch!

Technical Discussion:  Due to public appearance mentioned at start of discussion, won't be able to update until late afternoon or evening, much like yesterday's which is posted below.

Matt

Matt's Technical Discussion - Updated Tuesday, December 11, 2007 at 5:00 PM

Will spare you a rehashing of yesterday's disc since it's available below and focus on the few issues for revision/revisit.

Tonight:  Warm air riding far enough north across the board in progs to bring raindrops for most of Central/Srn NewEng.  Snow Nrn NewEng where colder air hangs on and precip has arrived first there and will fill in farther south.  One area of concern for icing tonight is the Lakes Region of NH and the other is the valleys of Ern VT into the Upper Valley where sn/fzra mix should go to fzra for a time, then to plain rn as Sly flow strengthens.  Other area of brief icing will be elevated terrain of Central MA, where ORH still subfreezing as precip approaches.  QPF of .10"-.25" in general, but good agreement on enhancement up the length of the Greens, and this is part of the reason for the 3-5" and 6" snowfall forecasts in the higher terrain there.

Thursday:  The battle of the guidance products rolls on here with the foreign guidance finding a friend in the NMM solution to keep precip largely suppressed south, tho it appears the NMM has gone a bit overboard to this regard.  Canadian Ensembles and operational favor carrying snow into most of SNE Thu afternoon into Thu eve with heaviest amts along the SCoast where .20-.40" QPF is possible.  The GFS is wetter and while I'm open to the possibility of this scenario the confluent flow does tend to argue for a bit more southward suppression.  What's important, tho, is the relatively good agreement on a lobe of moderate to strong vorticity advection moving from west to east Thu afternoon and eve even tho the core of the vort moves S of NewEng coast.  So, while a well-developed low level circulation may not develop, I think we have to give credit to the power of dynamic forcing here, and that's why, although I do think things look less snowy and the frontogenetic forcing is pushed much farther south than my thoughts yesterday, we're still looking at a plowable snow potential later Thu in at least CT/RI/SE MA.  Whether I'm being led down the primrose path here remains to be seen, but I think it's realistic to admit the snow will not ride as far north as I originally thought and also give credence to the vort moving overhead without blindly following the QPF fields of varying models and hoping the dart lands in the right place.

Weekend:  I laid out the ingredients in the General Wx Summary and made a few statements about the degree of certainty with this system. The longwave pattern was discussed in yesterday's techie, and I think an important point to make in an analysis of the weekend system is that there should be no question *whether* a storm will develop, nor should there be any question about *whether* the storm will strengthen.  The answers to those items are absolutely and yes, quickly.  It simply seems quite unlikely for the storm not to develop when considering all of the factors, and when the factors are all weighed, it seems almost equally unlikely that the storm could form without deepening quickly. Record warmth has been building over the Southeastern United States and is available.  Cold air will build over the Northeast during our break in the action on Saturday.  The corridor of moisture - quite moist, at that - owing to the prolonged flow off the Tropical Pacific will be moving east as the upper level trough shifts east, and the associated energy will bring falling heights to the coastline, where baroclinicity remains strong off the Mid-Atlantic and Southeast coast.  The Gulf Moisture feed remains established and though it will wane somewhat the end of this week with a weak high moving over the Northern Gulf of Mexico, as that high shifts east and weakens and the cyclone begins to take shape over the Southern US, the Gulf reopens.  Not only do we tap the Gulf for moisture, but the developing southerly flow also ingests - and this is agreed upon in a number of guidance solutions - a piece of subtropical storm Olga.  If that seems far fetched, I'd encourage you to follow progs of 850 mb vorticity, representative of low level center with this system, or even a simpler prog of 700 mb RH.  Though it seems uncertain whether Olga will maintain her identity and defined circulation, she will still represent a westward moving tropical wave of moisture and energy regardless, and the amplifying longwave trough with its associated strengthening southerly jet ahead of it will shear Olga and pull in moisture and energy across the Gulf, ingesting it ahead of the developing circulation as it pushes toward the coastline Saturday.  This is important on a number of levels, both with respect to the latent heat it represents, and the additional moisture input that will feed into the circulation and be thrust northward as positive theta-e advection.  This will enhance the potential for warm advection precip well in advance of the storm center Saturday late day or evening.  Because of the clash of airmasses that takes place with the cold antecedent airmass in New England, it seems wise to expect snow to overspread New England either late Saturday or Saturday night.  Of course, what every weather fanatic wants to know is, can I bank on a blockbuster storm as advertised by so many of the operational models?! I'm all for a good storm and I certainly see the potential here, largely for the reasons I've just stated that play in the storm development.  The ocean baroclinicity will also beg for a closer-to-the-coast solution and a doozie of a storm for New England. But, I'm not willing to pull out the stops yet and sound the alarm on a blockbuster.  My thoughts on this come not so much from my realization of uncertainty at day 5 (though that is always at the forefront of my mind in a forecast that far out) but rather because the longwave pattern is still unpredictable.  My cause for just a bit of hesitancy here comes from the fact that the operational GFS is much more defined aloft than the Ensemble members, which are arguing for a low staying farther offshore and strengthening just a bit later, which would yield a swath of accumulating snow in the huge shield of warm advection around the center, but also would keep a monster center from running the New England coast.  The Canadian Ensemble members are coming around to a stronger solution and most definitely to a wetter solution, but the 500 mb forecasts among the members are still dispersed and the mean MSLP forecast is very similar to the GFS Ensembles, even in the 12Z cycle, keeping the center farther off.  I guess the moral of the story here is that while I am certainly intrigued by the operational guidance and find them not only exciting as a snowlover but also rather difficult to argue with as one by one they come into line - and that is powerful evidence over time when the trend brings agreement and the longwave pattern we know is favorable for a storm to develop along the East Coast - the part of me responsible for sounding the alarm or not, for advising folks of a blockbuster storm or not, says give the heads up that the potential is there but tread carefully this time until we see the uncertainty borne out in the longwave pattern.  In summary, would I be surprised to see a blockbuster New England storm with blizzard conditions, a powerful coastal front and pounding wind, precipitation and surf?  No.  Would I be willing to bet the farm on it?  Not yet.  Keep an eye on the 500 mb pattern and especially the Ensemble members.  Get more of them on board in the 00Z run overnight tonight and then it's hard to deny.

Matt

Monday's Discussion, 11:25 AM

In the short term I went below statistical guidance given predawn wind shift to the north behind weak cold front and developing wave of low pressure along it.  Cold air is strong in Nrn NewEng where Nrn ME was below zero this AM and with persistent light N wind thru Srn NewEng into the afternoon this means a continued cold air drainage keeping temps below freezing MA Turnpike corridor northward.  With ample low level moisture drizzle will continue into this cold airmass - freezing drizzle - and light icing will continue in Nrn MA/Srn NH/Srn VT, but drier air moving southward is already bringing sun out in NCountry spots and that drier air eventually translates S by late afternoon and evening.  More breaks in clouds follow overnight and clearing allows temps to drop, refreezing road moisture for black ice.

Tue AM sun to fade as clouds thicken ahead of next vort max. Longwave pattern similar to what I expected last week, but subtle diffs are important to the forecast.  More amplified trough/ridge combo west to southeast means jet stream not a due SW to NE flow, but rather kinked over the Midwest then charging E over the Great Lakes to NewEng.  That changes timing of disturbances and means more intense western energy hangs back longer.  Therefore, vort max ejecting our way Tue not representative of deepest moisture or dynamic forcing, but will be enuf to spread precip back into NewEng Tue afternoon.  With light SW flow around bubble high moving over SCoast, and with morning sun to work on temps starting in the 20s, highs should reach near 40 in Srn NewEng.  This, combined with warm advection, will mean mostly rain as precip moves in across CT/RI/SE MA.  Farther N, some evaporative cooling will take place for perhaps a mix of ra/sn over to sn and back to ra as 925 mb warm advection pushes 0 C line north toward the NH border.  It's the MA/NH/VT border towns points north where mostly snow will fall, though QPF is limited to .10-.25" and ratios won't be all that exceptional with lower level warmth, so a general 1"-3" seems prudent for these spots Tue late afternoon thru Tue night.

Active wind on Wed from the NW allows for cold advection but it happens aloft first, and with Richardson numbers indicating mixing from 850 mb to the surface, adiabatic warming will assist diabatic warming to boost temps into the lower 40s SE NewEng, with temps some 15-20 degrees colder in the higher terrain of Northern NewEng.  This is largely a reflection of the failure for surface pressures to rise dramatically, meaning the cold stays aloft for a time until surface high strengthening across the Eastern Lakes builds into NewEng, finally more firmly entrenching the cold to the surface Wed Ngt into the start of Thu.

Cold start Thu and then we turn attention to ejecting vort max and deep tropical Pacific and Gulf moisture moving NE and poised to cross NewEng Thu Ngt.  Ahead of the vort, rather intense warm and moist advection will spread across the Ohio Valley and toward NewEng, and this will only become more intense when it actually moves into NewEng and encounters the, by that point, deep cold air that will be in place.  This will result in a corridor of intense isentropic lift and moderate to strong dynamic lift as the vort and substantial positive theta-e advection both spread north.  Given the amount of warmth assoc with the tropical nature of the incoming airmass and disturbance, there will be a very tight baroclinic zone that develops ahead of this disturbance and for some spots - like NYC to the SCoast NewEng to parts of Srn NewEng - this will mean a burst of intense snow for a few to several hours before a change to rain.  Farther N, the 0 C line will never make it through, and that's where we'll find our maximized frontogenetic forcing that will produce very intense snows from not long after precip onset and then lasting several hours.  Seems like a shot of half a foot of snow a real potential, but given the amount of warm and moist advection I wouldn't be surprised to see higher amounts in the areas of frontogenesis, with the limiting factor being the relatively quick shot, which is unlikely to last more than 8-12 hours since this is largely a glorified wave on a warm front.  Technically, that's an incorrect analysis as this wave actually is a triple point low, but practically we're looking at a wave that develops along the leading edge of warmth and moisture and behind it the moderated air will be in place on Fri.  Cold air is slow to follow but builds in for the weekend before we see another vort come to the coast late in the weekend.  With the trof becoming a more broad and flexible creature, flexing when the stronger shortwaves come thru, this does keep the East Coast in a favorable position for coastals when a strong enuf short comes thru.  The question each time will be the track of each low, and whether enough downstream ridging will build ahead of each one to drive it close enuf to NewEng.  ECMWF is going nuts on this storm, allowing the surface low to become caught beneath an upper low that wraps a rapidly intensifying storm south of NewEng coast for a virtual blizzard Sunday.  Of course, this can be offset by the thus-far reliable Canadian Ensembles, keeping all moisture offshore of NewEng, and the GFS Ensembles which are a middle of the road scenario with a glancing blow of snow to Eastern NewEng.  At this point, it does appear as though the trough is so soft it actually allows height falls to translate east across the Eastern US on Sat with the shortwave, allowing enough cold air to keep coming into the Ern US to keep significant storm development offshore, but this is something to keep watch on and interesting 12Z GFS solution of an inverted/Norlun trof potential on Sunday with the storm staying offshore.

As mentioned in the General Wx summary, flexible trough remains set in Central US into the last week of December, meaning the chance for coastals remains decent with each strong shortwave, but abundance of above normal temps after the 17th and until the 21st to 25th means a better chance of rain than snow if anything does develop close to the coast in that period.  White Xmas lovers may end up down to the wire in Srn NewEng.

Matt