Our storm continues to evolve over New England - a wave of low pressure along a cold front that is focusing moisture tapped from the tropics and feeding off of the clash of cold and warm air on each side of the front. Ahead of the counter-clockwise circulation of the storm, gusty southerly winds transport mild and moist air northward, while northwest winds behind the storm push cool air south. Enough cold air will continue to move south to change rain to snow across Northern and Western New England before blustery conditions sweep across New England through the upcoming weekend with a chance of light snow in parts of Southern New England Saturday night into Sunday.
Thursday morning began windy and mild in Eastern New England and rain has been gradually advancing eastward through the day, with heavier elements of downpours from time to time. One batch of heavy rain focused in a relatively narrow band along a prefrontal trough (a disturbance ahead of the main front) while the primary band of rain moves much more gradually to the east associated with the actual cold front and enhanced significantly by the wave of low pressure rippling northward across New England and along the slow moving cold front. The heaviest rain has been focused along and just north of the cold frontal boundary in Northern New England, and from Central and Northern New Hampshire to Central and Northern Maine two to three inches of rain are likely to fall. Amounts of this nature are unlikely to cause widespread flooding of any type, but a few brooks and small rivers in the mountains of Maine may exceed bankfull Thursday evening or night. Steady rain will produce one to two inches of rain through the remainder of Northern and Western New England, while communities in the Southeastern quarter of New England will receive only between half an inch to an inch - still a good soaking but less than the remainder of the region - as the majority of the jet stream disturbance causing this rainfall will be moving past New England by the time the front pushes into these Southeastern areas. Although rain amounts will be less, the timing of rain in places like Providence and Boston is likely to come during the evening commute, making a tough go of it. Winds have been cranking ahead of the front and may gust to 45 or 50 mph in a few isolated spots on the Outer Cape and along the coast of Maine out of the south as warm air streams north ahead of the front and its wave of low pressure. Behind the front, winds aren't quite as strong but still have been gusting to 20 mph out of the northwest as cold air streams southward around the back of the storm. Rain has changed to snow in the higher terrain of Vermont as of this writing, and that snow level will drop both in elevation and in latitude as it drops southward late Thursday through Thursday night. Snow amounts will, of course, be highest in the higher terrain, with all spots above about 2000 feet picking up 6" of snow, but as for the remainder of the North Country, remember that elevation will be key in determining snowfall amounts with the valleys picking up the least:
Amounts certainly will be higher in those places above 2500 feet in elevation, and in the Northern Green Mountains where "upslope flow" will promote continued snow through Friday as the air coming in on a west and northwest wind is forced up the mountainslopes, and rising air favors cloud and precipitation production. As a result of this process, places like Jay Peak may pick up as much as 9 or 10 inches of snow!
The expanding counter-clockwise circulation will carry cool air southward across New England on Friday, as the storm center wraps north of Maine and leaves lingering periods of snow early in Northern Maine and the mountains of the North Country, but continues to exert its influence as it delivers a shot of significantly below normal temperatures for Friday. Aloft, the presence of cold air and the contrast with warmer air near the ground will allow for plenty of gray bottomed cumulus clouds through the day, blanketing the sky and yielding a few rain and snow showers throughout the day, scattered across New England but most predominant in the mountains, where areas of snow will continue. A break in the action is expected Friday night and most of Saturday, though it's important to keep in mind that winds will remain active near the surface and this means that while it will be cold Friday night, it won't be as cold as it could be if winds died down, bottoming out in the 20s for most communities. Later Saturday, another energetic disturbance aloft will dive southeast in the jet stream winds aloft, caught in the very large counter-clockwise flow around the storm that was strengthening as it passed New England on Thursday, now part of a large cyclone over Eastern Canada that actually will help to funnel this new disturbance southeast toward New England. Clouds will increase later Saturday ahead of this disturbance, and a period of light snow is possible late Saturday night into Sunday as the energy center dives over Southern New England. Right now it looks as though the disturbance will begin to intensify directly over New England, as it readies itself for further strengthening over the ocean waters east of New England. Such an evolution would bring an opportunity for accumulating snow in Southern New England Saturday night into Sunday, but not much unless the system can strengthen quicker than it currently looks like it will. The key for snow lovers would be the development of a well-defined circulation in the lower levels of the atmosphere that would allow an easterly flow to establish, pulling in ocean moisture. In this instance, it seems like that circulation will develop about one hundred and fifty miles to far east to bring more than a dusting or solid coating to Southern New England, but the guidance and forecasts on this storm will likely come into better agreement as the current storm moving across New England as of this writing evolves.
Overall, below normal temperatures will continue to prevail through the end of the month, though substantial oscillations will be found as warmer air bumps northward from time to time, ahead of each storm that develops to our west. When enough cold air is in place, such surges of warmth will have the potential to create swaths of snow and rain, with precipitation type dependent upon how deep the cold air is, and a pattern like this is also sure to bring strong cold fronts with each push of anomalously cold air, meaning plenty of wind a periods of blustery, much below normal conditions lasting a few days at a time for the latter half of November.
Have a great day!
Matt
Matt's Technical Discussion - Updated Thursday, November 15, 2007 at 1:40 PM
Rushing around for appearances this afternoon but want to get some thoughts/reasoning out. Snow already falling high terrain of VT but MWN is still 39/39 as of this writing. Tight baroclinic zone that won't collapse SE until the low center moves east and north. Therefore, even tho the changeover is underway in VT, don't expect a quick changeover for the remainder of Northern NewEng - it will take time to gradually ease the rain/snow line east. Additionally, don't expect quick lowering of freezing line because the air draining in from the north is not exceptionally cold, nor exceptionally dry, meaning we can get the airmass into the middle 30s but in order to get those last few degrees in the valleys it's going to have to come from aloft - in other words, we're going to have to cool the atmosphere dynamically and through only slow advection, and while diabatic cooling will help, that doesn't come for awhile so a very slow descent of rain/snow line for now and until tonight. By then, much of the moisture has been expended which is why I don't expect much in the Champlain Valley, but of course the Greens have the help of upslope flow to keep precip coming, in addition to the widespread precip shield that is already falling as snow. Same phenomenon will spread into Whites overnight Thu Ngt while mountains of Maine get some upslope but also benefit more heartily from the surge of precip still hanging around as cold air comes in. Ratios for snow to water should be about 6:1 in the first batch, but improve to 18:1 for upslope snow as colder air invades all levels, allowing for dendritic crystal growth and good low level aggregation, rather than the marginal boundary layer temps that keep the snow wet and sticky to start. Used these ratios in calculating amounts in accumulation map in General Wx Summary (BTW - those of you who read the General Wx Summary yesterday may have noticed I neglected to include the accum map, even tho I referenced it - arrrggggg....so frustrating when I realized it last night, so I at least went back to add the snowfall forecast description from home...sorry about that).
Sat night/Sun storm has great upper level energy and boy when it closes off it'll be a great storm with a good easterly component but it's looking more and more like it closes off too late. Interesting that among the Canadian Ensemble members, however, 12 call for a miss but 4 call for light snow, 3 for moderate and 4 for heavy Southern NewEng snow. Highly doubt it'll develop in time for more than a dusting to a coating but something to watch and also will be some subtle interests like how much cloud cover to hold onto as the back edge of the cyclone develops and may try to hold a NE flow for Cape and maybe more of Ern MA on Sunday.
Out of time for today...out the door for another gathering.
Monday's Discussion:
I tried to be as explicit as possible in the General Wx Summary today so will touch up some of the points that may need expanding. Showers tonight are a bit trickier than they appear on the sfc, and one simplified hint of this is the extension of rain along SCoast Tue AM - trending toward a later finish in recent guidance runs. This comes as vort max assoc with shortwave trough taking a bit longer to move east and enhanced by precursor vort responsible for sct afternoon convection over OH/Wrn PA. This vort rides ahead of the shortwave trof axis and enhances available moisture. Orographic enhancement to bump up QPF amounts in NCountry where .20-.25 melted should fall into subfreezing air from the Nrn Whites and high terrain thru Coos county and ME mountains/Nrn ME. Ratios won't be superb with about -5 C at 850 mb good for aggregation but not quite cold enuf in max omega for dendrites so about an 11-13:1 ratio looks good, which would yield a solid 2" snowfall in Nrn ME and the ME mountains with an elevation dependent coating to 2" stretched SW through the Srn ME mountains and into the White Mountains. Elsewhere, cold raindrops expected with boundary layer temps above freezing, tho if breaks in the clouds migrate far enuf S and E, central/Srn NH and Srn ME may see some pre-dawn freezing of moisture on roads for patches of black ice, tho above freezing temps all day Mon and most of night suggest that threat would only really be for some rural back roads and perhaps some walkways/stairs. As for duration of precip, the slowing of the trof axis behind the weak southern input Tue AM implies precip lingering along and near SCoast until mid to late AM with clouds quite stubborn in these locales until good NVA and subsidence whisks the moisture offshore. Thereafter, shot of cool air is shallow and limited and west-northwest/west wind ready to bleed milder air in. Not sure how quickly we can do that on tue with the quick shot of cool behind the front needing to be offset, so either side of 50 may be the most reasonable option, but my AM fcst went with lower to perhaps middle 50s most of Central/Srn NewEng due to temps in that range MN to UP of MI Sunday and moving east Monday. Wed certainly looks warm with increasing SW flow ahead of digging trough, and this vort max is going to be a doozie. Big question is whether we see consolidation of vort maxes at longwave trough base allowing for development of coastal wave early in the game to extend and enhance precip Thu, or whether this waits until E of ME to strengthen. Canadian waits but then absolutely bombs the storm sub 970 mb to bring big shot of cold dry air, tho ECMWF and GFS are not so bullish, and rather strengthen storm quicker but not as intense, soaking NewEng and bringing threat for accumulating snow Nrn VT, then bringing just enuf cold air south to put frontal boundary south of NewEng to await next shortwave later Sat that would warm advect into cold dome = widespread Sat/Sat ngt snow. The latter forecast depends upon the former - that is, we have to resolve Thu situation before we have an idea of which way to go for the weekend. At this point, the Canadian seems overdone, tho it's agreed with my almost all of its Ensemble members and the Ensembles have been performing well of late. Still, GFS and ECMWF have been consistent in their solutions, with the exception of the 00Z 11/12 GFS that was quicker with the front. At this point, prefer to stick with the consistent ECMWF solution of cranking a coastal low enuf for windswept rain on Thu in most of NewEng with cold air and potential accum snow in VT. It's important to note...that while this is my preference, I did NOT feel strongly enough about this system yet to highlight it more than a mention of rain with rain or snow Nrn VT in my on-air forecasts. This was largely because I have great respect for the Canadian ensembles and the fact that the GFS was starting to change its tune in the 00Z run. GFS has stuck to the fast front forecast, tho now 12Z Canadian is cranking out precip with the coastal hugging the Ern MA coastline. Bottom line is that we're going to see some more problems with the culprit vort max still over the Pacific, and we should get more models online one way or the other in the coming 24 hours and perhaps by the 00Z run tonight.
The comments offered up on the second half of the month come from a persistent and sharp Ern US trof. I think we all know that means we'll continue to watch for coastals and that behind every blast of anomalous cold comes the chance for warm advection snow, the questions will simply hinge on how deep the cold is, and how far south its well established. As the month draws on, the cold blasts will become more intense and the snow line will, of course, shift farther south. One storm potential comes the week of Thanksgiving, tho it's interesting that the best chance of solid precip may come in the first part of the week as blasts of warm advection erode a cold dome, then perhaps warm sector before another cold blast. This can favor some warm advection snow that usually won't stick around but can actually build a beneficial and solid base of heavier, wetter snow for ski resorts. We'll have to see how this plays out.
Matt
Updated discussion coming as soon as possible posted to the New England Weather Analysis Page. In the meantime, updating accumulation map for noon show and figured I'd post it ASAP:
Matt,
In your previous blog I stated a 40% chance of a Boston area white Xmas. After comparing Canadian / Arctic snow and ice cover maps of Nov 15 2004 to the present, I'm convinced there's no chanbce of a white Xmas locally! Nada!!! There is a noticeable lag with 2004 and the water temperature in MA Bay has held stubbornly at 50 F. Looks like this winter will be tamer than 04-05 but I still have hopes it will be less mild with more snow than last winter. Very persistent mild signal in recent years. Last official white Xmas locally: 1997. Ironically, that was a full-bore El Nino winter.
Posted by: Tom | November 16, 2007 at 02:31 AM