It's the combination of a dull pattern for New England, but a complicated and intriguing pattern from a wide view. The interest comes from two strong upper level lows - one slowly spiraling toward the Southeast US coast from the Tennessee River Valley, and the other moving into the West Coast from the Pacific. The first of these will slow the progression of the pattern as it results in a strongly positively tilted trough (oriented from northeast to southwest) off the Eastern Seaboard. This is especially true on the southern end of the trough, while the proximity to the Westerlies will keep the northern end a bit more progressive. The remainder of the systems upstream follow suit, with the northern stream outrunning the southern stream, but interaction between them continuing in a trough-ridge-trough pattern over the contiguous 48 states. All of this will bring a pattern that will leave pockets of cool and disturbed weather near each upper level low that becomes cut off from the jet stream flow, which lifts north into Canada. This also means mild air will ride over and around - but not through - the pockets of cool air associated with each upper level storm.
As for what this means at the surface, it means if you're under one of the upper level cold pools - upper level lows - then you'll find lots of clouds and cold convective bursts. Away from the heart of these cold pools, conditions will be far better. As for New England, we're north of the first upper level low, and high pressure centered over the Upper Midwest features an eastward extension over Southeastern Canada that is expanding slowly into New England. Aloft, however, a shortwave dropping slowly south - and really heralding the beginning of the longwave trough axis - is bringing enough cold air aloft and resultant instability to breed plentiful cumulus clouds in Northern New England, and scattered to at times broken cumulus coverage elsewhere. This trough axis will move through New England Monday night, bringing clearing skies not only in the subsidence behind it, but equally owed to the absence of diurnal heating and therefore nocturnal stabilization of the lowest few thousand feet of the atmosphere. With low dewpoints in the 20s and teens overnight with some decoupling in deeper valleys away from the light NW wind, temsp will drop correspondingly. Of course, the addition of sunshine on Tuesday will bring quickly rebounding temps, and with a slight but constant modification of the airmass by a couple of degrees Celsius at 925 mb, this should result in almost the same direct temperature rise at the surface with another day of deep mixing to 850 mb, much like Monday. Warming continues for midweek as winds shift to blow from the west.
The next challenge comes for Thursday, as a series of surface low pressures continue to develop beneath the sluggish upper low meandering off the Atlantic coast. Though the first few eject east with little impact, Wednesday night and Thursday's surface storm starts southeast of the upper low, then gets wrapped northwest until it is nearly stacked beneath the upper low. There are a few reasons to pay close attention to this system, and expect more significant development from it than the guidance is initially indicating. The first is the relatively tight surface pressure gradient around it, with highs on either side and the understanding that air will flow from high to low pressure, meaning there will be excellent convergence to the storm center, giving it validity in the lower levels. Aloft, the ridge getting pinched over New England and oriented from northeast to southwest around the upper low has a name when it takes on such an appearance - in meteorology it's called a dynamically unstable ridge. This basically implies that while it's a ridge that typically would favor stability and subsidence, this particular setup features warm advection that starts on the west side of the ridge, but then wraps around the north side, and dumps southward. This feeds warm advection to the developing storm from both the north and the south, and is the mechanism by which some large ocean storms can develop. The relatively gradual advection around this particular dynamically unstable ridge is likely to limit the storm's potential for development relative to other systems developing beneath one of these ridges, but the storm should still feature and expanding wind field and precipitation shield with banding features around a well defined center come Thursday. So what does this mean for New England? Right now it means we sit on the fence, with some of New England likely to find a beautiful Thursday beneath the unstable ridge, and then there will be a point - be it in Eastern MA or just offshore - where the weather will instead feature a low altitude cloud deck and northeast wind with rain or showers. Right now, I'm keeping the chance of showers in the forecast for Eastern MA - with the greatest chance over Cape Cod - for Thursday, as I'm aware of the potential for the storm to develop in a bit more pronounced fashion than is currently being advertised on guidance products.
By Friday, this upper low finally starts to move as it's caught by a northern stream shortwave and is absorbed by the Westerlies, and while I'm not convinced we can get the approaching sliver of deeper warmth all the way to the coastline depending on the speed of departure, I do think most of New England will crack 70 on Friday. What about the next upper low? This has signs of another cutoff, so no making any weekend weather promises...
Matt
Matt,
What can I say? Your forecast like many others underscores, "So much for global cooling"
Posted by: Tom | Monday, April 14, 2008 at 03:58 PM