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
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