A weak cold front will continue to cross New England Wednesday afternoon and evening, bringing a slight wind shift from southwest to west, and a slightly cooler and drier airmass. The new air still won't be cold, however, and the region will recover back into the 40s and 50s on Thursday. Nonetheless, a storm with some snow likely looms for Friday.
Two low pressure centers are present in Southern Canada - one just north of Maine as of this writing and the other over Lake Superior. Both are associated with shortwaves (disturbances) that are traveling along the fast jet stream winds flowing near New England. With the jet stream nearby, there's not much impetus for a major change of airmasses, meaning no deep drive of air either north or south, but rather a wavering frontal boundary that will continue to see-saw a bit in the coming 48 to 72 hours, but generally will trend southward as a defining characteristic emerges onto the map - a strengthening high pressure dome that will build across Central and then Eastern Canada. The clockwise flow of air around this high pressure dome will force cold air southward behind a cold frontal passage on Thursday, associated with the second shortwave moving out of the Upper Great Lakes. Of course, the interaction of warm air to the south of New England and cool air to the north will continue to make for challenging forecasting. First, high temperatures Wednesday afternoon have warmed nicely owing to sunshine the first half of the day and an active west-southwest wind. That said, the cold front pushing gradually across New England from the northwest may be weak, but still is having a noticeable effect as it brings colder air both at the surface and a bit more impressively a couple of thousand feet above ground level, which has been the impetus for new cumulus cloud development above warm ground. This first front will settle south of New England where it will effectively die a slow and quiet death, but the cool air behind it will allow temperatures to drop below freezing in most New England communities overnight Wednesday night beneath mostly clear skies, though perhaps some clouds near the Canadian border ahead of the next shortwave.
The shortwave crossing New England on Thursday will straddle the Canadian border, but with a fast zonal flow there are so many shortwaves that another will ride toward the Southern half of New England later Thursday and with the approach of the surface cold front from the northern vorticity maximum (energy maximum), the surface convergence will help to focus afternoon showers in Southern New England - especially CT, RI and Southeast MA - while cold advection may touch off instability snow and rain showers in the North Country. In between, the most sun will be found tho clouds will increase with the confluence of disturbances as the day progresses. Of course, this front will be the main show, and as mentioned yesterday, I like the GFS depiction of driving the front thru New England to a position south of the South Coast by Thursday night into Friday. I support this scenario given the building anticyclone in Eastern Canada which will still be strengthening, and the propensity for a northeast wind to carry air southward effectively in New England. As discussed yesterday, this creates a path for the next surface low to travel on Friday, passing south of New England which will be important because of the ageostrophic flow that will maintain a northeast wind. For all of the moisture that plays into this system, it's become admittedly less impressive as we near it, largely because the timing as become quicker, owing largely to the southward surge of the cool air so effectively that the QPF and surface forecasts out of my preferred GFS model essentially supports one storm for the Southern half of New England on Friday and another for the Northern Mid-Atlantic with a Southern New England graze Friday night. But there's a huge problem with this line of thinking, which is that the 500 mb heights and vorticity (average temperature to abut 20,000 feet and energy at that level) prog doesn't support this. In fact, the GFS vorticity forecast takes the vorticity maximum directly over the MA/VT/NH border Friday night. Now, I can see how the front pushing south would separate the deep moisture southward from the core of the vorticity maximum, but I think trying to keep the precip almost entirely south of New England with the second round overnight Friday night is wrong. Whether this is owing to improper handling by the convective parameterization or an unfair weighting to other low level atmospheric conditions I don't know, but I do know that surface follows upper air pattern in a fast and strong jet stream flow, not the other way around. This cautions me against trusting in a miss Friday overnight, and therefore I'll keep a chance of precip in the forecast thru the overnight.
Precip type will be a challenge, of course, but rain/snow line should come pretty far south with good northeast cold advection drain. Have been approximating Central CT/RI/SE MA with a small zone of freezing rain north of that line and mostly snow farther north, but that is an estimate right now and is only based on my assumption that the historically difficult task of carrying a subfreezing boundary layer farther south than that will not be achieved. Where it will be mostly snow, I've been giving an early estimate of 2"-5", the lower end of which is close to a 10:1 ratio on forecasted QPF, and the upper range of which seems unlikely to verify at first glance. But keep in mind two things: 1) This is an early estimate and I am being tremendously honest about not just the possibility but rather the LIKELIHOOD that the forecast will be ammened, as is often the case in spring storms of vastly different airmasses so close together, and 2) If my hunch on the farther north track is right, these amounts *could* be vastly underdone when Friday night is figured in, so I figured this introduces the potential for plowable snow and leaves the door open for fine tuning.
Either way - a north or south track Friday night - this thing keeps moving and that means Saturday brings instability cumulus with scattered snow showers, especially in orographically favored areas. Cold surface air will be available from the anticyclone in Eastern Quebec and will do its thing. Sunday brings the ridge axis which means subsidence - equating to lighter wind, but cool conditions.
That's all for today.
Matt
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