General Weather Summary has been posted on WeatherNewEngland.com and can be linked to by following the General Weather Summary link at the top of this page.
Moisture loaded area of low-pressure moving northeast across the Midwest as of this writing is a 1005 mb low over Southern Illinois. After producing numerous severe thunderstorm and tornado reports ahead of the low last evening, heaviest precipitation for the time being has shifted to the north side of the circulation, on the cool side of a mostly stationary front that runs through Ohio and eventually off the Mid-Atlantic coast. I say "mostly stationary" because in the increasing pressure gradient squeeze between a bubble high over the Mid-coast of Maine, and the wave of low pressure over the Midwest (mostly driven by the high), a southerly wind has forced warmth and mositure northward into Pennsylvania and New York, including Syracuse, resulting in a tongue of high theta-e air. The leading edge to the warm and moist advection has prompted thunderstorm development in Upstate New York as the dynamic lift ahead of the developing shortwave moves east and combines with 1000-2000 J/kg of CAPE. In NewEng, after yesterday's few breaks of sun I expected the same if not a few more regionwide today, and while that's the case in Northern/Western areas, I think an objective analysis would find breaks equally sparse if not more so in Southern New England. This has been the product of low-level warm advection already underway, further solidifying the cap and also providing isentropic lift in the lowest 10K feet, as evidenced by the eruption of a cluster of morning thunderstorms south of Montauk. This warm advection will only strengthen with the approach of the low and the strengthening of the gradient, and this will prompt thick fog development along the South Coast tonight, especially just prior to midnight, and that fog will migrate north with the leading edge of deep warmth that will set up in Southern New England. Eventually, after midnight, temperatures will rise in Southern New England so that areas near and south of the MA Pike start in the 60s Fri AM, with thick fog into Central New England on the leading edge of the warmth and humidity.
There are so many dynamic factors converging for this event, but the most intriguing are in the lowest 10K feet. First, though, let's start higher with...dynamics! Dynamic lift will increase through the night with the passage of the shortwave. It's actually partly owing to the strong vorticity *minimum* that crossed New England today that we see such strong cyclonic vorticity advection in Western New England Thu Eve that will touch off showers and thunder in scattered form out of NY State and into far Western NewEng. The broad expansion of CVA/PVA will mean a broad expansion of lift, and this will promote deep cloud growth. The driving force here, though, is in the lower levels. At the surface, a frontal boundary extending into Upstae NY is the main focus on the weather map, but a careful hand-analysis today indicates both lower pressure and surface convergence in Northern VT to Central ME. This boundary will focus heavy precipitation tonight, and is the realistic basis for the 2"+ QPF numbers coming out of the NMM for Northern New England. This is below flash flood guidance but is likely to spark street and small stream flooding overnight. Locally higher amounts are possible. What is most impressive, however, is the tremendous convergence in the lower levels, evident beautifully at 850 mb. I'm including an image from the College of DuPage models page, which clearly shows the 850 mb low level jet punching across New England at 12Z Friday with 45-50 kt SW wind encountering a 5 kt NE wind!!! This is going to produce amazing lift. Add to this the tremendously tight theta-e gradient and resultant isentropic lift, and we're looking at a potent low-level setup for a system with a history of producing energetic thunderstorms. As the low level center crosses Central/Northern NewEng, the trough axis swings southeast across Southern New England Friday morning, and when it encouters the deep warmth and humidity in place, quite a show will let loose. Torrential rains will drop at least one to two inches of rain Friday AM, though some spots may see locally more, and this will come in a short time during the Fri AM commute. Additionally, the combo of low and mid level features will force lift so deep in the atmosphere - up o 300 mb - that cloud top temperatures may cool as low as -36 to -40 C. This is plenty cold enough (and then some!) for frequent lightning production from ice nuclei aloft. I think the real question will be how much of a damaging wind threat is there, and my guess is that the strongest cells will get damaging gusts to the ground, though it's going to be a touch and go situation based on radar analysis Fri AM. I'm on the fence here because dynamics are awesome and lift is incredible, but the column is saturated (not much dry air to work with, unless the negative theta-e advection settling in behind the trough axis sparks a squall line on the back side) which limits downdraft potential, and there will still be a shallow inversion in place, even though temps will have risen so much. The inversion is just that, though - shallow...and weak. A strong enough cell will bust through it and bring damaging wind to the ground, so that potential needs to be acknowledged. Finally, one has to at least consider in the back of his or her mind that, in a situation like this, there is going to be a strongly veering wind with height, and this favors embedded rotation. Especially if we develop the expected tropical tap, that becomes more of an issue to consider. At this point, I certainly wasn't anywhere near willing to mention it in the public package of forecast products, but I will be carefully watching the Hartford to Providence stretch north to the MA Pike (just far enough inland to be away from the stronger ocean stability, but far enough south to be unstable). This is not a surface based CAPE of LI situation, but rather an elevated event for many areas, but if the cap breaks and it becomes surface based, that's when things like this become important.
Rain is out by late Friday. The weekend looks like more showers/thunder thanks to multiple shortwaves. Sunday is still very uncertain in my opinion, but am playing first half of day optimistic with advancing storms blossoming afternoon. Will see.
Matt
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