With the month of July yielding summer in full-fledged beast mode with temperatures exceeding the 90F mark numerous times and very little (in fact hardly any) widespread rainfall the ground is bone dry yielding a substantial brush fire risk. With this we really need this rain and it looks like we'll get our wish on Friday as a developing area of low pressure is expected to track northeastward along an advancing cold front providing ample moisture and lift to generate widespread precipitation.
Both the NAM and GFS computer forecast models are indicating the potential for a quite a bit of precipitation to fall, beginning late overnight Thursday and lasting through the first half of Friday. In fact, both forecast models are indicating as much as 1-2'' of rainfall could fall across a large portion of the region with even potential for isolated higher amounts, especially in any embedded thunderstorms:
As the cold front approaches from the west, a warm front will be surging northward at the surface. With the area of low pressure expected to track right over southern New England or just south so the warm front will likely not make much progress into southern New England and actually may stall just north of Long Island Sound across coastal CT. South of the warm front the airmass is expected to become exceptionally moist with dewpoint temperatures rising well into the 70's and precipital water values (PWATS) rising well above 2''...and indication of an exceptionally moist airmass:
One interesting aspect with this system will be the strengthening of the lowe-level jet and winds within the lowest 5,000ft of the atmosphere. Both the NAM and GFS indicate that winds will strengthen to as much as 30-40 knots and be coming from the southeast. This flow will not only work to transport copious moisture in off the Atlantic but will actually also work to enhance the degree of directional shear in place. This will be very important for anyone south of the warm front as if any thunderstorms develop and tap into this shear they could begin to acquire rotation and pose a threat for a brief/weak tornado. The highest likelihood for this is across NW CJ into the NYC metro area and onto Long Island. In fact, many of the tornadoes which have occurred in the NYC area and on Long Island have occurred in similar setups to tomorrow. With the degree of instability isn't expected to be all that great, with the surge of low-level moisture (dewpoints near or above 70f), and rich presence of low-level moisture in the low-levels thanks to the southeasterly winds will create a very buoyant low-level airmass and an unstable layer here. While 500-800 J/KG of mixed-layer cape is not usually sufficient for severe weather, given how this setup would yield low-topped thunderstorms (thunderstorms which don't grow very tall) and they feed off of the low-level airmass and utilize those conditions. This combination of shear/cape would be sufficient to enhance storm rotation:
This point-and-click forecast sounding for a location on Long Island shows very good low-level directional shear along with strong winds (encircled area on bottom left) and looking at the hodograph you can see a curved looked indicating the good directional shear present (encircled on top right):
For those been asking for rain it appears Friday will finally deliver the goods and for a widespread across across southern New England where region wide as much as 1'' of rain could fall. Some localized areas could also see up to 2'' of rainfall with perhaps even a few locally higher amounts! This would certainly do wonders for the long-term rain deficit we're facing.
As for any severe weather potential, we have to watch where the warm front stalls as just south of the warm front atmospheric conditions could favor some of the thunderstorms becoming strong and posing a threat for strong to damaging wind gusts and even a brief/weak tornado.
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