AM Update: Heavy rain with localized flooding possible tonight

Good morning and Happy Thursday! As we move forward towards the end of the week, which seems to be approaching rapidly after Tuesday’s holiday, the weather has been continually pleasant throughout the majority of the Northeast States. Aside for a few local exceptions, seasonable weather has been the beat of the drum over the past 48 hours, with just a few isolated showers and storms in the Mid Atlantic up until Wednesday Night.

Things have already begun to change, as you may have noticed by a newly southeast wind direction along the shores which is pushing inland. This is known more affectionately as an “onshore flow” and it often signals changing weather in the Northeast, when high pressure systems are beginning to lose their grip or control on the area’s weather. Today is no different: Clouds will increase and the onshore flow will begin to aid in moisture advecting towards the region.

Mid level atmospheric disturbances approach the area starting Thursday, with a  significant one nearing the Northeast states by Friday morning.

Mid level atmospheric disturbances approach the area starting Thursday, with a significant one nearing the Northeast states by Friday morning.

Gradually increasing cloud cover this morning will eventually give way to the development of showers from south to north. These will become steady across the Mid Atlantic and Southern NJ this morning, gradually moving northward into Northern NJ and the NYC Metro Area. These showers are developing in response to a disturbance in the atmosphere’s mid levels, which is providing lift in a moist environment over the mentioned areas. Steady rain will spread into the Northern Mid Atlantic (Eastern PA, NJ, NYC, LI) during the early afternoon before tapering off by evening.

Waiting behind this initial disturbance, however, is an area of even more disturbed weather — and one that will approach the Northeast and Mid Atlantic with more vigor than its predecessor. A thermal gradient (boundary of temperature differential) is likely to set up from Eastern PA through NJ and towards Long Island, and this gradient will serve as a “highway” for the development of a low pressure system.

NAM model's simulated radar loop showing heavy rain Thursday Night into Friday across parts of the region.

NAM model’s simulated radar loop showing heavy rain Thursday Night into Friday across parts of the region.

There are a few things we look for in setups like these which can signal the potential for heavy rain: Ample moisture, good jet dynamics, and sufficient lift. This setup, as currently modeled, has all three. Short term forecast models suggest that a potent shortwave trough will provide sufficient lift, that ample moisture in the low levels will be in place (a very humid airmass), and that  favorable upper level jet dynamics will support heavy precipitation.

The main question with the forecast over the next 6-12 hours is the positioning of the mentioned boundary. These thermal gradients can serve to be critical in determining where heavy rain falls. As the surface low pressure forms, lift just to the north of it will be significantly enhanced. A process called frontogenesis (tightening of temperature gradients in multiple levels of the atmosphere) may further enhance the lift in the atmosphere in those regions, resulting in the potential for torrential rain and flooding.

Figuring out exactly where the boundary will set up will be quite a task, but we have begun formulating our best ideas at this current stage. Weighting high resolution forecast models along with ensemble guidance, the thermal gradient seems likely to set up in a position from Eastern PA through NJ and towards Long Island. With the heaviest rain just north of there, it appears likely that Central and Northern NJ, NYC, LI and parts of Southern New England will see the potential for torrential rains and localized flooding through Friday morning.

raingraphic

While our confidence in the overall setup is moderate to high, the exact location of this thermal gradient remains the key to the forecast. A slight adjustment could have major implications on the forecast. Our team is working hard this morning to pin down the boundaries location and will provide updates this afternoon with the very latest information. Stay tuned — remember, if you live in a flood-prone area within the green shaded region, have a plan in place in case a flood does occur. Never *ever* drive into flood waters.