Evening Zones: Chilly tonight, Heavy Rain on Friday

Good evening! After a chilly late-morning and early-afternoon filled with clouds, clouds quickly eroded and gave way to mostly sunny skies. This clearing of the clouds allowed high temperatures to reach the mid 60s. However, this clearing of the clouds was provided by an area of high pressure that moved into the area, which will help set the stage for a very chilly night tonight for NYC suburbs.

We already had a chilly airmass behind two cold fronts that have moved through over the past couple of days, and each cold front also provided drier air. 850mb temperatures fell to around 0C and dewpoints are generally in the 20s to the low 30s, and this area of high pressure will also assure that skies will remain clear as well as provide a large subsidence inversion.

This afternoon's GFS valid for Thursday morning shows low temperatures in the 30s to low 40s across most of the region.

This afternoon’s GFS valid for Thursday morning shows low temperatures in the 30s to low 40s across most of the region.

All of this leads to this afternoon’s forecast models still showing low temperatures generally in the 30s to low 40s across most of the region, and we agree with this. There is actually a frost advisory for Sussex, Warren and Orange counties, where low temperatures are expected to be in the low to mid 30s, with perhaps even an isolated upper 20s to around 30 reading in the elevated valleys. The city itself will have lows holding in the upper 40s due to the urban heat island effect.

Thanks to Thursday’s chilly start and increasing afternoon clouds out ahead of a large storm system, afternoon highs may only hold in the low to mid 60s. As the high pressure shifts offshore, the flow will also become more onshore, so areas from NYC and east may actually only hold around 60. Thursday night will be milder than Wednesday night thanks to the increased clouds as well as a good amount of warm air advection out ahead of the storm system, with lows generally around 50 degrees.

We expect some light rain to move in during the overnight — particularly for areas west and south of NYC — after 2:00am. The morning commute will have some light rain from C NJ and north, but S NJ will have some heavy rain.

This afternoon's 3km NAM model shows some very heavy rain with torrential downpours in the late-morning and early-afternoon (Tropical Tidbits).

This afternoon’s 3km NAM model shows some very heavy rain with torrential downpours in the late-morning and early-afternoon (Tropical Tidbits).

The AM Zones article has more details on the meteorology of the storm. But the above animation is the 3km NAM valid from 11am through 3:00pm, which shows a very heavy line of heavy rain via forced convection which could lead to flash-flooding. This is triggered by a very strong low-level jet that backs in from the Atlantic and has a tropical connection. Last night’s GFS model was somewhat weak with this low-level jet, but this afternoon’s runs have strengthened it and have also formed it a bit earlier. This allows more time for the tropical moisture to gather and converge along the coast, leading to an uptick in rainfall on today’s model runs. The orientation of this jet will allow for some training of the convection where it keeps reforming over the same areas. The timing, strength, and orientation of this jet is still very crucial to the forecast and we will be monitoring it closely. But as of now, we are expecting a general 1-2″ of rain with localized 3-4″ amounts where this training does occur.

Most of this rain will fall in a relatively short period of time from the late-morning through the mid-afternoon, before a mid-level dry-slot works its way into the area. This could lead to a dry-slot that lasts several hours. However, there will be a secondary maximum of jet energy that works its way in later in the evening, which may do two things: 1) keep areas of rain developing in eastern locations that never completely dried out and 2) reform some moderate to heavy batches of rain for the rest of the area later in the evening, though this rain will not be as heavy nor as widespread as the first batch.