A cutoff low pressure east of the region continued to slowly drift east, with continued strong northwesterly wind and highs in the upper 50s to low 60s. A slight warm up is expected through Saturday as a low pressure tracks through the region, producing heavy rain on Friday night and again on Saturday evening. Drier conditions will briefly return afterwards before rain returns by the middle of next week as a slow moving upper level low approaches the region.
Tonight – Saturday: Slightly Warmer, Some Rain
Posted above from left to right are the latest available surface analysis and radar composite from the NWS Weather Prediction Center, and the GFS initialized 500 millibar heights and vorticity from NCEP MAG; both are valid at 18z (2pm). The upper level trough which passed over the region yesterday has since split into a cutoff low south of Nova Scotia, with an inverted trough extending into Maine, having produced rain and wet snow in that region earlier today. A strong pressure gradient remained in place in between the departing cutoff low and an approaching area of high pressure, with strong northwesterly winds at 15-30 mph gusting up to 40 mph; despite the cold air mass aloft, the strong downsloping NW winds with mainly sunny skies allowed temperatures to warm up into the upper 50s to low 60s across the area.
With the strong NW wind last night, this morning’s temperatures generally fell into the mid 30s-low 40s across the area last night, including the Long Island Pine Barrens where strong winds prevented a radiation inversion from developing. As the high pressure builds in overnight, however, winds will quickly decrease and become nearly calm, which along with mostly clear skies will support strong radiative cooling across interior sections and the Long Island Pine Barrens, where lows are expected to fall into the mid to upper 20s. Elsewhere in the area, lows will fall into the low-mid 30s in the north/west suburbs of NYC into southern CT, and the mid-upper 30s near NYC and the immediate coast. Winds aloft will turn to the west overnight, supporting gradual warm air advection through Friday, with winds turning to the south at 5-10 mph as high temperatures rise into the mid-upper 50s near coastal areas and the low-mid 60s elsewhere.
18z NAM at hour 36, valid at 6z Saturday (2am), depicting a low pressure developing south of the area producing a narrow band of heavy rain tracking east (image credit: PSU e-Wall).
The shortwave trough currently near Oklahoma will quickly track ENE towards the region as the surface low reaches northern NY state on Friday night, bringing an occluded frontal boundary into the region where a second area of low pressure is expected to develop south of the area. With strong positive vorticity advection ahead of the shortwave, a narrow band of strong upward vertical motion is expected to move overhead around 9pm to 4am from west to east, resulting in a setup of a band of moderate to heavy rain moving through around these hours with rain totals likely around 1/2 to 1 inch. Meanwhile, the upper level low currently near North Dakota will trail behind, moving through on Saturday; partly sunny skies are initially expected, which along with a downsloping westerly wind at 5-15 mph support temperatures surging into the mid-upper 60s from NYC and north/west, possibly in the low 70s near and southwest of NYC, and mostly in the 50s in Long island and CT. With a pocket of cool air aloft associated with the upper level low leading to steep low-level lapse rates with a rapid decrease of temperature with height, scattered showers and thunderstorms are expected to develop over northeast Pennsylvania in the early afternoon, affecting the area mainly between 4-8pm especially from NYC and further north/west. While these showers are generally expected to remain non-severe with marginal instability parameters, these showers will likely be capable of producing heavy rainfall and possibly small hail.