April 24, 2014: Rain Expected on Friday and Saturday

Forecast Highlights:

nam-hires_namer_039_sim_reflectivityA 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


4.24.14Posted 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).

f36_namThe 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.


Sunday – Beyond: More Rain Next Week

As the upper level low shifts east of the region on Sunday and Monday, it will slow down and nearly stall south of Nova Scotia as high latitude ridging extends from Greenland through eastern Canada, connecting with amplified ridging in the central US ahead of an approaching upper level low in the western US to form a transient block-like feature over central Canada. This will result in a slower upper level flow next week, with most of the week highlighted by a slow moving upper level low producing severe weather in the Plains and rain in the region. Dry conditions are initially expected for Sunday and Monday as the upper level low stalls well offshore with a narrow area of high pressure extending into the region, resulting in mostly to partly sunny skies and highs in the 55-60 degree range on Sunday and the upper 50s-mid 60s on Monday. As the upper level low slowly tracks east through the US, cloud cover will increase on Tuesday with showers possible and highs back down into the 50s.

The model guidance has to get to gain a solid handle on the exact setup of the upper level low beyond Tuesday, although with a high pressure initially over New England, the region is expected to initially remain north of the warm front with a chilly rain expected especially around Wednesday. High temperatures are generally expected to reach the 50s, and may struggle to rise above the mid-upper 40s under the areas of heavier rain. Uncertainty is higher beyond the mid week, although temperatures are gradually expected to rise back into the 50s and later 60s towards the late week and next weekend with widespread clouds and scattered showers possible through Friday before possibly ending in time for next weekend. More details will be posted over the next several days.

4 thoughts on “April 24, 2014: Rain Expected on Friday and Saturday

  1. Eduardo Berroa Reply

    Some potential chance of thunderstorms are likelihood for tomorrow.

    • NYC Area Weather Post authorReply

      Yes, thunderstorms are likely to develop tomorrow afternoon and evening with a strong upper level low overhead and steep low-level lapse rates; probability of severe thunderstorms appears low, with minimal lift index/CAPE values, although heavy rain and possibly small hail do appear to be the main risks with these storms primarily north and west of NYC.

    • NYC Area Weather Post authorReply

      NYC and parts of western LI/CT are included in tomorrow’s thunderstorm risk; the strongest forcing appears to be focused north and west of NYC at this time where probability of stronger storms appears to be a bit higher than over the city itself, although storms that develop over the area in NJ/NY are likely to continue east/SE through NYC and Long Island before ultimately weakening overnight.

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