April 3, 2016: Winter Returns for Early April

Forecast Highlights:

ne_ptype_f7Despite a near record warm winter, and a lack of snow events in the Northeast US with the exception of a historic late January blizzard, cold and snow are set to make a comeback this coming week, and parts of the region could end up with their biggest snowstorm of the season in April. As a deep trough sets up near the region, multiple rounds of cold and storminess are expected, most notably tonight when thundersnow and strong wind gusts are possible in parts of the region, followed by additional rain and snow on Monday and a potentially significant rain event later in the week.




Pattern Shift: Anomalous Western US Ridge Impacts Downstream Flow

Over the last week, a highly amplified ridge evolved over the Northeast Pacific Ocean. The northward extent of this ridge, along with a deep upper level low passing to the north, resulted in a strong geopotential height gradient and a powerful northwesterly jet downstream of the ridge. The jet advected a frigid air mass from northern Canada in association with the upper level low into the northeast United States, a rare scenario this past winter where most air masses originated from the Pacific Ocean with additional warming due to downsloping off of the Rockies.

The strong northwesterly jet stream downstream of the ridge is associated with a shear vorticity maximum in the cyclonic side of the jet, which was advected southeastward by the northwesterly flow, resulting in significant trough amplification over the eastern United States. As the vorticity maximum reached the base of the trough and gained a negative tilt, curvature vorticity quickly increased, with an axis of very strong differential cyclonic vorticity advection expected in the short term just downstream of the trough, setting up from Pennsylvania into northern NJ, southern New York, and southern New England. The strong CVA is currently forcing rapid surface cyclogenesis to the east with the surface low pressure deepening to 984 hPa as it reaches Cape Cod.



Storm 1: Rain, T-Storms Tonight Transition To Snow

As of 2 AM this morning, the low pressure is currently tracking through the area. The shortwave trough is associated with anomalously low tropopause levels, implying steep low-mid tropospheric lapse rates, which in conjuncture with strong ascent due to cyclonic vorticity advection and surface convergence is providing forcing for the heavy rain and thunderstorms that passed through western parts of the tri-state area over the last few hours. The line was associated with lightning and even isolated hail accumulation over parts of Northeast New Jersey, with a report of 1″ of hail in Paramus.




As previously noted, an anomalously cold air mass will be advected into the region with the strong northwesterly flow in the wake of the frontal passage. Temperatures will rapidly cool down into the 40s and 30s, while temperatures aloft cool at a similarly rapid pace with 850 hPa temperatures near -10C. Especially towards northern NJ, southeast NY and southern CT, the thermal profile is expected to cool down sufficiently to allow for a changeover to snow to occur around 4-6 AM. Surface temperatures will remain high given the recent warmth, although initially heavy snow rates will likely allow for up to an inch of wet snow inland of NYC, especially over the higher terrain. The highest accumulations are expected over interior southeast New York and Connecticut, where up to 1 to 3 inches of snow are likely over the higher terrain due to heavier snow rates and cooler temperatures.

In addition to the snow, very strong winds are expected. With the low pressure rapidly deepening over the area with a minimum pressure below 990 hPa, a very strong pressure gradient is expected in it’s wake, resulting in sustained winds in the 30-40 mph range with gusts up to 50-60 mph at times. These strong winds will continue into Sunday, with continued cold air advection keeping daytime temperatures in the upper 30s to low 40s range along with wind chill values near 25-30 degrees.



Sunday Night – Monday: Clipper Passes North of NYC

The second wintry event of the month will occur on Sunday night into Monday, as a second vorticity maximum will quickly progress southeast around the base of the trough, with a clipper-like low pressure system tracking southeast from western Canada into Michigan and Pennsylvania. Given that the system is primarily driven by the northern stream with a lack of moisture interaction from the southern US, the system will remain fairly moisture starved, with most precipitation occurring north of the low pressure track in association with strong mid-level frontogenesis.

With the antecedent cold air mass in place, temperatures will be sufficiently cold to produce snow across much of New York State into southern New England, where at least 1 to 3 inches of wet snow are likely. Initially temperatures may be cold enough to support light snow north of NYC early on Monday morning, although the track near or just north of NYC supports warm air advection later in the event with a southerly flow and temperatures rising into the low 50s by the afternoon west and south of NYC. A sharp temperature gradient will be present near the frontal boundary, with temperatures peaking in the low to mid 40s farther north over southeast New York and southern Connecticut.

Such a low pressure track has not occurred yet this winter, as most low pressure systems either tracked near or southeast of the 40N/70W benchmark, or well inland; as a result, this will likely be one of, if not the biggest snowstorm of the winter for parts of New York State that fell in between the two mean storm tracks, most notably in Albany, NY which is currently well within the margin for its least snowiest winter on record (10.5″ to date). While probability of accumulating snow in NYC is low with both snow events this week, any accumulation would be notable for numerous reasons, especially considering the record warmth in early March when temperatures reached 79 degrees. In reliable records, only 1918, 1938, 1921 and 1990 saw maximum March temperatures above 75 degrees followed by an accumulating snow event in April.



Medium Range: Cold Midweek, Rainy Ending to Week

The peak of the cold is expected to occur on Tuesday with a secondary cold air advection episode in the wake of the low pressure, with a breezy northerly wind at 15-30 mph. Temperatures are expected to drop into the 20s throughout the area on Monday night, with high temperatures on Tuesday in the 35-40 degree range for most locations; for comparison purposes, the average high temperature in Central Park is 58 degrees.

Another trough will approach the region by the end of the week, although larger height rises downstream support more significant amplification of the trough, with the result likely a low pressure track west of the region with the potential for heavy rain on Thursday and Friday. Despite a strong southerly flow ahead of the system, temperatures are likely to remain below average during the daytime given the strong antecedent cold air mass, with high temperatures likely returning into the 50s for the event before falling into the 30s and 40s again by the weekend.

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