[notice]This is the final forecast discussion for the upcoming storm. Occasional storm updates will be posted on Wednesday afternoon and evening.[/notice]
Temperatures today continued the warming trend from yesterday and even exceeded the forecast, peaking in the low-mid 60s for most locations and even the upper 60s near NE NJ, marking the first widespread 60s since December 23. Temperatures will slightly cool down on Wednesday as a significant storm system affects the region; very little, if any snow is expected, although occasional showers are expected, followed by a narrow squall line and a flash freeze overnight with anomalously cold temperatures and strong wind gusts for Thursday, only to quickly warm up in time for next weekend.
Tonight – Wednesday Night: Significant Storm Produces Variety of Impacts
The most significant storm system since February 13-14 will affect the region tomorrow; despite the system failing to produce much snow over the area into the Hudson Valley and southern New England, it will nonetheless produce significant impacts across the area and the region, ranging from over 14-18 inches of snow and blizzard conditions in northern New England to strong thunderstorms near and south of Pennsylvania, a rapid flash freeze overnight and strong wind gusts.
Posted above from left to right are the latest available surface analysis and radar composite from WPC, and the initialized NAM 500 millibar heights and vorticity from NCEP MAG; both are valid at 0z (8pm). The two key features leading to the rapid deepening of this system consist of a shortwave over Kansas and a vort max over southern Manitoba, as seen in the 500mb map above. The former will continue tracking east while the latter dives SSE, with both expected to catch up over Ohio on Wednesday. At this point, phasing of the two shortwaves is anticipated as the system amplifies and tracks ENE with strong positive vorticity advection entering the region. Along with strong cold air advection from the north aiding in a strengthening temperature gradient, and the placement of the region under both the left exit and right entrance quadrants of two jet streaks, both which favor upper level divergence and aid in the lowering of surface pressures, a rapidly deepening low pressure is expected to track ENE through the region, with minimum pressure likely near 980mb over southern New England, producing widespread heavy precipitation across the northeast US.
Impact Analysis: Compared with yesterday’s model guidance, the latest initialized conditions depict both shortwaves as slightly weaker than modeled yesterday, with the northern stream originating from Canada positioned slightly east of yesterday’s model guidance. This results in a slightly less aggressive phase and a slightly weaker and flatter system than modeled yesterday, although these differences are small enough to have little impact on the final outcome in the area, with the main changes from this shift near NYC consisting of the northern extent of the warm front and thus the warmth during the day on Wednesday. This does little to result in a further south snow axis in the northeast US, however, as strong mid level warm air advection is still anticipated with the 850mb low tracking near Albany, NY and the 700mb low near the Adirondacks, compared with the forecast surface low pressure track from Pennsylvania into NYC/northern NJ and southern New England.
Even though this will not affect the area, it is still worth mentioning as this is always an important issue to consider with winter storms; in some cases the warm layer will be located above the 850 millibar pressure layer, decreasing the reliability of the 850mb freezing line as the placement of the rain/snow line. In such cases, locations with 850mb temperatures near or above freezing are nearly guaranteed to observe sleet/freezing rain or rain, but locations with 850mb temperatures below zero are not always guaranteed to observe snow, depending on the thickness of the warm layer aloft. In this case, this will be most noticeable along an axis from central NY state into southern NH/VT and coastal Maine, where 850mb temperatures are progged to remain near or below freezing. As the mid level low will be north of the surface low, strong cold air advection will occur near the surface while SW winds continue in the mid levels for a period of time, leading to a warm layer between 850mb and 700mb and a swath of sleet and freezing rain.
Assuming the track and intensity of the system verify close to the current forecast with a low track near or just north of NYC, the warm front is expected to extend up to at least central PA into northern NJ and southern CT. Most locations in the warm sector, especially along an axis from central PA to central NJ and further south, are expected to see little rain for the majority of the day with clearing skies, allowing for increasing instability as temperatures surge into the 60s close to the warm front and 70s as far north as Maryland and possibly southwestern NJ. This proximity of the warm front will set up for a relatively strong temperature gradient in the area, with mainly cloudy skies and temperatures likely struggling to rise above the low 40s in far NW NJ and interior SE NY, while peaking in the upper 40s near southern CT, and surging into the mid-upper 50s or possibly near 60 in NYC and northeast NJ. Light NE winds are expected in interior northern areas, with a southerly wind up to 10-20 mph elsewhere. This gradient could end up slightly south or north of the forecast, which presents a higher than normal bust risk regarding high temperatures.
18z run of the 4k NAM, depicting simulated reflectivity to the left at 23z (7pm), and forecast temperatures to the right at 01z (9pm) – image credit PSU e-Wall.
Strong cold air advection will become evident by the evening and early overnight hours as yet another anomalous cold air mass rapidly surges southeast into the region, aiding in a tightening temperature gradient; by 9pm, as depicted in the paragraph above from the 18z run of the 4k NAM from the Pennsylvania State University e-Wall, temperatures are expected to range from the 50s in New Jersey and eastern Pennsylvania to the 10s over central Pennsylvania. During the early-mid afternoon hours, a narrow but strong squall line consisting of heavy rain and thunderstorms is expected to develop near near West Virginia, progressing eastward and reaching central PA to Washington DC by at least 7pm and New Jersey by 9pm. Along with marginal instability, storms could become strong to locally severe from southern PA and further south, with the main risk of strong wind gusts. Very little instability is expected near the NYC area, with the risk of strong storms remaining southwest of the area, although the squall line will still be capable of producing a brief period of heavy rain and strong wind gusts between 9-11pm. As the cold front passes to the northeast of the area, a strong pressure gradient is expected over the region during the overnight hours, with northwesterly winds up to 20-30 mph with gusts near or over 40 mph expected. This will aid in advecting the cold temperatures into the area, with a flash freeze expected primarily from NYC and further west/south as temperatures crash from the 50s into the 20s in as little as 2-3 hours, leading to a risk of black ice continuing into Thursday morning. Overnight temperatures are expected to fall into the 10s north and west of NYC and the low-mid 20s near NYC into Long Island and coastal Connecticut, with wind chill values approaching zero for most locations and falling below zero inland.
Forecast Summary: Mainly cloudy skies are expected on Wednesday with scattered showers possible at times, especially north of NYC, as temperatures rise into the low 40s in interior SE NY, mid-upper 40s in NW NJ into southern CT, and the 50s elsewhere in the area, approaching 60 degrees southwest of NYC. A narrow but strong squall line with heavy rain, thunder and strong wind gusts is expected to move through between 9-11pm across the area, followed by a rapid flash freeze as temperatures crash into the 20s in only 2-3 hours, leading to a risk of black ice overnight into Thursday’s morning commute. Strong northwesterly winds up to 20-30 mph gusting to near/over 40 mph are expected, along with wind chill values in the single digits by Thursday morning and below zero inland. Rain amounts are expected to be relatively low, ranging from 0.25 inch in SE parts of the area to 0.75 inch further NW, with most of the rain expected from the evening line of heavy rain.
This is the final forecast discussion for this storm. Storm updates will be posted on Wednesday afternoon and evening.