Dec 26, 2013: Nor’easter Expected Sunday Night

[notice]The 8-Day Forecast is now updated on a daily basis again.[/notice]

Forecast Highlights:

f90A minor system moving through the region will produce isolated rain/snow showers across the area today, with temperatures remaining close to average through Friday before returning into the 40s on Saturday. The main change in the outlook is on Sunday, with a coastal low now expected to affect the area with widespread rain with up to an inch possible, before cold temperatures return for New Year’s Eve and beyond.


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Wednesday, December 25 Observations:

12.25.13Following Tuesday night’s light snow event that left up to 1.7″ of snow in parts of Suffolk county, cold conditions made a quick return into the region with mostly-partly sunny skies and highs only in the mid-upper 20s inland and the low 30s near NYC, nearly 40 degrees colder than just 3 days earlier when temperatures peaked in the 60s and low 70s. Generally dry conditions were observed, but with increasing clouds overnight as a weak system approached the region.

The highest temperature was 32 degrees in JFK airport, and the coolest high temperature was 25 degrees in Sussex and Andover, NJ.



Today – Saturday: Some Snow Today, Then Warming Up

A weak disturbance moving through the region is producing areas of moderate snow over PA and NY states, with up to 2″ of snow reported in east-central Pennsylvania exceeding the forecast. The snow has persisted further south and east than expected, with moderate snow currently spreading towards Philadelphia. The steadier precipitation will gradually lift northeast as a mid level shortwave tracks north of the area through southern NY state, but with the latest high resolution model guidance supporting light snow showers spreading through SE NY and northern NJ through 10-11am with up to a dusting of snow possible. A coastal low will begin to take shape off the coast late this morning into the afternoon, with scattered showers possible in eastern Long Island and rain/snow showers in SE CT towards 11am-2pm. High temperatures are expected to peak in the 35-40 degree range for most of the area.

Behind today’s system, a transient cold air mass will brush the region on Friday with highs remaining in the 35-40 degree range, along with lows in the low 20s inland and mid-upper 20s elsewhere. By Saturday, however, warmer temperatures will return into the region as the trough departs, with a stalled frontal boundary over the northern Northeast and a southwesterly flow bringing in warmer temperatures with highs expected to reach the mid to upper 40s across the area with mostly sunny skies.


Sunday’s Outlook: Nor’easter, Widespread Rain Expected

6z GFS hour 90 at 0z Monday (7pm Sunday), showing the anticipated nor’easter and widespread precipitation (Image credit: PSU e-Wall).

f90The most significant change in the forecast is on Sunday, with much higher probabilities of a low pressure affecting the region with widespread rain and some interior snow. Yesterday’s 6z GFS run initially introduced this scenario, with the rest of the model guidance catching up with yesterday’s 12z runs, and continuing to trend more amplified and west throughout last night. A shortwave trough currently over northern Mexico is expected to emerge into the southern US by Saturday and track up the east coast while interacting with the northern stream over the Great Lakes, associated with a surface low pressure over Wisconsin/Michigan, with a coastal low pressure expected to develop near the Gulf of Mexico and track up the East Coast. Over the last day, the trend has been for a more amplified southern shortwave and a slower and more neutrally tilted northern stream trough, leading to a stronger and further west coastal low pressure producing widespread moderate precipitation across the region.

Despite the time of the year, however, this is unlikely to be a widespread snowstorm for the area. As noted in the previous section, the departure of the late week trough along with a broad high pressure south of the area will lead to a southwesterly flow across the region with temperatures aloft in the 850mb layer above freezing both prior to and during the arrival of the system, as the cold air is held up northwest of the northern stream low pressure and is not expected to surge into the region until the system departs the region on Monday, thus keeping most of the precipitation from the coastal low under temperatures above freezing in the low-mid levels. Surface temperatures will be somewhat cold, however, as following a drop into the mid 20s-low 30s across most of the region on Saturday night, widespread cloud cover and precipitation on Sunday will prevent temperatures from rising above the mid-upper 30s for most locations; had it not been for this coastal low, this air mass would’ve been capable of highs in the low 50s near NYC. The main question is how much cold will be available in the region, which depends on the setup of the north stream and the positioning of the high pressure to the north; last night’s 18z GFS run for example depicted the northern stream surface low north of the Adirondacks, keeping almost the entire region with rain, while the latest 6z run depicts the low further southwest, near eastern Michigan, with a more suppressed high pressure and a further south extent of cold air leading to more widespread wet snow for the interior Northeast.

As this is still a new development, there remain uncertainties with the specific details, and a more amplified scenario than currently depicted remains a possibility. At this time, widespread rain is expected across the area between at least 2pm on Sunday to 2am on Monday, with highs in the mid-upper 30s and at least 1/2 to 1 inch of rain expected with the possibility of up to 1.5 inch totals. With the warm sector likely staying offshore, winds are not expected to be a significant issue at this time, although breezy north/NE winds are likely especially in Long Island/CT; this may become more of an issue for Long Island if the system is more amplified/inland than currently modeled. Given the lack of a strong cold air mass, the immediate NYC area and coastal CT/Long Island are expected to see rain out of this system, while NW NJ and interior SE NY may see some snow depending on the extent of the cold air, but with any significant snow likely staying northeast of the area into interior New England and Maine. More information will be posted on Sunday’s outlook with the next forecast discussion on Friday morning.


Next Week: Cold Returns

Cold temperatures will return behind Sunday’s system, with a cold start to 2014 expected. The polar vortex is expected to be displaced into southeastern Canada, keeping a frigid air mass just north of the US/Canada border, with some of the cold air entering the region at times next week. At this time, it does not appear a significant storm system will accompany the cold throughout most of next week. With the core of the cold air mass remaining to the north, anomalous cold is not anticipated at this time, although below average temperatures are still expected, generally in the 20s-low 30s for highs.

Temperatures on Monday are expected to reach the mid to upper 30s, and the upper 20s-low 30s on Tuesday. Forecast conditions for New Year’s Eve at 12am in NYC are currently partly cloudy skies and 25 degrees. The exact timing of the cold is still somewhat uncertain, with the GFS signaling a moderation in temperatures on Wednesday while the ECM signals the peak of the cold around the same time, although at the peak of the cold sometime towards the middle of next week, the air mass would be capable of supporting widespread highs at least in the mid 20s with widespread lows in the 10s for most and single digits for interior locations. A moderation in temperatures is possible towards next weekend, but with sustained warmth unlikely to develop at this time.

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