Dec 30, 2013: Snow, Mix Possible on Thurs-Fri

Forecast Highlights:

gfs_namer_096_850_temp_mslp_precipAs yesterday’s coastal low departed the region, cooler temperatures gradually entered the region as the trailing cold front from the northern low pressure is currently moving through. Temps will gradually fall throughout the day, with colder temps for Tuesday and Wednesday. The main highlight in the outlook is on Thursday into Friday, when a storm system is likely to produce widespread snow across the region but with uncertainty regarding its impact in the area, followed by a brief surge of very cold temperatures.




Today – Wednesday: Cooling Down, New Year’s Eve Flurries?

usfntsfc12wbgThe 7 AM WPC surface analysis has been posted to the left, depicting yesterday’s coastal low near Nova Scotia, and the cold front associated with the northern low currently moving through the area. Temperatures last night failed to fall as much as expected near NYC, remaining in the low 40s in NYC while falling into the 30s further inland. As the front moves through the area, a colder air mass will return with some clearing in the cloud cover and temperatures steadily falling throughout the day to reach the mid-upper 20s inland and upper 20s-low 30s elsewhere by 5pm. Partly cloudy skies are expected tonight with lows ranging from the mid 10s in interior areas to the low-mid 20s in NYC and upper 10s-low 20s elsewhere.

A weak disturbance will move through the region on Tuesday, producing widespread snow showers to the north of the area. While most of the precipitation will remain to the north of NYC, with mostly cloudy skies generally expected, isolated flurries can’t be ruled out primarily over SE NY and southern CT during the afternoon and evening hours. High temperatures are expected to reach the low to mid 30s. Windy conditions will develop briefly in the evening and early overnight hours, with a west wind at 10-20 mph sustained, with conditions at 12am in Times Square in NYC expected to consist of mostly cloudy skies, 26 degrees and a breezy west wind bringing wind chills into the upper 10s. Partly sunny skies are expected for Wednesday with highs in the upper 20s to low 30s.


Thursday – Friday: Storm Expected, Impact Uncertain

Attention turns to Thursday as the next storm potential exists, with widespread precipitation likely across the region. Two mid level shortwaves, the southern stream currently over Baja California and the northern stream currently in the northeast Pacific, are expected to interact as the north stream digs southward and picks up the southern shortwave over Texas on Thursday, aiding in the development of a low pressure which then tracks northeast and into the Atlantic Ocean while intensifying.


12z GFS at hour 54, for 18z Weds (1pm), depicting the setup with the two shortwaves involved, the southern stream near Texas and the northern stream near New Mexico (Image credit: PSU e-Wall).

f54Storm Analysis: The main uncertainty in the model guidance is with the handling of these two shortwaves, which has a significant impact on the forecast for the region. Yesterday evening’s update noted there has been a trend in the 500mb layer, especially notable on the GFS, for a slower and stronger southern stream shortwave, correcting earlier runs which depicted it as weaker and sheared it out too quickly, a bias which the GFS has displayed with some of the storms over the last month. This has reflected on the precipitation maps with a trend towards more precipitation over the southeastern US as the low develops and tracks northeast. Additional uncertainty rests with the northern stream, however, which is still over the Pacific where a lack of data sampling for the model guidance to use is partially responsible for the frequent variations in the outputs shown on the models. The shortwave should enter the northwestern US on Tuesday, when the spread in the model guidance is expected to narrow down.

The recent model runs trended towards both a stronger/slower southern shortwave and more digging of the northern shortwave, which supports more widespread precipitation coverage, but resulted in a further south and flatter low pressure; the GFS and CMC, for example, earlier showed moderate-heavy snow north/NE of the area with a wintry mix changing to some back end snow, while the latest runs depict lighter precipitation but a longer duration event with plain snow in the area. This trend has been the most evident on the UKMET, which shows a much flatter scenario and would keep most of the precipitation south of the area. The ECM, meanwhile, remains the most aggressive model depicting heavy snow north of the area which later shifts southward into the area in the back end of the system.

As the models continue to have some difficulty with the handling of the setup, the specific details in the forecast cannot be narrowed down with high confidence at this time, although some aspects of the forecast have higher confidence. Widespread light snow is expected to spread into the region on Thursday, either near or north of the NYC area, which is likely to support at least light-moderate accumulations during the day. Precipitation type is likely to fall as snow for the majority of the area, but with more uncertainty towards NYC and Long Island, where a stronger and/or further north system would support more of an initial warmth surge in the low-mid levels which would support a changeover to rain and/or sleet. As winds shift to the north overnight with a tightening pressure gradient, a changeover to snow is expected across the entire area with temperatures falling into the 20s and 10s, but with much higher uncertainty relating to the setup of the northern shortwave. More widespread precipitation is likely on Thursday night, but with the possibility most of it remains south or east of the area. At this time, I am siding towards a low pressure tracking southeast of the 40N/70W benchmark, with moderate snow overnight, although confidence is not very high for the overnight hours. With the uncertainty, I kept probability of precipitation at 80% for the area. The two main scenarios at this time are a stronger and further north system, supporting more widespread moderate-possibly heavy snow overnight, and a flatter/further south system, supporting lighter snow with most precipitation staying to the south.


Forecast for NYC area: Widespread light snow is likely to develop on Thursday morning, either near or north of NYC, and continue throughout the day with the possibility of light-moderate accumulations with high temperatures rising into the mid-upper 20s; by the evening, NYC and Long Island may change over to rain or sleet. As winds turn northerly with a tightening pressure gradient overnight, temperatures are expected to fall into the 20s and 10s with more widespread snow, ending by Friday morning with windy conditions and low wind chill values.

As previously noted, there remains a fair amount of uncertainty with the specific impacts relating to the handling of the two shortwave troughs involved. The current outlook is for at least moderate snow accumulations across the area, although the outlook is subject to some changes over the next 1-2 days. For an overall estimation of the precipitation and snow potential, refer to the precipitation/snow probability table. The main scenario potentials at this time are a stronger system supporting more snow accumulations over the area and possible mixing in NYC/Long Island, or a flatter and further south system keeping most of the snow south of the area. Stay tuned for more information over the next few days.


Friday – Beyond: Frigid Weekend, Then Some Moderation

Throughout the month, frigid temperatures have been present across Canada as the polar vortex was primarily focused over Canada this month, with some of the cold air occasionally dipping into parts of the Northeast US as Canadian high pressures entered the region, such as the December 14 and 17 storms when temperatures were in the single digits in the Northeast throughout most of the storm duration, but have failed to spread across the region. By the late week time frame, the polar vortex will have been displaced over southeast Canada, dragging the cold air further south, temporarily settling just north of the US/Canada border. While the polar vortex in its entirety is not expected to crash into the region, a surge of cold air is anticipated behind the late week storm system, with a strong northerly flow in the low-mid levels allowing the frigid temperatures to spill into the region, covering a larger area than earlier cold surges have done this month. How strong the surge of cold is depends partially on the intensity and track of the storm, with a stronger storm closer to the coast leading to a stronger northerly wind at the back end of the storm, which would support much colder than average temperatures for Friday and Saturday.

At this time, I maintained continuity with the late week outlook and sided towards the cold surging into the region, with highs on Friday only peaking in the upper 10s-low 20s, which along with strong northerly winds would lead to wind chills in the single digits. As a broad high pressure builds in overnight, clearing skies are expected with decreasing winds and temperatures likely falling into the single digits across most interior locations and the low 10s in NYC and the coast. This temperature outlook is somewhat conservative compared with the latest model guidance, which depict high temperatures in the 10s across the area and lows near or below zero for most locations north/west of NYC, and is subject to some changes. High temperatures on Saturday are likely to reach the mid-upper 20s, struggling to fall overnight as the cold air mass moves out with cloud cover building into the region.

More interesting scenarios have shown up on the latest model guidance beyond the weekend, depicting another displacement of the polar vortex into south central Canada, with scenarios ranging from the vortex plunging into the northern US as depicted with last night’s GFS and ECM runs showing extreme cold with near record cold temperatures, to the latest GFS run keeping the core of the cold north of the region but with widespread cold temperatures returning after Monday. At this time, the latter scenario is supported more with a gradual return to moderately below average temperatures by the middle of next week, although this time frame remains uncertain and additional changes are expected over the next few days.


<< December 27 | Dec 28 | Dec 29 | December 30 >>

Sunday, December 29 Observations:

12.29.13A deepening coastal low pressure tracked up the coast, bringing significant moisture from the Gulf of Mexico and producing widespread heavy rain across the area, mainly between the late morning and mid afternoon hours; despite the time of the year, precipitation type fell as rain as the cold was bottled up north of another low pressure near southern Canada. Rain totals of at least 3/4 to 1.25 inch were generally observed.

The highest temperature was 56 degrees in Westhampton Beach, NY, and the coolest high temperature was 41 degrees in multiple locations.

Saturday, December 28 Observations:

12.28.13As a westerly flow set up with the cold air mass departing the region, warmer temperatures returned across the region, with a high pressure nearby providing partial cloud cover along with temperatures surging into the 40s and 50s, reaching the upper 40s-low 50s in northwestern and coastal areas and the mid-upper 50s near NYC and northeast NJ. Cloud cover increased overnight, however, as a coastal low approached from the south.

The highest temperature was 57 degrees in Newark, NJ, and the coolest high temperature was 48 degrees in Bridgeport and New Haven, CT.

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