Feb 26, 2014: Cold Friday, Watching Next Week

Forecast Highlights:

temp48A light snow event is currently affecting the area after a week-long break from snow, with up to 1 inch expected. A brief surge of much colder temperatures is expected Thursday night into Friday, falling into the single digits for most locations overnight and reaching the mid 20s for highs. More widespread clouds will build into the region as a widespread storm system affects the region on Sunday and Monday, with a high probability of snow despite the time of the year entering March (Image credit: PSU e-Wall).



12pm Short Term Update

nerad5As of 12pm, most of the snow showers have progressed offshore, but with an area of moderate snow squalls over Long Island which will continue to track east through 1-2pm, producing areas of moderate to locally heavy snow and lower visibility. These squalls are expected to produce up to 1 inch of snow, with locally higher totals under the heavier snow squalls. Another back end line of light-moderate snow showers is developing over north central NJ into SE NY, and will continue to progress ESE as well with light to moderate snow over the next hour expected to affect NE NJ, SE NY/southern CT and NYC.



Today – Saturday: Light Snow Today, Cold Ending To Week


Posted above from left to right are the latest surface analysis and radar composite from WPC, and the initialized GFS 500 millibar heights and vorticity from NCEP MAG; both are valid at 12z (7am). After a week-long pattern relaxation, winter has returned into the northeast US, first marked by the return of colder than average temperatures beginning Monday. Focusing on the eastern half of the US, widespread light snow can be seen extending from Washington DC into New Jersey, associated with a relatively weak shortwave trough over Ohio. This shortwave will continue to race eastward, with scattered snow showers over the NYC area tapering off by 1-2pm with up to 1/2 inch expected, locally up to 1 inch east and south of NYC. Clearing skies are expected later today with high temperatures peaking early in the upper 20s to low 30s, falling into the upper 10s-mid 20s by the evening hours.

With a highly amplified ridge over the western US extending up to Alaska supporting a strong northerly flow extending close to the North Pole, a strong piece of the polar vortex has surged southwards over the last two days and is currently positioned near central Canada. With the usage of the term polar vortex emerging again, especially following the early January cold outbreak, it is important to remember that the polar vortex is not a feature which forms and dissipates at random times, nor is it a feature that can be visibly observed, but is rather a strong upper level cyclone associated with cold surface temperatures that always exists in some form, and is typically centered near the north pole region but occasionally splits and/or shifts away from the polar regions. In this case, the upper level low over Canada is a strong piece of the polar vortex which split several days ago from a strong upper level low over Siberia, associated with some of the coldest temperatures in the northern hemisphere.

12z run of the high-resolution 4k NAM at hour 48, for 12z Friday (7am), depicting the modeled overnight low temperatures, which are over 20 degrees below average. This model may be too cold for NYC and coastal locations (image credit: PSU e-Wall).

temp48While the core of the cold will remain over southern Canada, the region will again experience a brief round of well below average temperatures. A strong shortwave southwest of the Hudson Bay, associated with this strong piece of the polar vortex, will continue to rapidly progress south, then southeast, reaching New York state on Thursday, aiding in the development of a relatively strong but moisture starved low pressure near northern NY state on Thursday. While overnight lows tonight will fall into the mid-upper 0s inland and the lower half of the 10s elsewhere due to brief clearing with a period of radiational cooling, the southwesterly flow ahead of this low pressure will lead to Thursday’s high temperatures reaching the upper 20s-low 30s again. With a strong pressure gradient behind this low pressure, a period of strong cold air advection is expected on Thursday evening and night with winds from the west-NW at 15-25 mph, gusting up to 40 mph at times. With frigid temperatures just north of the US/Canada border, very cold temperatures are expected on Thursday night, likely falling into the 0 to -5 degree range in interior NW NJ/SE NY, low 10s in NYC and the immediate coast, and the single digits elsewhere, generally in the upper half of the 0s closer to NYC and lower half further inland. This will be accompanied by wind chill values near to below zero, especially away from the coast. Winds will subside on Friday as a high pressure moves overhead, with mostly sunny skies and highs in the low-mid 20s across the area. This is nearly 20 degrees below the Central Park average of a high of 44 and low of 31 for February 28. Central Park’s record low of 5 degrees, set in 1934, is likely to remain unchallenged, but with other record lows likely to be approached and/or surpassed, including 15 degrees in JFK (1950), 12 degrees in LaGuardia (1980), 12 degrees in Islip, NY (2008), and 10 in Bridgeport, CT (1950). Warmer temperatures will return for Saturday with mostly cloudy skies and highs in the low-mid 30s.


Sunday – Monday: Snow, Mix Expected, Exact Impact Uncertain

12z GFS at hour 126, at 18z Monday (1pm), depicting widespread moderate snow affecting the area (image credit: PSU e-Wall).

f126Despite Saturday marking the first day of meteorological spring, the actively cold and snowy winter of 2013-14 is not ending just yet, with cold and snow to continue into the beginning of March as well. As the polar vortex becomes elongated over southern Canada, stretching into southwestern Canada, the northern half of the western North American ridging will split into a transient block over Alaska, with a strong shortwave trough currently over the central Pacific Ocean expected to slide underneath the block and reach California by Friday into Saturday, producing a widespread major rain event for locations affected by the severe drought this winter. Meanwhile, as the piece of the polar vortex near southeast Canada retreats further north, a southwesterly flow aloft will set up from the southwestern US into the region, allowing a frontal boundary to extend close to the area by Sunday advecting moisture into the region as the strong shortwave over California continues ENE into the region. Along with the cold air mass available over the northern parts of the region, widespread precipitation is expected across the region, including snow in the area.

As this is several days away, and the main storm system is still in the Pacific Ocean with no upper air data sampling, there remain uncertainties with the specific details in this time frame. Last night’s 0z model guidance continues to show a lack of solid consensus, with the GFS siding towards a more sheared and elongated system with a long duration moderate snow event from Sunday into Tuesday, the CMC depicting a further north frontal boundary with most of the snow north of the area, and the ECM depicting a more consolidated and somewhat faster system producing some mixed precipitation followed by heavy snow during Monday. This uncertainty will likely continue until the model guidance gains a better handle on the key players in the development of this system, including the southern shortwave still in the Pacific Ocean, and the setup of the northern stream over western Canada and the extent of its interaction with the southern system. The northern stream and the positioning of the polar vortex over Canada will also determine the extent of suppression of the southern shortwave, with a further south suppression of height over Canada aiding in a weaker and further south system, and thus a colder outcome for the area. As a result, despite the relative agreement on the synoptic features of the storm supporting widespread snowfall, it is too early to speculate on exact snow totals at this time, although given the coverage area and magnitude of this system, more significant snow totals are expected than the recent storms which were fairly weak.

The GFS tends to have a recurring bias to underestimate the southern stream, and at this time I am siding towards a somewhat more consolidated system affecting the region with some precipitation beginning on Sunday afternoon, continuing overnight with the peak of the storm during Monday, ending by the evening. Besides the CMC, most of the model guidance currently depicts a mostly, if not plain snow event for the area; at this time, I opted to keep a chance of mixed precipitation in the 8-day outlook, although should current trends continue, future outlooks may be revised to keep plain snow in the forecast. The timing is uncertain as well, and the possibility is there for precipitation to extend longer than currently forecast, perhaps into early Tuesday. The specific details will continue to be narrowed down over the next few days as the storm approaches the short range.

Beyond the departure of the early week system, the model guidance shows a variety of solutions depending on the handling of the Monday system, with high temperatures on Tuesday ranging from the 30s on the GFS to the low 20s on the ECMWF, although the operational model guidance has consistently hinted at the potential for another storm system to affect the region on Thursday and Friday, but without the strong ensemble member support which accompanied the early week storm at this time range. This is still over a week away, a time range when the model guidance can significantly change with each run, although this time frame will be monitored for the possibility of precipitation.

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