Feb 7, 2014: Light Snow Sunday Night

Forecast Highlights:

f60Following the two consecutive significant winter storms a few days ago, the week has ended on a much quieter note with dry and chilly conditions. While no major snowstorm currently appears to be on the way, several potentials for accumulating snow exist in the coming week, including a light snow event on Sunday night and another storm potential on Thursday which may involve rain as well, while temperatures generally remain below average.




Tonight – Monday: Dry Start, Snowier Ending To Weekend


The latest sea level pressure analysis and national radar composite from the Weather Prediction Center (WPC) has been posted to the left, with the 18z GFS initialized 500 millibar geopotential height and vorticity map posted above to the right from NCEP Model Analyses and Guidance. Unlike the last few days, the pattern over the eastern half of the US is fairly inactive as a broad 1032 millibar high pressure is centered near the Ohio Valley. A broad southwesterly flow is present in the mid and upper levels, but with a fairly cold air mass remaining overhead with 850mb temperatures near -10C supporting temperatures remaining below average, having peaked in the low 30s for most of the area today. More activity is present in the western US, however, which will expand into the eastern half of the US by Saturday into Monday, bringing more widespread cloud cover and precipitation. The first feature to note is a shortwave trough over western Texas, which is enhancing mid level lifting to the east and aiding in the persistence of showers over eastern Texas. With a progressive WSW flow in the upper levels, the shortwave trough will rapidly track northeast towards the southeast US as a weak low pressure develops off the coast, enhancing more widespread rain over the southeast US on Saturday extending into southern Virginia before tracking offshore. This event is less than 24 hours away, and given an absence of any notable short range trend on the model guidance or correction towards a much stronger or amplified shortwave, this system will remain south of the area, with mostly cloudy skies on Saturday and highs in the upper 20s to low 30s.

18z GFS at hour 60, for 6z Monday (1am), depicting light snow covering the area. The GFS is currently snowier than the CMC and ECMWF models (image credit: PSU e-Wall).

f60With a fast, progressive and relatively zonal flow aloft over the eastern US, implying a lack of significant amplification of any trough or ridge, and which is depicted fairly well with the 500mb chart above with the height contours aligned almost due west to east, the second system which is currently producing rain and snow over the western US will quickly track to the east as well, and will reach the area on Sunday night into early Monday. This is the same storm that a week ago was modeled as a major snowstorm on the medium range model guidance; due to reasons explained in yesterday’s discussion, there will be no major snowstorm this weekend. As the shortwave trough currently over the NW US quickly reaches the north central US on Saturday night, an area of light to moderate snow is expected to develop near the Ohio Valley, which then reaches the region late on Sunday into the overnight hours. The model guidance over the last day has generally indicated only scattered snow showers remaining to the south on Sunday night, but have trended slightly stronger and more amplified with the shortwave, especially the 18z GFS which produces at least 2-3 inches of snow in the area on Sunday night.

While the fairly zonal flow will prevent a major snowstorm from developing, this system has the potential to continue trending towards a somewhat stronger and wetter system than currently depicted, which would produce widespread light snow accumulations across the region. This is particularly supported by the GFS ensemble members, with the majority of the members depicting more precipitation than the operational run, and the NAVGEM model, which is known to have a progressive and weak/suppressed bias with storms, but in this case is noticeably slower, stronger and more amplified with the system; while its exact output may not verify, it is often a signal that the remainder of the model guidance may trend more amplified with a system. The last time the NAVGEM was well to the north of the rest of the models was a few days prior to the 2/3 storm, which ultimately trended north on the remainder of the model guidance. The outlook at this time is for periods of light snow on Sunday night into Monday morning with at least 1 to 3 inches across the area; currently, the lower end of this range is supported by most models except for the GFS and is the most likely to verify, although the potential for the higher end totals of this range would be more likely should the system trend wetter over the next day; should the trend towards a wetter system end up more significant than currently anticipated, snow totals over 3 inches may be possible. More information will be posted with Saturday’s update.


Next Week’s Outlook: Chilly/Dry Start, Another Storm Wed-Thu

The time period early next week will continue with the progressive and relatively zonal flow aloft, with no precipitation anticipated through Tuesday; a few models hint at snow showers south of the area on Tuesday, although the upper level low appears too flat to support significant amplification of this system to bring snow into the area. A slightly cooler air mass will slide ESE from the Midwest into the region for Monday as a high pressure builds eastward, with partly sunny skies and highs in the mid to upper 20s. Overnight lows are expected to fall into the upper 0s inland and 10s elsewhere, with mostly sunny skies on Tuesday and high temperatures peaking in the mid 20s. The high pressure will be placed over the area on Tuesday, supporting mostly clear skies and light winds, which along with the remaining snow cover is likely to support radiational cooling, leading to cold overnight lows especially away from urban areas and the coast. Lows near NYC and the coast are expected to fall into the low 10s, but with much colder temperatures elsewhere falling into the single digits in the north/west suburbs of NYC, southern CT and most of Long Island, and likely falling below zero in NW NJ, interior SE NY and possibly the Pine Barrens in Long Island.

The next storm system will approach on Wednesday night into Thursday, with the latest model guidance already indicating a significant spread of possible solutions, leading to lower confidence regarding the exact impacts in the region. The most aggressive model in developing the storm was the 12z ECM, depicting a widespread snowstorm along the I-95 corridor; it is important to note, however, especially considering that just a few days ago the model guidance supported a major snowstorm for this Sunday, that any exact model output in the medium to long range especially beyond 5 days needs to be taken with caution, especially if there is a lack of run-to-run continuity as there is with this case. The origins of this system can be traced back to the northeast Pacific, with a strong upper level low currently positioned south of Alaska, which progresses southeast into southern Canada by the middle of next week. As this is in a region with poor data sampling, with full sampling of the shortwaves involved in this setup unlikely until at least Tuesday, significant changes are still possible in the exact setup, which is also highlighted by the GFS ensemble members which depict a wide variety of possible scenarios. Given a southwesterly flow from the surface to the mid-upper levels ahead of this system supporting the removal of the early week strong cold air mass, a lack of a strong cold air source appears likely to be of concern in this time frame, which supports the potential for a mixed precipitation and/or rain event. At this time, it is too early to determine the specific details with more confidence, although the potential exists for a wintry mix event in this time period with the possibility for some snow but with lower probabilities of a major snowstorm than with previous events.

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