Feb 22, 2014: Snow Returns This Week

NOTE: The original version of this article featured an incorrect image of the initialized NAM 500mb heights. This has been fixed as of Sunday.

Forecast Highlights:

f96Mostly sunny skies and mild temperatures made a return today and will continue into Sunday, which while typically unremarkable, is a rarity for this winter’s pattern consisting of frequent cold and snow. These conditions will not last, however, as a wintry pattern makes a gradual but significant return into the region beginning on Sunday night with a light rain/snow event, continuing into Wednesday with more snow possible, ultimately peaking by next weekend with well below average temperatures resembling January more than March.




Pattern Overview: Winter To Blast Into The Northeast

f96Over the last week, a relaxation in the global pattern which has dominated the winter flow over North America allowed for a return of warmer than average temperatures as persistent ridging in the northern Pacific collapsed and was temporarily replaced by a strong upper level low, enhanching a more zonal flow near the western US and removing the strong cold air supply from Canada. This proved to be only a temporary change, however, as partially with the aid of continued above normal sea surface temperatures in the northern Pacific and tropical forcing in the equatorial Pacific, strong ridging is currently rebuilding over the northeast Pacific, which will continue to expand north as can be seen in the GEFS 500 millibar geopotential height anomaly map to the left, modeled for Wednesday, from the Pennsylvania State University e-Wall. The northern half of the ridge will split over the next 1-2 days, thus acting a block-like feature sitting over Alaska over the next few days; this will result in a strong northerly upper level flow developing near northwestern Canada, helping to drag a strong piece of the polar vortex currently near the North Pole, associated with frigid temperatures near the surface, southwards into Canada during the first half of next week and marking a return of winter into the region.


Tonight – Monday: Mild Temperatures End With Overnight Snow


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 0z (7pm). In the shorter term, while the significant cold will remain on hold until the second half of next week, the pattern over the northeast US will gradually step down towards more cold and some snow over the next few days. The region is currently under a southwesterly flow in the mid to upper levels; with no colder air mass to advect from the central US northeastward into the region, conditions on Sunday will fail to deviate much from today’s weather across the area, with partly sunny skies and highs in the upper 40s to low 50s.

18z GFS hour 42, for 12z Monday (7am), depicting the low pressure east of Boston with a narrow band of light snow over the area (image credit: PSU e-Wall).

f42This will begin to change on Sunday night, however, as a shortwave trough currently over the northwest US will rapidly track east towards the region, gradually amplifying in the process and aiding in the development of a surface low pressure southeast of the area, which rapidly intensifies as it tracks towards Atlantic Canada on Monday. Increasing upward vertical motion ahead of this shortwave trough is likely to support the development of a narrow band of light to moderate precipitation on Sunday night into Monday morning over the area into southern New England, especially between 11pm and 6am. With temperatures near the boundary layer initially above freezing as colder air slowly filters into the area, precipitation type is likely to initially begin as rain except for interior northwest areas, gradually changing over to snow after 12-2am. The intensity of the snow is still uncertain at this point, along with the coverage area, although given the anticipated mid-upper level forcing, I am siding with the model guidance depicting at least some precipitation for this time frame rather than none. The ensemble members of the GEFS are currently the wettest for this time frame, depicting a much wetter and further inland system, although recently they have not had a solid track record by following the output of the GFS very closely, including offering strong support to the operational GFS run yesterday when it depicted a light-moderate snow event on Tuesday which is no longer anticipated to develop; as such, I am currently anticipating preciptation totals up to 0.10 inch, which would support less than an inch of wet snow for most locations.

The above forecast will be further narrowed down on Sunday; the latest model guidance does not offer full support to this potential, with the latest 0z NAM run failing to depict much, if any precipitation. Given the anticipated setup, the potential outcome in the event that the current forecast does not verify would favor higher odds of a drier solution than a wetter solution, although both scenarios cannot be ruled out at this time.


Tuesday – Wednesday: Temperature Step-Down #1, Snow Likely on Wednesday

The current 500 millibar map in the previous section features a strong upper level low just south of the Hudson Bay, the same storm system which two days ago produced a blizzard in the Midwest and later rain/thunderstorms in the northeast US. This upper level low has been prevented from advancing further north by a transient area of higher heights aloft to its north, acting as a block-like feature in this case. This upper level low is associated with a cold air mass, which currently remains stationed underneath the low over southern Canada. As the polar vortex gradually surges southward through Canada this week, this upper level low will be forced eastward, with the northwesterly flow aloft over the northeast US behind the passage of Monday’s shortwave trough allowing this colder air mass to track southeast into the region, marking the first step down in temperatures, which will return to below average levels beginning on Monday. Partly sunny skies are expected with highs in the upper 20s to low 30s; along with a breezy WNW wind at 10-20 mph, wind chill values in the 10s are expected throughout the day. Similar temperatures will continue into Tuesday but with lighter winds.

12z ECM at hour 96, for 12z Weds (7am), depicting a strong shortwave trough over Ohio with a developing low pressure close to the northeast US coast. The ECM is currently the only model to depict widespread accumulating snow on Wednesday (image credit: PSU e-Wall).

f96_2The next feature to make note of is on Wednesday, as a strong shortwave trough currently near far northwest Canada will dive southwards just ahead of the polar vortex, entering the northern US on Tuesday and reaching the region on Wednesday due to a persistent northwesterly upper level flow. With a lack of upper level data over Canada for the model guidance to ingest in order to more accurately reflect the shortwave trough, there is a large amount of uncertainty at this stage, with most of the model guidance keeping the shortwave relatively flat and weak, leading to a weak low pressure developing offshore, while the ECMWF is notably stronger and more amplified with this shortwave trough, aiding in the development of a stronger low pressure closer to the coast leading to a widespread 3-6 inch snowstorm across the area into New England. This is interesting to note, considering that during the late January time period with a northern stream-dominated upper level flow, when the model guidance often tended to underestimate the strength and amplitude of shortwaves entering the US and trended towards stronger and further northwest low pressures, the GFS was often most aggressive in identifying this trend while the ECM took too long to catch on. In this case, the ECM has support from its ensemble mean, which is often an indication offering more support to the ECM than the remainder of the model guidance, but with no support from the remainder of the model guidance except for a handful of GFS individual ensemble members.

Taking the above into consideration, I am currently siding with a solution slightly less amplified than the ECMWF, which would support light snow accumulations during most of the day with highs in the upper 20s to low 30s; at this stage it is too early to place a high confidence forecast regarding snow totals, although such a scenario would support accumulations up to at least 2-4 inches. This time frame will continue to be monitored over the next few days, considering that most of this winter’s storms tended to feature a short range northwest trend.


Thursday – Beyond: Temperature Step-Down #2, Well Below Average Temperatures

The peak of the cold air mass will affect the region in the late week into the weekend, likely resulting in temperatures even colder than typical January averages. The polar vortex will make its closest approach to the region on Thursday and Friday before likely retreating to the north, bringing a frigid air mass with 850mb temperatures below -30C close to the US/Canada border. Unlike the early January cold outbreak, in this case the core of the cold air mass is not expected to spill over into the US with a lack of significant blocking over Canada to aid in more southward displacement of the strong upper level low, and a lack of more amplified ridging to the west which would help to aid in a more southward track to the cold air aloft. This will keep the coldest temperatures to the north of the region, but will nonetheless support well below average temperatures in the region.

The main uncertainty is exactly how cold it will get; considering that the model guidance often tends to exaggerate the intensity of cold in the medium range, along with an ECMWF cold bias regarding overnight lows especially with the presence of snow cover, I am siding towards somewhat warmer temperatures than the latest models, with partly sunny skies on Thursday and Friday with highs in the mid-upper 20s across the area, perhaps the low 20s in interior northwest locations, and lows in the 10s for most of the area and single digits in northwest NJ/interior SE NY. Depending on the intensification and proximity of Wednesday’s storm, breezy westerly winds are possible on Thursday and perhaps on Friday, which would lead to wind chills in the single digits and perhaps below zero for parts of the area.

The next uncertainty is regarding Saturday’s outlook, as the strong upper level low over Canada retreats further north, with a potential shortwave trough emerging out of Canada as a southern stream shortwave tracks through the southern US. Earlier runs of the ECMWF two days ago developed this into a snowstorm over the region, but have since backed away, with only the 18z GFS depicting light snow affecting the region in this time period. While there is not enough support to firmly side with any storm scenario at this stage, considering the upper level flow in this time period, along with year’s trends and the fact that this is almost a week away, this time period will continue to be monitored for the potential for snow to affect at least parts of the region. In the longer term, below average temperatures are likely to continue into the beginning of March, but with indications towards a more sustained warming trend showing up towards the second week of March.

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