Mar 1, 2014: Snow Returns Monday

Forecast Highlights:

gfs_namer_048_850_temp_mslp_precipVery cold temperatures were again observed this morning outside of NYC, falling to -2 degrees in Sussex, NJ and 0 degrees in Westhampton Beach, NY. A southwesterly flow will lead to temperatures warming up into the mid 30s today, with the frontal boundary then pushing southwards on Sunday. This will set up for some snow to affect the area on Sunday night into Monday, but with major snow accumulations decreasing in likelihood. Frigid temperatures will return for the early week, but with a gradual warming trend by the end of next week.




Short Term: Snow Expected Monday, Heaviest Snow Likely South


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). The significant storm system which has affected California with heavy rain over the last day continues to weaken as it approaches the coast, with a minimum pressure of 987 hPa southwest of San Francisco, and will continue to track ESE as both the upper level low and its associated surface low pressure gradually weaken. The upper level low is expected to open up into a shortwave trough as it passes through the southwestern US tonight, and will continue ENE from there into the central US with an upper level WSW flow. The next feature to consider is the polar vortex over Canada; the center of the vortex can be seen in the 500mb geopotential height map over the Hudson Bay, with an elongated vorticity maximum stretching into southern Canada, just north of North Dakota. The strong polar vortex will help to compress heights over the northern US and push anomalously cold air, especially for this time of the year, further south into the region, which is expected to result in some shearing of the southern shortwave as it approaches the region, with an elongated wave of low pressure approaching the region by Sunday. Despite the shearing of the system, however, moderate to heavy precipitation is expected to develop along this wave of low pressure with the tight contrast in temperatures between the very cold air to the north and warm, moist air to the south, but with the exact positioning and strength of the earlier noted features to determine exactly how much precipitation falls and how much snow accumulates in the NYC area.

Model Analysis: As yesterday’s update noted, the model guidance, after holding fairly steady with a very snowy outcome for the area, have significantly backed away, and as of the latest 12z set of model guidance continues to depict a weaker and more suppressed outcome. Before determining whether this trend is valid or just a typical model swing, some of the reasoning behind this trend needs to be analyzed. Yesterday’s runs, especially in the morning, depicted the polar vortex remaining slightly north and more elongated, with less suppression ahead of the southern shortwave and a further north frontal boundary allowing for a stronger and further north system to track into the region with significant snow totals over Pennsylvania and New Jersey into southern New England. The CMC, however, has been less in agreement with the GFS and ECMWF, and has generally depicted a more disorganized and suppressed system.

From left to right are the latest 12z GFS initialization (hour 0), and the 2/27 18z run of the GFS at hour 42 for the same time frame, depicting the differences in the model runs over the last two days, most notably with a stronger polar vortex than earlier modeled (image credit: NCEP MAG).


Over the last two days, however, the model guidance has continued to slightly narrow down on the exact handling of the key players, which despite only seemingly minimal shifts, have had a more significant impact on the development of the system downstream over the area. The handling of the Pacific system had some slight inconsistencies, especially with the GFS trending slightly slower and flatter with the shortwave as it enters the southwestern US, but the more significant impacts come from the handling of the polar vortex, which has initialized stronger than earlier modeled, especially with a stronger vorticity maximum near the northwestern periphery of the vortex. This stronger vort max then swings further south than earlier model runs depicted, further suppressing heights over the northeastern US ahead of the southern shortwave, leading to some additional shearing of the system, and pushing the frontal boundary on Sunday further south, leading to a more elongated and suppressed southern stream shortwave, with this trend towards a weaker and more strung-out system also leading to a slightly drier outcome with precipitation stretched out across a larger coverage area as opposed to a single, consolidated strong wave of low pressure.

12z GFS at hour 48, at 12z Monday (7am), depicting a suppressed wave of low pressure with the area near the northern end of the steady snow. The suppressed heights can be seen over the northeast US, with the southern shortwave approaching Kentucky (image credit: PSU e-Wall).

f48With the trends above noted, the latest runs of the model guidance have reflected towards a more suppressed, weaker, elongated and slightly drier system affecting the region. The GFS and ECMWF, previously having been in solid agreement for over a foot of snow in the area, have trended significantly drier with the 0z ECMWF and 12z GFS depicting below 6-7 inches across the area, with accumulations as low as 1-3 inches in SE NY and southern CT. The next step is to consider whether this is the final outcome or if a change back to the earlier scenario is possible; considering that the latest initialization of the upper levels has had minor but significant differences which make a return to the earlier depiction of over 12 inches over the area much less likely, there is likely validity to this trend towards a further south and strung-out system. It is, however, possible that the extent may have gone too far south on some of the model runs. Even so, the sharp precipitation gradient in the northern end of the precipitation shield serves to only increase the level of uncertainty, as a relatively minor shift would mean the difference between 1-3 inch accumulations across the area and accumulations up to 8-10 inches reaching at least the I-80 corridor. As such, I lowered my forecast accumulations and decreased the forecast range compared with yesterday’s update, but with some uncertainty remaining with the exact handling of the upper level features, with some additional revisions possible later today and on Sunday; at this time, I did not lower totals to the full extent of the latest model guidance, but will continue to monitor the trends for further adjustments.


Forecast for NYC Area: As the frontal boundary gradually shifts south through the area, temperatures are expected to peak in the low-mid 30s on Saturday with light rain/snow/sleet developing in the mid-late afternoon hours. As temperatures steadily cool down overnight, falling into the low-mid 10s inland and the upper 10s-low 20s elsewhere, periods of light snow are expected overnight into Monday morning, with a northerly wind around 10-15mph. Snow is then expected to gradually taper off from NW to SE, starting in the mid-late morning hours inland and ending in the mid afternoon hours towards Long Island. High temperatures on Monday will be unseasonably cold, peaking only in the upper 10s-low 20s inland and the low-mid 20s elsewhere; this is expected to be the coldest March high temperature since 2009 or 2007.

Given the latest developments, the latest snow outlook is for 2-5 inches in southeast NY and southern CT, and 4-8 inches elsewhere across the area, perhaps slightly lower further north, with lower totals towards northern NJ and higher totals towards southern parts of Long Island/NYC into the I-78 corridor in NJ. As previously mentioned, this remains a difficult forecast, especially given the tight precipitation gradient near the area and the small changes in the handling of the upper level features which lead to more significant impacts downstream, and some changes in the forecast are possible later today and on Sunday. The possibility for a slightly further north system than currently modeled can’t be ruled out, in which case snow totals may be slightly increased; at this time, however, I sided slightly higher than the latest 12z GFS and 0z ECMWF models, and in the event that the latest models verify, snow totals may be slightly lower than currently forecast. If necessary, an evening update may be posted, otherwise the next update will be on Sunday.


Later Next Week: Frigid Midweek, Warmer And Possibly Wet Ending

Frigid temperatures are expected in the wake of Monday’s storm, with overnight lows expected to fall to near zero towards interior NW NJ/SE NY, low 10s near NYC, the immediate suburbs and Long Island, and the single digits elsewhere. Highs on Wednesday are expected to peak in the mid 20s for most of the area, generally remaining 20 degrees below average. Towards Wednesday into Thursday morning, a strong high pressure is expected to build into the region, providing clearing skies and another opportunity for lows to approach zero degrees in NW NJ/SE NY and the Long Island Pine Barrens overnight. As this occurs near the surface, some changes will be taking place aloft as a shortwave trough is expected to reach the southeast US with a low pressure developing near the Gulf Coast, but with significant model differences regarding the handling of this system. These uncertainties preclude high confidence regarding its future track, although this system needs to be monitored for the potential to produce precipitation over the region on Thursday night into Friday, but with much less cold air available given the departure of the polar vortex and the cold air mass over the region, leading to marginally cool temperatures. At this time, a slight chance of rain/snow has been added into the forecast for Friday, and this time frame will continue to be monitored.

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