Mar 23, 2014: Early Week Cold, Then Warming Up

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Forecast Highlights:

gfs_namer_066_850_temp_mslp_precipThe brief warmth surge over the last few days has been replaced once again by a cold air mass, with temperatures tomorrow to peak in the low-mid 30s, nearly 15-20 degrees below average. A low pressure will undergo explosive deepening offshore on Tuesday but will only brush the region with light snow, especially near the coast, followed by another warm up for the late week with highs returning into the 50s and possibly 60s.

 

 


 

Tonight – Monday: Cold Returns

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Posted above from left to right are the latest available surface analysis and radar composite from WPC, and the NAM initialized 500 millibar heights and vorticity from NCEP MAG; both are valid at 18z (2pm). The frontal boundary which passed through the region yesterday has progressed well to the southeast, with a weak wave of low pressure offshore and an area of rain/snow over Virginia associated with a shortwave trough near the Ohio Valley continuing ENE and offshore. This trough is associated with yet another anomalously cold air mass, a recurring theme this winter, accompanied by a broad surface high pressure near the Midwest which will extend into the region tonight. This will provide the area with mostly clear skies tonight and on Monday, with temperatures tonight falling into the 10s inland and low-mid 20s near NYC and the immediate coast, with highs on Monday only peaking in the low-mid 30s across most of the area and mid to possibly upper 30s near NYC, despite full sunshine. Average high temperatures for this time of the year are generally in the low 50s, with average lows in the upper 20s-mid 30s.

 

Tuesday – Wednesday: Storm Deepens Offshore, Some Snow Expected

What appears to be the last snow potential in the foreseeable future, in a sign that winter is slowly approaching its end, is anticipated to affect the region on Tuesday night into Wednesday, marking an end to a month which has seen numerous moderate to significant snowstorms across the region, but remarkably with every snowstorm remaining either south or north of the area, despite well below average temperatures. With a transient spike in ridging in the western US, a deepening trough is expected over the eastern US with three shortwave troughs, one currently near Montana, another near the Northwest Territories in NW Canada, and another near north central Canada, meeting up near the Ohio Valley with phasing expected to occur as the trough becomes negatively tilted. Along with an impressive upper level jet structure with rapid acceleration near the right entrance and left exit quadrants of two jet streaks, a low pressure will develop off the southeastern US coast and explosively deepen overnight, falling from about 1000mb to 964mb in just 18 hours as it approaches Nova Scotia, likely reaching the low-end criteria for explosive deepening of a drop of 2.5mb per hour.

While this is expected to end up as a major storm system capable of producing blizzard conditions with significant snowfall, the positioning of the system will be too far east to affect most of the region with such conditions. As previously noted, the spike in ridging in the western US will remain transient, as an approaching upper level low entering the NW US along with a vorticity lobe approaching from western Canada help to flatten out the ridge, leading to a more zonal flow setting up which later results in a warmer ending to the week. Additionally, the latest set of model runs has generally trended towards slightly weaker vorticity with the mid level low taking longer to close off, with the low pressure rapidly intensifying further east and later than modeled yesterday. This leads to much higher confidence that the worst impacts of the storm, including the heavy snowfall and strong winds, will remain east of the region, instead affecting Nova Scotia. With this considered, however, uncertainty remains regarding some minor details, which will continue to gradually narrow down as the shortwaves responsible for the development of the system enter regions of better upper level data sampling. While the heaviest snow is expected to stay offshore, east-central Long Island and SE CT are close to the axis of moderate snowfall, and a slight westward shift would support higher totals near or over 6″ in these locations. At the same rate, a slight eastward shift would place the moderate snow offshore, with coastal regions, even eastern LI and Cape Cod, only experiencing light snowfall directly associated with the coastal low. Additionally, some of the model runs hint at an inverted trough feature enhancing snow totals close to the area, but these features are often difficult to narrow down with high confidence in advance, specifically with regards to the placement of the inverted trough and snow totals, assuming one does take shape.

With the above taken into consideration, along with the anomalously cold air mass supporting precipitation type falling as snow but with marginal surface temperatures in the 30s perhaps a limiting factor for accumulations for any snow that falls before Thursday evening, latest thinking for snow totals is generally up to 2 inches across the area, perhaps higher in eastern LI and SE CT. Some uncertainty remains as earlier noted, especially regarding the possibility of an inverted trough which would enhance snow totals across most of the area, and a further west precipitation shield directly associated with the coastal low which would lead to higher totals over SE CT and eastern Long Island than currently forecast.

 

Late Next Week: Warming Up Again

As previously noted, with the breakdown of the transient western US ridging, a more zonal flow will resume for the late week as a low pressure tracks east from the Midwest into SE Canada, leading to warm air advection across the region with temperatures rising into the 40s on Thursday and well into the 50s on Friday, possibly in the low 60s depending on the timing of showers on Thursday night. Uncertainty increases on Saturday with the ECMWF depicting a low pressure well to the north of the area and the GFS and CMC depicting the low pressure near the area, which would support a moderate to heavy rain event; at this time, I am siding closer to the ECM, which would support some showers on Friday night-Saturday morning with highs rising again well into the 50s and possibly low 60s. Another transient cool down is expected on Sunday with highs in the 40s, but with a more significant warm up into the 50s and possibly 60s likely by early next week.

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