Mar 2, 2014: Light Snow Tomorrow

Forecast Highlights:

snowmap_2A widespread storm system will affect the region today into tomorrow, but with the majority of the snow remaining south of the area with only light accumulations generally expected tonight into tomorrow morning. Besides the snow, however, the main highlight will be near record cold returning once again from Monday through Wednesday, with temperatures over 20 degrees below average, with a gradual warming trend anticipated towards the end of the week.




Today – Monday: Light Snow Near NYC, Heaviest Snow Far to the 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). A slow moving frontal boundary can be seen extending from coastal Maine into Pennsylvania, the southern Ohio Valley and Texas, with widespread precipitation to its north, falling mostly in the form of snow and ice, along the temperature contrast between an anomalously cold air mass to the north and a warm and moist air mass to its south. The key players involved in the setup can be seen in the 500mb map, consisting of the southern shortwave, which has weakened since affecting California with heavy rain and is positioned near New Mexico, and the polar vortex, which is centered east of the Hudson Bay but with a vorticity maximum extending to its southwest near northern Ontario. This vort max has continuously trended stronger than previously modeled throughout the last 2-3 days of model runs, which then swings further south than earlier runs depicted and suppresses geopotential heights over the northeast US ahead of the southern shortwave while aiding to push the frontal boundary further south, leading to additional shearing of the system and a weaker, more suppressed and slightly drier system affecting the region tonight into Monday. Given that the latest observed conditions better reflect those of the latest initialization of the model guidance than the model runs 2-3 days ago, this southern trend is valid and decreases the likelihood of a 6-8+ inch event affecting the area.

As of yesterday’s evening update, the main question was whether the south trend was completed or if additional south shifts were still possible. The latest model runs as of this morning have either held steady or shifted slightly further south, with additional minor north or south revisions possible as the exact development of the system becomes clearer. As of yesterday’s update, I kept snow totals higher than the latest guidance to account for the uncertainty, and to avoid significant discontinuities between updates, although taking the latest data into consideration, I opted to slightly decrease snow totals from yesterday’s outlook.

This case is an example of the problems with taking the model guidance output as certainty, considering that as recently as the morning runs of 2/28, most of the models, especially the ECMWF and GFS, were in strong agreement regarding over 10-12 inches of snow from Pennsylvania into the NYC area and southern New England. Despite this model agreement, however, at no point was a 10+ inch scenario considered a guarantee for the area; as many cases have shown in the past, recently including the February 9-10 storm potential that failed to materialize to its potential, model consistency does not necessarily imply accuracy, regardless of the number of runs which have continuously depicted a similar output. Additionally, during the time period that this outcome was depicted on the models, both key features, the southern shortwave and the polar vortex, were still located in regions of poor upper level data sampling with little or no RAOB data from weather balloons; this also likely played some role in the model errors as the handling of the polar vortex was inconsistent, having been modeled as weaker and more elongated than actually observed, and the southern shortwave was also slightly slower and ultimately flatter over the US than originally indicated. This was reflected in the lower than usual confidence in the forecast discussions leading up to this storm, as earlier forecasts noted the significant snow totals as a potential rather than a high-likelihood expectation. Some of my more recent updates noted a drier and suppressed scenario to be the main bust risk, but underestimated the magnitude of the trends which were observed with the latest model runs, and I was too slow to lower the snow outlook; even the latest forecast may still be a little too far on the high end. While the exact outcome is not certain yet, thus making it too early to determine if the outlook was correct or not, the original modeled scenario of over 6-8 inches across the area is no longer a realistic scenario.


snowmap_2Forecast for NYC Area: Light snow/sleet showers are expected to develop later this afternoon and evening, with occasional periods of light snow tonight as temperatures gradually drop into the low-mid 10s inland and the upper 10s elsewhere. Snow will gradually end in the morning hours from NW to SE, with most of the area expected to be dry by at least 10am-12pm. Mostly cloudy skies are expected later on Monday with high temperatures in the upper 10s-low 20s inland and the low-mid 20s elsewhere, with most of, if not all of the snow from the second wave of precipitation during the day on Monday remaining south of the area.

Snow accumulations of at least 1-3 inches are expected in northern NJ north of I-80, SE NY and southern CT, and 2-4 inches elsewhere across the area; current thinking is leaning towards the lower end of this range. As previously noted, the exact snow totals may end up slightly higher or lower than currently forecast, especially across southern parts of the area which are likely to be just north of a tight precipitation gradient. Considering the continued trends towards a stronger polar vortex diving further south into the NE US, however, the main bust risk at this point is regarding even less snow than currently forecast, which would support less than 2-3 inches across the entire area and less than 1 inch in northern regions. A few evening updates will be posted as precipitation affects the region.


Next Week – Weekend: Frigid Temps, Then Warming Up

Besides Monday’s light snow event, the main highlight this week will be the frigid temperatures expected for Monday through Wednesday, with temperatures approaching but not as likely to break record lows. Overnight lows on Monday are expected to fall to near zero in interior NW NJ/SE NY, low 10s near NYC and most of Long Island, and the single digits elsewhere. High temperatures on Tuesday and Wednesday will remain 15-20 degrees below average, peaking in the 25-30 degree range for most of the area. Mostly cloudy skies on Tuesday night will keep low temperatures mostly in the 10s, while a high pressure sliding to the north on Wednesday night will provide a brief period of clearing skies and temperatures likely falling back into the single digits towards interior NW NJ, SE CT and central CT, perhaps including the Long Island Pine Barrens.

As the polar vortex departs Canada by late Tuesday, the frigid air mass will move out of the region, with a seasonably cool air mass remaining in place which is likely to be further moderated by a southwesterly flow ahead of a shortwave trough over southern Canada. This will support temperatures on Thursday gradually warming up, likely peaking in the upper 20s to low 30s. Attention then turns to the southeast US, where a fairly strong shortwave trough, perhaps closing off into an upper level low, passes near the Gulf of Mexico, where a surface low pressure is expected to develop and track northeast. The exact track remains uncertain, with most of the model guidance keeping precipitation mostly, if not entirely east of the area, while the moderated cool air mass will likely lead to marginal temperatures supporting a mix or perhaps rain if precipitation does reach the region. At this time, a slight chance of rain/snow remains in place for Thursday night into Friday, and this time period will continue to be monitored. Beyond the late week, slightly below average temperatures are likely to continue into next weekend and next week, with signs of a possible warm up towards average but with a sustained, more prolonged period of above average temperatures unlikely at this time.

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