Mar 22, 2014: Cold Returns Next Week

Forecast Highlights:

f96Warmer than average temperatures continued yesterday with highs in the upper 40s to mid 50s, and warmed up even more today into the low to mid 60s. A cold air mass will return again for the first half of the week with highs returning into the 30s along with the possibility of some snow on Tuesday night associated with a rapidly deepening low pressure offshore, followed by another quick warmup into late next week accompanied by additional rain.




Today – Monday: Warm Start, Then Cooling Down


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). Widespread cloud cover was present earlier in the day associated with the low pressure currently near northern NY state, with scattered rain/snow showers mostly remaining north of the area. The cold front passed through earlier this afternoon, but with fairly warm mid level temperatures above freezing at the 850 millibar pressure layer (approximately 1.5km altitude), clearing skies in the early afternoon and an increasing westerly wind at 10-20 mph, gusting up to 30-35 mph, temperatures peaked in the low to mid 60s across most of the area.

This winter has featured a high frequency of cold outbreaks keeping warmth surges short lived, and despite the time of the year in late March, this will be no exception as a broad longwave trough remains situated over the eastern half of Canada, with an upper level block over Alaska aiding in a northerly flow over NW Canada helping to displace the cold air masses into central-southern Canada. There is a broad cold air mass currently centered near southern Canada, accompanied by a strong high pressure near western Canada which can be seen in the surface map above, which will continue to slide east and expand towards the region, leading to colder temperatures returning by Sunday peaking in the low to mid 40s, and crashing into the 10s inland and low-mid 20s near NYC and the coast overnight as a wave of low pressure likely produces some snow south of the area. Unseasonable cold will continue on Monday, with highs peaking in the low-mid 30s inland and the mid to possibly upper 30s elsewhere.


Tuesday – Wednesday: Monitoring Coastal Low Track

The long and active winter is not over just yet with the returning cold next week, and the region is still not done with snow with the model guidance depicting a rapidly intensifying low pressure with an anomalously cold air mass producing snow across parts of the region. This system remains at least 3-4 days away, precluding higher confidence with the exact outcome, but while latest indications suggest the snow will mostly remain to the east, this time frame needs to be monitored for significant impacts across at least parts of the region.


f66Storm Synopsis: Despite the model guidance differing with the track and to some extent the timing, there is remarkable agreement supporting a low pressure developing off the southeastern US coast and undergoing rapid intensification east of the region. The 6z GFS at hour 66 from the Pennsylvania State University e-Wall has been posted to the left, valid at 0z Tuesday (8pm Monday), depicting the key features involved in the development of the storm, depicting a ridge in the western US and three vort maxes, near Missouri, Minnesota, and central Manitoba in Canada. A strong upper level low currently near the northern Pacific will gradually track eastward, sliding underneath the upper level block near Alaska and supporting a transient spike in ridging over the western US. With a strengthening northwesterly upper level flow ahead of the ridging, the central shortwave will quickly race SE towards the Midwest US, later interacting and phasing with the southern shortwave ahead of it as the trough axis becomes neutral and later negatively tilted. The third shortwave from Canada will then interact with the system, with a cold air mass continuing to expand into the region and filter into the storm as it enters New England. With the anticipated phase and negative tilt, as well as a strong jet streak placed over the northeastern US supporting divergence aloft, rapid deepening of the low pressure is expected off the coast.

According to the 6z GFS, the minimum pressure falls from 990.8mb to 973.1mb in only 6 hours, and to 960.5mb another 6 hours later, a total drop of 17.7mb in 6 hours and 30.3mb in 12 hours. Explosive deepening is often defined as a drop in minimum pressure of 2.5 millibars per hour for at least a duration of 12 hours, totaling to a drop of 30 millibars in 12 hours; if the latest model guidance was to verify, this storm would meet the qualifications for explosive deepening. This is not highly anomalous especially in recent years; the most recent case of such rapid deepening was on February 17, 2013, with a low pressure deepening from 1001mb to 954mb in 24 hours as it stayed east of the region before reaching Nova Scotia, as well as February 4, 2013 with a low deepening 46mb in 24 hours to 952mb as it entered southeast Canada, although such rates of intensification are more unusual for storm systems as close to the northeastern US as currently modeled.


12z GFS hour 96, at 6z Wednesday (2am), depicting the rapidly intensifying low pressure east of New Jersey with light snow brushing NYC and heavy snow and strong winds near southeast New England (image credit: PSU e-Wall).

f96Model Analysis: As this is still 3-4 days away, the model guidance is struggling to narrow down the fine details in the setup, which results in slight shifts with each run. The GFS keeps the heaviest precipitation offshore, brushing parts of southeast Massachusetts and downeast Maine, with otherwise light snow accumulations extending into the I-95 corridor. The CMC and UKMET, meanwhile, are further west with moderate snow into the area and major snow accumulations towards eastern LI into SE New England. The ECMWF, meanwhile, shows a different evolution with the 12z run, with the low pressure tracking much further east but with a second area of low pressure extending to its northwest, allowing for moderate precipitation to extend towards the area and parts of New England, but with the low pressure becoming occluded near Cape Cod’s latitude, briefly stalling before tracking due east, as opposed to the GFS and CMC which continue to take the low on a steady northeast trajectory into Nova Scotia without stalling or drifting east.

The model guidance is still anticipated to change over the next few days as the key features are still in regions of poor upper level data sampling, including the southern shortwave originating in the Pacific Ocean which enters the NW US in about 2 days, the Canadian shortwave trough currently north of Alaska which will reach the US late on Monday, and the trailing northern shortwave currently near the polar regions, which will track south through Canada before reaching the US on Tuesday. Given the discontinuity among the model guidance, confidence remains relatively low for this time frame. Preliminary thinking at this time based on the latest data and the upper level flow, including the transient nature of the western US ridging, would support a further east scenario keeping the heavy precipitation offshore with lighter snowfall in the area, but only relatively small revisions would allow for a further west storm with much more significant impacts in coastal parts of the region, or a further east storm which would fail to produce much precipitation. Given the cold air mass in place, precipitation type is expected to fall as snow, but marginal surface temperatures may lead to more of a wet snow setup. More information will be posted over the next few days as the exact setup becomes clearer.


Thursday – Beyond: Warming Up Again

As the trough departs the region on Wednesday, another trough will gradually track east through the northern US, which along with rising heights aloft supports more significant warm air advection into the region as temperatures quickly warm into the 40s on Thursday and well into the 50s on Friday. An area of low pressure is expected to track north of the region before passing to the east around Saturday; depending on its track and the timing of rain, temperatures may reach the 60s on Friday. Another transient cool down is anticipated behind the system, although no major cool down appears to be on the way for the beginning of April at this time.

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