Feb 24, 2014: Colder Week Ahead, Some Snow

Forecast Highlights:

gfs_namer_054_850_temp_mslp_precipA cold front moved through the area last night, accompanied by light rain showers, bringing an end to the warmer than average temperatures and a resumption of the colder than average pattern through early March. Temperatures will generally remain in the mid 20s-low 30s for highs with lows in the 10s to low-mid 20s, falling into the single digits inland at times. This week will be fairly dry with light snow on Wednesday, but with increasing precipitation chances by next week.




[notice]Note: The 8-day outlook is now scheduled to be updated on a daily basis again.[/notice]

Today – Wednesday: Cooling Down, Light Snow Wednesday


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). Conditions are fairly calm across the eastern half of the US with a broad area of high pressure extending from the upper Midwest into the Ohio Valley, while an upper level trough over the northeast US signals the cold air mass having returned into the region, with temperatures today expected to peak in the mid 30s for most locations. With an upper level WNW flow, a shortwave trough currently north of the Dakotas will rapidly progress ESE towards the region, flattening out in the process with a lack of height rises to its east, leading to scattered snow showers mostly remaining south of the area on Tuesday with high temperatures in the upper 20s to low 30s.

12z GFS at hour 54, at 18z Weds (1pm), depicting a weak low pressure developing offshore with the shortwave trough over Massachusetts (image credit: PSU e-Wall).

f54The next feature to monitor is a strong shortwave trough near the northwest Hudson Bay in Canada. With a strong northerly flow over Canada as a result of a strong block-like feature over Alaska, this shortwave trough will continue south, then SE into the north central and northeast US, where slight amplification is expected with weak height rises ahead of the departing trough currently over the region. Along with some involvement from moisture near the southern US, this will aid in the development of light snow over the southern Ohio Valley on Tuesday night, rapidly progressing NE on Wednesday as a low pressure develops offshore and gradually strengthens. The main uncertainty is the handling of this northern stream feature and the intensity/track of the developing low pressure which will determine how far west snow extends and how much accumulates. Since the previous update, most of the model guidance has slowly shifted west, supporting at least light snow over the area, while the ECMWF, previously the most amplified model, backed down from its earlier 3-6 inch output with its latest runs suggesting a lighter 1-3 inch snow event. Forecasting these north stream features can be difficult as the model guidance occasionally tends to underestimate the strength and amplitudes of shortwaves emerging out of Canada, although it is important to note that despite the high frequency of short range northwest trends within the 60-84 hour range, the model guidance overestimated the extent of this trend with most recent cases and slightly backed down.

At this time, there is support for a quick 6-hour snow event during Monday morning, mostly ending by 12pm, with light snow rates generally expected. Snow totals up to at least 1-2 inches are likely, with lower totals further west and higher totals further east. In the event that the storm does end up more amplified than currently forecast, snow totals over 2 inches would be possible south and east of NYC. High temperatures are expected to reach the upper 20s to low 30s.


Thursday – Sunday: Cold Continues, Some Snow Saturday

Following the departure of Wednesday’s system, the polar vortex will make its closest approach to the region with well below average temperatures expected. A brief period of clearing skies is likely on Wednesday night, but with a progressive upper level flow preventing a high pressure from persisting for a longer period of time, supporting low temperatures in the low 10s, perhaps the upper single digits, in interior NW NJ/SE NY and the mid 10s to near 20 degrees for the rest of the area. A weak clipper-like system will pass through northern New England on Thursday, with high temperatures in the upper 20s to low 30s expected, while a stronger northwesterly flow in its wake will help to bring in colder temperatures overnight into Friday, while the stronger pressure gradient will lead to windy conditions for a period of time on Thursday evening into the early overnight hours. These strong winds will initially prevent significant cooling to the coldest potential, although cold temperatures are expected nonetheless, falling into the mid 0s-low 10s inland and the low-upper 10s for the rest of the area, with highs on Friday mostly in the mid to upper 20s; average temperatures for February 28 in Central Park are 44 for the high and 30 for the low.

The major highlight in the US in the medium range is the return of widespread rain to California, which has been affected by a drought for the majority of the winter, with two southern stream shortwaves tracking through California in the late week underneath continued upper level ridging over Alaska. The first shortwave will move through California on Wednesday, producing widespread rain over the northern half of California before rapidly tracking eastward through the central US. Given a fast upper level flow over the northern US in association with the retreating polar vortex, however, significant interaction with the northern stream appears unlikely, which is likely to prevent a significant storm system from developing. While how much precipitation affects the region remains uncertain, the potential exists for a light snow event, perhaps including the area, during Saturday. A transient period of stronger northwesterly winds behind this system will help to drag in colder temperatures again for Saturday night into Sunday, with temperatures similar to those of Friday; this is likely to be the first March to feature a high temperature below 30 since 2009, when high temperatures on 3/3 peaked at 26 degrees.


Next Week: More Widespread Rain/Snow Possible

The second and stronger shortwave trough is likely to land onshore in California on Friday night into Saturday; if the track verifies close to the latest model projections, this would produce a significant rain event in southern California. By this time frame, the strong polar vortex over Canada will have retreated further north with a weaker geopotential height gradient allowing for more significant height rises ahead of this shortwave and perhaps more interaction with the northern stream. This type of setup often supports a high probability of precipitation affecting parts of the northeast US in the longer range as this system tracks east through the US in the following days. As this is a week away, many uncertainties remain at this stage, including the exact positioning of the southern stream shortwave over California, which is often difficult for the model guidance to properly handle with a lack of data sampling over the Pacific Ocean, along with the setup of the northern stream flow over Canada, although this time period currently has strong support from the model guidance and the ensembles for precipitation to affect the region, and with an antecedent cold air mass in place, the potential for snow exists as well despite the time of the year entering March. More information will be posted on this time period over the next few days as details become clearer.

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