Nov 21, 2013: Winter-Like Cold For Sunday-Monday

Forecast Highlights:

gfs_namer_063_500_vort_htTemperatures will continue to gradually warm up today and on Friday, returning into the mid 50s for highs, along with scattered showers on Friday-Saturday but with minimal rain totals. A strong cold surge will enter the region on Sunday and Monday, bringing the coldest temperatures of this fall with winter-like temperatures, barely reaching 32 degrees for highs and falling into the mid 10s-low 20s away from NYC and the coast. This cold will be transient, however, with moderating temperatures by the mid week followed by a possibility of rain by the middle of next week.



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Wednesday, November 20 Observations:

11.20.13With the cold air mass in place following Tuesday’s midnight high temperatures, colder maximum temperatures were recorded on Wednesday, in the low to mid 40s across the area, at least 6-10 degrees below average. Mostly sunny skies were generally observed with a strong high pressure nearby, with a decreasing northerly wind. The highest temperature was 47 degrees in Somerville, NJ, and the coolest high temperature was 41 degrees in Montgomery, NY and Sussex, NJ.

 

 


 

Today – Saturday: Warmer Temps, Some Showers

With a strong high pressure moving through last night, cold temperatures were observed this morning away from NYC and the coast, with lows near 16 degrees in Sussex, NJ and Montgomery, NY. With strong radiational cooling due to clear skies and light winds, Long Island was able to significantly cool down as well, with a low of 19 degrees in Westhampton Beach, NY. Temperatures have quickly rebounded afterwards, however, with light southerly winds expected today along with partly sunny skies and highs reaching the upper 40s to low 50s. Cloud cover will increase overnight into Friday, however, as a fast moving cold front associated with a weak wave of low pressure near northern Maine passes through the region.

12z run of the high resolution 4k NAM, for 12z Friday (7 AM), depicting an area of light rain focused mostly north of NYC (image credit: NCEP MAG).

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The timing of the front has slightly changed from the previous forecasts, however, with occasional showers now expected on Thursday night into Friday morning ahead of the front, followed by another round on Friday night associated with the actual front. Rain amounts are generally expected to be minimal, less than 0.1 to 0.2 inch, with the highest totals north of NYC. In between the two rounds of light rain showers, temperatures are expected to rise into the low to mid 50s across the area, perhaps above 55 degrees near and/or southwest of NYC. Saturday is now expected to be dry, with partly sunny skies, a breezy NW wind at 10-15 mph, and highs reaching the low-mid 40s inland and mid-upper 40s for the rest of the area.

 

Sunday – Tuesday: Very Cold, Windy, Possible Snow Showers

While temperatures will cool down on Saturday, the strongest cold won’t arrive until Sunday, however, behind a second cold front moving through on Saturday night. A strong cold air mass has been building up over western Canada over the last few days, and along with ridging building over the western US, this air mass will be able to spill southeast into the eastern US behind Saturday night’s cold front, bringing the coldest temperatures of this fall. Widespread unseasonably cold 850 millibar temperatures of -14C to -18C are expected across the region, typical of an average wintertime cold surge, which along with a tight pressure gradient on Sunday setting up for strong cold air advection and windy conditions, will make Sunday feel more like January than late November. With the trough axis moving through the area on Sunday, partly cloudy skies are expected with the possibility of isolated snow showers across the area.

12z NAM for Sunday, showing the tight pressure gradient, strong NW winds, and cold high temperatures; this run may be 1-2 degrees too cold for some (image credit: PSU e-Wall).

f75The 24-hour high temperatures on Sunday are expected to occur at midnight, peaking in the mid-upper 20s inland and the low-mid 30s from NYC and further east/south. This will be as warm as temperatures get, however, as with the aforementioned strong cold air advection, partly sunny skies and a strong NW wind at 20-30 mph, gusting up to 40-45 mph, very cold temperatures are expected with unseasonably cold afternoon high temperatures, peaking in the mid to upper 20s inland and upper 20s to low 30s for the rest of the area; based on latest indications, it is possible the entire area may fail to rise above 32 degrees during the day. Along with the strong winds, wind chill values during the day are expected to end up in the 15-25 degree range. These temperatures are approximately 20 degrees below average, and are even colder than the average high temperatures in January.

The coldest temperatures are expected overnight, with the high pressure moving overhead resulting in clearing cloud cover along with very cold temperatures aloft. A continued NW wind will somewhat limit how cold temperatures will end up, especially for interior locations, but with the immediate NYC area likely approaching record lows. Temperatures at this time are likely to fall into the low to mid 10s in interior NW NJ/SE NY and interior CT, mid 10s to low 20s in the north/west suburbs of NYC, southern CT and Long Island, and the mid 20s in NYC and the immediate coast. While winds won’t be as strong as during the day, wind chill values in the single digits to low 10s are likely inland. Temperatures will slightly warm up on Monday into the low-mid 30s inland and mid-upper 30s for the rest of the area with mostly sunny skies as the cold air mass begins to moderate.

 

Tuesday – Thanksgiving Weekend: Still Cold; Rain Possible?

Storm Analysis: While temperatures from Sunday through next week are expected to average out to below to well below average, the cold air mass from Sunday and Monday will fail to become sustained as the progressive flow continues aloft, and will quickly exit by Monday into Monday night ahead of another trough entering the north central US with a surface low pressure in southern Canada. This will lead to a brief moderation in temperatures, rising into the low to mid 40s for highs, which despite being quite a bit warmer than Sunday’s temperatures, is still below average.

The main uncertainty in the outlook is towards Wednesday, when some of the model guidance continues to hint at a storm potential but with uncertainty regarding the details. This potential arises from a strong upper level low stationed over southern California during the weekend, which will then exit the southwest region and track east towards the south central US by the early-mid week, picking up moisture from the Gulf of Mexico. Beyond that point, the future track and intensity of the storm depend on multiple variables, such as the location, amplitude and timing of the northern stream and how they both interact, which also depends on how strong the western US ridge ends up, as well as the amplitude of ridging ahead of the southern shortwave, especially if it misses the connection with the north stream until tracking offshore. Yesterday’s models showed more interaction and had the northern trough picking up the southern shortwave further west but not fully phasing, allowing the storm to track up the coast, leading to a widespread rain/snow event, but most of today’s models have backed away from this solution, leaving the storm as a mostly southern stream-driven event with widespread rain south of the area and little cold air available until the system tracks well offshore and begins to phase with the trough. The model guidance continues to depict different storm evolutions, however, and will likely continue to do so until the key players for this setup are better sampled in the shorter range.

72-hour accumulated precipitation from the 12z run of the operational GFS (top left image) and the 11 GEFS individual members. Most members are north of the operational GFS run (image credit: PSU e-Wall).

f180_2The GFS and the latest CMC runs side with the suppressed outcome, although the latest 12z ECMWF and UKMET runs show more interaction of the northern trough and the southern shortwave occurring further west than the rest of today’s runs, with an intensifying low pressure tracking up the coast, resulting in a widespread heavy rain event on Tuesday night into Wednesday. It is far from the first time this fall that the ECMWF was much more amplified with a low pressure over the northeast US in its medium range, and most times this has occurred this fall the system was much weaker and less amplified than the ECM indicated, the most recent example being this coming Saturday where just 2 days ago, the ECM supported a stronger low pressure overhead with heavy rain. Solely going by the progressive trend this fall and model verification, a weaker, suppressed storm would be preferred, although in the medium range with such a large model spread it is often useful to refer to the ensemble guidance; if the majority of a model’s ensemble members show a significantly different outcome, it may be an indicator that the operational model run could be off. Currently, the majority of the GFS’ ensemble members are further north than the operational run with at least some rain over the area, with a handful that show the northern stream picking up the storm further west with a strong low pressure overhead. While using the GEFS alone is no guarantee the storm will affect the area with heavy rain, it gives more credibility to the stormy scenario than using the ECM alone, which has not been very accurate this fall.

Preliminary Outlook: While there are many uncertainties to the outlook, based on the setup with a lack of cold air supply given the partial phasing, it appears likely that should the storm directly affect the area, the main impact would be rain, with timing likely between Tuesday night and Wednesday night. As any full phasing does not occur until the storm has exited the region given a lack of blocking in the northern Atlantic, a reasonable setup would be a broad low pressure tracking NE or NNE, either nearby or to the east, with a cold surge likely behind the storm by late next week but with its intensity also dependent on the outcome of the midweek storm. At this time, I maintained at 50% chance of rain in the 7-day outlook, with the highest probability east of NYC; as previously noted, however, this outlook is still subject to change, with the two main scenarios at this time a moderate-possibly heavy rain event on Wednesday, or a storm mostly suppressed south of the area. Despite uncertainty with the storm, however, there is higher confidence of another strong cold surge for Thursday and Friday, with temperatures again peaking in the 30s for highs with widespread lows in the 10s possible inland in the colder case scenario. Stay tuned for more information over the next few days.

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