[notice]The 2013-14 winter outlook is planned to be posted by the second half of next week. Additionally, the 7-day forecast has been slightly extended to 8 days.[/notice]
The fall of 2013 has been unusually dry, as despite frequent temperature swings, little rain fell. This fall looks to end on a much more active note, however, as two strong cold surges will affect the region on Sunday-Monday and again late next week with near record cold temperatures possible, falling into the 10s for lows across a large part of the area, along with windy conditions leading to low wind chill values, accompanied by a potential storm on Wednesday.
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Thursday, November 21 Observations:
As the early week strong cold air mass exited the region, temperatures moderated again with the high pressure moving through the region, starting out with mostly to partly sunny skies but with increasing clouds as a weak storm approached from the west. High temperatures were warmer as a result, peaking in the upper 40s to low 50s across most of the area. The highest temperature was 54 degrees in Somerville, NJ, and the coolest high temperature was 45 degrees in Montgomery, NY.
Tonight – Tuesday: Few Showers Tonight, Then First Cold Blast
Earlier this morning, a round of light rain moved through the region, mostly staying north of NYC, with minimal rain totals again below 0.1 inch. Since then, there has been slight clearing in the cloud cover with temperatures rising into the low to mid 50s. Clouds will increase again this evening as the cold front approaches, with scattered showers expected again in the evening to early overnight hours. A brief increase in winds is likely tonight, with a NW wind at 5-15 mph on Saturday with partly sunny skies and highs in the low to mid 40s for most.
18z GFS at hour 54, showing the 850mb temperatures in the bottom right in Celsius. 850mb temps are near -16C, which is unseasonably cold for late November (image credit: PSU e-Wall).
The stronger cold air mass, however, won’t come in until Saturday night behind a second, dry cold front, with an unseasonably cold air mass that has been building up in Canada spilling southward into the eastern US. 850 millibar temperatures of -14C to -18C are expected across the region, typical of a mid-winter cold surge, which along with a tight pressure gradient and strong winds will lead to very low wind chill values for this time of the year, along with the possibility of isolated snow showers on Sunday as the vort max moves through the area. Daytime high temperatures on Sunday are expected to barely reach the mid-upper 20s inland and the upper 20s to near 30 degrees elsewhere, likely remaining below 32 degrees across the entire area. These temperatures are nearly 20 degrees below the average high temperatures for late November. With the strong NW winds at 20-30 mph, gusting up to 40-45 mph, wind chill values are expected to remain in the 10s to low 20s throughout the day.
The coldest temperatures are expected overnight as winds gradually decrease with the high pressure moving nearby, along with the cold air mass positioned overhead. Temperatures are expected to fall into the low to mid 10s inland, upper 10s in the north/west suburbs of NYC, southern CT and interior Long Island, low 20s for coastal CT and the rest of Long Island, and the low to mid 20s in NYC. Temperatures may approach record lows for part of the area. Mostly sunny skies are expected on Monday with winds shifting to the southwest and high temperatures reaching the low 30s inland and mid 30s elsewhere. As a clipper system moves through southern Canada, the southwesterly flow will help to quickly remove the cold air mass, however, with high temperatures moderating into the low 40s on Wednesday with partly sunny skies.
Tuesday Night – Wednesday: Analysis of Storm Potential
12z ECMWF at hour 96, showing the southern stream (over Texas) and the northern stream (over the Midwest), before the system begins to affect the region. This specific run produces heavy rain for the area (image credit: PSU e-Wall).
Storm Setup: The current upper level setup consists of a stalled upper level low just off the coast of southern California, with a ridge over the northwestern US and a strong low pressure to its north, near coastal Alaska. Further downstream, a strong upper level low will move into the northeast US region with 500mb heights nearly 48 decameters lower than average, in the 522-510dm range, associated with the unseasonably strong cold surge entering the region for Sunday and Monday. With ridging persisting over the northwestern US, the aforementioned system near Alaska will dive southeast through Canada, reaching the north central US on Tuesday, while the upper level low near California slowly drifts onshore during the next 24 hours, allowing it to become better sampled, and will then steadily track east, reaching the south central US on Tuesday where a surface low pressure is expected to develop, picking up moisture from the Gulf of Mexico. From there, the timing, location and amplitude of the southern and northern streams and their interaction will determine the outcome to what could be the first significant storm to affect the region in over 2 months.
18z GFS at hour 114, showing the developing low pressure producing heavy rain for the area and snow inland. The GFS is among the most amplified models for this storm (image credit: NCEP MAG).
As the main features in the setup have not been fully sampled yet, the model guidance continues to show several different outcomes, and will likely continue to do so for the next few days until a consensus will ultimately be reached. Yesterday’s forecast update noted how the GFS and CMC were suppressed, but that the more amplified GFS ensemble members strongly hinted that the operational run may be off with its southern outcome; the operational GFS run ended up trending towards the ensemble members and showed a cleaner phase between the northern and southern streams with its 0z, 6z and 18z runs today, resulting in a strong coastal low pressure intensifying as it tracks parallel to the coast, producing widespread heavy rain for coastal areas and heavy snow inland. The GFS is among the most aggressive models with the phasing scenario, however, as the ECM shows a messier partial phase, with two areas of low pressures, one moving through on Wednesday morning producing widespread heavy rain, the other staying south and east of the area overnight. While the UKMET has limited data beyond hour 72, it appears to resemble yesterday’s 12z ECMWF scenario showing heavy rain for the area, and the CMC is even more amplified than the GFS but is not as much of a reliable model especially in its medium-long range. The ensemble guidance, meanwhile, continues to consistently support a more significant storm; when the 12z GFS run temporarily showed a weak and suppressed storm, it was the least amplified out of its ensemble members.
While there remains uncertainty with the exact details, which will likely continue for at least another 2 days until the model guidance becomes more consistent, there are some clues regarding the potential outcome. As the phasing or partial phasing begins too late for the storm to fully mature until it is near Maine or has exited the region, the low is likely to be in its developing stages while moving through the area, with precipitation types likely rain for most of the area, and possibly snow for part of the storm further inland but with mixing issues due to a mid level warm layer around 850-800mb with initial southwesterly winds aloft. Timing is likely to be between the early overnight hours on Tuesday and Wednesday afternoon/evening. Depending on the exact track of the storm, heavy precipitation is expected along and close to the storm track, with the potential for 1-2 inches of rain should the low pressure track near the area. Windy conditions at this time do not appear to be a significant concern with the first half of the storm, although stronger wind gusts are likely at the back end of the storm as it departs with a tightening pressure gradient.
Preliminary Forecast: While there remains uncertainty regarding the exact impact, there is higher probability of at least some rain affecting the area, and I increased probability of rain to 70% in the 8-day forecast, recently extended from 7 days. Should the storm affect the area as currently forecast, rain would likely develop late on Tuesday evening, possibly mixing with light snow towards NW NJ/SE NY initially, before changing over to rain across the area through Wednesday afternoon. The storm is likely to end by Wednesday afternoon/evening with clearing skies and increasing winds. As previously mentioned, there remains some uncertainty with the exact setup, and some changes are still possible to the forecast. Should the significant storm scenario verify, rain totals of 1-2 inches are possible during this time frame. The possibility still remains for a weaker and more suppressed storm that has little impact in the area, however, in which case scattered rain or snow showers would be possible. Stay tuned for more information on Wednesday’s outlook.