Oct 21, 2011: Colder Pattern Gradually Develops

Behind Wednesday night’s cold front, a stronger cold air mass moved into the region, bringing slightly below average high temperatures across the area. Due to increased cloud cover as well as 850 mb temperatures slightly cooler than expected, high temperatures were colder than last night’s expectation, reaching the mid 50s inland, mid to upper 50s in the immediate NYC area, and the lower 60s in Long Island and southern Connecticut.

With more sunshine expected for tomorrow, temperatures will slightly warm up, reaching the lower 60s across more parts of the area than today. Temperatures will continue to slowly warm up ahead of a weak low pressure with light rain on Monday, with more widespread rain returning into the region on Wednesday. Afterwards, however, there is more uncertainty in the forecast as the potential is there for another low pressure by the end of next week along with stronger cold air in the Northeast US.

Tomorrow’s Outlook:

With the cold air mass persisting over the region, chilly temperatures will continue through tomorrow as well. Unlike today, when mostly cloudy skies were observed, tomorrow will bring less cloud cover, which combined with 850 mb temperatures similar to those of today, near 1-3 degrees celsius, high temperatures are expected to peak in the upper 50s inland and the upper 50s to lower 60s across the rest of the area. A west wind is expected.

The coldest temperatures of this cold spell are expected for tomorrow night, with areas of frost expected for the interior parts of the area. With mostly clear skies and light winds, temperatures will drop into the lower to mid 30s in NW NJ and interior SE NY, which will produce areas of frost. Otherwise, low temperatures will reach the mid 30s to lower 40s in the north/west suburbs of NYC, upper 30s to lower 40s in Long Island/S CT, and the mid 40s in NYC.

Sunday – Tuesday: Slightly Warming Up

As the cold air mass slowly starts to weaken, temperatures will warm up across the area as well. High temperatures on Sunday are expected to reach the upper 50s to lower 60s inland and the lower to mid 60s across the rest of the area with partly sunny skies. As a weak low pressure in southern Canada moves east on Monday, temperatures will warm up ahead of the storm, with scattered showers affecting parts of the area. For tonight’s 5-Day Forecast, I went with the warmer case scenario, where most of the area ends up in the mid 60s with upper 60s in parts of the immediate NYC area, getting very close to the 70 degree mark. Behind this storm, temperatures on Tuesday will slightly cool down, reaching the lower to mid 60s across the area.

Wednesday – Friday: Rain Returns Again

The latest models continue to show a storm around the second half of next week, but the set up for this storm became more interesting today when the majority of the model guidance showed a wet snowstorm for parts of the northern Mid Atlantic and southern NE, with the latest GFS run bringing widespread snow in northern New Jersey. While there is stronger evidence of more sustained cold air for the last few days of October, the cold rain/interior snow scenario shown on the models is questionable.

The low pressure that will affect the region with light rain on Monday evening will move towards southeastern Canada and will become negatively tilted, which combined with a dropping trough will place weak yet widespread cold across southern Canada. Further south, a moderately intense wave of low pressure is expected to mvoe through the northern Ohio Valley/Great Lakes region and into the Northeast, bringing warmer temperatures into the area reaching the mid to upper 60s, and the potential may be there for lower 70s if the storm is far north enough. The cold front will then approach the region as cold air drops further south behind this low pressure, bringing rain across the area for Wednesday and Wednesday night.

The forecast then becomes more uncertain for Thursday, once the front approaches the area. The models that show a snowstorm take the front through the area, and as the front drops to the south of NYC, a wave of low pressure moves along the stalled front, with precipitation pushing north into the cold air to produce cold rain and snow in the area. October snow, however, is uncommon in the NYC area, and would verify if there is strong cold air mass with 850 and 925 mb temperatures below freezing and near freezing surface temperatures. If the temperatures are cold enough, elevation has to be factored in as well when considering where snow falls and how much of it, if any, does fall. At this time, the pattern and set up is not an ideal one for October snowfall in the NYC area, and the source of cold air isn’t as strong as it was with previous October snowstorms.

There is still a relatively low confidence level for the forecast on Thursday and Friday, but at this time, based on the pattern, the trends we have seen this fall with longer range outlooks, and the set up with a slightly -NAO, -AO and a slightly negative PNA, the potential is there for a secondary low pressure to form after the first low pressure, and the secondary low pressure may end up further inland than the models currently show, potentially tracking over the eastern US instead of staying offshore, bringing widespread rain across the area with relatively chilly temperatures along with the potential for snow in parts of the interior Northeast. Once again, this is still a relatively low confidence forecast, and it is possible that Wednesday’s storm simply brings a cold front through with cold air moving in, or we may see a stronger secondary low pressure develop either inland or offshore. Stay tuned for more information on the outlook for mid-late next week.

Longer Range: Tomorrow’s update will discuss the longer range in more details, including the existing potential for a stronger cold spell for the last few days of October.

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