Oct 23, 2010: Monday Night, Wednesday Thunderstorms

This morning was the coldest of the year so far, with lows ranging from the upper 2 s inland to the lower-mid 30s for most places north and west of NYC, with lows in NYC in the lower to mid 40s. The colder spots in eastern Long Island also reached the lower 30s this morning. Temperatures warmed up quickly in the afternoon, and the highs were slightly warmer than expected, peaking in the mid 60s across most of the area.

During the next few days, a much more active pattern will unfold, including the potential for severe weather on Wednesday with thunderstorms and heavy rain.

Sunday/Monday Forecast:
Tomorrow’s outlook hasn’t changed much, please refer to yesterday’s discussion for today’s expected temperatures. Monday is expected to be mostly cloudy with highs warming up, reaching the mid 70s in the immediate NYC area. A weak low pressure is going to bring widespread rain and thunderstorms on Monday night, however rainfall amounts for the main part will be light, ranging from 1/4 to 1/2 inch. Locally heavier rainfall amounts are still possible, potentially up to 3/4 inch.

Tuesday And Wednesday: Major Storm Expected

As I previously mentioned with the expected set up for this time frame, a low pressure entering Canada tomorrow will be in southern Canada on Monday, merging overnight with a rapidly developing low pressure to its south to form an intense storm in Minnesota on Tuesday.

This storm is expected to peak near 960 mb, the equivalent of a Category 2 hurricane, which will lead to strong winds in the Midwest along with heavy rainfall, as well as some snow at the end of the storm as colder air gets drawn into the NW side of the storm. Meanwhile, the storm will draw in much warmer air into the Eastern US, which will lead to widespread thunderstorms south of the storm, potentially strong.

The area will see mostly cloudy skies on Tuesday with highs in the lower to mid 70s, and as the warmest air mass moves through overnight, with 850 mb temperatures near 18c, low temperatures will be unusually mild, even warmer than the average high temperatures, reaching the mid to upper 60s in the immediate NYC area.

Wednesday: The storm is expected to peak in intensity on Tuesday night between 960 and 965 mb, and will remain stationary while starting to weaken and drifting ENE. The rain, however, will expand, with a strong cold front leading to a long line of heavy rain and thunderstorms moving through the Ohio Valley, with strong wind gusts being a threat with these storms.

On Wednesday, the cold front will move through the area. As the expected scenario has not changed, I am using yesterday’s scenario map for this section. The strong cold front, with a much colder air mass behind it, will run into the very warm air mass in the region, with 850 mb temperatures between 15-18c, which will lead to thunderstorms. With precipitable water values near 2 inches, these storms could produce locally heavy rainfall. CAPE is expected to be up to 750-1000, LI between -2 and -4, and bulk shear will be near 50 knots, which will lead to the potential of strong to severe storms, with strong wind gusts being a risk. As with yesterday’s update, I am mentioning the potential of heavy rain/gusty winds for Wednesday in the 5-Day Forecast.

Longer Range: Colder, Then Warming Up For Halloween

For the longer range, it now appears that the pattern will be too progressive to allow a storm to form and move up the coast. Thursday will bring slightly cooler temperatures but with drier conditions, though as the trough enters on Thursday night, temperatures will start to cool down, with low temperatures returning into the 40s, and even potentially 30s for the interior. Friday will then bring colder temperatures with highs in the mid 50s to lower 60s possible.

The trough will already be in the area on Friday, however it will have significantly moderated from when it comes into the United States in the middle of the week, which means that the cool down won’t be as strong as the current one, but still strong enough to cause a large change in temperatures from the early-mid week. Friday night could be the coldest night, though there is some uncertainty with the trough’s intensity and how far south it gets.

Looking at the general pattern, however, a storm should enter the NW US on Thursday, and start to move east, and if it’s far north enough as currently expected, it will quickly push out the cold air mass by Saturday night, leaving most of the United States with a mild air mass again, and leading to chilly but not as cold conditions for Halloween. If this verifies, this could leave us with a set up quite similar to the one we will be dealing with this week, as the GFS model is already hinting of a storm that may form in the central US on November 1st, affecting the region between November 2-3. Stay tuned for more details on this time frame.

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