Sept 20, 2011: Week Of Rain Starts Tomorrow

The stretch of mostly dry weather since the remnants of Tropical Storm Lee finally ended today as a cold front approached the area, bringing light rain up to 0.1 inch across the area. High temperatures were chilly due to the cloud cover and rain, peaking in the mid to upper 60s inland, upper 60s to lower 70s in the immediate NYC area, and the upper 60s across most of Long Island and southern Connecticut.

Today is just the start of what will be a long stretch of cloudy skies and rain, with rain expected to fall at some point during each day for the next 8 days. It will not rain consistently during this entire time frame; the rain is expected to come in waves, with light to moderate rain on Wednesday night, moderate/heavy rain on Friday, and another potential round of moderate/heavy rain next week. When combined, rain totals may exceed 3-5 inches across parts of the area, resulting in flooding potentials once again.
Tomorrow’s Outlook:

With the rain out of the area, tomorrow will bring mostly cloudy skies with the risk of an isolated showers. Temperatures aloft will be warmer, however, with 850 mb temperatures near 14 degrees celsius. A SSE wind is expected, which combined with mostly cloudy skies, will result in temperatures warming up into the 70s across the area for the first time since last Thursday. Temperatures tomorrow will reach the mid to upper 70s from NYC and further west, and the lower to mid 70s in Long Island and southern Connecticut due to the onshore wind.

Wednesday Night – Friday: 3+ Inches Of Rain Possible

A low pressure near the Great Lakes will bring a cold front towards the region, but this cold front will slow down as a cut off low pressure develops near the Ohio Valley on Thursday and Friday. This will bring moisture up the coast, with high precipitable water values bringing the potential for heavy rainfall. Rain is not expected to fall in this entire time frame, but the rain is expected to affect the area in waves. The first wave of rain will affect the NYC area on Wednesday night, bringing widespread light to moderate rain across the area. Rainfall amounts between 1/4 and 1/2 inch are expected across the area.

Behind this wave of rain, mostly cloudy to cloudy skies with isolated showers are expected for Thursday and Thursday night. With 850 mb temperatures warming to 15-16 degrees celsius, surface temperatures will be slightly warmer than those of tomorrow, reaching the upper 70s to potentially the lower 80s in the immediate NYC area.
Mostly dry conditions will continue through most of Thursday night. By Friday morning, however, a wave of low pressure will begin to move up the coast towards the area, and with moisture moving up the coast as well with precipitable water values nearing 2 inches, heavy rain is expected across the area. Rain will begin to develop around Friday morning and intensify, with moderate to heavy rain expected through most of Friday into the early overnight hours. There is some uncertainty with the exact timing, but the overall idea is there that the heaviest rain will fall around Friday. Thunder is expected as well. By the time that this wave of rain will end on Friday night, a total of 1.5 to 3 inches of rain are expected across most of the area, with amounts locally as high as 4 inches of rain. The heaviest rain is expected to fall in the immediate NYC area, Long Island and southern Connecticut. Stay tuned for more information on Friday’s rain event.

Saturday – Next Week: Clouds, Rain Continue

The heavier rain will end by Friday night, with drier conditions returning for the weekend. Mostly cloudy skies are expected for Saturday and Sunday, but the risk of isolated showers will continue throughout the weekend. The cut off low previously mentioned will remain stuck over the Ohio Valley, however, and with the main flow of storms moving through central Canada, well north of the US, there will be no feature to take the cut off low out of the region, and it will remain stationary over the Ohio Valley through next week. Due to the cut off low staying to the west of the area, the colder air mass will be stuck over the Ohio Valley, and mild 850 mb temperatures will continue across the area, with temperatures peaking in the mid to upper 70s across most of the area.
With the cut off low slow to exit the region, rain will continue into early next week as well. The latest models show another low pressure developing around Monday/Tuesday, which may bring another round of moderate to potentially heavy rain across the area between Monday and Wednesday. The cut off low is then modeled to slowly begin to exit the region, finally leaving the United States around Thursday with drier conditions returning. There is more uncertainty for next week, however, as the models differ with how they handle the potential storm early next week ,as well as when and how the cut off low exits the region. Stay tuned for more information on next week with tomorrow’s update.

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