Sept 19, 2011: Heavy Rain Potential For Friday

As a high pressure spread from New England into the NYC area, today brought mostly sunny skies to the area. Temperatures were chilly as the leftovers of the cold air mass stayed in place, with high temperatures reaching the mid 60s inland, upper 60s to 70 degrees in the immediate NYC area, and the upper 60s across most of Long Island and southern Connecticut. Due to the mainly clear cloud cover and the high pressure in place, temperatures this morning were chilly as well, with parts of NW NJ and SE NY dropping into the upper 30s for lows.

An approaching front will bring an end to the stretch of dry weather, however. Almost half a month after the remnants of Tropical Storm Lee brought heavy rain into the region, a slow moving front combined with a cut off low will affect the region, producing moderate to locally heavy rain for the second half of this week, with the potential for 1 to 3 inches of rain to affect the area. With the pattern in place, the cut off low will stay stationary, keeping a risk of showers in place through this weekend and into early next week.

Tomorrow’s Outlook:

A front will approach the area tomorrow, bringing cloudy skies along with scattered showers. Most of the rain will fall west of NYC, as the rain will start to weaken once it reaches NYC and is expected to fall apart by the time that it reaches Long Island and southern Connecticut. Mostly light rainfall amounts are expected, with 0.1 to 1/4 inch of rain from NYC and further west with lighter amounts east of NYC. Due to the cloud cover, rain, and an expected SSW wind, temperatures will stay chilly, peaking in the mid to upper 60s inland and the upper 60s to lower 70s in the immediate NYC area, Long Island, and southern Connecticut.
Wednesday – Friday: Cutoff Low Brings Heavy Rain Potential
Cloud cover will clear on Tuesday night as the rain dissipates, with partly to mostly cloudy skies expected for Wednesday with a SSE wind expected. Combined with warmer temperatures aloft, with 850 mb temperatures reaching 10-14 degrees celsius, temperatures will warm up across the area, reaching the mid to upper 70s from NYC and further west, and the lower to mid 70s in Long Island and southern Connecticut. Parts of NE NJ may get close to the 80 degree mark.

Isolated showers are possible across the area on Wednesday, but chances for rain will increase by the end of the week. A low pressure will move through the northern Great Lakes on Wednesday night before moving into Canada, bringing its cold front towards the Ohio Valley. Meanwhile, a cut off low will form over the Ohio Valley by Thursday/Friday. This will bring moisture up the East Coast, with precipitable water values reaching 1.5 to 2 inches, capable of producing heavy rainfall.
There is uncertainty with the individual details with this time frame, as rain is not expected to fall in the entire time frame from Wednesday through Saturday. As with Tropical Storm Lee’s remnants earlier this month, occasional periods of moderate to locally heavy rain are expected to affect the area, with most of the models focusing the rain between Thursday night and Friday night. The models are split with the rain totals, with the GFS and NAM/DGEX models showing lighter totals, between 3/4 and 1.5 inch, with the ECMWF and CMC showing wetter solutions, with up to 2-4 inches of rain. With the amount of moisture available and considering that this is a long duration rain event, I decided to side with the wetter models, expecting at least 1 to 2 inches across most of the area with amounts locally as high as 3-4 inches of rain. There is still some uncertainty with this time frame, however, and this forecast is not final. Stay tuned for more information on this storm and how it will affect the NYC area.

Next Weekend – Next Week: Clouds, Rain Stick Around

As previously mentioned, a cut off low is expected to develop over the Ohio Valley around Friday. The main flow of storms, however, will be stuck over central Canada, and there does not appear to be any feature pushing southwards towards the US to pick up this upper level low. As a reuslt, the cut off low is expected to remain stationary over the Ohio Valley through the weekend and into early next week as well. This scenario will keep average temperatures in place as the cold air mass remains stuck over the Ohio Valley, with temperatures reaching the lower to mid 70s across most of the area for the weekend into early next week, potentially getting into the upper 70s near and south of NYC in the warmer case scenario. There is more uncertainty with the rain potential, however, as some models keep dry conditions in place, while others show scattered showers with another storm developing around early next week. Stay tuned for more information on the outlook for next weekend and next week with tomorrow night’s update.

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