Sept 20, 2013: Rain, Storms Sat Night, Then Chilly

[notice]After a 2-week experimental run, and based on feedback received, the 5-day forecast has been officially extended to a 7-day forecast.[/notice]

Forecast Highlights:

nam-hires_namer_042_sim_reflectivityThe warming trend observed this week continues, with temperatures today peaking well into the 70s across the area. A cold front will move through the area on Saturday night, bringing more windy conditions, a brief increase in humidity and a line of heavy rain and thunderstorms overnight. No storm is expected to occur next week as breezy and much cooler conditions return for Sunday and Monday, with a gradual warm up through next week.




This Weekend: Cold Front Brings Rain, Thunder For Saturday Night

Pattern Analysis: The latest synoptic pattern consists of ridging aloft over the eastern US, with a strong low pressure currently located over central Canada near the Hudson Bay pushing a cold front east through the Great Lakes and Ohio Valley. A high pressure sat over the region for the last few days, bringing sunny and seasonable daytime temperatures with chilly overnight lows and areas of fog. A warm air mass is present just ahead of the cold front, west of the area, with temperatures well into the 80s and dew points in the 60s; this air mass will moderate as it reaches the region with temperatures expected to slightly cool down on Saturday, but with an increase in humidity tonight into Saturday as a southerly flow brings dew points into the low-mid 60s.

The previous forecast discussion noted the possibility of an early week storm in parts of the region, partly related to the tropics, along with several scenarios in which either the tropical system stays south or the troughing setting up differently that would lead to the setup failing to occur with sunny and chilly conditions instead. This storm turned out to only have been a brief possibility as the model guidance quickly backed away from this scenario with yesterday’s runs. Several changes in the anticipated setup led to this potential failing to occur; Invest 95L remained disorganized with moderate shear, and as a result failed to develop into a tropical cyclone, instead staying weak and further south, quickly moving northwest towards the Mexican coastline where it is expected to make landfall today without having developed further. Additionally, the trough currently approaching the region is further south than modeled, and less energy splits from the trough than previously shown; a small area of higher vorticity is expected to stall near the Gulf of Mexico coast, but without a well defined upper level cutoff low.

While the significant heavy rainfall will stay well to the south, near the Gulf of Mexico coast, a wave of low pressure is expected to develop along the cold front on Saturday, moving through the area overnight and exiting the region on Sunday, with a line of rain and thunderstorms ahead of the front affecting the area on Saturday night (more information on the timing can be found in the next section). A southerly flow will develop over the region, which will pull tropical moisture up the East Coast with precipitable water values rising to nearly 2 inches, leading to heavy rainfall rates with the line, especially along and north/west of the wave of low pressure track where near to over 1 inch of rain is expected. Behind the cold front, a much cooler air mass will return into the region with temperatures below average again for Sunday and Monday, as the low pressure slows down over Atlantic Canada and likely becomes a cutoff low.


12z run of the 4km NAM, showing the frontal passage at hour 42 with heavy rain over NYC (image credit: NCEP Model Analyses and Guidance).

nam-hires_namer_042_sim_reflectivityForecast for NYC Area: Cloud cover will gradually increase tonight ahead of an approaching cold front as dew points slowly rise, with slightly more humid conditions tomorrow as dew points peak in the low to mid 60s on Saturday. Partly to mostly cloudy skies are expected on Saturday with a breezy SSE wind at 10-15 mph, gusting up to 20-25 mph along the coast, along with a risk of isolated showers over NYC, Long Island and Connecticut. Highs are expected to peak in the low-mid 70s in Long Island/CT and the mid to upper 70s from NYC and further north/west.

The cold front will move through overnight, with a line of heavy rain and thunderstorms expected to move through the area, starting to affect interior locations (western NJ, Orange county in NY) around 7-8 PM, immediate NYC area around 9-10 PM, and Long Island/CT between 11 PM and 1 AM. Heavy rain and thunderstorms are expected, with strong wind gusts possible as well. The line will last for a few hours, gradually ending from west to east as temperatures cool down. Partly cloudy skies are expected on Sunday with a breezy NW wind at 10-15 mph and highs peaking in the mid-upper 60s inland (west NJ, Orange county in NY) and the low-mid 70s for the rest of the area.

Storm updates will be posted tomorrow evening on the storms, both on the blog and on Twitter.


Next Week: Chilly Start, Then Warming Up

Next week will start off not unlike the beginning of this week, with a strong trough over the region bringing a round of unseasonably cool temperatures. Clearing skies are expected overnight with chilly lows; wind is expected to persist overnight, however, from the north at 5-10 mph, which is expected to prevent significant cooling away from the coast as was observed with this week’s cool spell, keeping temperatures in the mid to upper 40s, while NYC cools down again into the low 50s; while Central Park failed to drop below 50 degrees so far this month, temperatures did reach 50 degrees on Tuesday morning. Mostly sunny skies are expected on Monday with highs in the low to mid 60s inland and the mid to upper 60s for the rest of the area, with chilly overnight lows again especially with winds calming down, as lows fall into the upper 30s inland, low to mid 40s in the north/west suburbs of NYC, south CT and the Long Island Pine Barrens, mid to upper 40s for the rest of Long Island, and the upper 40s to low 50s in NYC.

The weekend cold front is expected to stall to the east with a cutoff low developing over Atlantic Canada, with temperatures gradually moderating through the middle of next week. Mainly sunny skies are expected on Tuesday with highs in the upper 60s to low 70s, with another chilly night but slightly warmer than Monday night, with partly sunny skies on Wednesday and highs peaking in the low to mid 70s. A weak shortwave is likely to move through on Wednesday into Wednesday night, which is likely to result in slightly lower heights aloft near/east of the area as ridging amplifies over the central US in response to a strong trough digging in the western US along with a developing low pressure. While the warmest temperatures will stay west of the region, the gradual warming trend will continue through the late week, with highs on Thursday likely to peak in the mid 70s and Friday in the mid-upper 70s.

Uncertainty increases in the longer range beyond Friday regarding the specific details in the forecast, although the next cold front is likely to affect the area towards next Sunday and/or Monday, with another cool down in temperatures likely towards early next week. More information will be posted with the next medium range outlook, to be posted on Sunday night.



<< Sept 17 | Sept 18 | Sept 19 | Sept 20 >>

Wednesday, September 18 Observations:

9.18.13A strong high pressure remained overhead, with cold morning temperatures observed inland and near the coast, with Sussex, NJ down to 39 degrees; Danbury, CT to 37 degrees; and Westhampton Beach, NY to 34 degrees, with areas of patchy frost observed. A high pressure remained parked overhead, with areas of morning fog giving way to sunny skies and highs in the upper 60s to low 70s across the area. The highest temperature was 75 degrees in Teterboro, NJ, and the coolest high temperature was 68 degrees in Montauk, NY and New London, CT.

Thursday, September 19 Observations:


The strong high pressure remained over the area, with another chilly morning with lows in the 40s away from NYC and the coast, areas of fog in the morning, with sunny skies for the rest of the day and temperatures continuing to gradually warm up. High temperatures reached the mid 70s across most of the area, with low to mid 70s east of NYC. The highest temperature was 78 degrees in Newark, NJ and Central Park, NY, and the coolest high temperature was 71 degrees in New London, CT.

2 thoughts on “Sept 20, 2013: Rain, Storms Sat Night, Then Chilly

    • NYC Area Weather Post authorReply

      Scattered showers are possible at any time of the day, especially in the morning/early afternoon, with the main line of heavy rain and thunderstorms expected to arrive towards 11 PM-12 AM.

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