Aug 13, 2013: Cooler Temps Through Weekend

Forecast Highlights:

temp34A wave of low pressure moved through the region today, producing widespread moderate-heavy rain in the morning, followed by a cold front in the late evening which produced additional scattered thunderstorms. The front will continue to clear the area tonight, with a cooler air mass bringing temperatures into the mid-upper 70s for highs and 50s-low 60s for lows for the rest of the week, with a gradual increase in temperatures and humidity going into next week. (Image credit: PSU e-Wall)

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Monday, August 12 Observations:

8.12.13 - CopyMostly cloudy skies were observed across the region with isolated thunderstorms affecting northern NJ, NYC and parts of Long Island in the morning; these showers produced very localized rain totals near/over 1/2 inch, with otherwise less than 1/4 inch of rain. Temperatures reached the low to mid 80s from NYC and further north/west, but were slightly cooler than expected in Long Island and coastal CT, reaching the mid to upper 70s. The highest temperature was 86 degrees in Teterboro, NJ, and the coolest high temperature was 73 degrees in Montauk, NY.

Tuesday, August 13 Observation:

8.13.13A pre-frontal trough moved through the region this morning, producing widespread heavy rain and flash flooding in southern-central Pennsylvania, where totals were locally as high as 5-6 inches. Severe thunderstorms were observed in the southern end of this rain area, primarily producing gusty winds and flash flooding. These storms spread into New Jersey, but struggled to reach NYC, weakening as they moved in, with the heavy rain staying entirely south of Connecticut with much less rain than expected; the severe storms that fueled the morning heavy rain over PA collapsed while additional storms relocated slightly further south, over south-central NJ where a tornado was reported, with the northern end of the heavy rain shield collapsing as it shifted further south as well. The rain cleared the area by 11 AM to 1 PM, which was 2-3 hours earlier than expected, with partly to mostly cloudy skies continuing afterwards, with additional scattered thunderstorms affecting northern NJ, NYC and western Long Island between 8-11 PM.

Temperatures were cooler than those of Monday; highs reached the low to mid 70s in Long Island, mid to upper 70s in NYC, SE NY and south CT, and upper 70s to low 80s in northern NJ. The highest temperature was 82 degrees in Somerville, NJ, and the coolest high temperature was 71 degrees in Montauk, NY.



Tonight – Friday: Cooler, Drier Conditions Return

A strong shortwave at 500mb is currently moving through the Great Lakes and will move into northern New England on Wednesday while becoming negatively tilted and lifting north into Canada, with the resulting westerly flow aloft delivering a moderated Canadian cooler air mass into the area, bringing 850mb temperatures down from 16C today to 8C on Wednesday. A surface high pressure will spread into the region as well, setting up for more sunshine for the rest of the week while temperatures remain cooler than average.

Temperatures tonight are expected to fall into the mid to upper 50s inland, upper 50s to low 60s in the north/west suburbs, and the mid 60s in NYC. A relatively tight pressure gradient between the departing cold front and the incoming high pressure is initially expected, which will result in a breezy NW wind on Wednesday, up to 10-20 mph with gusts up to 25-30 mph possible. With the cold air advection and breezy NW wind, cooler high temperatures are expected, peaking in the low to mid 70s inland and the mid to upper 70s for the rest of the area with mostly sunny skies. The coolest lows are expected on Wednesday night, falling into the upper 40s-low 50s inland, mid 50s in the north/west suburbs, southern CT and eastern Long Island, and upper 50s to low 60s in NYC and the immediate coast.

The high pressure will remain stationary overhead through Thursday and Friday, keeping mainly sunny skies in place; the cool air mass will gradually moderate as the northern stream trough lifts back into Canada resulting in a gradual but slow warming trend in temperatures, but with cooler than average temps continuing, with highs expected to reach the mid 70s-low 80s and lows ranging from the low 50s inland to the low 60s in NYC, a few degrees warmer than Wednesday night.

Weekend – Next Week: Slowly Warming Up

By the weekend, the trough previously over the Northeast US will lift back north into Canada, while a weak trough digs into the central US, placing the region under a weak southwest flow aloft which will slowly advect a warmer air mass into the region, while also allowing a stalled frontal boundary to the south to slowly advance north. The surface high pressure will slide east, focusing offshore but still extending west into the region, however, which should keep clouds and showers associated with the front and a potential tropical disturbance from the Gulf of Mexico south of the area, with partly to mostly cloudy skies and highs reaching the low to mid 80s across the area.

Uncertainty increases going into next week regarding rain probabilities in the region. The central US trough will gradually weaken with a westerly flow setting up aloft as western Atlantic ridging gradually expands back west into the eastern US; some models keep the region mostly dry through this time period, while others show a better defined low pressure tracking north from the Gulf of Mexico into the Ohio Valley, which would then spread east into the Northeast with increased cloud cover and showers/thunderstorms for the middle of next week. The latter scenario is especially supported by the 18z GFS, while earlier GFS runs and other models support the former scenario. A tropical wave officially labeled invest 92L is currently located in the western Caribbean; the 18z GFS is most bullish with developing this wave of low pressure, although at this time conditions in the Gulf of Mexico do not appear very favorable for tropical cyclone development. At this time, I am siding with partly-mostly cloudy skies and a risk of scattered showers/storms towards the middle of next week, although this outlook is subject to change.

Warmer temperatures will make a comeback as well during this time frame, as the combination of the western Atlantic ridging spreading westward and ridging slightly weakening over the western US resulting in a relatively zonal flow should allow for a warmer and somewhat more humid air mass to return into the region, likely bringing temperatures back above average again, with highs in the mid-upper 80s possible in the warmer case scenario next week. The overall pattern does not appear favorable for any period of sustained or significant heat, although given the large scale pattern, additional 90+ degree days are still possible late this month into early September. Stay tuned for more information on the longer range.

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