Aug 30, 2013: Stormy Weekend, Labor Day

Forecast Highlights:

gfs_namer_075_850_temp_mslp_precipAs the strong central US ridge gradually shifts west, a trough will dig into the eastern US region, with increased clouds and scattered storms from Saturday through Tuesday, focusing especially on Monday, with even warmer temperatures and more humidity expected as heat index values approach 90 degrees. A cold front will move through on Tuesday, bringing drier conditions and a more seasonable air mass for the mid-late week, with a stronger trough for next weekend possibly bringing the first taste of fall to the region.

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Thursday, August 29 Observations:

8.29.13A back door cold front moved into New England, with more widespread cloud cover and much cooler temperatures towards Massachusetts, New Hampshire and Maine. Eastern parts of the area were in the southern end of the cooler air mass, with highs in the mid to upper 70s, while NYC and further north/west reached the upper 70s to low 80s. The high temperature was 82 degrees in JFK Airport, NY and Andover, NJ, and the coolest high temperature was 74 degrees in Montauk, NY.




Tonight – Monday: Occasional Storms

The strong ridge responsible for the widespread central US heat wave is slowly shifting to the west towards the western US, which will result in a gradual lowering of 500 millibar heights aloft as several weak shortwaves move through the region during the weekend, triggering more widespread cloud cover and scattered showers/thunderstorms, mainly in the afternoon and evening. Unlike this week, when the region was under a northwest flow, in this case winds will remain from the south-SW, advecting a warmer and more moist air mass into the region, with 850mb temperatures rising to near 16-18C along with dew points climbing into the upper 60s to low 70s, bringing a much more summer-like feel relative to the rest of the month for the end of the meteorological summer season, which ends tomorrow, August 31.

The cloud cover and storms will prevent temperatures from reaching the 90s this weekend, which would have been possible had there been more sunshine, although temperatures are still expected to climb into the mid 80s away from the coast, possibly upper 80s near and southwest of NYC, which along with humid conditions will result in heat index values approaching 90 degrees. With the aforementioned shortwaves moving through, scattered thunderstorms are expected on Saturday and Sunday across the area; while severe thunderstorms are not anticipated, storms may be capable of producing heavy rainfall and localized strong wind gusts. The highest risk of storms is to the north of NYC, towards the lower Hudson Valley and NW NJ, although the entire area has a risk of storms at some point during the weekend. Along with the increased humidity, overnight lows will be much warmer, ranging from the mid-upper 60s inland to the mid 70s near NYC.

A stronger trough will dive into the region on Monday with a more organized low pressure moving through, bringing more widespread thunderstorms. The first round of storms is likely on Sunday night into Monday morning; some models indicate the possibility of heavy rainfall and localized flash flooding north of NYC, although this time frame is a little too far out for high confidence on the exact timing and rainfall amounts. Scattered storms are likely throughout the rest of the day and overnight hours with slightly cooler temperatures, peaking in the upper 70s to low 80s for highs.


Tuesday – Next Weekend: Drier, Cooler, Less Humid

As the upper level trough lifts back north into Canada, the surface cold front will slow down, moving through during Tuesday morning or afternoon with additional scattered showers and thunderstorms likely until the frontal passage. The cool air mass behind the trough will moderate as it reaches the region by Wednesday, with temperatures only slightly cooler in the mid-upper 70s inland and upper 70s-low 80s for the rest of the area with a west wind, but with the most noticeable change being the lower humidity, with dew points back into the upper 50s-low 60s.

With ridging focused over western North America, a strong upper level low will dive southeast through Canada, approaching the Northeast US on Thursday, bringing a modified polar air mass into the region along with it. A dry frontal passage is expected on Thursday, followed by the cold air mass entering for Friday and Saturday with 850mb temperatures in the area falling into the 6-10C range and as low as zero degrees Celsius towards central-northern New England. The coolest temperatures will remain north of the area, where overnight lows may fall into the 30s with frost possible in parts of New England, although temperatures in the area are expected to be cooler as well with highs rising into the 70s and lows in the 40s-50s away from the coast. Temperatures are then likely to moderate again towards the weekend into early next week, warming back to near-above average with 80s likely, although widespread humid conditions such as this weekend are not likely at this time. Stay tuned for more information on the longer range.

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