July 19, 2013: Cooler Weather Returns Sunday

Forecast Highlights:

The heat wave that affected the region since last Sunday reached its peak today for most, with widespread highs in the mid 90s to low 100s away from the coast, where a sea breeze and isolated storms kept temperatures cooler. Along with humidity, heat index values got up to the mid 100s to even low 110s. A weak cold front will produce some storms tomorrow as it comes through, ending the heat wave, with temperatures returning into the 80s with some rain chances next week.

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Thursday, July 18 Observations:

Some of the hottest temperatures of the heat wave took place on Thursday as a westerly flow set up along with mostly-partly sunny skies and warmer temperatures aloft, reaching 20-22C at 850mb. High temperatures reached the low to mid 90s in southern CT and the mid to upper 90s for the rest of the area, with parts of NE NJ and NYC reaching the low 100s. Along with humid conditions with dew points in the mid 60s to low 70s, heat index values reached the low-mid 100s. Newark, LaGuardia and JFK both reached the low 100s, with the hottest temperature recorded in Newark with 101 degrees.

Record highs on Thursday, July 18, 2013:

JFK Airport, NY
100 degrees (previous record: 96 degrees - 2012)

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Tonight - Sunday: Heat Ends, Some Storms

A cold front is currently producing severe thunderstorms in southern Canada and northern NY state as it slowly sinks to the southeast, approaching the region. Although there was significant instability across the region today, there was a lack of trigger for widespread storm development, with a few isolated storms developing in eastern Suffolk county early this afternoon. This cold front will continue to slowly slide southeast, moving through the region on Saturday while pushing the hot and humid air mass to the south. This cold front will be weakening on Saturday, however, with the best forcing staying north of the area. Due to increased cloud cover, less significant instability is expected with CAPE less than 2000 J/kg and a lift index generally under -3 to -5, and mid level lapse rates will be lower than today as well. Bulk shear during the time of the storms is expected to remain under 30 knots, with the higher shear near/over 30 knots staying to the north.

Overall, another hot and humid day is expected on Saturday with highs in the upper 80s to low 90s, continuing the heat wave for some which as of today stands at 6 consecutive days, with thunderstorms developing in the afternoon and evening, focusing over Pennsylvania, New York and south-central New England where a slightly higher risk of some storms reaching strong or severe criteria is expected. Later into the day, storms are generally likely to weaken as they reach the area with lower bulk shear and the marginal parameters previously mentioned, although some storms may be strong with locally heavy rain or gusty winds. Storms are expected to end overnight as the cold front pushes through.

As the cold front will be weak, it will cool temperatures down but will fail to bring temperatures below average; highs on Sunday are still expected to reach the mid to upper 80s with partly sunny skies, and some locations near and southwest of NYC may even reach 90 degrees again. Overnight lows are expected to fall into the upper 50s to mid 60s away from the coast.


Next Week: Seasonable, Some Rain Expected

The pattern setup for next week will be different from this week, when a massive ridge set up over the majority of the US with 500mb heights as high as 600 decameters. During next week, strong ridging will set up over the central Atlantic Ocean, southeast of Newfoundland, where similar 500mb heights are expected, keeping the mean trough stuck over the eastern half of the US. A southwest flow will set up aloft, bringing a relatively moist air mass into the region with precipitable water values up to 1.5 to 2 inches, which along with a frontal system generally remaining over the region is expected to produce scattered thunderstorms, locally heavy, across the region each day. As with previous times such a setup unfolded recently, such as late June-early July, not every day will be rainy, and there will still be sunshine mixed with partial cloud cover and scattered storms especially in the afternoon and evening hours, although each day, especially Monday and Tuesday, have at least a risk of scattered storms in the area. High temperatures are expected to remain close to average, generally in the mid to upper 80s, while the rain and warmer air mass keep overnight lows mostly in the mid 60s to low 70s. Going into the longer range, the long range models keeps temperatures close to average through the next 10 days, although some signals from the GEFS show early August as the next potential for a heat surge. Stay tuned for more information on the longer range outlook.