July 18, 2013: 100 Degrees Today And Tomorrow

Forecast Highlights:

The peak of the heat wave takes place today and tomorrow, as more westerly winds and warmer temperatures aloft bring temperatures into the mid to upper 90s across the area, with low 100s possible in parts of NE NJ or NYC. Along with humid conditions, heat index values will reach the low-mid 100s, possibly even higher locally. A cold front will produce thunderstorms on Saturday evening, ending the heat wave, although the severity of the storms may not be as significant as originally forecast.

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Wednesday, July 17 Observations:

Hot and humid conditions continued for a 4th consecutive day as a massive ridge continued to dominate the weather pattern. Mostly to partly sunny skies were observed with light W-NW winds and high temperatures reaching the low to mid 90s in Long Island/south CT and the mid to upper 90s for the rest of the area, with parts of far eastern Long Island and SE CT staying below 90 degrees. The hottest temperatures were 97 degrees in LaGuardia and Central Park and 100 degrees in Teterboro.

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Today - Friday: Peak Of Heat Wave

A shortwave trough passed through southern Canada, and the massive ridge that covered nearly half of the US with 500mb heights as high as 600dm is starting to slowly flatten and shift south, setting up for a more westerly flow today and a SW flow tomorrow as a cold front gradually approaches from the northwest. With the W-SW flow, a warmer and more humid air mass will be advected into the region, with 850mb temperatures reaching the 19-22C range, bringing the peak of the heat wave into the region.

Temperatures last night struggled to cool down in the immediate NYC area, only falling into the upper 60s to low 80s. Mostly sunny skies are expected today across the area; along with the aforementioned setup, high temperatures will reach the low to mid 90s near the coast and the mid to upper 90s for the rest of the area. The hottest temperatures are expected in NE NJ and NYC, with highs in the upper 90s to low 100s. Along with humid conditions, heat index values will reach the upper 90s to mid 100s across the area. Although temperatures aloft will be warmer on Saturday, slightly more cloud cover is expected along with isolated late afternoon-evening thunderstorms inland, with high temperatures again reaching the low to mid 90s near the coast and the mid to upper 90s for the rest of the area, nearing 100 degrees in the immediate NYC area. More humidity is expected compared to Friday, however, with heat index values in the low-mid 100s, possibly exceeding 105 degrees near NE NJ and NYC.


Weekend: Some Storms, Then Cooler

A cold front will move through the region on Saturday, ending the heat wave, with the last day of heat as high temperatures reach the upper 80s to mid 90s. This front has been mentioned to produce widespread thunderstorms, some strong to severe, across the region, although there are some limiting factors regarding the intensity of the storms. The air mass ahead of the front is humid and unstable, with CAPE up to 2000 J/kg and LI around -5 to -10 across the region. Precipitable water values are high as well, near 2 inches, also setting up for heavy rainfall with the storms. Despite the high instability, however, bulk shear is lacking, near 25-30 knots in the NYC area, with the higher shear of 40+ knots displaced to the north of the highest instability. Mid level lapse rates are marginally sufficient, near 6 to 6.5 C/km, although the best forcing will be north of the area and less significant compared to Friday in the Great Lakes.

Storm development is still anticipated during the early afternoon hours over central NY state and central New England, shifting southeast by the late afternoon and evening hours, although with the aforementioned factors, the strongest storms are expected north of the area, primarily over southern NY into south-central New England, and are expected to weaken with less severity once reaching the area in the late afternoon or evening, although locally heavy rain and gusty winds are still possible especially north of NYC. Storms are expected to end overnight as the cold front moves offshore and pushes the heat and humidity offshore.

Behind the cold front, a drier, cooler and less humid air mass will return, with ridging kept offshore for a more prolonged period of time. Below average temperatures are expected, peaking in the low to mid 80s across most of the area, along with partly sunny skies. Overnight lows will be much cooler as well, falling into the upper 50s to mid 60s for most, a significant change compared to the first half of the month when lows were frequently in the upper 60s and 70s. Beyond the early week, models show an increased risk of rain and storms towards the mid-late week as temperatures gradually increase but with no additional heat wave forecast at least in the next 8-10 days.