00Z GFS Ensemble forecast map of temperature anomalies for late Saturday with coast-to-coast heat; map courtesy tropicaltidbits.com, NOAA
We are now halfway through the summer season time period of June, July and August and temperatures have averaged at slightly above normal levels in the I-95 corridor (Philly: +0.9° June, +1.9° July; DCA: +1.0° June, +1.1° July; NYC: +0.8° June, +0.4° July) and there is no reason to believe this overall warmer-than-normal pattern will stop anytime soon despite a nice break coming here for the mid-week. In fact, after a break in the heat and humidity on Tuesday, Wednesday and Thursday, another hot spell appears to be headed our way for the Friday, Saturday, Sunday, Monday time frame and this one could actually result in 100° readings for DC, Philly and NYC. Indeed, the nation as a whole will be quite hot by the weekend which is rather typical weather for a summer featuring an El Nino-to-La Nina transition in the tropical Pacific Ocean. The nice break in the heat and humidity for the mid-week time period will be brought about by the passage late tonight of a cold frontal system that will generate scattered strong-to-severe thunderstorms in the DC-to-Philly-to-NYC corridor during the 3-9 PM time period with possible damaging wind gusts.
Late morning NEXRAD radar showing scattered showers/storms across western portions of the Mid-Atlantic region; map courtesy University of Wisconsin, NOAA
3-9 PM thunderstorm threat
A cold frontal system is sliding eastward this afternoon from the Ohio Valley towards the east coast. Ahead of the front, hot and steamy conditions are creating unstable atmospheric conditions and this combination along with an upper-level short wave is generating showers and thunderstorms across the western portion of the Mid-Atlantic region. This activity will arrive in the I-95 corridor in the 3-9PM time period and there are likely to be some locally strong-to-severe storms producing wind gusts of 50 mph or greater. The greatest chance for strong thunderstorm activity will be to the north and west of the big cities as there will be a tendency for this system to slowly weaken as it slides closer the east coast. Once this front clears through the area by late tonight, high pressure and a new air mass will edge into the area on Tuesday and humidity levels will drop noticeably tomorrow so that it should feel better later in the day than it does early. Large and strong high pressure will sit right on top of the region on Wednesday and this day promises to be about as good as it gets around here in mid-July with very comfortable conditions.
00Z GFS Ensemble forecast map of 500 mb height anomalies for late Saturday; map courtesy tropicaltidbits.com, NOAA
Weekend heat wave
Once this high pressure system pushes off the east coast late in the week, the stage will be set for hot air to pour into the eastern US from the middle of the country. By the weekend, high pressure will dominate all levels of the atmosphere across much of the nation and temperatures will soar to well above normal levels. Last night’s 00Z GFS ensemble model run shows widespread above normal temperatures across much of the nation late Saturday (oranges) along with above normal 500 millibars height anomalies in most areas (oranges). The upcoming weekend heat wave could actually result in 100° readings in DC, Philly and NYC, but broken records are somewhat difficult to come by in mid-July. This kind of widespread hot weather pattern in the US is rather typical in a summer that features the transition of an El Nino-to-La Nina in the tropical Pacific Ocean. Indeed, the overall warmer-than-normal weather pattern in the Mid-Atlantic region is likely to continue through the rest of the summer - and perhaps even well into the fall season.
Meteorologist Paul Dorian