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Posted at 02:00 PM ET, 07/27/2011

Sweat Ceiling, the sequel: Next bout of extreme heat and humidity nears

GFS model for temperatures at 2 p.m. Friday. Temps range from 95-100+ over much of interior mid-Atlantic.
Although the current heat wave, defined as any stretch of at least three consecutive days at or above 90, has reached its tenth day, the drop in humidity since yesterday has provided a reprieve of sorts. Smitten by today’s low humidity, Twitter follower @aimster215 wrote: “Today=best 90 ever.”

But tonight, the sinister southerly wind returns, amping up the humidity Thursday through Saturday. How hot and humid are we talking about? When will it end? And what’s causing the heat?

How hot and humid?

Print out of GFS model forecast for next seven days. Top row of numbers is the forecast high and low each day. Middle row is forecast temperature at 8 a.m. and 8 p.m. each day. Lower row is forecast dew point at 8 a.m. and 8 p.m. each day.
Thursday, the GFS model (print-out shown above) predicts a rather modest high of 91, but increases dew points to near 70 - producing an uncomfortable, but relatively maneagable peak heat index of 97. The NAM model, not shown, predicts more or less the same thing.

My Thursday forecast (for Reagan National): High 94, low 75. Peak heat index 99.

Friday is the scorcher. The GFS forecasts a high of 99, with a dew point of 71, producing a heat index of 107. That’s quite high, and meets heat advisory criteria. But at least it falls well short of last week’s excessive heat indices of 110-120. The NAM model, while it’s been running too low of late, predicts a high of just 94 and a dew point of 72, producing a peak heat index of just 102.

All of the models may be underdoing the heat a bit on Friday as winds are likely to be from the west and southwest - a direction favorable for downsloping winds off the spine of the Appalachians - which aids compressional heating of the air as it comes over the mountains. On the flip side, downsloping can dry the air out - so humidity levels may not be as high as forecast.

My Friday forecast: High 101, low 79. Peak heat index 108.

Saturday, the GFS forecasts another hot and humid day (the NAM doesn’t extend that far), with highs of 95 and dew points near 70, which results in a peak heat index around 101.

My Saturday forecast: High 95, low 80. Peak heat index, 102.

The GFS simulates just a modest drop in heat and humidity Sunday through next Wednesday. Highs remain in the low 90s with dew points in the highs 60s. If it’s right, we’ll be dealing with heat indices in the mid-to-upper 90s for the forseeable future.

What about the Euro model?

The latest Euro model is similar to or slightly less hot than the GFS, predicting low 90s for Thursday, mid-to-upper 90s for Friday, and low-to-mid 90s for Saturday. Humidity levels are similar to the GFS.

Will more records be set?

There’s little to no chance of record-setting heat Thursday, but on Friday and Saturday, we could make a run at a few. I’ll bold those I think we may tie/break.

Friday’s records:

Reagan National (DCA): Hi max - 99 (1993); Hi min - 80 (1949)
BWI: Hi max - 99 (1954); Hi min - 82 (1949)
IAD: Hi max - 97 (1993); Hi min - 74 (2002)

Saturday’s records:

Reagan National (DCA): Hi max - 99 (1953); Hi min - 78 (2008)
BWI: Hi max - 98 (1940); Hi min - 78 (1943)
IAD: Hi max - 98 (1998); Hi min - 75 (1963)

The cause?

Upper level wind and pressure (300 mb and 30,000) showing large high pressure system over Southeast U.S. late Friday, also known as heat dome. Blue shades areas over northern U.S. coincides with the jet stream - effectively the border between the heat dome and cooler air. (
A very large area of upper level high pressure ( frequently referred to as a heat dome or heat bubble) that has stalked the south central U.S. will drift just to our south. Its circulation, extending down to the surface, will act like a heat pump, drawing hot, humid air from the south and southwest. The exact same scenario occurred last week. This week, this heat bubble won’t extend quite as far north due to the position of the jet stream, which should temper its intensity a bit at our latitude.

Bottom line?

Sweat Ceiling the sequel is unlikely to be as potent as the original. For a city that has recently endured the third hottest June and hottest July on record (to date), that is good news. But the combination of heat and humidity will still be oppressive Thursday through Saturday, so take it easy.

Related: Heat wave and hot weather guide

By  |  02:00 PM ET, 07/27/2011

Categories:  Extreme Heat, Local Climate, Latest

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