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Posted at 10:00 AM ET, 08/ 8/2009

Forecast: Heat Wave One Day Away

By Jason Samenow

* NatCast | Radar, Temps, and More: Weather Wall *
* NOAA Says No to Hurricane Modification | July Recap: 5th Driest *
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Today: Partly sunny, 20% chance of a p.m. t'storm. 85-88. | Tonight: Partly cloudy, 20% chance of t'storm. 69-74. | Tomorrow: Mostly sunny and hot. 94-98. A Look Ahead


If you thought we might make it through summer without a heat wave, think again. A trio of days when the mercury reaches well into the 90s will scorch the region starting tomorrow, along with increasing humidity. On the plus side, we should avoid 90 today and the humidity will be bearable.

Radar: Latest regional radar loop shows movement of precipitation over past two hours. Powered by HAMweather. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall.

Today (Saturday): Though it won't be as pleasantly dry as yesterday, we'll have pretty nice summer weather today. Temperatures will max out in the mid-to-upper 80s with moderate humidity levels. Towards evening an isolated thunderstorm could pop up (20% chance) as a warm front advances toward the region. Winds will be from the southwest at about 5-10 mph. Confidence: Medium-High

Probability: 20%
Coverage: Isolated

Tonight: The warm front -- aka the harbinger of our coming heat wave -- will roll through the area. It might (20% chance) set off a thunderstorm or two in the evening, but more likely north of the District. Once the front passes by, temperatures won't fall much, just bottoming out in the upper 60s in the cooler suburbs to the mid-70s downtown. Confidence: Medium-High

Keep reading for the forecast for tomorrow and into early next week...

Tomorrow (Sunday): Here comes the heat. Abundant sunshine coupled with a very warm airmass building in from the southwest will send temperatures into the mid- and possibly upper 90s during the afternoon. Factoring in the humidity (in the medium to high range given dew points of 61-65), it will feel like 100 degrees at times.

There are a number of major sporting events that will coincide with the heat of the afternoon including the Legg Mason Tennis Final, the Nationals game against the Diamondbacks and the D.C. United showdown against Real Madrid. Athletes and fans should plan for very hot conditions and stay hydrated. A breeze blowing at only 10 mph or so from the west won't offer much relief. Confidence: Medium-High

Tomorrow Night: Partly cloudy and very warm and muggy. Lows only fall to the upper 70s in the city, with low 70s in the suburbs. Confidence: Medium-High


Monday may be the most oppressive day of the upcoming heat wave. Humidity will be higher than Sunday (dew points in the upper 60s) and indications are that temperatures will rise into the upper 90s. So it will probably feel like between 100 and 105 during the heat of the afternoon. Breezes from the west will continue at around 5-10 mph. Uncomfortably warm overnight, with lows only in the mid-to-upper 70s. Confidence: Medium-High

Tuesday may not be quite as hot as Monday as clouds increase in advance of an approaching cold front, but humidity levels may peak (with dew points at or even above 70). High temperatures will probably be in the mid-90s, but it may feel just as hot (i.e. above 100) as Monday factoring in the humidity. During the late afternoon and into the evening, thunderstorms are possible (30% chance) which may offer a little relief from the heat. Confidence: Medium

By Jason Samenow  | August 8, 2009; 10:00 AM ET
Categories:  Forecasts  
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What are the chances of us beating high temps this go around?

Posted by: hereandnow1 | August 8, 2009 10:19 AM | Report abuse

Chances are rather good if it's mainly sunny; not so good if decaying MCS's or other factors promote cloudiness.

The projected 850 mb temperature is 20-22C or 68-72F. This translates to >96-102F at the surface in fair weather, indicating a "capped" high-pressure ridge.

The worst of the heat wave is tomorrow through Tuesday.

The tropics today are largely quiet on the Atlantic side. The Bermuda High remains strong. The ITCZ is at or just below 10N latitude, with a low forming over the Isthmus of Panama. On the Pacific side, Hurricane Felicia continues to track towards Hawaii, but encounters strong westerly wind shear. It should weaken considerably over the next 48 hours. Farther west, typhoons are acting up between Guam and China, with Taiwan taking the brunt of the recent action.

Posted by: Bombo47jea | August 8, 2009 12:51 PM | Report abuse

Bombo47.........with the type of heat wave expected here the next couple of days (and a westerly wind) the conventional Bermuda High positioning is not a factor. We usually get our hottest temperatures not from the high off the coast, which brings southerly to southwesterly winds (and higher dew points, which dampen extreme temperatures) but from either a sub-tropical High cented inland over the Southeastern states or a westward extension of the Bermuda High ridging inland. When a big summer High is over the Southeast, it gives us west-to-northwest winds, which descend the east slopes of the Blue Ridge, warm up, dry out and compress, blow away any cloud cover, and give us roasting hot temperatures and usually little if any rain.

Posted by: MMCarhelp | August 8, 2009 9:46 PM | Report abuse

Today's high was 87 at DCA.


If you're asking if we have a chance to break any records, I'd say the best chance is Monday, when the record high at DCA is 98.

Posted by: Jason-CapitalWeatherGang | August 9, 2009 12:38 AM | Report abuse

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