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Posted at 5:00 AM ET, 07/21/2008

Forecast: Two More Days of Big Heat

By Jason Samenow

Our streak of consecutive 90+ degree days grows to six as we endure another scorcher today. On the plus side, the streak may end at seven tomorrow with modest relief mid-to-late week. The other piece of good news is that between Tuesday and Thursday, we should pick up some rain. After a wet May and June, July rain is about 0.75" below average and the vegetation is starting to show it.


Air Quality: Latest air quality based on ozone levels. Courtesy AIRNow. Click on image to expand. Refresh page to update. Take action to protect your health and reduce air pollution.

Hazy and hot. Mid 90s. A very weak front which passed through the region overnight (triggering a few storms) has switched the winds from southerly (from the south) to northwesterly (from the northwest). The (weak) northwest flow will result in a slight drop in humidity levels relative to the weekend, but temperatures will be just as hot under mostly sunny skies. We'll have some haze and the air quality index (shown to the right) is forecast to peak at Code Orange -- unhealthy for sensitive groups.

Overnight, skies will likely be partly cloudy with lows in the mid 70s downtown and near 70 in the cooler suburbs. An isolated thunderstorm can't be ruled out (20%) especially in western suburbs.

Confidence: Medium-High

Keep reading for the forecast through the week.

Storms east of the District produce lightning last night. By Capital Weather Gang photographer Ian Livingston.


Hot and humid. T'storm risk late. Mid 90s. The humidity, which dropped slightly on Monday, will return to oppressive levels. The afternoon heat index will near 100. By late in the afternoon and during the evening, a few thunderstorms are possible (35% chance) as a slow moving cold front approaches.

A few storms could linger through midnight or so, otherwise skies will be partly cloudy. It will remain humid with lows near 70 in the cooler suburbs to the mid 70s downtown.

Confidence: Medium-High


Tuesday's cold front will stall over the region Wednesday, producing mostly cloudy skies and a decent chance (50%) of afternoon/evening showers and thunderstorms. The lack of sunshine may hold high temperatures below 90 for the first time in a week. Confidence: Medium-High

On Thursday, the front may get a push offshore from an area of high pressure to the northwest or it may linger over the region. If it's pushed offshore, we'll have partly to mostly sunny skies and noticeably lower humidity. If it stays put, we'll have mostly cloudy skies and a good chance of showers and storms. The most likely scenario may be a compromise where areas east of I-95 have a storm threat, whereas areas to the west start to dry out. For any of the scenarios, highs will probably be in the mid to upper 80s. Confidence: Low-Medium

On Friday, high pressure is likely to influence our weather, meaning mostly sunny skies and seasonably warm high temperatures in the upper 80s. Confidence: Medium-High

Saturday should be partly cloudy with a 30% chance of afternoon/evening storms due to a possible approaching front. Highs should be in upper 80s to low 90s. Confidence: Medium

If Saturday's front clears, Sunday should be a pretty nice day, with mostly sunny skies, comfortable humidity, and highs in the mid 80s. Confidence: Medium

By Jason Samenow  | July 21, 2008; 5:00 AM ET
Categories:  Forecasts  
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Next: Freedman: First Half of 2008 Was Warm Worldwide


West Springfield (Accotink Lk/Mixing Bowl): Storm Total = 0.08" 7/20-7/21 6am

Posted by: Bikerjohn | July 21, 2008 9:25 AM | Report abuse

My view may be colored by my intense dislike of DC summers, but why does it seem that there tend to be more warmer-than-average days in summer than cooler-than-average? Media meteorologists usually keep tallies of 90+ degree days, but there's never a need to keep logs of 70 degree streaks. I'd appreciate your thoughts on this.

Posted by: dcreader | July 21, 2008 10:00 AM | Report abuse

Bombocast: Current: wispy cumulus, 87F, b.p. 29.90.

Forecast: This afternoon. Hot, humid, high 95. Tonight. Slight chance (10%) airmass/outflow thunderstorm, low 73, cooler outside beltway.

DCReader, the dewpoints seem to stay near 65-70 this time of year even when the temperature is below average. After Labor Day, the dewpoints start dropping, though we have had some very damp Septembers when the hurricane season was active around here. Thus it's muggy around here even when the temperature is "below" normal.

Posted by: El Bombo | July 21, 2008 11:34 AM | Report abuse

Question about air quality...what meteorological factors influence this? i've been experiencing some major respiratory issues for the past 2 weeks and am hoping for a reprieve...does rain help?

Posted by: Rishi | July 21, 2008 11:48 AM | Report abuse

It's a matter of arithmetic. In July, a high of 90 or more is at most 3° above average. A high of 79 or less is at least 8° below average.

Posted by: CapitalClimate | July 21, 2008 12:26 PM | Report abuse

To DC Reader

90 degrees is pretty normal for DC summers

Average Highs

July 88
August 86

Expecting it to be 70 is unrealistic until September at the earliest

Or to put it another way in july and august the propability of having a 75 degree day is the same as having a 99 degree. Both extremely rare events. Even below 80 only happens as many days as it being above 95. These days happen occassionally but usually not more than 5-6 days per summer.

Posted by: its normal | July 21, 2008 12:38 PM | Report abuse

dcreader: CapitalClimate is right. Would add that if we had consecutive days in the 70s in July, we'd definitely be writing about that streak.

Rishi: Check out this link for more on how heat and air stagnation make pollution worse.

Posted by: Capital Weather Gang | July 21, 2008 1:15 PM | Report abuse

To put some numbers on the previous point,
July days since 1929:
High 79 or less: 210 (8.6%)
High 90 or above: 1009 (41.2%)
Longest streak of 79 or less: 5 (1933)
Longest streak of 90 or more: 16 (1993)

Posted by: CapitalClimate | July 21, 2008 2:39 PM | Report abuse is forecasting a high on Wednesday of 79. Capital Weather Gang is not going that far. Usually the two sources are pretty close in their predictions. What high does CWG predict for Wednesday? Could it really get that low?

Posted by: Reston Reader | July 21, 2008 4:15 PM | Report abuse

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