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Posted at 10:45 AM ET, 08/ 7/2009

Forecast: Delightful Today, Major Heat by Sunday

By Josh Larson

* NatCast | July Recap: Summer Finally Gets Real | T-Storm Photos *
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Today: Sunny & very pleasant. Mid-80s. | Tonight: Mostly clear early. Partly cloudy late. 65-70. | Tomorrow: Mix of clouds & sun. Near 90. | Sunday: Partly sunny, hot & humid. Mid-90s. | A Look Ahead


In stark contrast to yesterday's dreary weather, high pressure overhead today brings brilliant blue skies to our region, with low humidity levels and afternoon highs popping back into the mid-80s. In a word: Delightful!... And in stark contrast to today's pleasant weather, big-time heat and humidity builds into the area for the second half of the weekend into at least early next week, with highs likely well into the 90s Sunday, Monday and possibly Tuesday along with truly oppressive humidity.

Today (Friday): High pressure overhead today puts an abrupt end to yesterday's clouds and replaces it with a full serving of sunshine. Afternoon highs will reach the mid-80s, accompanied by negligible humidity.

Tonight: Mostly clear skies are on tap for the evening and overnight hours tonight, with lows dropping to the mid-60s across the cooler 'burbs to near 70 downtown. Confidence: High

Keep reading for the downright hot weekend forecast...

Tomorrow (Saturday): As a warm front lifts north through the area on Saturday we crank up the heat and humidity a bit, but to merely normal levels (maybe even slightly below) for this time of year. Expect a blend of clouds and sun, with afternoon highs around 90. Confidence: Medium-High

Tomorrow Night: It won't be nearly as comfortable Saturday night as it will be tonight. Expect partly to mostly cloudy skies, increasingly muggy conditions, and overnight lows ranging from the low-to-mid 70s. Confidence: Medium-High

Sunday: Sunday will be a crank-the-A/C and stay-indoors-if-at-all-possible type of day. Though we'll have a blend of clouds and sun and low chances for precipitation, afternoon highs will likely soar to the mid-90s with high humidity pushing afternoon heat indices into the upper 90s to near 100. Keep the A/C cranking overnight as lows probably won't get below the mid-to-upper 70s. Confidence: Medium


Back-to-work Monday presents us with a blend of sun and clouds and the possibility of the hottest day of the summer thus far. Morning temperatures will rise into and through the 80s with afternoon highs at least into the mid-90s and only a slight chance of a late-day thunderstorm. Downright oppressive humidity should foster heat indices near 100. Ick! Overnight lows once again in the mid-to-upper 70s. Confidence: Medium

The outlook for Tuesday calls for a continuation of high heat and humidity. I'm expecting partly sunny conditions, afternoon highs in the 90s, and an increased chance of late-day showers and thunderstorms. Confidence: Low-Medium

By Josh Larson  | August 7, 2009; 10:45 AM ET
Categories:  Forecasts  
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Hey, I liked yesterday's "dreary" weather! It was a nice break, and the rides to and from work were really pleasant.

Posted by: debiguity | August 7, 2009 6:11 AM | Report abuse

So... what you're saying is that Sunday will be a great day for 70,000 futbol fanaticos to converge on FedEx Field for D.C. United vs. Real Madrid.

... Right? Bueller?

Posted by: TheAMT | August 7, 2009 9:07 AM | Report abuse

Ugh - is there any chance that it will be cooler than 98 on Sunday and Monday or is it more likely to be warmer? My complex's AC seems to break down every time it gets that hot and I'm just dreading those two days.

Posted by: hereandnow1 | August 7, 2009 10:15 AM | Report abuse

NBC Channel 4.2 is forecasting a high of 99 on Sunday.

My weather station recorded a low of 58.8F last night here east of Front Royal.

Posted by: spgass1 | August 7, 2009 10:27 AM | Report abuse

@hereandnow1, I personally think that mid-90s are probably more likely than upper 90s Sunday and Monday, but the latter definitely cannot be ruled out at this point. Anecdotally, in my experience, these heat waves from a few days out generally under perform as opposed to over perform...

Posted by: Josh-CapitalWeatherGang | August 7, 2009 11:04 AM | Report abuse

Josh, sort of agree that we see a fair amount of underperforming heat, though I think this time may be different thanks to source region. We have not seen a setup like this all summer. I would not be surprised to see either sun/mon (or both) go into the upper 90s, though would lean hottest Monday given extra warm start. I think the previous high of the yr at National (96) should fall.

Posted by: Ian-CapitalWeatherGang | August 7, 2009 11:50 AM | Report abuse

Ian, you have a valid argument. I wouldn't even be shocked if we hit the 100 degree mark on Monday, but for now I'm still thinking mid-90s...though I do realize MOS guidance has climbed up a couple degrees

Posted by: Josh-CapitalWeatherGang | August 7, 2009 12:02 PM | Report abuse

What is so unique about the source of the heat and what is the MOS guidance and how does that impact forecasting?

Posted by: hereandnow1 | August 7, 2009 12:14 PM | Report abuse

If the airmass is well-capped and no clouds form, Sunday's and Monday's temperatures could over-perform [98-102F].
If convective cumulus congestus DO form then we could well under-perform [93-95F], and get a "popcorn" thunderstorm or two.

Predictions for the 850 mb temps in degrees Celsius generally provide a good indicator of projected maximum temps for the day.

This morning we had a good display of cirrus clouds; wonder if anyone got any or two formations tried to form Kelvin-Helmholtz waves on top.

Northern areas could get one or two MCS, tempering the heat wave over the weekend into Monday.

The tropics are quiet; even the low-pressure systems have vanished. Pacific Hurricane Felicia is beginning to weaken, from 140 to 115 mph.

Posted by: Bombo47jea | August 7, 2009 12:36 PM | Report abuse

hereandnow1, it originated in the desert southwest and is migrating east. I may have been wrong though, perhaps the day we hit 96 was somewhat similar.. can't fully recall right now. It's certainly different than the pattern we have seen most of the summer. Either way, this looks like the most prolonged heat of the summer (even if only 3 or 4 days). MOS guidance is just raw temperature outputs from the American models.

Posted by: Ian-CapitalWeatherGang | August 7, 2009 12:43 PM | Report abuse

This comment will only make sense to the meteorologically-savvy out there, but I like to see 850 temps predicted at or above 21C before committing to an upper 90s (or higher) forecast. As of now, it looks like we have a chance at that for both Sunday and Monday, but it's still borderline enough to cast doubt.

Posted by: Dan-CapitalWeatherGang | August 7, 2009 2:07 PM | Report abuse

Don't rule out 105 - 110 degrees on Monday either!

Posted by: Axel2 | August 7, 2009 3:10 PM | Report abuse

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