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Posted at 11:00 AM ET, 07/21/2010

In focus: The heat & humidity double whammy

By Jason Samenow

* Extreme heat coming, then going? Full Forecast | Tropical tracking *

An analysis of the average humidity through a portion of the atmosphere from July 7 (from the NAM model) and a simulation of the predicted humidity for this Saturday, July 24 (from the GFS model). The humidity forecast for this Saturday is much higher than the humidity analyzed during the early July heat wave. Imagery courtesy Unisys Weather, adapted by CWG.

Back on July 6 and 7, when we suffered through the hottest days of the summer thus far, you may recall it wasn't all that humid. Though Reagan National sizzled to 102 both of those days and BWI touched a jaw-dropping 105 on 7/6, afternoon relative humidity values averaged just over 20% and dew points were in the relatively comfortable mid-to-upper 50s.

Compare those conditions to what we're predicting for this Saturday - when we think highs will reach the upper 90s to near 100 but relative humidity values will likely be closer to 40% with dew points around 70. Due to the added humidity, the coming heat may feel more oppressive than the record-breaking heat of just two weeks ago.

So what's the reason for the difference in the character of these two hot spells?

Keep reading...

A weather map showing high pressure centered over the mid-Atlantic on July 7. This high developed under a ridge in the jet stream that developed over the Upper Midwest. The colored areas show temperatures about 5,000 feet above the surface. Image courtesy Unisys Weather.

It all has to do with the origin of the two air masses.

The air mass responsible for the record heat in early July originated in the Upper Midwest and Great Lakes as a giant ridge of high pressure developed and then intensified as it headed towards the East Coast. Because the hot air mass was born over the interior of the North American continent and away from the ocean - it was relatively dry.

Such an air mass is classified as "Dry Tropical" as it was produced by rapidly descending air via strong subsidence (sinking) which tends to dry out the air. The intensity of the heat over our region and lack of humidity may have been amplified by a lack of soil moisture resulting from the rainfall deficit at the time.

Forecast weather map for Saturday from ECMWF model showing Bermuda high pumping up warm air over mid-Atlantic and enhanced by approaching front. The colored areas show temperatures about 5,000 feet above the surface. Courtesy College of Dupage.

Fast forward to this week and we're dealing with an air mass with a totally different origin. This week's air mass is originating from high pressure over the western Atlantic ocean, also known as the classic Bermuda High. The clockwise circulation around the Bermuda High brings warm and humid air mass from the south and southwest over the mid-Atlantic and is typically classified as "Moist Tropical" (MT). According to its description the MT air mass "is typically found in warm sectors of mid-latitude cyclones [low pressure] or in a return flow on the western side of an anticyclone [high pressure]." On any given day, you can determine what the air mass type is by visiting the Spatial Synoptic Classification Web site.

This coming Saturday, the potential for extreme heat and humidity from this MT air mass is elevated as the Bermuda high pumps in juicy warm air while we're simultaneously in the warm sector of an approaching area of low pressure system to the north. We could enter MT+ territory, reserved for the statistically most oppressive days.

The bottom line: this Saturday has the potential to be the most uncomfortably (and dangerously?) hot day of 2010. It may also break records. Saturday's record highs are: 96 at DCA, and 97 at IAD and BWI, all set in 1987.

By Jason Samenow  | July 21, 2010; 11:00 AM ET
Categories:  Extreme Heat, Science  
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Next: PM Update: After a week of heat, bigger coming


The Euro (which I'd probably weight toward in this time range) looks like we may stay with the big heat Sunday and even Monday. One thing I'll say is that heat has been very well modeled this summer (perhaps because it's been mostly hot). This pattern was showing up 7-10 days ago at least.

Posted by: Ian-CapitalWeatherGang | July 21, 2010 11:46 AM | Report abuse

Wow - you guys really know how to sicken your readers with honesty. Just once I'd like to hear something like, 'we are 41 days away from the start of meteorological fall,' or, 'in something like 50 days, our average high will be ____!' If I wanted this much brutal honesty I'd have a conversation with my wife.

Posted by: authorofpoetry | July 21, 2010 11:48 AM | Report abuse

Sounds awful. Can't wait!

Posted by: FIREDRAGON47 | July 21, 2010 11:50 AM | Report abuse

This reminds me of a song from my fellow Texan, Lyle Lovett. From the "Road to Ensenada" album, the song "It Ought to be Easier." This line always reminds me of a hot Houston summer day:

See the sun come up on the pavement
The pavement it starts to sweat
The steam rises up from the water
And the hotter it is you know the harder it gets

Posted by: erbele | July 21, 2010 1:07 PM | Report abuse

Thanks for this post! I think the greatest part about this site is the explanations of the big picture affecting the weather.

Posted by: CM_in_Fairfax | July 21, 2010 2:02 PM | Report abuse

Continental Tropical [cT] dry air is rather rare in the continental U.S. except in the region from Texas to Arizona/California, and rarely moves northward or eastward. It did during the Dust Bowl era of the 1930's and is evidently doing so again this summer.

We should be lucky we're not in the friagem-boosted Maritime Equatorial [mE] airmass sitting over the Panama Canal. Equatorial air is just like wet tropical air but even more so; the humidity is even deeper. Our current tropical airmass is capped by a cooler drier "superior" layer riding above it on the jet stream, but this dry air is increasing the severe weather threat, since it produces MCS' which ride eastward on the jet.

Posted by: Bombo47jea | July 21, 2010 2:05 PM | Report abuse

Summer of 1980 was much worse. Lot more humid from June through August. And it was even more fun for me I was in cast from the toes to top of my left leg from June 13 until Aug 6.

Posted by: sheepherder | July 21, 2010 2:18 PM | Report abuse

Tell me why this is so much better than snow again?

Posted by: celestun100 | July 21, 2010 3:34 PM | Report abuse

I think I am going to do some kind of ceremony dance around my heat pump to make sure the air conditioner keeps working...remember when we were pulling mountains of snow off our heat pumps?

Posted by: ThinkGreen | July 21, 2010 3:36 PM | Report abuse

Ok...I just "googled" heat index and my head is still swimming with facts that I don't understand. Would someone please explain in simple terms what it is, and why is it important?

Posted by: worldtraveler83 | July 21, 2010 6:37 PM | Report abuse

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