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Posted at 3:15 PM ET, 04/28/2009

PM Update: Heat and Pollen On the Downswing

By Ian Livingston

Much cooler Wednesday with showers likely

* Heat Wave by the Numbers | Greenpeace Protest Not So Green *

Sick of the early heat wave? Good news! It's on its way out. Highs have reached the 80s to near 90 for the fourth straight day. But clouds and showers cool things off starting tomorrow. And the rain will likely push the pollen count down, at least temporarily, after coming in at 3,947 grains per cubic meter of air today (1,500 is considered high) and reaching 4,539 on Saturday (the highest daily count since 1998), as measured at Walter Reed.

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

Tonight: Thin high clouds of today will be replaced by thicker and lowering clouds as the evening progresses. After mild evening temperatures, upper 50s to low 60s should do it for overnight lows. As the front approaches, some showers (and maybe a rumble of thunder) should arrive across northern and western parts of the area around midnight, before settling south through the area by sunrise.

Tomorrow (Wednesday): Wednesday looks cloudy, cool and occasionally showery. Showers will be possible throughout the day, but the heaviest rain should come in the first half. Many locations should see around .25-.5" of rain by the end of the day. Daytime highs near 60 will be about 30 degrees cooler than the past few days, and readings should actually drop throughout much of the day, into the 50s by afternoon on an easterly wind.

See Matt Rogers' full forecast through a cooler rest of the week.

Air Quality Awareness: As you may have noticed, this is Air Quality Awareness Week. Today's focus: What Affects my Air Quality? High pressure, like we've seen the past few days, is a leading factor in poor air quality for several reasons. Toping the list are associated high temperatures, low wind speeds, high humidity and temperature inversions that trap air near the ground. For more details, see today's fact page.

By Ian Livingston  | April 28, 2009; 3:15 PM ET
Categories:  Forecasts  
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Next: Forecast: Abrupt Reversal to Cool, Damp Weather


At least one 100+ year monthly record high for April appears to have been broken.

Posted by: CapitalClimate | April 28, 2009 5:27 PM | Report abuse

Current guidance seems to indicate lighter rather than heavier amounts of rain in the next few days. Most will fall as showers rather than downpours. Thursday could be drizzly rather than rainy. More risk of thunderstorms is possible Friday.

I wonder if there is a climate connection to the "swine flu" outbreak. Normally the flu doesn't hit during the summer.

Posted by: Bombo47jea | April 28, 2009 6:06 PM | Report abuse

I'd like Mother Nature to send showers sufficient to wash all that green powder away!

As far as I'm concerned the trees have had enough sex for this year!

Posted by: RedBird27 | April 28, 2009 7:19 PM | Report abuse

As far as I'm concerned the trees have had enough sex for this year!

Do what now?

Posted by: Axel2 | April 28, 2009 10:07 PM | Report abuse

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