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Posted at 02:30 PM ET, 04/21/2011

Will rain Friday wash away pollen?


Two rainfall projections for Friday morning into Saturday morning, ranging from about 0.1-0.5” across the metro region.
If there’s any good news for pollen sufferers, it’s that some rain is likely late tomorrow into tomorrow night, which should knock levels down some. Tree pollen counts today have surged to 2177 grains/cubic meter which is very high according to Walter Reed Army Medical Center. So how much rain are we talking about?

NOAA’s Hydrometeorological Prediction Center is projecting over 0.75” between Friday morning and Saturday morning which we think is overdone as the rain will move along a front positioned just to our north. The latest output from the NAM and GFS indicate 0.1-0.5” is more likely. Around 0.25” (or even a little less) sounds like a good bet to us for the immediate metro area, with upwards of 0.5” in the northwestern suburbs (where a Flood Watch is in effect, map) and lighter amounts south.

The rain may take its time in getting started, and steady rain is not necessarily a sure thing during the day Friday especially from the District and points south. In other words, rain that falls Friday afternoon may be very light and patchy and some spots could even stay dry.


Irrespective of the rain timing and exact total, it will be cloudy and unseasonably cold, with highs only in the low 50s. Friday night stands to be especially raw, with a better chance of (mainly light) rain and temps in the 40s.

Getting back to tree pollen, though this week’s levels over 2,000 at times have certainly been miserable, we still haven’t come close to rivaling last year’s 4,000+ levels. As I discussed Tuesday, the fact that we haven’t experienced a long dry streak has a lot to do with it.

This pattern where we see rain every two or three days is really helping to dampen what would be an otherwise even more difficult pollen season.

In between rainy periods, we may continue to see high to very high tree pollen counts over the next week, but historically they level off and start to decline in May.

By  |  02:30 PM ET, 04/21/2011

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