What happened to the rain?
After a very soggy spring, somebody has finally shut off the rain valve. Baltimore has recorded no measurable rain for more than two weeks now. The grass that grew so happily through April, May and June has suddenly choked and turned brown. My thickest lawn in 12 years is suddenly begging for moisture.
With some luck, we may get some showers and thunderstorms during the next week or so. The best shot seems like Friday night and Saturday, which forecasters give a 50 percent chance of delivering some precipitation.
For now, we're in for a real 
summery day, with highs likely to top out in the 90s. West winds are bringing air down the eastern slope of the mountains, which in addition to heating things up, is also drying them out some. The relative humidity here at The Sun is holding in the 50s, so it doesn't feel quite as miserable as it can at this time of year. It only starts to feel uncomfortably humid when the dewpoint reaches 70 degrees or so.
But this won't last long. Forecasters are expecting a series of weak cold fronts, the first arriving here overnight from the Ohio Valley and stalling just to our south. Low-pressure systems tracking along that front could will deliver some showers and thunderstorms tonight or tomorrow. Then, a second, reinforcing front will slide by on Saturday with a higher risk of showers and storms, and cooler temperatures - back into the low 80s by Sunday and Monday.
After a mostly cloudy Sunday, rain chances will begin to rise a bit early in the work week, to 30 percent.
Since June 20, BWI-Marshall has received just 0.48 inch of rain, with nothing measurable since July 1. The dry air has also been relatively cool. Of the last 18 days (June 28 through Wednesday), 16 have averaged cooler than the long-term norm for Baltimore. One matched the average, while just one day - July 12 - was warmer than the norm.
Cooling degree-days so far in July are running at just 65 percent of the long-term average, saving us real money on our cooling bills. The average temperature for July so far at BWI is 72.4 degrees. We still have two weeks to go, but if this were to hold it would be the second-coolest July on record. Only 1891 was cooler, with an average July temperature in Baltimore of 71.6 degrees.
As dry as it has been, the Drought Monitor map still shows all of Maryland with normal moisture. although unusually dry conditions exist in southeastern Virginia. Streamflow in Maryland remains mostly normal for this time of year.
(SUN PHOTO/Colby Ware 2006)
So just how dry has it been? Steve Zubrick, the science and operations officer at the National Weather Service forecast office in Sterling, Va., has done some unofficial sleuthing through the records for BWI and found this:
On Tuesday, July 14, the dewpoint at BWI reached a remarkable 39 degrees at 3:54 p.m. (The dewpoint is the temperature at which the air becomes saturated with water vapor and it begins to precipitate. The lower the dewpoint, the drier the air.)
Combined with the temperature at that moment on Tuesday, the relative humidity in Baltimore was just 21 percent, "which is probably as low as you'll ever see in July," Zubrick said.
Unfortunately, the NWS doesn't keep comprehensive records on relative humidity. But the lowest July dewpoint that turned up in Zubrick's search of the records from 1962 to 2009 was 36 degrees, reached twice, in 1966 and 1980.
Between 1962 and 2009, Zubrick found just 16 observations with dewpoints less than 40 degrees. The last time was on July 2, 2002. There were four consecutive hourly readings with dew points below 40 degrees on July 7, 1980, and on July 21, 1966, BWI reported 9 consecutive hours with sub-40-degree dewpoints.
Comments
Re relative humidity in the 50s:
Did you mean to refer instead to the dewpoint? The dewpoints in the upper 50s to low 60s have held the relative humidity to 34% at 5:00 at BWI. A relative humidity of 50% would result in an uncomfortable heat index well over 100 at these temperatures, and 70% would be unbearable.
FR: Yes. Thanks for the CX.
Posted by: CapitalClimate | July 16, 2009 6:40 PM
Just as I predicted back in June......back then people bailing out basements, stalled in traffic on flooded roads, proms & graduations (yes - both for our poor daughter) held during monsoons, farmers whining about TOO MUCH rain after praying for buckets of it and.....I predicted back then if we had just 2 weeks of dry weather we'd start hearing the whining again.....well.....there you go........right on cue....I'll likely have fun slogging through a soggy week at the beach next week......good grief....is no one happy unless it pours buckets 4 times a week and the precip surplus is at plus 4 or more?
FR: Hey, it's a WEATHER blog. We talk about Maryland weather, which at the moment is dry. Happy or unhappy or oblivious, that's the weather. Besides, people love to complain about things they can't control. It just feels better. And if you really want to know the truth, whenever we write about a weather trend here, it ends. So, to save my lawn, I write about how dry the weather is, and it rains. Watch the skies.
Posted by: Rich | July 17, 2009 1:47 PM
Frank,
Were we ever in a drought this year?
FR: Yes, for a few weeks in late March to mid-April a half to 80 percent of the state was in "moderate drought" conditions after a very dry fall and winter. The entire state was at least "moderately dry" on the national Drought Monitor map. Then the heavens opened up. April, May and the first half of June were very wet, but then the rains stopped again, and we have had very little since. But despite the brown lawns, the entire state (as of last week) remains officially free of any dry or drought conditions.http://tr.im/tnom
Posted by: Voldermort | July 21, 2009 11:29 AM
Hey Frank, any idea what the record high dewpoint is for the Baltimore area?
FR: No idea. I doubt the NWS tracks it. It's only significant when you know the actual temperature.
Posted by: Greg | July 29, 2009 4:28 AM