Good Ol' Summertime

A Severe Thunderstorm Watch is in effect for the area until 11pm.

OK, you know the drill by now. It's summer, it's warm and humid (although a little less so than yesterday), there's an extra source of moisture from the high ground water content, and there are a couple of triggering mechanisms in the form of two weak frontal boundaries in the region and some upper-level instability rolling through this evening.

Accordingly, the NWS has issued a severe thunderstorm watch for the area until 11pm. The Flash Flood Warning for the Lake Needwood Dam in MoCo also continues. By 4pm, radar showed only isolated activity, some of which was locally intense, ranging over the map in location from upper MoCo (weakening as it moved eastward) to nearing Culpeper, to north of Charlottesville, and to west of Ocean City. The forecast for tonight through the weekend and beyond are below the fold.

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Climate Corner: Comedy Central Coverage

Last night's Daily Show on Comedy Central devoted half the program to an interview with ex-next President Al Gore on the subject of his climate-change documentary "An Inconvenient Truth". The show will be repeated this evening at 8pm and most likely during their holiday break next week.

And Now For Something Completely Different . . .

Right Now

Garden-variety (as opposed to the industrial grade of recent days) thunderstorms with some areas of heavy rain are affecting parts of the area this evening.

With the threat of storms, a Flash Flood Warning remains in effect for Montgomery County where seepage at a Rockville dam is a concern.

Tonight and Tomorrow

Tonight, it will be warm and humid with scattered thunderstorms and lows near 71° in the city, mid to upper 60s in the cooler 'burbs. Tomorrow will be mostly sunny and a little less humid with seasonable highs in the upper 80s and a slight chance of afternoon or evening thunderstorms.

For the outlook into the weekend, see Dan's post below.

Deficit Demolished

Water-weary Washingtonians woke Wednesday with wonder: Sunshine! Blue Sky! After days of near-Biblical deluges, the rain finally stopped. At National, the rain ended at 3:48 am this morning, to be followed by a few minutes of drizzle. The excess precipitation since Friday was more than enough to wipe out the accumulated deficit for the year. In fact, what had been a deficit over 30% quickly reversed to an even larger surplus of 32% through yesterday.

CapitalWeather.com chart from NWS data, photo © Kevin Ambrose

Metaphorical Meteorological Meanings

Tom Toles finds a symbolic interpretation of the recent storms in today's WaPo editorial cartoon. (Note to online WaPo: That omnipresent Sprint/Nextel Flash ad is a CPU HOG! It gets your window closed immediately on the PM Update computer every time. Talk about negative advertising!)

Moderate to Monsoon: State of Emergency

Flash Flood Watch is in effect for the entire DC area until Wednesday morning

State of Emergency declared in Washington, DC

9pm update: Heaviest rain is west of the beltway where the highest rainfall totals are likely tonight. Generally 1-3" should fall across the area (with isolated higher amounts--especially west) with rainfall tapering off late tonight. Rain may be enhanced as tropical low moves northward for a time between about 10pm and 3am. Washington Post: More Rain Heading to Washington.

6:45 Update:Flash Flood Warnings have now been issued for most of the immediate DC metro area.

5:30 Update: The low pressure area in the Atlantic moved inland near Morehead City NC before it could develop. However, it is spreading heavy rains and gusty winds across the Outer Banks as it moves northward.

To follow the current storm progress in the Washington area, periodically refresh the radar image from wunderground.com.
For storm related links and a timeline through tomorrow, scroll down to Matt's earlier post below.

With the lower half of the atmosphere almost completely saturated, storms can break out just about anywhere in the region the rest of this afternoon and evening, and storms now in central Virginia will continue moving north or west of north toward the DC area. Although some downpours may be brief, they can also be quite heavy.

After 2.56" of rain fell at National Airport in the 24 hours ending this morning, by early this afternoon, only 0.02" fell, but a heavy shower has raised that by 0.21" in only 14 minutes in the past hour. In an almost complete reversal of the usual thunderstorm path in this area, that storm moved from Prince George's County across the District from Southeast to Northwest, and is now pushing into Montgomery County, where rain is ranging from moderate to monsoon outside the window of Afternoon Blog Central. The main focus of activity so far today, however, is somewhat to the west of the immediate metro area. The most widespread heavy storms extend from near Hagerstown to south of Charlottesville.

Another issue of concern is a weak low pressure area about 35 miles off the lower North Carolina coast. The National Hurricane Center reported this afternoon that a reconnaissance flight found some gale force winds in the eastern portion of the area, but there was no closed circulation; therefore, it is not a tropical cyclone, at least yet. It does have the potential to develop into a tropical storm, however. Depending on the exact track of this disturbance as it moves northward, it could inject even more moisture into the DC region, or it could remain closer to the coast. In any case, the models are predicting several more inches of rain through the next day or so.

Flood Threat Continues

FLASH FLOOD WATCHES AND WARNINGS are in effect for the entire DC metro area. See details in the Current Watches and Warnings box above. Headed out for the commute home? Listen live to WTOP for the latest road conditions.

6:30pm Update: Light to moderate rain now falling in much of the metro area will become heavier at times in many places over the next couple of hours as stronger storms move in from the south.

For the outlook through tomorrow, and a recap of events since yesterday night, scroll down to Jason's post below.

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5:00pm Update: The Flash Flood Warning has been extended for DC and the immediate area except for Prince George's County until 11pm. An area of moderate to heavy rain extends from Springfield northward along much of the VA Beltway and is moving into western Montgomery County.

3:30pm Update: Storm activity is now located almost completely outside the Beltway. The heaviest showers are in southern Montgomery and western Fairfax Counties. Another area of moderate to heavy rain is in southwestern Prince George's County. Dulles has recorded 1.43" of new rain since 8am today. A band of storms extends southward just west of I-95 in Virginia to south of Richmond.

1:30pm Update: Right now, the local areas most affected by showers are in a narrow band from near Bowie southwestward across the eastern portion of DC through Alexandria and along I-95. Don't be fooled if you're seeing sunshine and some blue sky, as we are here in west-central Montgomery County. The air is extremely juicy; dewpoints in the area are in the low to mid 70s, and Stafford is reporting a super-tropical 81°. The sun just adds fuel to the mix, and bands of showers and storms extend (with some breaks) from north of Baltimore southward well into southern Virginia. These storms are moving a little east of north and will impact various parts of the DC region intermittently through the afternoon and evening, some with heavy downpours.

Drought Busters

Tonight and Tomorrow

There is a Flash Flood Watch in effect for the entire DC region which has now been extended from 8pm tonight through Sunday morning. Conditions will be mostly cloudy and humid with an 80% chance of showers or thunderstorms tonight (mostly after 7pm) through tomorrow. Lows overnight will be in the low 70s, and tomorrow's highs will be in the low 80s.

For the outlook through Sunday, including weekend event forecasts, see Camden's post below.

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Last night's late thunderstorms put on a spectacular light show in much of the Washington DC area, but heavy rain was again, like Monday, mainly limited to southern portions of the region. National picked up 0.71" of rain, and Dulles had 1.51", most of which fell in one hour. This was more than enough to set a new record for June 23 at Dulles. Most of the Maryland suburbs north of the Beltway, however, had very light amounts.

This afternoon's activity has been concentrated around the lower Potomac, Northern Neck of Virginia, and the Eastern Shore. Salisbury MD has had over 2.6" of rain, and Wallops Island VA was walloped with just under 3" in one hour. The heaviest storms are now slowly working their way into the Virginia Tidewater area.

As a cold front approaches and stalls out over the Mid Atlantic region, showers and thunderstorms are likely through tomorrow. As noted above, there is a Flash Flood Watch in effect for the entire area through Sunday morning. Rainfall amounts are expected to average 2 to 4 inches, with locally higher amounts. For an example of what 5"+ of rain can do in the wrong place, check out the photo of the parking deck at the Crabtree Valley Mall in Raleigh during a recent flash flood. Note the 7-foot clearance barrier.

CapitalWeather.com photo by Sarah Scolnik

Climate Corner

The National Center for Atmospheric Research (NCAR) yesterday announced the results of a study by NCAR scientists Kevin Trenberth and Dennis Shea on trends in Atlantic sea surface temperatures (SSTs), which have been associated with increased tropical storm activity. The researchers found that global warming explained 0.8° of the rise in tropical Atlantic SSTs, but the so-called Atlantic multidecadal oscillation (AMO) accounted for only 0.2°. This raises serious doubts about theories proposed by Colorado State's William Gray, among others, that the recent increase in hurricane activity is due strictly to the AMO.

The NCAR results are consistent with an article, "Atlantic Hurricane Trends Linked to Climate Change", (subscription required) in the June 13 issue of the American Geophysical Union publication EOS. The article, by Michael Mann of Penn State and Kerry Emanuel of MIT, describes a statistical (regression) analysis of tropical Atlantic SSTs. The authors conclude:

In short, there is no evidence that a natural climate oscillation such as the AMO contributes to long-term tropical North Atlantic SST variations.

In fact, they find that the apparent effect of the AMO may actually be the result of a partial masking of the global warming trend by aerosol cooling effects ("solar dimming").

Air You Can Wear

Tonight and Tomorrow

Tonight will be warm and humid, with lows only in the mid 70s downtown, near 70 in the 'burbs. The chance of thunderstorms through this evening is 20%. Tomorrow will again be warm and humid, but more cloudiness will keep highs around 87. The chance of thunderstorms, especially in the late afternoon and evening, is 60%.

For the extended outlook, see Josh's post below.

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The remnants of some storms which had deluged parts of Ohio yesterday moved through the Washington DC metro area this morning, limiting early temperature rises. The breakout of sunshine, however, has pushed the muggometer setting to "Sticky". The most oppressive readings are in the southern fringes of the region, where Fredericksburg and Stafford have reported heat indices over 100. Both of these stations, along with Culpeper, reported temperatures of 99° at 4pm.

On this anniversary of the drenching from the remnants of Agnes in 1972 (see Today in Weather History to the right), we finally have a chance over the next 5 days to put a serious dent in the area's precipitation deficit, especially if a possible tropical tap develops. Scroll down to Josh's earlier post for details.

A Finger in the Dike

The experience of New Orleans with Katrina has created greater awareness of the threat of flooding from tropical storms. Yesterday's WaPo had an article, "An 800-Pound Gorilla Waits to Escape in S. Florida", which described the risk of flooding from a breach of the dike around Lake Okeechobee in southern Florida. The dike, originally built in reaction to a devastating flood from a hurricane in 1928 which killed over 2000 people, has deteriorated according to a recent engineering study. The Palm Beach Post published an earlier special report on the subject, including a number of web extras and links to related stories.

Praying for Rain

Tonight and Tomorrow

Lows tonight will be near 70 in the city, down to the low 60s in the cooler 'burbs with moderate humidity. Tomorrow will be sunny with highs near 89 and increasing humidity.

For the extended outlook, see Jason's post yesterday, and check back for Dan's post tomorrow.

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Be careful what you wish for. With the Washington metro area needing rain to help close a widening precipitation deficit this year, some places got more than they wanted yesterday. Matt showed a radar picture of the total precipitation in his earlier post. Here is a wider regional view from the National Weather Service experimental precipitation analysis. The heaviest precipitation of 1.5" and above (yellow and gold areas) extended from just south of the District across central Prince George's County and eastward to the middle of the Chesapeake Bay and the Eastern Shore. Most of Montgomery County outside the Beltway, on the other hand, had 0.10" or less.

New Reign Begins

Here's someone who probably knows how to seriously pray for rain. Before this week, it's not likely you would have seen "Episcopal" and "oceanographer" in the same sentence. Katharine Jefferts Schori is not only the first woman elected to head the Episcopal church in the U.S.; she also has a doctorate in oceanography, making her the first earth scientist to hold that office. Her specialty is critters rather than currents, however. She studied oysters and squids and worked for a while for NOAA's National Marine Fisheries Service.