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Posted at 11:15 AM ET, 08/12/2010

Where did wild morning storms come from?

By Dan Stillman

*** Storm impacts, power outages & more: ***

* Our Full Forecast | Earlier storm updates & tweets | NatCast *

Satellite shot of this morning's thunderstorms. Loop created by CWG's Ian Livingston from NASA images.

Some forecasts issued yesterday, including ours here, did include a chance of showers or showers and storms for the D.C. area early today. But the severity -- the incredible rain output, damaging winds and seemingly non-stop lightning -- was a surprise to pretty much everyone.

So where did the storms come from and why were they so strong?

Keep reading for some answers...

Gridlock on Connecticut Ave., NW, after this morning's storms. By CWG photographer Ian Livingston.

Graphic from early-morning Storm Prediction Center discussion shows temperature/humidity boundary (marked as "Thermal Axis") mentioned in the adjacent text, and area of severe potential.

The cluster of thunderstorms that wreaked havoc this morning was in the Pittsburgh area late yesterday evening and moved southeast toward the D.C. area overnight. As anticipated by the Storm Prediction Center in a weather discussion issued about 7 a.m., the southern extent of these storms intensified as they neared a boundary separating warm-and-humid air to the north from very-warm-and-very-humid air to the south.

Not helping matters was some drier air high up in the atmosphere courtesy of upper-level winds from the northwest. The mixture of drier air aloft with saturated air below can lead to pockets of cooler, more dense air that falls faster toward the ground, creating stronger winds when the air spreads horizontally upon reaching the surface. Here's a more detailed discussion of this phenomenon.

The National Weather Service noted the dry air's potential influence in a forecast discussion yesterday:


Usually the best chance of severe storms is during the afternoon into evening, when the destabilizing effects of daytime heating are at their greatest. But severe storms do occasionally occur during the late-night and early-morning hours, especially when conditions near the ground and high above are as well aligned for severe weather as they were this morning.

By Dan Stillman  | August 12, 2010; 11:15 AM ET
Categories:  Thunderstorms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Morning storms gone; more to come?
Next: Quieter night ahead, still more rain for some



Posted by: trollboy69 | August 12, 2010 11:22 AM | Report abuse

On a more serious note...where can we find a radar loop of the storm that struck our area this morning?

Posted by: trollboy69 | August 12, 2010 11:26 AM | Report abuse

Posted by: Ian-CapitalWeatherGang | August 12, 2010 11:30 AM | Report abuse

CAPE at DCA at 7:00 am was huge at 5291 J/kg.

Posted by: psilosome | August 12, 2010 11:53 AM | Report abuse

aha! so it was all pittsburgh's fault....jerks

Posted by: hockeypunk | August 12, 2010 12:06 PM | Report abuse

My cat and I were asleep here in Annandale, windows open (no A/C) when what sounded like early morning construction work woke us. As it grew more continuous, it occurred to me it could be thunder, but, nah, it was the wrong time of day and besides it didn't sound like ominous like thunder. Suddenly all curtains blew straight into the room held aloft by ahhhh, blessedly coooool gusts, nice, so nice that when the rain began to fall I couldn't bear to close the windows. It was a surprising gentle rain, so gentle my 12 yr old cat relocated to the open window sill to better enjoy the drop in temperatures. More kindly storms, like that, please and fewer cruel downpours.

Posted by: jhbyer | August 12, 2010 12:14 PM | Report abuse

Rapid Climate Change............certainly abnormal, warmest July on record.

Posted by: MajorFacemask | August 12, 2010 12:21 PM | Report abuse

Just curious... is there a site with archived watches/warnings? Listening to wtop this morning, it sounded like they were forecasting the storms for northern suburbs and the warnings for places like Alexandria and Clinton may have come a bit on the late side. I guess the southern intensification wasn't fully expected.

Posted by: spgass1 | August 12, 2010 12:32 PM | Report abuse

Now at work where I have power, I'm starting to realize just how insane that storm was. As a dorky, avid weather fan... I'll admit that I was pretty scared. I thought my windows were going to blow in and I woke up to intense lightning right outside my apartment. According to WTOP, I was in one of the hardest hit areas - 16th St in Silver Spring. Scary!

Posted by: CuseFan07 | August 12, 2010 12:34 PM | Report abuse

@psilosome: where do get a local cape value exceeding 5000? The 8AM Dulles sounding shows 2500. That's impressive enough - 5000 around here is incredibly rare and pretty much impossible in the early morning.

Posted by: foul_throw | August 12, 2010 12:34 PM | Report abuse

It was dark as night at 7am this morning south of Fairfax City on 123, but no rain or thunder. Driving north on the FXCO Parkway it quickly cleared and by Route 50 it was a beautiful day with plenty of sunshine. Hard to believe the reports on the radio that Canal Road was flooded and people were being rescued from their vehicles.

Posted by: TheOneWhoHurtsMost | August 12, 2010 12:42 PM | Report abuse


Doesn't have the graphics showing the exact outlines of the warning areas, but at the very least the archived history of our dcweatheralerts twitter feed -- -- helps show the timeline/location of warnings this morning.

Posted by: CapitalWeatherGang | August 12, 2010 12:47 PM | Report abuse

Wasn't this an MCS, not just an ordinary straight line thunderstorm? At least ours wasn't as bad as this one: Also it seemed to get some energy by riding the front boundary. The steering at 700MB was from the NW this morning, but now I realize that picture was from overnight. The current plot at seems to show the effects from the MCS as it passed through.

Posted by: eric654 | August 12, 2010 1:07 PM | Report abuse

@hockeypunk, Washington will be revenged for the storms Pittsburgh keeps sending our way, when the Caps smite the Penguins during the Winter Classic.

Several of my colleagues had their cars smashed by treees this a.m., including a person driving on Canal Road. Fortunately the big tree rolled off the top of his car and the only damage was a cracked windshield.

Posted by: JerryFloyd1 | August 12, 2010 1:19 PM | Report abuse

what's a cape value? when are storms coming this afternoon to Arlington and DC?

Posted by: saracooper | August 12, 2010 1:25 PM | Report abuse

Once again NW Spotsy left high & dry. 2.8" of rain since June 3. Please send me the rain minus the wind.

Posted by: VaTechBob | August 12, 2010 1:35 PM | Report abuse

CAPE stands for "Convective Available Potential Energy." I had to look it up myself. Here's what I found:

"The convective available potential energy or CAPE, is used by weather officials and storm chasers to understand what the potential might be for severe weather, and how powerful those storms might become if they do materialize. The potential energy available for convection is expressed mathematically using a standard measurement of energy represented as Joules Per Kilogram. A high CAPE value might also be expressed by storm chasers and weather experts by using the term "high instability". When chasers talk about a highly unstable atmosphere, CAPE values are usually in excess of 2500 J/kg's which would supply ample energy for strong updrafts and violent storms should they develop."

Anything above 2500 is "very unstable" and above 3500 is "extremely unstable." If this morning was a 5000, that's "holy crap! unstable" or "Katie, bar the doors."

Posted by: hisownfool1 | August 12, 2010 1:37 PM | Report abuse

thakns, hisownfool1! even i understood that.

Posted by: saracooper | August 12, 2010 1:43 PM | Report abuse

Here's the current CAPE plot which I believe is for 7 or 8 this morning (12Z).

Posted by: eric654 | August 12, 2010 1:46 PM | Report abuse

I see this sounding from 12Z at Dulles says CAPE:2647

Posted by: eric654 | August 12, 2010 1:58 PM | Report abuse

Maps appear to show that the evening commute is not going to be a good look.

Posted by: keithrjackson | August 12, 2010 2:01 PM | Report abuse

I was at work this morning when the storm hit SE DC. One of my co-workers had just come in, and said she just missed the rain. Then someone else said, "wow, it's getting really dark outside!" We watched the brunt of the storm, and dealt with the resulting ceiling leaks - it was a fun morning. Hopefully I have power at home!

Posted by: akchild | August 12, 2010 2:13 PM | Report abuse

I got the CAPE value here
Put in DCA and 11:00 Zulu time (7:00 am local):

Posted by: psilosome | August 12, 2010 2:17 PM | Report abuse

The only time I had seen it that dark before, was under the cover of a super-cell. What was the potential for tornados out of that storm?

Posted by: akchild | August 12, 2010 2:19 PM | Report abuse

CWG - I heard that a lot of global weather issues (hot US east coast, draughts in Russia, floods in Afganistan) might be a result of "frozen" jetstreams. Any thoughts?

Posted by: davidcc | August 12, 2010 2:22 PM | Report abuse

psilosome that is interesting. I wonder if extremely high CAPE is a plausible local effect? Maybe right before the storm? If you take your full URL and change dca to iad there is a CAPE of 1738 at the same time.

Posted by: eric654 | August 12, 2010 2:45 PM | Report abuse

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