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Posted at 11:45 AM ET, 11/17/2010

Fierce midnight storms of Nov. 17: what happened?

By Jason Samenow
wind-damage-11710.jpg
Reports of wind damage from NOAA's Storm Prediction Center.

During the summer months, middle of the night severe thunderstorms are not uncommon even if startling. But we seldom witness a post-midnight squall line tearing through the heart of the metro region in mid-November. So how did this occur last night?

The intensity of the thunderstorm outbreak and the damage its 60 mph winds inflicted can be traced to the convergence of a number of atmospheric ingredients:

1) The approach of a fast moving, strong cold front
2) The presence of a powerful low level jet stream (or river of air) ahead of the cold front that pushed warm, unstable air into the region
3) A screaming upper level jet stream perfectly positioned to support very strong vertical motions in the atmosphere for thunderstorm growth.
4) Turning of the wind with height which generated spin further enhancing the storms.

Let's take a closer look...

cold-front-111710.jpg
Surface frontal map from late last night. Source: National Weather Service.

The cold front, shown to the right (just after it had passed the D.C. metro area), served as the trigger for the storms. This particular front wasn't unusually strong for this time of year but strong enough to fire off storms when it interacted with warm, humid stream of air racing out ahead of it. Just before midnight last night, the temperature shot up to 64 degrees which turned out to be yesterday's (Tuesday's) high. So where did the warm air come from?

Hours before the cold front approached, a warm front (shown near New York City) in the above map lifted through the region around 4 p.m. when winds shifted from the east to from the south. It was around this time that the steady rain started to taper off and the precipitation became more showery.

llj-11710.jpg
Low level jet stream at low altitudes (about 5,000 feet or 850 mb). Dark red shades indicate winds of at least 50 knots. Source: College of Dupage.

Once the warm front passed, an intense low level jet stream funneled through the region from the south carrying warm, moist air. Coinciding with the arrival of this jet, you may have noticed the character of the air change from cool and clammy (during the day) to more tropical (in the evening). The low level jet is apparent in the image to the right which shows winds at around 5,000 feet (or 850 mb) last night. The red shaded areas indicate winds of 50 knots or around 60 mph blowing from the south to north at that level. When the cold front interacted with this raging river of warm air, it created an unstable situation.

ulj-111710.jpg
High altitude jet stream (about 35,000 feet or 250 mb) winds. Dark red shades indicate winds of at least 120 knots. Source: College of Dupage

Another primary factor supporting last night's storms was a screaming upper level jet stream at about 35,000 feet (or 250 mb). A very fast moving portion of this jet stream (often referred to as a jet streak), racing from southwest to northeast at 140 mph (shown in the image to the right), was moving over the region at exactly the time the storms fired up. Importantly, it was situated in such a way to promote strong vertical motions (weather wonks: we were in the left exit region of the jet streak). Effectively, these upper winds acted like a vacuum sucking the air up, feeding the fierce thunderstorms forming along the cold front.

vort111710.jpg
Vorticity (or spin) at about 18,000 feet last night. Yellows, oranges reds, and purples indicate high levels of spin. Source: College of Dupage

Finally, the changing of the wind with height, from the south at low altitudes to more from the west at high altitudes, generated some spin in the atmosphere - which further enhanced rising air motions. The spin in the atmosphere present at an altitude of about 18,000 feet (5000 mb) is depicted in the image to the right using a metric known as vorticity. In this particular image the vorticity is highest to the west and southwest (as illustrated by the yellow, orange, red and purple shades), but this region of spin was carried by the winds right over the D.C. area (weather wonks: this is known as positive vorticity advection).

nov17-radar.jpg
Radar image from 1 a.m. last night. Source: Weather Underground

So, with all of these ingredients in place, why didn't forecasters see it coming? Good question as the forecast models simulated all of them. While we and the National Weather Service did mention the possibility of thunder in our forecasts, I think we all were surprised (and caught off guard) by how strong the storms became given both the season and time of day.

Usually, to forecast severe thunderstorms, we like to see sunshine to destabilize the atmosphere and warmer temperatures and higher moisture levels. But the lesson we should learn from this event is that atmospheric dynamics - if the right ingredients are in place - can overcome this...

By Jason Samenow  | November 17, 2010; 11:45 AM ET
Categories:  Latest, Thunderstorms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   Del.icio.us   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Cool & calm after Wednesday winds
Next: A look ahead toward Thanksgiving and beyond

Comments

Jason:

Great write-up! And done so quickly!

Thank you for the clear explanation for last night's squall line.

Best,

Posted by: ubimea | November 17, 2010 10:52 AM | Report abuse

I might have missed it, but did you post rain totals anywhere? It rained all day in Alexandria yesterday and then we got buckets and buckets from that insane storm. I would think we got at least an inch from Monday through this morning.

Posted by: egengle | November 17, 2010 11:02 AM | Report abuse

@egengle

We had just 0.31" of rain at National but 1.02" at Dulles yesterday - before the big storms after midnight. After midnight, looks like (unofficial) National picked up .12" and Dulles 0.04" - the storms were intense -- but very fast moving -- around 50 mph.

Posted by: Jason-CapitalWeatherGang | November 17, 2010 11:17 AM | Report abuse

Great breakdown of the storms. Thanks, Jason.

Posted by: Ann-CapitalWeatherGang | November 17, 2010 11:24 AM | Report abuse

Phrase of the Day - "positive vorticity advection".
Thanks for the explanation & graphics.
Another freak weather event in 2010: six more weeks left for even more forecasting fun.

Posted by: FIREDRAGON47 | November 17, 2010 11:35 AM | Report abuse

The squall line, itself, was only about 10 miles wide or so (that clearly showed on radar). Radar data indicated tops to only 14,000 feet...but I question that....I suspect they were substantially higher. The data also indicated hail to 1 inch, but I didn't see any hail at all(in Vienna/Oakton). Nor did I see any strong winds.....where I was, the turbulence was (apparantly) mostly aloft.

You guys at CWG...and NWS....don't have to apologize for anything. The forecast was actually pretty accurate. We knew, in the morning, that we would probably get into the warm sector, and, with the rapidly deepening low over Ohio, the strong cold front, tropical air at the surface, and jet stream, that there was a significant chance of thunderstorms after dark. In fact, Bombo747, early in the day, noted the possible high CAPE value and the thunderstorm concern.

Posted by: MMCarhelp | November 17, 2010 11:39 AM | Report abuse

My north facing front door was a decoupage of wet leaves and pine needles this morning.

Same for the car.

The leaf removal company left a 4 foot pile on the common grounds Monday, presumably to vacuum up on the next dry day. Bummer for them!

Posted by: ASColletti | November 17, 2010 12:22 PM | Report abuse

I agree there were some indicators for weather watchers, even rank amateurs like me who rely chiefly on commonsense. I saw a big mess on radar earlier in the day heading our way, wind advisories and tornado watches popping up well south & west but in the path, and noticed it was awfully warm here.

So I went into Severe Storm Preparedness mode. Expected to get zero sleep, but actually managed 2-3 hours. It was scary when winds were sustained at 43 gusting to 51--it's quite a roar at that speed.

Posted by: tinkerbelle | November 17, 2010 12:30 PM | Report abuse

I think I saw a possible warm-sector setup a couple of days ago/wasn't too surprised.

Actually intense overnight thunderstorms are quite rare around here even in summer, but are very common during the warm season in my native west-central Wisconsin. The trigger mechanism is mighty similar to what we saw last night...usually a rather strong low moving eastward or northeastward towards Lake Superior and southern Ontario...we get caught right under the jet stream in the warm sector with a low-level jet moving up the Mississippi Valley underneath the upper jet, sometimes all the way up from the Gulf of Mexico. In extreme situations tornado outbreaks have occurred.

Posted by: Bombo47jea | November 17, 2010 1:33 PM | Report abuse

Tom Skilling appears to be signing on.

Right now it's -26 in the central Yukon...Dawson or Whitehorse. The sun also is setting at Barrow, AK, not to rise again till late January [SAD, anyone???]. Ought to check on Verkhoyansk and Oymyakon.

Thanks for noting my earlier post, MMCarhelp.

Posted by: Bombo47jea | November 17, 2010 1:47 PM | Report abuse

I'm so sorry I slept through these! I bet they were amazing!

Posted by: sigmagrrl | November 17, 2010 2:32 PM | Report abuse

sigmagrrl, sadly, I slept through them too. Bummer!

Posted by: Camden-CapitalWeatherGang | November 17, 2010 2:47 PM | Report abuse

Even as an amateur, I knew it was possible to see storms like that. There's no need to think you dropped the ball on the prediction. I was surprised it came in the middle of the night and was so intense, but most fall storms seem to have a touch of the instability as the front passes. Perhaps this one was more intense, but it's not like we don't see a few of these setups every year. These storms are one of the parts of fall that I love. It seems to me to be part of the transition to winter and it reminds me just how powerful and beautiful nature is.

In Greenbelt last night, we certainly didn't get the worst of the storm, but the most interesting characteristic was the sustained high winds for about a minute. It was howling so steadily with some gusts that both my bf and I considered whether it was a tornado. The rock solid, 60 year old oak tree out my window was swaying, which I almost never see it do. It was truly remarkable and I was so very sad I didn't have a weather station set up to record it. It only lasted a minute or so (though it seemed longer), and that was about it for the storm. It was certainly flying by.

Posted by: r3hsad | November 17, 2010 3:20 PM | Report abuse

Up in Columbia it was the rain that woke me up. .22" in 5 minutes. Usually when rain "tapers off", it doesn't end with a monsoon. When I saw a flash of lightning I checked the radar to see what the heck was going on. Quite impressive! Now can we get this in a thundersnow in a month or so?

Posted by: MaltyCharacter | November 17, 2010 3:32 PM | Report abuse

Posted by: Jason-CapitalWeatherGang | November 17, 2010 3:35 PM | Report abuse

I echo what r3hsad said: the winds came through Greenbelt like a freight train - the roar made me wonder "Is that what a tornado sounds like?" I was surprised not to see much in the way of tree damage, but given that we were hit hard by both the snow and the spring thunderstorm, whatever's left much be pretty sturdy.

Posted by: kevinwparker | November 17, 2010 5:07 PM | Report abuse

Jason, just had a chance to read this now. Thanks for another great post! You guys always do an awesome job providing comprehensive updates on DC's mercurial weather.

Around 12:30 last night I heard violent thunder just south of Alexandria and within minutes there was torrential rain and wind. In retrospect, it made sense that something "was in the air," given that it felt unusually balmy out when I got home at midnight.

Posted by: meteorolinguist | November 17, 2010 9:51 PM | Report abuse

@meteorolinguist

Thanks for the nice feedback.

Posted by: Jason-CapitalWeatherGang | November 18, 2010 3:04 PM | Report abuse

The comments to this entry are closed.

 
 
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