Fierce midnight storms of Nov. 17: what happened?
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...
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.
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.
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.
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).
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...
| November 17, 2010; 11:45 AM ET
Categories: Latest, Thunderstorms
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