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Posted at 10:15 AM ET, 08/08/2012

Late-week severe weather? Thursday and Friday offer increasing odds for storminess in D.C.


Outlooks for severe storm chances from the Storm Prediction Center as of early August 8 for day 2 and day 3. The D.C. area is on the fringe of a slight risk on Thursday and well within it in Friday.
The Storm Prediction Center began highlighting the region for late-week severe storm potential quiet early (starting Monday for Friday) compared to normal, and seemingly for good reason. While both Thursday and Friday may present storm possibilities, Friday is arguably the main day to watch as an unusually strong upper-level low pressure for August moves by to our north.

This plunge in the jet stream should help create a clash between the hot and humid air mass we’ve been sitting in for what seems like eternity, and a cooler one to the north.

All severe weather modes—wind, hail, and even isolated tornadoes— appear possible across the broader mid-Atlantic to close the week, particularly on Friday. Additionally, once the system begins interacting with the area as soon as late Thursday, it’ll be in no major hurry to leave. That means rain, while uncertain in coverage and timing, may come in large quantities.

Although it’s been a dry year, we’ve still had our fair share of storminess during the summer—starting with the June 1 tornado outbreak, and of course including the June 29 derecho, as well as a number of other smaller-scale events. Part of this frequent storm return-rate has been thanks to our location primarily near the edge of the main central U.S. heat ridge.

With the ridge even further west for now, there is room for a stronger surge of energy into the mid-Atlantic and Northeast.


A radar simulation from the 0z 8/8 run of the North American Model (NAM) for Thursday evening shows a line of scattered showers and storms passing through the area. (instantweathermaps.com)
By tomorrow (Thursday), we’ll have a frontal zone pushing toward us from the north through Pennsylvania. How far it gets is in question, and for that reason it appears the greatest storm threat should try to stay north.

However, storms may fire up on a trough (zone of wind shift) near the mountains and cause a line of storms capable of isolated severe weather, including wind and hail, to pass across at least part of the area.

Friday is when things might get more interesting. I say might, because while some ingredients for a more significant or widespread severe weather episode appear to be coming together, others are not quite there. Also, at this range, there is question as to where the worst sets up (here, or to the north in Pa. seem the main targets for now).

Strong upper-level systems passing where this one is forecast to pass have a history of producing severe weather, including tornadoes. There are even a few similarities (primarily the trough location) to the June 1, 2012 event.


An upper-level low is seen over the Great Lakes late Friday to the northwest of our area. (NCEP)

There is little reason to believe this event could produce the same results as June 1, given weaker surface features and a less progressive pattern. The point here is that turning and increasing winds with height, from south or southeast at the surface to west well above the surface, are a reasonable possibility.

By Friday afternoon, storms should begin developing locally while others approach and potentially spill over the Appalachians. How much sunshine we get is in question due to a tropical-like air mass, as well as possible debris clouds from earlier activity. In short, more sun equals more potential, but either way it appears much of the area should see sufficient convective available potential energy or CAPE (~1,000+) for storminess.

Another potentially lacking ingredient to give storms an extra kick is mid-level dry air which helps in rapid storm formation. Some models have “enough,” others don’t. Also, the speed of the larger system may hinder development during prime heating in the afternoon into evening. The upper low wants to slow down considerably as it nears and passes. For our area to reach true potential, we can’t have it slow down too much.


Prior outlooks for severe weather on Friday from the Storm Prediction Center. Yellow dashed line was Day5 (from Monday), while light blue was issued Day4 (Tuesday). Outlooks this far in advance are somewhat unusual.
Regardless of qualifiers, anything from line segments with damaging winds and potential spinups, to periodic batches of storms, to isolated low-topped supercells capable of producing tornadoes appears possible on Friday.

The predominant risks might end up as scattered wind damage, hail, and/or rain, but we’ll need to carefully watch smaller-scale features like the surface low pressure track and any boundaries which might be conduits for what would likely be weak tornado formation.

Where these features set up should become clearer over the next day or two. It is indeed possible that the worst of storminess could miss or skirt the area.

Because the upper-level system is expected to be moving rather slowly, a ~24-48 hour window where heavy rain threatens periodically is possible across the mid-Atlantic. Overall coverage of this type of rain is highly uncertain this far out, but the signal is there for a dousing—and even potential isolated flooding—in any particularly hard-hit spots.

By  |  10:15 AM ET, 08/08/2012

Categories:  Latest, Thunderstorms

 
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