Friday, April 19, 2013

April 19, 2013: Severe Tstorms Possible This Evening

The NWS has issued a Tornado Watch - which also includes the probability of severe tstorms (i.e. damaging wind gusts, heavy rain, small hail) - for the entire D.C.-Baltimore region until 10 pm tonight.

What we are tracking:  A broad band of showers and thunderstorms, forming ahead of a cold front approaching from the west.  The front is presently crossing the Appalachians.   Individual storm cells will move in from the southwest at about 40 mph.  The timing is most likely between 6-10 pm.

What are the odds?   Severe tstorms require strong wind shear and an unstable atmosphere.   This afternoon, because of the overcast, the sun has not been able to heat the surface, and this has diminished the instability.   However, uplift along the front is quite vigorous, and the increase in wind speed and wind direction changes with altitude (wind shear) are very strong.   When severe thunderstorms form in this type of setting (weak instability and strong shear) they tend to be low-topped supercells or short, bowing line segments.   The "mini-supercells" can produce a weak or moderate intensity tornado.  Bowing lines (called bow echos) can create strong straight-line winds called downbursts, and isolated small tornadoes.

Heavy rain is likely because the moisture content of this air mass is very high, and storms have the potential to "train" or repeatedly hit the same location over and over again.   One thing that may catch folks by surprise - there may be little or no lightning associated with these "thunderstorms" because of the limited instability.

Here are the 4 pm radar and lightning maps, showing intense storm cells crossing the central Appalachians (top panel).   At the same time, the nearest cluster of lightning is along the NC-VA border (bottom panel).  There are active severe thunderstorm and tornado warnings several hundred miles to our southwest.

The NWS Storm Prediction Center feels that the greatest likelihood of severe weather is over central NC and southern VA.  This is where the air mass is most unstable, and overlaps with the region of strong wind shear.  They state the probability of a tornado for any one location in our region is 5% and damaging winds is 15%.

The most likely weather impact at any one location:  2-3 hours of heavy showers and strong wind gusts, with some limbs down, and a few rumbles of thunder.  Isolated pockets of power outages.  I would not be surprised to see the Tornado Watch canceled earlier than 10 pm.

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