As noted in the SPC discussion, destabilization ahead of a shortwave in the Ohio Valley spawned prolific downburst-producing storms, in an environment characterized by 20-30 kt of shear and strong low-level lapse rates.<\/p>\n\n\n\n
...Ohio Valley into the Mid-Atlantic and Southern New England...
On the southern fringe of upper troughing over eastern Canada and
the Great Lakes, a weak\/convectively augmented mid-level shortwave
trough will progress northeastward across the OH Valley and central
Appalachians through the day, eventually reaching the Mid-Atlantic
tonight. Robust daytime heating of a modestly moist low-level
airmass (surface dewpoints generally in the low to mid 60s) ahead of
a surface cold front will aid in the development of moderate
instability by early afternoon. Ongoing convection across OH\/KY this
morning may eventually strengthen as it encounters this
destabilizing airmass, and additional thunderstorms are expected to
develop along\/near the cold front by mid afternoon from the central
Appalachians into portions of the Mid-Atlantic and southern New
England.
Stronger flow aloft will tend to lag\/remain north of the cold front,
but modest enhancement to the west-southwesterly mid-level flow
along\/near the front should support around 20-30 kt of deep-layer
shear. This will be sufficient for some updraft organization with
mainly multicells expected, although marginal supercell structures
may occur. Low-level lapse rates are expected to become quite steep
through the day, evidenced by a general lack of clouds on recent
visible satellite imagery from central\/eastern WV into the
Mid-Atlantic. This will likely aid efficient downward momentum
transfer in convective downdrafts, with scattered severe\/damaging
winds possible as multiple clusters spread east-northeastward
through the afternoon\/evening. Isolated hail may also occur with the
stronger cores. A Slight Risk for severe\/damaging winds has been
introduced from parts of WV to southern New England given increased
confidence in multiple clusters traversing a narrow zone along\/ahead
of the front.<\/pre>\n\n\n\nIn the animation below, reports are valid for the next 45 minutes<\/em>. Thus, the ProbSevere v4 predictions are matched up with what the model is trying to predict (any severe weather in the next 45 minutes and within 15 km). After playing the animation, you can see that PSv4 does an excellent job predicting the severe downbursts in northern Virginia, West Virginia, Maryland, Pennsylvania, and New Jersey. A few reports are missed (at the 25% threshold) in Virginia and New York.<\/p>\n\n\n\n