Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Sat Aug 01 2020
Day 1
Valid 2016Z Sat Aug 01 2020 - 12Z Sun Aug 02 2020
..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE OHIO
VALLEY AND ADJACENT PARTS OF THE EASTERN U.S...
...Ohio Valley into the Mid Atlantic...
We continue with a broad Slight Risk area that encompasses various
threat areas associated with an anomalously deep upper trough. A
well defined surface wave will lift through the Ohio Valley this
evening. Cells in the warm sector will be moving fairly quickly,
but the combination of moderate instability and precipitable water
values well over 1.50 inches will yield heavy short term rain
rates capable of producing flash flooding in sensitive basins. Out
along the warm front, storms initiating off the Blue Ridge or more
generally in the very moist and unstable air mass pooled along the
Virginia portion of the warm front will have potential to produce
very heavy downpours. Increasing inflow speeds could yield pockets
of training or propagation back into the inflow if organized
clusters can persist into the evening hours. Overnight the
emphasis will shift to the deformation zone of the deep layer
cyclone approaching northwest Ohio and southeast Michigan. Strong
lift will work with instability rooted at mid levels to produce
some slow moving convection, with local inch per hour rates likely.
Finally, a threat exists over eastern North Carolina and southeast
Virginia for the next few hours, as the sea breeze and presence of
a lingering frontal zone will yield a local maximum of storm
initiation, allowing for cell mergers within a very moist and
unstable environment. The latest 12Z HREF exceedance probabilities
are quite impressive -- with probs of >1"/hr rates peaking over
80% across this area between 20-00Z, with probs of >2"/hr rates
equally as impressive during the same time (40-60%).