Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Wed Apr 9 2025

Day 1
Valid 16Z Wed Apr 09 2025 - 12Z Thu Apr 10 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Thu Apr 10 2025 - 12Z Fri Apr 11 2025

A broad area of instability is forecast to develop across the
Lower Ohio Valley and portions of the Mid-South which shifts into
the Southern Appalachians with time. Moisture availability appears
sufficient for atmospheric saturation within a cool atmosphere
under a digging upper level trough. However, the area of sufficient
moisture/moisture plume appears to be on the move. This should
limit the overall excessive rainfall/flash flood potential. The
probability of rainfall exceeding flash flood guidance may be non-
zero, but it is considered to be less than 5 percent.

Roth


Day 3
Valid 12Z Fri Apr 11 2025 - 12Z Sat Apr 12 2025

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
MID-ATLANTIC STATES AND SOUTHERN NEW ENGLAND...

A deep layer cyclone slows down as it moves across the Mid-Atlantic
States on Friday. The 1000-500 hPa thickness values are quite low
across the region, so one would only need 0.75-1" precipitable
water values to saturate the atmosphere. When combined with slow-
moving low- to mid-level frontogenesis across PA in particular,
heavy rainfall with possible areas of snow through dynamic cooling could
ensue. MU CAPE is only sufficient for issues from a vertically-
derived instability standpoint down near the VA/NC border, but
slantwise convection could be a problem for areas farther north. In
theory, hourly rain totals up to 1" with local amounts to 3" are
possible. In such situations in the past, the convective band that
forms could be quite narrow which makes placement especially
difficult to discern at this time horizon. Not helping matters are
mass field differences between the guidance as the mid- to upper-
level low is attempting to close off as the upstream ridge is
beginning to falter in strength, which would normally side against
a more southerly solution like the 00z NAM.  Even so, there is
enough agreement on the heavy rain potential through the various
pieces of guidance, when combined with the ingredients above, to
maintain the Marginal Risk from continuity. Should model mass
fields converge and snow becomes less of a factor, an upgrade to a
Slight Risk may be worth considering as some areas of the northern
Mid- Atlantic States have quite low flash flood guidance (northern
NJ, west- central PA, and the Southern Tier of NY).

Roth


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt