Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Thu Sep 28 2023

Day 1
Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC, NORTHEAST, FLORIDA AND THE OHIO VALLEY...

...Mid-Atlantic into Northeast...
We added a Marginal risk across portions of eastern PA into NJ and
southeast NY with this update. Lower confidence than normal for a
day 1 forecast as an area of low pressure develops offshore and
attempts to push an inverted trough/coastal front inland. The
extent of the flash flood risk really comes down to the
positioning of this front/convergence axis later tonight into
Friday morning. If this axis stays offshore then any instability
will also remain offshore, resulting in just some stratiform
rainfall moving inland. However if the axis is along the coast or
inland, then some weak instability should get ashore as well, and
set the stage for the potential of shallow efficient convection.
Most of the HREF members do bring some heavier rain onshore, with
the highest probabilities focused over NJ. However each HREF
member has a slightly different location, and most of the 00z
global models keep the bulk of the heaviest rain offshore through
12z Friday. Given this spread and uncertainty opted to stick with
a Marginal risk for now. The main threat for day 1 is the last 6
hours (06z-12z Fri), so will let the day shift take a look at the
12z HREF and make a call on whether to upgrade to a Slight or not
at that time.

...Ohio Valley...
A Marginal risk was maintained across portions of TN/KY into
southern OH. Convection will be ongoing at 12z this morning, with
some brief training/backbuilding remaining possible. We might end
up with a downward trend in convective activity by mid to late
morning...but with an elongated area of vorticity continuing to
traverse the area through the day good synoptic ascent will remain
in place. Thus once instability recovers from the morning activity
we could see some additional convective development across the
region this afternoon/evening. The magnitude/organization of this
additional activity is conditional on the amount of recovery we
get....but if we area able to ramp instability back up then some
additional brief training could occur given the persistent low
level inflow and convergence in place. Isolated flash flooding
will be possible, both this morning and potentially again this
afternoon/evening.

...Florida...
Only minimal changes to the inherited Marginal risk area over FL.
Anomalous moisture will remain in place, with a stationary front
and stronger upper level flow helping trigger above average
convective coverage again today. High rainfall rates will drive a
localized flood risk over any more susceptible urban locations.

Chenard


Day 2

The Day 2 outlook will be updated by 0830Z.


Day 3

The Day 3 outlook will be updated by 0830Z.


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