961
FOUS30 KWBC 071920
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Mon Apr 7 2025

Day 1
Valid 16Z Mon Apr 07 2025 - 12Z Tue Apr 08 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHEAST AND LOWER MID ATLANTIC REGION...

...1600 UTC Update...
Latest surface analysis indicates the cold front has now moved east
of the western portions of the FL Panhandle, as has the axis of
deeper convection/higher cloud tops per longwave IR associated with
the Warm Conveyor Belt (WCB). With the diminishing deep-layer
instability and negative TPW behind the front, we`ve been able to
remove the Slight Risk across south-central AL and the far western
portions of the FL Panhandle.


Otherwise, the Marginal Risk remains in place across parts of the
eastern Gulf Coast into the Southeast and Lower Mid Atlantic
region. The main hindrance from a flash flood perspective is the
progressive nature of the front as time marches on this afternoon
and evening. Considered a Slight Risk for areas farther east near
the Apalachicola River in FL which could see an outflow boundary
reach the region and stall while convectively active, waiting for
the cold front to catch up before picking up the pace. However, QPF
amongst the global and mesoscale guidance showed low chances for
3"+, so left the risk farther east as Marginal, which was
coordinated with the TAE/Tallahassee FL forecast office.

Hurley/Roth


Day 2
Valid 12Z Tue Apr 08 2025 - 12Z Wed Apr 09 2025

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN SOUTHERN
FLORIDA...


...1930 UTC Update...
Based on the 12Z guidance, particularly the HREF and its suite of
especially the exceedance probabilities, have trimmed the Marginal
Risk a little to include far southeast FL from Homestead to West
Palm Beach. This given the anticipated frontal progression and the
fact that the bulk of the heaviest rainfall after 12Z Tue will be
east towards the northern Bahamas. 12Z model QPFs follow suit,
especially the CAMs, as the 12Z HREF probabilities of QPF greater
than 3" in 24 hours (12Z Tue-12Z Wed) now shows only a sliver of
30-40% right along the coast along and north of Pompano Beach.


...0800 UTC Discussion...
Deep southwest flow is expected ahead of an advancing cold front
across the FL Peninsula. Precipitable water values of 1.75-2" are
forecast to reside here, and ML CAPE should maximize in the
2000-3000 J/kg range per NAM guidance. Low- level inflow and
effective bulk shear are sufficient for convective organization.
Whether or not a flash flood risk fully materializes depends upon
whether or not an East Coast seabreeze front can develop and focus
convection, which could anchor convection in the large urban area
of South FL as it is unlikely to progress inland within this
synoptic pattern. In this sort of atmosphere, hourly rain totals to
2.5" and overall totals to 5" are possible. The 00z HREF does have
some non- zero probabilities of 5"+ totals across sections of the
northern Keys, while the 3"+ probabilities are more prominent along
the Gold Coast. Considering the urban environment, ingredients
available, and mesoscale guidance signal, added a Marginal Risk to
South FL and the Upper Keys as a precaution as isolated to widely
scattered flash flooding appears possible.

Roth


Day 3
Valid 12Z Wed Apr 09 2025 - 12Z Thu Apr 10 2025

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

...Southern Illinois...
Conceptually, there is a non-zero threat of heavy rainfall in and
near southern IL Wednesday evening as precipitable water values
rise near to above 1" within a region with 1000-500 hPa thickness
near 5550 meters, implying a completely saturated troposphere.  MU
CAPE is expected to rise above 250 J/kg per NAM guidance, which
tends to be under-forecast at this time range. The area will still
be attempting to dry out from recent heavy rainfall/ saturated
soils. However, there is only about a six hour window for heavy
rain concerns to materialize near the parent cyclone`s associated
frontal boundary before it moves through. There is the potential
for hourly rain totals to maximize near 1.5". The guidance is this
time range QPF-wise is underwhelming, so no risk area was
depicted. However, this may change as we move closer in time into
the high resolution guidance window.


Roth


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