Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Sep 27 2023
Valid 12Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AND OHIO VALLEY...
A slow moving mid- and upper-low will continue to make its way
eastward from the Mid-Mississippi Valley early this morning into
Indiana or far southern Michigan by late this coming night. Weak
and difficult to time shortwave energy...but significant in
helping trigger and focus convection...will be rotating around the
low as it tracks eastward during the period. South to southwest
low level winds will help draw deeper moisture back into the area
with precipitable water values increasing to around 1.25 inches
within the outlook area ahead of the approaching dynamics.
Deterministic guidance still shows a ribbon of elevated MLCAPE
between 1000-1500 J/kg. The 27/00Z HREF mean showed another
increase in QPF along an east to west axis across parts of
Kentucky on Wednesday evening...which looks plausible for a region
of training on the south side of the low. As a result...expanded
the Marginal risk a bit south and westward here.
Overall...though...the not many changes were needed to the
previous Day 2 ERO as it becomes the current Day 1 ERO.
A near carbon-copy outlook of recent days given a pool of 2-2.5"
PWs to go along with a quasi-stationary front draped over Florida
and an upper trough over the Gulf of Mexico. Wednesday features a
little more instability across the Gulf side of the Florida rather
than the Atlantic for now, maintained the focus for Excessive
Rainfall from southwest Florida on north to southern Georgia and
southern Alabama. As much as 500-1,000 J/kg of MLCAPE will be
present across central and northern Florida, and with such a
tropical air-mass in place, rainfall rates could approach 3"/hr in
the strongest storms. A Marginal Risk for Excessive Rainfall
remains in place as urbanized communities and poor drainage areas
are most vulnerable to possible flash flooding.
Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
FLORIDA AND OHIO VALLEY...
Maintained the Marginal Risk area that was introduced on Tuesday
afternoon with only a few adjustments. The latest runs of
numerical guidance continue to show increasing ascent ahead of an
approaching longwave trough over the northern Midwest/Ohio Valley
with some overlap with areas expected to get locally heavy
rainfall on Day 1. Global deterministic guidance became much more
aggressive compared to previous runs in terms of QPF...with local
1-2+" totals focused within the region...and that largely
persisted into the most recent model runs. The HREF probabilities
that are available...through the first 12-hours of the Day 2
period during this outlook cycle...tends to focus higher amounts
in the southern Ohio Valley closer to the better instability and
better precipitable water values. Made a southward
nudge/expansion as a result.
Locally heavy rainfall across the coastal areas of central and
south FL will pose a non-zero risk for flash flooding within a
tropical environment entrenched over the Sunshine state. Best
convergence is still being depicted over the adjacent waters, but
elevated instability across both coasts will pose a threat for
enhanced rainfall rates where thunderstorm activity is prevalent.
The model consensus is that most activity will be off-shore...but
the model consensus was for more inland convection than shown by
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