Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
133 PM EST Wed Dec 18 2024

Day 1
Valid 16Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF
THE SOUTHEAST FLORIDA COAST...


...16Z update...

...Southeast FL...
Weak 0-6 km AGL and LFC-EL mean layer winds (both ~5 kt on the 12Z
MFL sounding, but slightly stronger flow to north) will continue to
support the potential of slow moving showers/thunderstorms during
the day today. While the earlier cell near FLL has weakened since
this morning, a low-end threat for mainly urban flooding will
linger into peak heating. A convectively enhanced mid-level trough
just east of the FL Peninsula is expected to continue weakening
with an eastward translation today. Low level flow has become more
southerly and while the bulk of convection today is expected to
remain offshore, an isolated chance for a slow moving cell later
this afternoon will be possible with daytime heating within the
anomalous moisture over southeastern FL. Will keep the Marginal
Risk, though the threat appears to be diminished compared to
earlier today.

...Parts of the Mid-South to Lower Ohio Valley...
Opted to remove the Marginal Risk for this portion of the country
with this update. A SW to NE axis of locally heavy rain extended
from southeastern KY through middle TN and northwestern AL at
1545Z, moving off toward the east. There has been brief training
within this line with MRMS- estimated rainfall rates near 1 in/hr,
but overall, the line has been progressive with some possible
minor flooding earlier this morning along the western KY/TN border.
As a surface low over KY tracks northeastward today, the front and
associated precipitation axis will advance steadily off toward the
east with some weakening due in part to a lack of
moisture/instability downstream. Additional spotty rainfall of 1
to 1.5 inches will be possible on a localized basis.

Otto

...previous discussion follows...


...Parts of the Mid-South to Lower Ohio Valley...
Maintained the Marginal Risk area across parts of western and central
Tennessee into southern Kentucky as models still advertise an
upper-level trough amplifying to the west which allows for a more
curved upper level jet streak on the lee side of the trough. That
configuration may result in some enhanced rainfall rates...an idea
supported by the 18/00Z HREF neighborhood probabilities showing
potential for rainfall rates exceeding an inch per hour between
12-18Z today (mainly in Tennessee). With rainfall blossoming to
the west of the outlook area in the pre-dawn hours this morning
helping to prime the soils a bit more and a general slowing of the
eastward progression across the Tennessee Valley compared with
earlier forecasts...opted to extend the Marginal risk area a bit
farther westward than previously indicated. The HREF probability of
exceeding flash flood guidance remained low but did reflect
somewhat higher probabilities west of the same guidance from the
17/12Z and 17/18Z runs. The decision to expand westward slightly
was also driven by the observed rainfall during the past day or two
over parts of northwest Tennessee and southwest Kentucky which has
resulted in the latest 1-hour and 3- hour flash flood guidance
coming down and the 0-40km soil moisture percentiles per NASA SPoRT
have climbed to 60-80% for most areas.

...Southeast FL...
Post frontal easterly low level flow off the Atlantic will linger
allowing for a possible repeat of convective clusters over and
near the southeast Florida coastline today. The 00Z runs of the
high-res CAMs continued to show onshore low-level moisture
flux/convergence and sufficient deep-layer instability to support
localized downpours. The 18/00Z HREF supports rates of 2+ inch per
hour peaking around 10 percent during the afternoon with and a
50-70 percent 40km neighborhood probabilities of at least 5 inches
of additional rain and 30-50 percent probabilities of 8+ inches.
Given that these numbers are comparable with the 17/12Z and 17/18Z
runs of the HREF...saw little reason to make too many changes other
than nudges to the boundary of the previously issued Marginal risk
area.


Bann

Day 2
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

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

Pereira

Day 3
Valid 12Z Fri Dec 20 2024 - 12Z Sat Dec 21 2024

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

Pereira


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