Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Tue Sep 19 2023

Day 1
Valid 12Z Tue Sep 19 2023 - 12Z Wed Sep 20 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS
PORTIONS OF THE SOUTHERN PLAINS AND MISSOURI VALLEY...

...Southern Plains/Missouri Valley...

WV satellite imagery this evening shows our next disturbance
traversing the southern Rockies with convection already in-of the
TX Panhandle. This shortwave will become the primary focus for
later this afternoon and evening as large scale ascent within a
favorable thermodynamic environment will trigger a round of strong
to severe thunderstorms across the Southern Plains, migrating
eastward during an expected upscale growth convective mode for the
back end of the D1 time frame. A sharp increase in regional
moisture is already occurring thanks to a prominent 35-40 kt LLJ
bisecting the plains as the area lies on the western flank of a
sprawling surface ridge analyzed over the Tennessee Valley, and
the eastern side of developing low pressure along the lee of the
Rockies. PWATs running between 0.75-1" over the plains east of a
line from SJT to DDC will see a rapid increase towards 1 standard
deviation above climatological norm by the time we reach this
afternoon. This will greatly increase regional instability over
much of east TX up through OK and KS which will play a role in the
convective pattern later today. 2000-2500 J/kg of MLCAPE is
forecast to the east of the dryline centered over the western
rolling plains up through southwest KS. As the mid-level
perturbation exits the Rockies to the plains, this will garner the
triggering mechanism to initiate the first round of thunderstorms
across the southern plains along and to the east of the dryline
(western edge of the theta-E gradient). Multi-cell and discrete
supercells will be the initial modes of convection when the storms
begin, but increasing LLJ pattern over central and eastern OK up
into eastern KS will promote a shift to more organized cell
clusters promoting heavy rainfall coverage over a larger area.
This evening and overnight into Wednesday AM will be the primary
time frame of interest with regards to flash flooding potential as
the expected complex will provide the coverage and magnitude
necessary to bring more widespread impacts to the areas above.

Hi-res ensemble (HREF) and associated deterministic are all fairly
aggressive with the totals across the aforementioned area of
interest, especially over east OK up through southeast KS where
HREF neighborhood probabilities of >2"/24 hrs have shot up near
40% with a signal for 1-2"/3 hrs between 00-06z Wednesday up to a
large area of 20-40%. Deterministic totals are obviously very high
with a multi-model consensus of a 4-6" maximum somewhere in the
vicinity of the risk area. Bias-corrected ensemble means are also
fairly robust with over 1.75" bulls-eyed over NE OK up to the KS
line. This actually is in agreement with CIPS analogs placement of
max QPF based on the top 5 analogs within the 00z run, which would
place the QPF centroid right in the middle of the forecast Slight
Risk. The northern and southern expansions are for multi-model
consensus on secondary peaks within the QPF field based on
convective development and propagation within the spread. There is
some deviation on where exactly the complex, or remnant MCV goes
by the end of the forecast period, so there could be some shifts
in the overall SLGT and adjacent MRGL in subsequent updates.
However, will maintain the current positioning based on HREF
blended mean QPF and outline of the 20% probability of 2" or
greater within the ensemble forecast.  

...South Florida...

Another day with a stalled front nearby, another day for
thunderstorms capable of producing excessive rainfall rates of
2-3"/hr. Area averaged guidance soundings over South Florida
showed MLCAPE of 2,000 J/kg available, as well as PWs near 2" and
averaged low-mid level RH values near 80%. 00z HREF probabilities
are once again fairly robust, even for FL standards with signals
for 3+" and 5+" totals within the D1 time frame (50-60% and 20-30%
respectively). The enhancement of the front and sea breeze
convergence creates a favorable scenario for localized flooding,
especially in urbanized corridors along the coast(s). There is
more of a signal today for the southwest coast being impacted by
heavy rain today in due part to a progressive sea breeze
propagation from the east and heavy storms forming in-of the
Everglades creating subsequent outflows. This is the biggest
difference from the last few days, but the impact potential still
stands leading to a maintenance of the MRGL risk from previous
forecast.  

Kleebauer


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