Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 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
Valid 12Z Wed Sep 20 2023 - 12Z Thu Sep 21 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN PLAINS, MISSISSIPPI VALLEY, NORTHERN MIDWEST, AND
INTER-MOUNTAIN WEST...


...Eastern ID, Western WY, northern UT/NV...

A deepening upper trough over the western CONUS will create an
enhanced area of difluence and attendant vorticity maxima rotating
around the general upper circulation positioned over the PAC
Northwest. Modest low to mid-level moisture advection within the
confines of the Snake River and adjacent terrain will create a
pocket of better convective potential Wednesday afternoon and
evening as a stronger vorticity maximum pivots through the region
providing deep layer ascent during peak diurnal destabilization.
Despite the limit on potential due to only a modest moisture
profile, enough instability (~500-1000 J/kg SBCAPE) and focused
ascent over the topography should provide some heavy rain
potential within a zone that is highly prone to flooding due to
steep terrain and already elevated soil moisture content as
pinpointed by the NASA SPoRT data set indicating areal coverage of
70-90th percentile of climatological norms within the confines of
the outlined area. Only minor adjustments were made to the MRGL
risk, mainly attributed to the QPF reflection within the NBM and
bias-corrected ensemble forecast over the area of best positive
vorticity advection (PVA).

...Upper Midwest...

MRGL risk from previous forecast was maintained with a slight
expansion over the Dakotas and northwest MN to reflect the
uncertainty of model guidance where the best convection will
develop Wednesday evening. The ingredients for locally heavy
rainfall with a zone of lower FFG's is still very much probable as
modest theta-E advection into the northern plains will lay the
ground work for a zone of potential, mainly along and south of a
stalled frontal boundary positioned over the northern plains.
Increasing difluence aloft with small vorticity perturbations will
provide enough large scale ascent for a round of thunderstorms to
develop across central ND over to the MN border. Ensemble means
are fairly low with regards to top-end potential for the setup,
but some deterministic are fairly robust for localized heavy
rainfall within a corridor of low FFG's thanks to a prolonged
drought in place. PWATS will also be steadily increasing with GEFS
standardized anomalies approaching 1.5-2 standard deviations above
normal by the end of the D2 period. Instability in-of the stalled
boundary is tame compared to other periods of interest, but
increased forcing ahead of a deepening upper trough to the west
could provide enough destabilization to generate a period of
thunderstorms, mainly Wednesday night into Thursday AM when the
best impacts would occur. Thus, will maintain the previous
forecast for continuity purposes and assess in subsequent updates.

...Southern Plains/Mississippi Valley...

Remnant MCV is anticipated the vicinity of the southern plains up
through the mid-Mississippi Valley on Wednesday. There is some
discrepancy on the track of the circulation, as well as the
convective impacts that are possible during its propagation.
There's a mix of deterministic outputs with a maximum focused over
eastern OK to western AR, but there's another set of guidance
tracking the MCV up into Missouri creating a heavy precip field
near Kansas City and the northern half of MO. 00z HREF
neighborhood probabilities are actually the most aggressive for
totals across eastern OK, primarily in the morning when the
complex is still in its infancy compared to where it would be in
the afternoon and evening. The 500mb mean height field on ensemble
guidance shows a decent signal of at a least a stronger shortwave
moving out of OK into MO later Wednesday afternoon with some
indications it may close off at points. The guidance on the
northern precip maximum have the best 500mb reflection, which
makes sense given the more aggressive nature of the evolution. In
any case, the MRGL risk was expanded to include both the southern
and northern maximum potential as regardless of evolution, the
environment remains favorable within a corridor of higher PWATs
and large scale forcing associated with the shortwave trough.

Kleebauer

Day 3
Valid 12Z Thu Sep 21 2023 - 12Z Fri Sep 22 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS NORTHERN
PLAINS...


...Northern Plains...

Deep moisture advection pattern continues into D3 across the
northern plains where standard anomalies for PWATs over the
Dakotas are beginning to breach 2.5-3 deviations above normal
which is a great reflection on the ambient environment available
across the above area. Large scale ascent on the eastern flank of
a deep closed circulation over the west will aid in initiating a
large swath of precipitation over the Dakotas, especially focused
along a stalled frontal boundary forecast over the region.
Bias-corrected ensemble means are already showing a large areal
extent of 1-1.5" of precipitation falling on Thursday afternoon
and evening with maximum(s) positioned over the area of lowest
FFG's. Strong convergence signal on the surface and 850mb
frontogen fields across the northern plains from Bismarck,
northeast to the Canadian border show the focal point of where the
heaviest precipitation would occur. With the highly moist
environment in place, heavy rain potential will be at
climatological maximum given the expected forcing and overall
synoptic scale setup across the plains. Ensemble probabilities of
>1" are beginning to spur up towards 20-30% at lead with some
non-zero probability for >2" located over northern ND. Considering
the large scale synoptics and focused area along the stalled front
within a zone of low FFG's with potential priming the day before,
have maintained a Slight Risk with some expansion to the Canadian
border to adjust for the latest ensemble QPF forecasts and
probabilistic data. The southern edge of the SLGT risk over SD was
also expanded to the NE border given the increased convective
threat late Thursday, but considering the Sand Hill region of NE
and it's very high FFG indicators, did not want to extend beyond
the border as impacts will likely be muted within that zone.

Will make note that another area of interest will be across
northern NV as a cold front and increased upper forcing from a
strong vorticity maxima providing PVA to the region could pose
some increased flash flooding concerns on Thursday morning and
afternoon before NVA and dry air advection sweep the precip focus
further to the northeast. There's still a bit of uncertainty on
the prospects for widespread convective impacts that would
necessitate a higher risk for the area, but the enhanced upper
forcing within the pattern could very well lead to enough of a
signal for an upgrade to a SLGT. 

Kleebauer


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