FOUS30 KWBC 170758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Wed Sep 17 2025

Day 1  Valid 12Z Wed Sep 17 2025 - 12Z Thu Sep 18 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR WESTERN AND
CENTRAL SOUTH DAKOTA...

...Northern and Central Plains...

A broad mid-latitude cyclone will continue to impact a large
portion of the Plains over the course of Wednesday into Thursday
with multiple areas of heavy rain plausible given a variety of
dynamical and thermodynamic processes. The area with the greatest
concern is likely within the well-defined axis of deformation that
is forecast to materialize across the western half of SD leading to
scattered flash flood concerns thanks to wet antecedent conditions
in wake of a previous disturbance that distributed copious amounts
of rain to areas along and west of the Missouri River. Guidance is
in agreement on the threat for 1-3" over the area extending from 
the river over to the SD/WY state line, delineating the deformation
structure forecast to materialize within the northern and western
half of the cyclone situated over NE. FFG's on the order of 0.5-2"
between 1/3/6 hr. intervals are running much lower than climo
leading to a lower threshold to breach that could very well cause
issues across that broad area of western SD. 5 and 10-year ARI
exceedance probs are running between 50-80% and 30-60%,
respectively between the two intervals, a solid representation of
at least some scattered flash flood concerns with perhaps a broader
scope of impact considering the areal footprint of the
probabilities in question. Rates will be ~1"/hr at peak intensity,
but could very well last for multiple hours due to the anticipated synoptic
scale evolution of the surface lows slower forward propagation
speed leading to training over a large area of SD down into
northwest NE. In this case, the previous SLGT risk was maintained
with some minor expansion on the edges to account for the latest
probabilities and HREF blended mean QPF output.  

By the afternoon and evening, defined warm conveyor belt (WCB) will
be situated across much of the Missouri River basin extending from
OK/AR up into eastern SD. A warm front will lift north and
transition to a quasi-stationary boundary across east-central SD
through south-central MN leading to a focused frontal alignment for
storms to materialize later in the period. This has been a trend
within the recent CAMs as convective initiation will likely be
spurred by the arrival of a shortwave pivoting around the eastern
flank of the primary ULL, generating a better large scale ascent
pattern focused downstream of the center of circulation. With dew
points expected to be in the 60s for locations within the defined
WCB, there will be room for scattered to widespread convective
development during afternoon and evening time frame as the
environment ripens with the typical diurnal heat flux. The frontal
positioning over the Northern Plains and Upper Midwest will be a
target for training convection as cells can become anchored to the
front and lead to localized areas of heavy rainfall that could drop
a quick 2-4" over a span of a few hrs. Rates between 1-2"/hr in
convection are likely in this setup for anywhere within the WCB
leading to a general maintenance of the MRGL risk across the
Plains, but have now expanded the MRGL risk eastward into MN to
account for the frontal alignment and growing threat of convection
spawning near and within the boundaries forecast location. 

...Southern High Plains...

Multi-round convective episode anticipated across the southeast CO
Front Range down into the OK/TX Panhandles and adjacent northeast
NM beginning later this afternoon, carrying through the evening
hours. Lee side low over eastern CO/western KS will materialize
within the broad synoptic scale evolution occurring over the center
of the CONUS. A trailing cold front will drag south and southwest
through the above areas with an initial flare up of convective
activity this afternoon along the front creating a moment of
priming before a larger shortwave ejecting out of the Central
Rockies becomes the focus for later. Models are in agreement on
quite a strong mid-level vorticity ejection into the Front Range
late-afternoon and early evening today leading to a blossoming
convective initiation in-of the terrain just west of I-25 in
southeast CO. Storms will grow upscale and migrate eastward into
the Front Range, continuing to slide east-southeast as they deliver
locally heavy rainfall in their wake leading to an isolated to
scattered flash flood concern through the second half of the
forecast. This setup has been documented in some way over the past
several forecast iterations, but the threat is now clear in the
main show focused within the strong mid-level ascent pattern
generated by the shortwave rounding the trough base. Expect
localized totals of 2-3" in the hardest hit locations with a max of
up to 4" across the far southeast corner of CO down into the
northern TX Panhandle. This is reflected by the modest >3"
neighborhood probabilities in the latest 00z HREF suite, a solid
indicator for a MRGL risk maintenance from previous forecasts. 

...Southwest...

Little change from the previous forecast as guidance maintains a
signal for another period of isolated to scattered convection 
over the terrain encompassing southern AZ up through the central 
and eastern portions of NM with the primary threat being heavy 
rainfall situated over more complex terrain and burn scar remnants.
Assessment of forecast soundings and hodographs indicate 
relatively slower cell motions, a classic characteristic for flash 
flood prospects over the desert southwest. Guidance reflects local 
1-2" totals scattered across the region extending from southeast AZ
into NM with some of the better signals focused within those 
mountain ranges like the Huachucas and Sacramento's. The previous
MRGL risk was relatively unchanged considering the favorable run
to run continuity. 

...Southeast Florida...

Broad cyclonic pattern across the eastern CONUS will aid in
advecting a relatively stout mid-level shortwave currently centered
along the northern coast of Cuba. IR satellite this evening shows
the disturbance is very well-defined with a centroid of cooler
cloud tops indicating scattered to widespread convective coverage
in-of the western Florida Straits at this hour. Considering the
south to southwesterly flow between the 850-300mb layer, the
expectation is for convection to move generally northeast with the
mean flow, ushering in a more focused area of forcing capable of
impacting all of South FL, including the Keys with heavy rainfall
at any point. PWATs are on the rise according to the latest 00z
RAOB's out of KMFL and KEYW this evening with a broad expanse of
1.9-2.2" PWATs over the region. Forecast is for PWATs to continue
to increase with an expectation of indices to settle between
2-2.3", a solid 1-1.5 deviations above normal when assessing the
latest NAEFS output. Guidance is keying in on the FL Keys to be the
primary focus for heaviest QPF during the forecast period, however
the environment is favored for any area situated south of I-4,
especially over southeast FL where sea breeze components and deep
moisture presence will lend to pockets of very heavy rainfall from
convection that develops over the area. These cells will be
scattered in nature, but the prospects for 2-3"/hr rates with
higher intra-hour rates across the urban corridor from Melbourne
down to Miami will make for at least a modest MRGL risk for
excessive rainfall over the next 24+ hours. The previous risk was
expanded north to include KMLB as hi-res trends are keying on a
convergent area north of where the previous risk forecast(s) have
been made.

Kleebauer
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