Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 PM EDT Sat May 17 2025
Day 2
Valid 12Z Sun May 18 2025 - 12Z Mon May 19 2025

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE
CENTRAL PLAINS INTO THE LOWER AND MIDDLE MISSISSIPPI VALLEY...

20Z Update: The overall synoptic pattern remains steady within the
run to run variance as the longwave evolution still indicates a
broad upper trough migrating east with large scale ascent
increasing over much of the Southern Plains to Mississippi Valley.
The northern extent of the convective threat and subsequent flash
flood concerns are still locked in as secondary surface reflection
over the Western High Plains will induce a regional axis of
sfc-700mb convergence with the best threat of convection aligned
within a defined theta_E ridge positioned over the Front Range up
through south-central MT. Slower storm motions and terrain
influences from local topography within the Big Horns will lead to
general training with heavier convective cores likely within the
broad instability and low-level ascent pattern. 12z HREF
neighborhood probs for >2" is between 30-60% over a large area
between the northwest Sandhills up through Central MT, a signal
that is coincident with elevated flash flood concerns due to the
local topography and complex drainage in-of those mountains/hills.
A targeted SLGT was contemplated, but the coverage of heavy
convection and the current FFG exceedance probs for the region
above are still on the lower end with the best probabilities
aligned within the Big Horns up through the Crazy and Little/Big
Belt Mountains. The MRGL was sufficient for this update, but will
monitor trends for a potential upgrade in future updates.

Across the Southern Plains to Mid-Mississippi Valley, the threat is
largely influenced by the broad large scale evolution upstream
creating a significant diffluent pattern within a corridor of
elevated instability and deep moisture presence. As of the 12z
suite, the jury is still out on exactly how the more organized
convective scheme across the Central Plains and Mid-Mississippi
Valley will transpire with the general deterministic variance
positioning the threat from as far north as Northeast KS and
Southeast NE to as far south as Southern MO, a solid 200 mile
deviation from one member to the next. The CAMs suite provides a
little more favor in-between with the ML output sort of edging into
that corridor from Northeast Oklahoma up into Central MO.
Considering some overlap with the CAMs on the alignment, the threat
for flash flooding is likely highest in that general zone with a
margin of error within 50 miles in any direction. The SLGT given
the above information was relatively unchanged with a high-end SLGT
still focused over the region extending from Northeast KS to points
southeast with the southern edge over the Ozarks of AR/MO. A
targeted MDT risk is possible, but with still some discrepancy
in the exact placement of the expected MCS, the SLGT was
maintained.

The MRGL risk into the Southern Plains seems locked in as well
with a potential for a targeted SLGT pending any overlap with
convection that occurs tonight. The DFW metro is the main
population center of focus in this setup as convection fires
within a zone of very unstable air. Rates of 1-2"/hr will be
possible with forward propagation speeds another deterrent for an
upgrade at this time. The MRGL risk across New England was adjusted
a touch south to include some lingering convective concerns over
north-central VT/NH. Otherwise, the setup didn't offer any
significant run to run discrepancies.

Kleebauer

..Previous Discussions..

...Plains, MS Valley and Southeast...
A favorable pattern for widespread convection and heavy rainfall
on Sunday across the middle portion of the country. A longwave
trough and embedded shortwaves pushing eastward across the Rockies
and into the Plains will provide ascent over a broad region. The
frontal pattern will likely feature a developing low moving into
the High Plains of CO/NE, a warm or stationary front extending east
of this low across portions of OK/KS into MO/AR, and a dryline
across OK/TX. A large pool of instability is expected east of the
dryline and south of the stationary front, with values upwards of
4000-5000 j/kg. Some convection may be ongoing near the
warm/stationary front Sunday morning, but more robust development
is expected by Sunday afternoon into the overnight hours as
stronger forcing ejects into the Plains.

The main area of focus for flash flooding appears to be near the
aforementioned warm/stationary front where some training of convection
is probable. The overall expectation is that robust convective
development occurs along/ahead of the dryline over central OK/KS
during the afternoon hours. Strong deep layer southwesterly flow
will likely quickly push this activity northeastward towards the
warm/stationary front. Activity will likely grow upscale Sunday
evening into the overnight, potentially into an MCS, at which point
it should feel the impact of the strong southwesterly low level jet
and begin turning easterly and then southeasterly aligned with the
upwind Corfidi Vectors. As this process occurs some
training/backbuilding on the south/southwest extent is probable
resulting in a scattered flash flood threat.

The exact MCS location/track remains a bit uncertain. All but
certain that the 00z NAM is too far north, as its QPF evolution is
an outlier and does not align with pattern recognition. The GFS
and GEFS mean are also probably too far to the northeast, with the
aforementioned expected MCS propagation taking it on a farther
south track. The 00z Gem Reg and RRFS are very far south, tracking
across eastern OK into central/northern AR. This may be too far
south, but can not be ruled out as easily as the farther north NAM
can. The AIFS has a track record of pinpointing organized
convection location quite well, and the 00z AIFS favors northeast
OK into southeast KS and southwest MO. This also happens to
generally be the middle ground of the other available models, and
so does appear to represent the most likely outcome at this time.
Thus a higher end Slight risk will remain in place across this
corridor, where scattered flash flooding is expected. The Slight
risk area was cut back a bit on its northern extent and expanded
to the southwest into more of OK and AR to account for the southern
trend. Can not rule out additional southward shifts/expansion..but
for now think the Slight risk area represents the general region
of most concern well.

Another area of interest will be farther northwest into central and
western NE, closer to the low track. A multi-model signal exists
for a second QPF max over this area...which makes sense given what
should be impressive synoptic driven ascent and instability along
the stationary/occluded front. Considered expanding the Slight risk
into this area...but opted against that for now. This area remains
in severe to extreme drought and possible the rainfall max ends up
over the high FFG sand hills. But we will continue to monitor
trends and an additional focused Slight risk may eventually be
needed.

A localized flash flood risk also exists both along the dryline from
central TX into OK...and over the Southeast where a lingering
boundary and substantial instability will support heavy rainfall
with any storms that area able to develop.

...Northeast...
A Marginal risk continues over portions of northern New England on
Sunday as a deep layered low remains overhead. This will be the
3rd straight day of localized heavy rainfall, and thus some areas
may be more sensitive by this time. Generally thinking both
instability and moisture will be lower by Sunday, however cold air
aloft under the low will likely still support some heavier
convective cells and an isolated flash flood risk.

Chenard


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt