FOUS30 KWBC 160057
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Mon Sep 15 2025

Day 1
Valid 01Z Tue Sep 16 2025 - 12Z Tue Sep 16 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN NORTH CAROLINA AND SOUTHEASTERN VIRGINIA...

...Eastern North Carolina and Southeast Virginia...
An increasingly hybrid cyclone, with recent persistent deep
convection, lies offshore NC. While the convection is being sheared
from the south, it has yet to fully decouple from the center and
seems to be drifting a little father offshore rather than closer
to the coast. Both the 18z HREF and 12z REFS have ~50% chance of
5"+ across sections of eastern NC, but they are assuming a steady
approach ashore which doesn't seem to be happening yet.

Conceptual models with these sort of systems suggest that they
remain convectively coupled and shift right of the model guidance
for a time before the convection shears off and you ultimately end
up with a comma head pattern which has low- to mid- level
frontogenesis, which happened for a short period with Chantal when
it came ashore over two months ago near the SC/NC border. Since the
wet mesoscale guidance doesn't match recent radar/satellite
imagery trends, held off on a Moderate Risk after coordination with
MHX/Newport NC and AKQ/Wakefield VA forecast offices, but do think
this is a higher end Slight Risk. Despite mostly marsh being
impacted and the region being dry lately, the combination of
available moisture and instability moving in from the Atlantic
should allow for hourly amounts to 3" with local totals to 7" where
any convective banding trains for a couple hours or so, which
would overwhelm the limited urban areas that exist in east-central
NC. Nudged the Slight Risk more northward to more fully encompass
the VA Capes as that's where satellite imagery appears to suggest
such a comma head would try to focus heavy rainfall overnight.


Roth


Day 2
Valid 12Z Tue Sep 16 2025 - 12Z Wed Sep 17 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EASTERN NORTH CAROLINA AND VIRGINIA...

...Virginia and North Carolina...
The surface low moving onshore NC from Monday will occlude and then
begin to return to the east as the system vertically stacks beneath
an upper low. Despite the slow filling and opening of the parent
mid-level low, continued ascent through PVA (weak vorticity maxima
rotating around the low) and periods of enhanced upper diffluence,
especially north of the low, will result in sufficient ascent to
maintain heavy rainfall, especially the first half of Tuesday.
Although synoptic lift will gradually wane, continued impressive
onshore fetch on 850mb winds of 25-35 kts (and above the mean wind
by almost 1.5x) will continue to transport elevated PWs above 1.5
inches onshore while producing additional ascent through
confluence. This will result in persistent training of moderate
rainfall with embedded convective elements leading to rainfall
rates for which the HREF neighborhood probabilities indicate a
30-40% chance of exceeding 1"/hr at times.

Where these rain rates train most efficiently through anti-
parallel Corfidi vectors, additional rainfall on D2 could reach an
additional 2-4" with locally higher amounts, especially near the
NC/VA border. This is also the area most likely to receive heavy
rainfall on D1, so 48-hr rainfall could reach 5-8" with local 10"
amounts possible. The REFS is a bit farther north than the HREF
today, but is supported by the AIFS and the area of maximum upper
divergence, so the MRGL risk was expanded a bit to the north, and a
targeted SLGT risk was added where D2 rainfall most efficiently
overlaps D1, leading to the enhanced flash flood risk. This was
coordinated with the local WFOs.


...Southwest...
A weak shortwave will emerge beneath the elongated mid-level ridge
Tuesday evening, helping to enhance the otherwise typical diurnal
convective activity across parts of AZ/NM. This weak impulse will
impinge into increasingly favorable thermodynamics as low-level
S/SE flow pushes PWs to 1-1.25 inches collocated with MUCAPE that
will exceed 500 J/kg. This suggests a local maxima in convective
coverage across southern AZ and into southern NM. Outside of any
clusters that develop thanks to modest bulk shear as storms dive
off the terrain later in the period, convection will slow moving on
0-6km mean winds of just around 5-10 kts, leading to locally heavy
rainfall capable of producing isolated flash flooding.


...Central High Plains...
Amplifying trough across the Intermountain West will shed a
shortwave into the Central Plains during the latter half of Tuesday
into Tuesday night. This shortwave will impinge into a favorable
airmass to spawn convection, reflected by PW forecasts of around
1.25 inches overlapping MUCAPE exceeding 2000 J/kg. The primary
longwave trough and the accompanying height falls/PVA combined with
modest but intensifying upper diffluence will likely result in
surface low developing in the lee of the Rockies, with a
downstream warm front pushing into Nebraska serving as a convergent
boundary for additional ascent. This warm front will waffle
east/west in response to low-level jet energy pushing NW from
OK/KS, resupplying the favorable thermodynamics to support
convection.

Thunderstorms that develop across this area will likely be heavy
rain producers with 1-2"/hr rain rates supported by 30-40%
probabilities from the HREF. Storms that develop will be supported
as well by 24-40 kts of bulk shear to help organize convection into
clusters, with some training along boundaries also expected. Where
this rain is most pronounced, total rainfall could exceed 3 inches
in a few locations, with the greatest potential focused near the
KS/NE/CO juncture. Here, rainfall has been above normal the past 7
days as well, compromising soils and creating FFG as low as
2"/3hrs, so any additional heavy rain could cause runoff leading to
instances of flash flooding. The threat still appears MRGL as
inherited, but some cosmetic adjustments were made to refine the
area based on new guidance.

Weiss


Day 3
Valid 12Z Wed Sep 17 2025 - 12Z Thu Sep 18 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
NORTHERN AND CENTRAL PLAINS, SOUTHERN HIGH PLAINS, AND SOUTHEAST
FLORIDA...

20Z Update:
Changes to the forecast ERO areas were minimal with this update.

The MRGL risk across southeast Florida was maintained with little
changes needed in response to good model agreement and trends in a
surge of PWs lifting into the southern peninsula. With the previous
discussion highlighting well the risk, and the coordination
completed overnight with WFO MFL, only minor cosmetic adjustments
were required.

In the Northern Plains, the MRGL risk was expanded just slightly
downstream of an amplifying trough axis digging out of the
Intermountain West. However, the carving out of the Sand Hills was
continued in response to the overall modest probabilities for
exceedance of 3" QPF over higher FFG. The inherited MRGL risk was
just expanded slightly to cover the highest axes of the new model
QPF fields.

Finally, in the Southern Plains, did make some modest southward
adjustment to the inherited MRGL risk. The guidance has trended a
bit faster/farther southwest with the trailing cold front and
accompanying convection. This suggests that the heavier rainfall
may occur atop drier soils with higher FFG, limiting the flash
flood risk. There are still some low-end probabilities for heavy
rain exceeding FFG, especially in the NW portion of the adjusted
MRGL risk, so while the threat appears minimal, it was enough to
continue the risk area at this time.

Previous Discussions:

...Northern and Central Plains...
A strong mid-level shortwave trough will eject into the WY Front
Range and eventually close off across NE generating a maturing lee
cyclone across the Central High Plains. The setup will likely cause
quite a robust deformation signature on the northwest flank of the
cyclone with ensemble mean QPF placement situated over western NE
up through southwestern SD, arcing back into the nose of the warm-
conveyor belt on the northeast side of the system. This is a
classic mid-latitude lee cyclogenesis setup with a strong jet
coupling leading to broad large scale forcing and widespread precip
across the Plains. Eastern side of the cyclone will exhibit the
best convective potential while the deformation axis provides a
solid precip field with moderate to locally heavy rates for several
hrs. in that northwest side of the low. Totals >2" are forecast
within the means across western NE into SD with scattered elevated
QPF across the Missouri River basin located in eastern SD and NE.
This setup is conducive for at least some isolated to scattered
flash flood prospects considering the synoptic evolution. This was
enough to warrant a MRGL risk in those areas deemed the best chance
for heavy rain prospects.

...Southern High Plains...
Maturing lee side low to the north will help drag a trailing cold
front through the Central and Southern High Plains on Wednesday.
The front will slip south and southwest once down near the latitude
of the OK Panhandle, leading to a more textbook backdoor frontal
progression as it enters into the northern TX Panhandle and
northeast NM. Flow begins to run more parallel to the boundary in
these zones when assessing forecast soundings from relevant global
deterministic. The key in the setup is a relatively solid
isentropic ascent pattern in a small zone bordering CO/OK/NM/TX
with cross-section views over the area displaying a classic sloped
low-level surface FGEN with strong omega correlating to rapid
ascent as the front progresses through the area. Locally enhanced
corridor of precip will transpire along and in wake of the front
with local maxima of 1.5" to as much as 3" plausible during the
event. This could be enough for an isolated flash flood threat,
especially in those smaller urban areas situated across the 4 state
intersection. A MRGL risk has been added to account for the
threat.

...Southeast Florida...
Stalled front with an enhanced moisture return across South FL
will lead to a heightened threat for flash flooding over the urban
centers of southeast FL. Deterministic output is scattered in the
exact location of the heaviest precip, ranging from the Keys up to
West Palm Beach and everywhere in- between. The setup is
historically favorable for flash flood concerns over the urban
zones of southeast FL with PWATs hovering between 2-2.3", a solid
1/1.5 deviations above normal. In coordination with the Miami WFO,
a MRGL was maintained for that urbanized coastal corridor between
West Palm down to Miami proper since this is the most likely
location for flash flooding in this type of setup. Will monitor
convective trends as we get closer to see if this necessitates any
expansion, or even a targeted upgrade.

...Mid-Atlantic States...
Although no MRGL risk was added at this time, the coastal low
plaguing NC/VA will continue to move slowly northward Wednesday,
spreading moisture via onshore flow into VA/MD/DE. The
probabilistic guidance suggests only a very low-end threat for 3"
of rainfall, but admittedly the guidance did not really pick up on
the current heavy rain signal for NC/VA until within 3 days either.
After coordination with AKQ, no MRGL risk was added at this time,
but with some history of "over-performing" rainfall upstream, it is
possible a targeted MRGL risk may be needed with later issuances.

Kleebauer/Weiss


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt