FOUS30 KWBC 160808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
408 AM EDT Tue Sep 16 2025

Day 1  Valid 12Z Tue Sep 16 2025 - 12Z Wed Sep 17 2025

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

...Northeastern North Carolina and Southeast Virginia...

Well-defined surface reflection off the NC coast will continue to
slowly move to the north over the course of the overnight and
morning hours before parking off the VA Capes as the initial
occlusion process takes shape. There's been a bit of a deviation
in the overall precip pattern from what has been forecast over the
past few forecast cycles, a lot of it driven from complex
convective dynamics that thwarted the initial low-level moisture
surge that was supposed to have already occurred according to older
forecasts. The previous convective cluster has since dissipated,
indicated via the rapidly warming cloud tops located just to the
north of the surface low center. As a result, have finally seen
bands of precip rotate inland with relatively modest rates as the
instability pool remains parked just offshore when assessing the
latest mesoanalysis. The trend is for a slow advection of
increasingly favorable instability to penetrate inland with the
alignment of the greatest forecast MUCAPE to be situated over far
northeast NC into the VA Tidewater. This is the area of interest
for the D1 when it comes to potential slow-moving convective
elements within the smaller deformation axis transposed on the
western flank of the cyclone. Some of the CAMs have been way too
aggressive with this feature and has allowed the HREF probs fields
to be heavily skewed into a more robust precip depiction compared
to what is current ground truth. 

The HRRR/RAP combo has been handling the current evolution the best
of any CAMs member, and has certainly been outperforming most
global deterministic with the CMC the closest to what is occurring.
Considering the variables and complexity of the forecast, there
were some changes from the inherited SLGT risk across NC where the
western extension being removed allowing only the northeastern
corner of NC up into the VA Tidewater the only remaining locations
within the SLGT. This is subject to change pending on the
instability advection regime as rates will need to pick up in order
to secure a significant flash flood threat with the Hampton
Roads/VA Beach area the most prone to the risk due to urbanization
factors. Anticipating 2-4" with perhaps up to 5" within the
targeted SLGT risk as this remains the area with the best chance to
see heavy rain longevity with less of a threat the further away
from the coastal plain. Hourly rates maxed between 1-2"/hr are the
most likely scenario at this juncture, putting mainly a small area
of potential for flash flooding as sandier soils located outside
the VA Tidewater are likely to negate widespread flash flood
concerns. Considering how poorly the guidance is handling this 
setup, near term monitoring will be employed over the course of the
period with the best threat likely occurring between 12-00z, 
waning quickly in the evening as the low becomes vertically stacked
and favors a rapid occlusion leading to a degrading heavy precip threat.

...Southwest...

Scattered convective pattern will initiate later this afternoon 
and the early evening hours before fading with the loss of diurnal
heating. Current progs are for convection to be tied closer to the
terrain across southeast AZ into southern NM with isolated 
signatures of >1"/hr plausible in the stronger cell cores. These 
areas generally prone to runoff due to terrain complexities, as 
well as remnant burn scars present in some locations. There was
very little change from the inherited MRGL risk with the forecast
running close to general continuity. 

...Central High Plains...

Strong shortwave trough ejection out of the interior Northwest will
lead to convective development and regional enhancement across the
Central High Plains. Greatest ascent is aligned with the diffluent
region of the mid-level shortwave with left-exit region dynamics
likely to maintain a prevalent heavy rain signature across the
western third of NE down into neighboring northeast CO and
northwest KS. Guidance has come into alignment with the axis of
heaviest precip centered near and just north of North Platte, NE
with the corridor extending south into southwestern NE and
northwest KS by the second half of the forecast period. 00z HREF
neighborhood probs for >3" are running between 20-40% across the
referenced area near North Platte and points south. This still
aligns well with the previous forecast MRGL risk leading to little
deviation in the current thinking. The areas of highest risk will
likely be smaller towns and urbanized areas, like North Platte that
are more susceptible to runoff. 

Kleebauer
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