Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM EDT Wed May 14 2025
Day 1
Valid 16Z Wed May 14 2025 - 12Z Thu May 15 2025
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC INTO THE CENTRAL APPALACHIANS...
...Mid-Atlantic...
16Z Update: Portions of the Mid Atlantic extending from the
Tidewater of VA to the northwest across the Central and Northern
Blue Ridge to the Appalachians Front are now in a SLGT risk after
coordination with surrounding WFO's. The overall synoptic pattern
remains unchanged with a slow progression of a negatively tilted
500mb trough currently centered over the eastern Ohio Valley when
assessing WV satellite and recent UA analysis. Compared to
yesterday, widespread elevated totals are not likely to be seen
with the anticipated setup as the primary precip schema will lean
to more scattered variety thunderstorms with potential for
localized heavy maxima given the 90th percentile PWATs situated
over the area (12z KIAD sounding of 1.31"). Convection is slowly
beginning to initiate downstream of the mean trough across Southern
VA into NC with a greater coverage anticipated between 18-00z
during peak diurnal heating. MUCAPE between 500-1500 J/kg will be
co-located within the terrain extending through the Blue Ridge and
adjacent Appalachians with 1000-2000 J/kg bisecting the VA
Tidewater up into Northern VA. This instability presence will be
plenty to help maintain some stronger convective cores capable of
localized rainfall rates between 1-2"/hr at peak intensity. Coupled
with very low 1/3/6 hr FFG intervals after yesterdays heavy rain
episode, this is sufficient for short term rates to exacerbate
already compromised soils as NASA SPoRT maintains a signature of
>90% soil moisture presence across pretty much the entire Central
Mid Atlantic.
12z HREF neighborhood probs for >2" of precip indicates a broad
footprint of 50 to 90% over the aforementioned area of focus with
modest probs (20-40%) for >3" in the same zone. The most concerning
depiction is within the 3 and 6-hr exceedance probability output
showing a large chunk of the Blue Ridge, Central Appalachians, and
Southern Laurels around 40-80% and 50-80% respectively. This is
typically a good indicator for a locally more significant event
potential in most areas of the CONUS, but especially over terrain
with lower FFG indices starting out. This allowed for an upgrade to
a SLGT risk in those locations extending from the VA Tidewater,
northwest through parts of Northern VA, Blue Ridge and neighboring
Shenandoah, into the Central Appalachians. A broad MRGL
encompasses the metro corridor over MD along with Western and
Central PA down towards NC as coverage and impacts are anticipated
to be more isolated in these zones with greatest threat likely
within urban areas due to higher runoff risk.
Kleebauer
...Northern Plains...
16Z Update: After last nights shift of the QPF maxima further west,
the guidance has come into agreement with little run to run
variability when assessing the two main CAMs periods. The
antecedent dry soils will be the "scale tipper" to the MRGL side of
the threat as convective premise will likely be more of a welcomed
sight, initially with some localized flood prospects in the Western
High Plains most likely incurring due to hydrophobic soils if
rainfall rates breach 2"/hr, or similar rates impacting a town with
more runoff potential given urbanization factors. The nocturnal MCS
development off the Front Range into SD/NE will be worth
monitoring, but if the convection leans closer to the Sandhill
domain along those borders into NE, the threat for flash flooding
will likely be muted outside of direct impacts on any towns in the
path of the complex. The MRGL risk was relatively unchanged given
the above factors, but pending the output from the first round of
convection along the lingering surface trough in conjunction with
the secondary convective onslaught anticipated this evening, a
targeted upgrade could be plausible. This is something we will
monitor as the day progresses.
Kleebauer
..Previous Discussion..
A deep upper level trough centered over the Intermountain West this
morning will become negatively tilted as an impressive shortwave
draws plentiful upper level energy into the northern Plains.
Surface cyclogenesis is expected ahead of this trough over Kansas,
which will track NNE into south-central North Dakota by Thursday
morning. A line of convection associated with the leading/eastern
edge of the trough is moving into far western South Dakota from
Wyoming, and this line of storms will track NNE over the western
Dakotas through the morning. The primary flooding threat will be
from a new line of storms which will develop over western South
Dakota with peak heating this afternoon, then track very slowly
eastward, as embedded cells move north, parallel to the line. This
will result in some training of convection, leading to an isolated
flash flooding threat.
In coordination with UNR/Rapid City, SD; FSD/Sioux Falls, SD; and
ABR/Aberdeen, SD forecast offices, the inherited Slight Risk was
removed with this update, and the surrounding Marginal shifted well
to the west of inherited. The westward shift was due largely to a
shift in the guidance slowing the forward motion of the trough.
While this increased forecast rainfall amounts some, the area of
the Dakotas now highlighted is in Moderate to in some localized
cases Severe drought. NASA SPoRT soil moisture imagery shows the
area has next to no moisture. Thus, despite the potential for heavy
rainfall at times in some areas, the flooding threat was
determined to be isolated.
Wegman
...Ohio Valley...
Scattered convective activity within an elevated moisture advection
regime will occur this afternoon and evening with the greatest
coverage anticipated in the evening thanks to added forcing from a
mid-level perturbation moving up from the southwest. Sufficient low
to mid-level buoyancy coupled with elevated PWATs will help with
locally heavy rainfall potential across areas extending from
Southern IN down into KY with totals potentially reaching between
1-3" in the strongest convective impacts. 12z neighborhood probs
for >2" are modest (20-40%) across the above area with some
elevated probs for >1" (50-70%) in the same locale. Considering the
nature of the soil anomalies >80% within areas of Eastern KY into
the Ohio River basin in conjunction with the heavier rainfall
rates, a MRGL risk was expanded westward to encompass the threat.
Kleebauer
...Michigan...
Near record daily PWATs (12z KAPX sounding of 1.37") and
anticipated lake breeze initiation thanks to strong differential
heat flux off Lake Michigan will generate a period of slow-moving
strong convective cores capable of rainfall rates of 1-2"/hr during
anticipated impact. Modest 10-year ARI exceedance probs based on
the latest hi-res ensemble suite (15-30%) across Northwest MI
signal a modest threat for localized flash flood concerns over any
urban zones within stronger convective cores. The key is the slower
mean storm motions anticipated with much of the convective
development likely anchored to the lake breeze and subsequent
outflow generation. Ensemble mean areal average QPF around 0.5-1"
with deterministic maxima between 2-3" was enough to warrant a
targeted MRGL risk issuance within a zone situated west of I-75 and
just north of I-96 in Lower MI. This is the primary area of concern
with the greatest threat up closer to Traverse City and points just
south.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
