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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Wed Aug 13 2025

Day 1
Valid 12Z Wed Aug 13 2025 - 12Z Thu Aug 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN
AND CENTRAL APPALACHIANS...


...Southern Appalachians...

A cold front over the Great Lakes to mid-Mississippi Valley early
Wednesday morning is forecast to advance southeastward into New
York, the central Appalachians and western Kentucky/Tennessee by
Thursday morning. High moisture with precipitable water (PW)
anomalies of 1 to 2+ are forecast to extend ahead of the cold front
from central Texas to western New England and much of the East
Coast. Across the southern Appalachians, there may be an early
round of locally heavy rainfall early Wednesday, related to
lingering nocturnal convection. The greater concern will be later
today with peak afternoon heating and scattered coverage of
thunderstorms, ahead of a synoptic scale mid-level trough axis
approaching from the northwest and a low to mid- level impulse
tracking around a ridge offshore of the Southeast, with some
locally enhanced southerly flow into the Appalachians. 00Z HREF and
18Z REFS neighborhood probabilities of exceeding flash flood
guidance (FFG) were a bit higher here than other areas of the U.S.
with potential for some 2-4 inch totals (locally higher) through
Thursday morning.

...Remainder of Eastern U.S. into Lower Mississippi Valley...
Confluent and locally stronger 925-850 mb winds of 20-30 kt are
expected across Virginia/North Carolina as the region sits between
ridging to the south and troughing to the north today. There will
be some enhanced potential for training and backbuilding cells in
the west to east flow, coupled with PWs over 2 inches, supporting
potential for a few 3 to 5 inch totals.

Back toward Texas and much of the Lower Mississippi Valley, high
PWs and slow storm motions could support some widely scattered
instances of flash flooding during peak heating, with dissipation
into the overnight.

...Northern Plains...
A few thunderstorms will be possible over South Dakota in the
first 6 to 9 hours of the period, ahead of an advancing mid-level
impulse tracking eastward from southeastern Montana. Nocturnal
strengthening of the low level jet is expected overnight, ahead of
a shortwave approaching from the west, with southerly winds of 35
to 45 kt at 850 mb forecast by the 00Z GFS through the central
Plains into the Dakotas. A small MCS is possible near a warm front
across the region overnight but details on exact location and
rainfall totals remain a bit uncertain. The Marginal Risk covers
the probabilities for 2+ inches highlighted in the 00Z HREF and 18Z
REFS.

...Southwest...
A ridge aloft over the Four Corners region is forecast to slowly
shift east into eastern New Mexico by Thursday morning, ahead of an
approaching upper trough into California. Some increase in PWs are
expected into Arizona and southwestern New Mexico during the period
but values will climb to near average by 00Z Thursday. An
expectation of decent solar insolation with limited cloud cover
should support seasonable instability values into the region for
this afternoon/evening and Localized flash flooding will be
possible with diurnally driven thunderstorms, especially across
more vulnerable terrain features or burn scars.

Otto


Day 2
Valid 12Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY TO THE MID-ATLANITC COAST AND NORTHEAST...AS
WELL AS THE SOUTHWEST AND THE UPPER MISSISSIPPI VALLEY...

...East Coast to Lower Mississippi Valley...
The cold front from Wednesday will continue to advance toward the
southeast with lingering high moisture ahead of the boundary from
New England into the Mid-Atlantic Coast. Potential for localized
excessive rainfall within the anomalous PW axis and with forcing
ahead of the front, but cloud cover may limit the potential for
higher instability values. There should be at least an isolated
flash flood threat with thunderstorms ahead of the front from
southern New England to the Mid-Atlantic coast.

Farther south, from the Southeast to the Lower Mississippi Valley,
there does not appear to be a substantial focus area for excessive
rainfall potential, hence the broad Marginal Risk, but high
moisture and daytime heating will result in scattered thunderstorms
with peak rainfall rates in the 1 to 3 in/hr range. Similar to
Wednesday, parts of the Lower Mississippi Valley will have high PWs
and slow cell motions given placement between a mid- level ridge
center over KS and another over the northeastern Gulf.

...Southwest...
The upper trough over California on Wednesday will continue to move
east on Thursday with increased moisture advection into Arizona and
parts of Utah into the Four Corners region. PW values are forecast
to increase to near +1 standardized anomaly for Arizona into
southern Utah. While cloud cover is a bit uncertain for Thursday
into Friday, ahead of the western trough and from any lingering
disturbances from the previous day, A similar sized, perhaps
slightly larger, Marginal Risk is drawn for Thursday into Friday
following the model QPF consensus and Arizona Regional WRF
simulated reflectivity, which showed the potential for more
widespread coverage of thunderstorms on Thursday compared to
Wednesday.

..Upper Mississippi Valley...
A cold front will move into the north-central U.S. on Thursday with
a steady enhancement to southerly/southwesterly 850 mb winds ahead
of the boundary. Even after diurnal weakening of the nocturnal low
level jet, 00Z GFS forecasts show 20-40 kt continuing from the
central Plains into the eastern Dakotas and Minnesota through
Thursday evening. Locally heavy rain may be ongoing Thursday
morning with decent potential for another round Thursday night with
favorable jet diffluence/divergence aloft, possible moderate
instability and standardized PW anomalies of +1 to +2. Confidence
on the exact location remains uncertain however, with hopefully
increasing confidence occurring as we move into the hires range.

Otto


Day 3
Valid 12Z Fri Aug 15 2025 - 12Z Sat Aug 16 2025

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FROM THE LOWER
MISSISSIPPI VALLEY TO THE SOUTHEAST COAST...AS WELL AS THE
SOUTHWEST/FOUR CORNERS REGION AND THE UPPER MISSISSIPPI VALLEY...

...Southeast Coast to Lower Mississippi Valley...

Lingering high moisture will be in place on Friday, south of a
southward sagging frontal boundary with standardized PW anomalies
of +1 to +2 from the Lower Mississippi Valley to the Southeast
Coast. With an upper ridge centered over the northeastern Gulf,
there will be potential for mesoscale impulses to ridge around the
ridge and help focus diurnally driven thunderstorms during peak
heating. Similar to previous days, locally high rain rates with 1
to 3 in/hr may occur with localized training.

...Upper Mississippi Valley...
A cold front will continue to advance downstream through the Upper
Mississippi Valley but slow/stall back toward the west. PWs will
remain above average in the vicinity of the front and organized
thunderstorm potential will exist within the mean westerly flow
aloft. Similar to previous days, nocturnal strengthening of the low
level jet at night will have the potential to generate an MCS or
loosely-organized convective cluster with localized training
potential. Confidence is rather low at 3 days out, but the Marginal
Risk was drawn to cover the various model solutions and conceptual
thinking for greatest rainfall potential near the front and low
level jet axis intersection.

...Southwest...
Active southwesterly flow across much of the western U.S. will
encourage the further expansion of above average moisture into the
Four Corners region and northern Great Basin. While standardized PW
anomalies will generally peak in the +1 to +2 range, not everyone
within these anomalies will see the same flash flood potential. The
focus appears to be best tied to the Mogollon Rim into the Wasatch
and central to southern Rockies, ahead of a number of shortwave
impulses moving in from southern California. Tied to the relatively
stronger belt of westerlies could be jet-induced divergence maxima
aloft to help in the production of locally heavy rainfall.

Otto


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