FOUS30 KWBC 022000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Sat Aug 2 2025

Day 1
Valid 16Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST, SOUTHERN PLAINS, AND NORTHERN PLAINS...

...Southern Appalachians through the Gulf Coast...
A wavering hybrid cold/stationary front will remain draped from
eastern Texas through South Carolina today, with a slow southern
translation of this feature expected into Sunday morning. This
front will waver (with the gradual southern progression) in a
region of broad troughing extending down from the Northeast, with
weak flow across the Gulf Coast leading to the slow movement.
However, the front will be entrenched within a plume of elevated
tropical PWs (measured via 12Z U/A soundings of 2.2 to 2.4 inches,
nearing daily records across the area.) This will support
widespread showers and thunderstorms along the low-level
convergent boundary of the front, with generally weak west-to-east
0-6km mean winds of just 5-10 kts supporting slow storm motions,
and training thanks to this boundary parallel wind. Weak impulses
within the flow will enhance ascent leading to locally clustered
convection with rainfall rates above 2"/hr (briefly 4"/hr likely as
reflected by HRRR 15-min rainfall above 1", and there are likely
to be two primary areas of higher excessive rain risk.

The first area is across the Southern Appalachians from northern
Georgia through eastern TN and far southwest NC. Here, a narrow
ribbon of instability 750-1000 J/kg (MUCAPE) will extend northward
and interact with increasing moist upslope flow as 925-850mb winds
veer more E/SE late this aftn into the evening. Although PW
anomalies are somewhat lower here compared to points south,
increased bulk shear and weak mean winds will support heavy
rainfall rates of 2-3"/hr within what could be nearly stationary
cells at times. 0-10cm soils in this region are saturated above
the 95th percentile according to NASA SPoRT, leading to compromised
FFG and a 60-80% chance of exceedance from the HREF.

Farther south from eastern AL though GA and onto the SC coast,
training of cells which develop along the front and within the
greatest plume of PW will support an increased excessive rainfall
risk, especially through peak heating. Although soils here are a
bit drier due to a lack of recent rainfall (7 day rainfall
generally 25-50% of normal), a 60-80% chance of 2"/hr rain rates
from the HREF and REFS could lead to stripes of rainfall exceeding
5 inches (40-70% chance from both HREF and REFS.) After
consideration of the new probabilities and 12Z CAMs, the two slight
risk areas were merged into one larger SLGT risk which encompasses
more of GA/eastern AL as signals from both the HREF and REFS were similar.

...Northern Rockies through the Southern Plains...
An elongated north-south stationary front will drape from Montana
through Texas today, while multiple mid-level impulses embedded
within generally zonal flow from the Rockies into the Plains
interact with this surface feature as they emerge from the west. At
the surface, flow downstream (east) of this stationary front will
become enhanced as return flow persists out of the Gulf, driving an
850mb LLJ to 30 kts across the Central Plains. This surging LLJ
will help draw impressive thermodynamics northward, characterized
by PWs of 1 to 1.25 inches (locally higher) overlapped with
1000-2000 J/kg MUCAPE. This will support widespread convection
developing beneath any of these shortwaves, with development most
likely along and just east of the wavering stationary front into
the more impressive thermodynamics.

Although convection is expected to move briskly eastward within any
clusters that develop thanks to 30-40 kts of bulk shear, rainfall
rates of 1-2"/hr are anticipated (30-50% chance from the HREF)
which could cause at least isolated excessive rainfall impacts,
especially across any urban areas or more sensitive soils with
lower FFG. The greatest risk appears to be across the TX panhandle
where an MCS may form tonight on the periphery of the greater
instability plume and then dive into the moisture beneath one of
these shortwaves, with a subtly higher training risk along its SW
quadrant, and both HREF and REFS 3"/24 hr probabilities peak above
40%, and the inherited SLGT risk was adjusted cosmetically. There
may be a secondary area of heavier rainfall across eastern SD and
into ND where some repeating or backbuilding convection may occur
later tonight as westerly flow in the vicinity of an MCV/shortwave
results in repeating development into the higher instability west,
and convection moves into some more sensitive soils in eastern SD
tonight. This has prompted a targeted SLGT risk for tonight as
well, which is supported somewhat by the CSU UFVS first guess field
which has a SLGT risk in the same general area, although displaced
from the HREF/REFS probabilities which drove this new excessive area.

Weiss



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