Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
729 PM EDT Thu Aug 27 2026
Day 2
Valid 12Z Fri Aug 28 2026 - 12Z Sat Aug 29 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
CAROLINAS INTO SOUTHEAST VIRGINIA...

...Southeast/Mid-Atlantic...

Minor expansion made to the previous Slight Risk in the Mid-
Atlantic based on the latest HREF and REFS guidance. The upper-
level trough continues to linger in place while continuing to dig.
A sharpening upper-level jet streak on the lee side of the upper
trough will enhance deep-layer ascent as well and low-level FGEN,
increasing the convective coverage and intensity during the peak
heating hours Friday afternoon and evening. This forcing combined
with PWs exceeding 2", 1000-2000 J/kg MUCAPE, and a stalled front
will likely lead to multiple rounds of heavy rainfall within the
Slight Risk area, with the strongest cells capable of hourly rain
rates of up to 3". Heavy rainfall is also expected in the surrounding
areas extending from the Gulf Coast to southern Virginia, in which
isolated instances of flash flooding are possible. 12Z guidance
has a signal for a weak circulation developing off of the Florida
Panhandle over the next 24 hours, which focuses convection near the
coastline. While FFGs in the are are high, a categorical upgrade
could be needed if the heavy rainfall advertised from this feature
trends further inland.

...Northern Plains...

A shortwave trough and associated surface warm front will quickly
punch through/somewhat flatten the upper ridge and move through the
eastern Dakotas and western Minnesota and Iowa. This will bring a
progressive complex of thunderstorms into the area, with some
potential for heavy rainfall. Healthy moisture and solid
instability will be present with PWs exceeding 1.5" and MUCAPE
exceeding 2000 J/kg, respectively. However, given the progressive
nature of thunderstorms, the Marginal risk area was maintained.

...Southwest and Portions of the Great Basin...

Monsoonal moisture continues the fuel diurnally drive convection
along the southwest. A plume of moisture will drive PWATs around
1.5-1.75" northward along much of the Southwest. This will lead to
renewed chances of isolated flash flooding, as has been similar for
much of the monsoon season.

Over the Great Basin, anomalous mid-level moisture, on the order
of 4-5 standard deviations above average, will stream northward
from Post-T.C. Iselle and get swept into a deep mid-level trough
propagating through the Pacific NW. MUCAPE values near 500 J/Kg
working their way north through the southern Great Basin/Sierra
should be enough to produce heavy rainfall and at least isolated
flash flooding out of the available moisture. The Marginal Risk
area was adjusted southward to reflect the highest HREF
probability of MUCAPE exceeding 500 J/Kg -- where rainfall rates
would be most intense.

Asherman/Hurley


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt