Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM 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...

Some changes were made to the Slight Risk area inherited from
yesterday's Day 3 ERO, most notably to extend the northern portion
to include SE VA based on the latest guidance and (in particular)
the bump in REFS 24hr QPF exceedance probabilities. The upper-
level trough mentioned in the Day 1 discussion continues to linger
in place while continuing to dig. A sharpening upper-level jet
streak on the lee side of the upper trough will boost deep-layer
ascent as well as low-level FGEN, increasing the convective
coverage and intensity during the peak heating hours Friday
afternoon into the evening hours. PWATs meanwhile area expected to
range between 2.00-2.25" per the ECMWF and GFS. High PWATs, decent
dynamical support, and a stalling surface 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". As
such, scattered instances of flash flooding will be possible along
the coast. 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.

...Northern Plains...

A plume of upper-level energy by way of 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 potential for heavy rainfall.
Healthy moisture and solid instability will be present, with PWATs
exceeding 1.5", and MUCAPE exceeding 2000 J/kg. 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.

A Marginal Risk was retained over portions of the Sierra Nevada up
into the Great Basin for Day 2 (Friday-Friday night). Anomalous
mid-level moisture, on the order of 4-5 standard deviations above
average, will stream northward from Tropical Storm Iselle and get
swept into a deep mid- level trough propagating through the
Northwestern U.S. on Friday. MUCAPE values between 250-500 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.

Hurley/Kebede/Blanco-Alcala


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