Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Sat Jul 5 2025
Day 1
Valid 01Z Sun Jul 06 2025 - 12Z Sun Jul 06 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
CENTRAL TEXAS...

...Central Texas...
A new Moderate Risk area is out for a portion of central Texas.
The remains of Barry aloft and resultant "new" 850 hPa cyclone are
parting ways in a sheared environment, with the 850 hPa low moving
west while the mid-level trough moves east. Normally the night
after this happens is the last night/subsequent morning of heavy
rain potential. Effective bulk shear due to the opposing low-level
and upper-level flow is sufficient for organized convection and
cells being stuck in place. There's an instability gradient near
29.5N latitude, seen on SPC mesoanalyses, that's creeping north and
likely to be the better focus for heavy rainfall this period as
low-level inflow intercepts it -- used this as the southern bound
of the risk areas. The retrograding upper low is also a possible
spot for such...used its track as the northern bound of the risk
area. Some retrograding convection is possible on its west side as
outflow boundaries try to instigate new convection on that side of
the 850 hPa circulation/trough. Precipitable water values remain
near and above 2.25". ML CAPE south of the instability gradient is
2000+ J/kg. Convective environments with this much moisture in
northeast shear since 2016 (August 2016 convective low in Louisiana
and Imelda in 2019) have favored the wetter mesoscale guidance, so
leaned a bit on the high- resolution NAM, HRRR, and RRFS. Hourly
rain amounts to 4", with local amounts to 8", appear the best bet.
This on its own is problematic, but the heavy rains could also fall
on already saturated ground. Coordination with the TX forecast
offices -- SJT/San Angelo, EWX/New Braunfels, and FWD/Fort Worth --
as well as the National Water Center led to the new Moderate Risk
area. As a side note, the remains of Barry are the 20th tropical
cyclone or remnant since 1913 to produce 15"+ across interior
portions of Texas.


...Carolinas/T.S. Chantal...
Much of the high resolution guidance for Chantal keeps the focus of
heavy rainfall into the early morning hours Sunday near the NC/SC
border. The guidance has had a westward bias thus far, and radar
trends show that what passes as a CDO/Central Dense Overcast keeps
moving north more towards NC. Both the HREF/RRFS insist on a
northwest turn with an occluded look overnight, which would be
something and not befit a tropical cyclone that's supposed to be in
a less sheared environment. Tightened up the western gradient of
the risk areas as a precaution -- the SC portion of the risk areas
remains the most uncertain.


...Midwest...
Trimmed the area per radar reflectivity trends and HREF output.
Activity should forward propagate and fade overnight.


...Northern High Plains...
A well-defined shortwave moving into the northern High Plains is
leading to widely scattered thunderstorms. Forecast PWs are
expected of 1 to 1.25 inches (1 to 1.5 std dev above normal) are
high enough to imply saturation in the High Plains, implying
rainfall efficiency. While a widespread heavy rainfall event is
not expected, heavy rainfall rates and some potential for
redeveloping storms may produce locally heavy amounts (greater than
an inch) and isolated runoff concerns.


...Southeast Arizona...
See Mesoscale Precipitation Discussion #598 for more details.


...Western Florida...
Pared back the Marginal Risk area to western FL per radar
reflectivity trends and 18z HREF output. Convection should fade
and/or move westward into the Gulf.

Roth


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt