FOUS30 KWBC 170814
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Thu Jul 17 2025

Day 1
Valid 12Z Thu Jul 17 2025 - 12Z Fri Jul 18 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE LOWER
ATCHAFALAYA BASIN IN SOUTH-CENTRAL LOUISIANA...

...Northern Gulf Coast...

A highly disorganized tropical disturbance continues tracking west
across the far northern Gulf this morning, with a center off the
coast off Mississippi as of the time of this writing. This 
disturbance is responsible for a larger area of thunderstorms well 
west of the center, moving across southern Louisiana. The storms 
will turn southward and move out into the Gulf through the morning.
More widespread, albeit widely scattered thunderstorm activity 
will develop today as the center moves into southeast Louisiana.
Much of the strongest activity is expected to focus west of the
center of circulation, but will push westward as it does so. 

Beyond ample atmospheric moisture is in place, with New Orleans
sounding measuring PWATs of 2.27 inches from the 00Z sounding.
Thus, while the storms ongoing over Louisiana are largely sticking
to rainfall rates under 1.5 inches per hour...with ample
instability developing with daytime heating today, any storms that
form will have more than enough instability and moisture available to
locally produce rainfall rates in excess of 3 inches per hour.
However, since the storms will be moving westward, any one location
will see rainfall rates that heavy for only a short time. With FFGs
near their average at 3.5 to 4 inches per hour over much of
Louisiana, it will take repeating rounds of storms to exceed FFGs
enough to induce significant flash flooding. This is likely to be a
high bar to overcome in most areas, as despite the ample moisture
and instability, the forcing associated with this weak, slanted,
and overall disorganized low is very lacking in most areas.

The Moderate Risk includes the Lafayette area and the Lower
Atchafalaya Basin in large part because of the V-shaped nature of
the river basin, heavy rain could funnel into flood prone areas
quickly if the heavy rain over the basin is persistent enough. In
coordination with LCH/Lake Charles, LA forecast office, the
Moderate Risk previously in the Day 2/Friday period was transferred
to today, as the likelihood for more organized convection across
Louisiana will be significantly greater today, resulting in a
greater coverage of heavier rain today than is now expected
tomorrow/Friday.

The surrounding Slight Risk area extends from the far western
Florida Panhandle west along the coast to the Texas border. Many of
the same thunderstorms impacting southeastern Louisiana will
continue west into southwestern Louisiana before dissipating,
resulting in the continuation of the Slight Risk. Any heavy rain
will have to contend with very high FFG values across all of the
Louisiana coast. Meanwhile further east from coastal Mississippi to
the far western Florida Panhandle, the combination of urban areas,
and southerly onshore flow off the Gulf will support a northward
convective motion, favoring training in these areas.

...Ozarks to the Ohio River Valley...

A stalled out front over this region acting as a barrier between
much drier air to the north over the Great Lakes and a hot and very
humid air mass from the Ohio Valley south will act as a focus for
eastward-moving complexes of showers and thunderstorms to develop
and push quickly east as new thunderstorms from to their west. This
will occur over a long line from the central Plains to the central
Appalachians this afternoon and tonight. Persistent southerly flow
of moisture-laden air will support numerous thunderstorms occurring
over these areas today as that southerly flow runs into the stalled
out front and turns eastward. While portions of the Slight Risk
area have seen more prior rains than others...during the day today
expect PWATs routinely over 2 inches to support thunderstorms
capable of up to 3 inch per hour rainfall rates. These heavy
rainfall rates will be offset by their fast movement, but given the
stationary front in place directing all the storms along the same
track...training is likely.

The Slight Risk area was expanded a bit into southeastern Virginia
and northwestern North Carolina with this update, in coordination
with RNK/Blacksburg, VA forecast office. Elsewhere few changes have
been made since the pattern remains steady state.

...Northeastern New Mexico and Adjacent Areas...

Only a very modest eastward shift was made to the previous Slight
Risk area, which is largely centered over northeastern New Mexico,
a southerly LLJ running into both terrain and a thermal low will
act as a focus for rather fast moving thunderstorms to track ESE
out of CO/NM and eventually into the OK/TX Panhandles. Like the
other higher risk areas, southerly flow of moisture will support
backbuilding and additional convective development behind the
initial round of storms. Training over flood-sensitive areas may
result in widely scattered instances of flash flooding this
afternoon into this evening.

Wegman


$$
--- SBBSecho 3.28-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)