Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
455 AM EDT Thu Jul 4 2024
Day 3
Valid 12Z Sat Jul 06 2024 - 12Z Sun Jul 07 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN AND EASTERN U.S. AND OVER THE CENTRAL PLAINS TO TO
THE MID/UPPER MISSISSIPPI VALLEY...

The front mentioned in the day 2 discussion shifts off to the
southeast on Saturday. Would again expect to see at least
scattered convective development along the boundary from TX into
the eastern Mid-Atlantic. There is a bit more uncertainty on the
location of the front by this time...but have noted a bit of a
quicker trend in the newer models/ensembles. This would push the
threat closer to the Gulf Coast on the southern extent, and closer
to the East Coast on the eastern side. It is possible that an
isolated risk could exist over New England, but there is even more
uncertainty on the timing of the front and thus location of better
instability and moisture...so opted not to include these areas in
the Marginal at this time. Direct impacts from Beryl should not
impact the country through Saturday night...however some moisture
ahead of the system could start to interact with the frontal
boundary over TX. Thus will need to keep an eye on the potential
for an enhancement of convection and QPF along the southwestern
flank of this front.

A Marginal risk also extends from portions of NE into southern MN.
Broad troughing remains over the central U.S, and guidance
indicates a mid level shortwave should be diving south within the
long wave trough. There is also a response in the upper jet, and
overall large scale synoptic support for ascent looks pretty good
by Saturday night. Not surprising that we also see a low level
response to this increased forcing, with 850mb moisture transport
seeing a notable uptick. Moisture looks to be a limiting factor,
as still not seeing a great deep layer moisture connection, and so
PWs are only forecast around average levels. Absent wet antecedent
conditions this setup may not warrant any risk at this
point...however with much of this part of the country well above
average for rainfall of late...any organized convective setup
likely poses at least some flash flood risk. And despite only
modest moisture...the strong forcing should make up for this and
allow for some potentially heavy rainfall totals. The GFS is
displaced a bit north of the better model clustering seen in the
ECMWF/CMC/UKMET...so relied more on these latter models for the
placement of the Marginal risk. Some adjustments are likely as the
event nears.

Chenard

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt