FOUS30 KWBC 270828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 27 2024

Day 1
Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TEXAS GULF COAST...

...Texas Coast to Southeast US...

Maintained the Slight Risk area with onshore southerly flow of 
deep tropical moisture off the Gulf set to bring another day of 
scattered convection capable of producing heavy rain over an area
where flash flood guidance has been lowered by several days of
moderate to heavy rainfall...with the expectation that the 
convective organization should be less today compared with previous
days.

Elsewhere, only modest adjustments were made to the expansive 
Marginal Risks surrounding the Slight Risk area and eastward to the
mid-Mississippi Valley given persistent south to southeasterly 
flow on the east side of an upper level trough/closed low. The
trough/upper low could lead to enough upper support to focus and
sustain storms that produce localized downpours...but even 
convective initiation along local outflow boundaries will be 
forming in an environment supportive of downpours from the 
Mississippi Valley into parts of the Southeast U.S...so isolated 
instances of multi- inch rainfall totals are quite possible.

...Southwest United States...
Signals from the 27/00Z suite of numerical guidance continues to
show decreasing threat of excessive rainfall as mid level flow
increases across the Intermountain region. The exception remains
over parts of Arizona and New Mexico where enough
moisture/instability linger in a region with meager southwesterly
steering flow...so will be maintain at Marginal risk area.

...Upper Midwest...
Maintained the Marginal Risk area mainly over northern Minnesota as
given the presence of increasing southwesterly low level flow 
feeding an airmass with precipitable water values locally in 
excess of 1.75 inches ahead of a cold front making its way across 
the area. Given spread in forward speed of the front...expanded the
Marginal Risk area but largely maintained continuity in terms of 
the area covered.

Bann


Day 2
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST...

...Northern Minnesota...

Rain associated with a cold front moving across the northern tier
of the country should be ongoing across northern Minnesota. While
there is likely to diminish in intensity for a period early in the
period...there should be a renewed risk of excessive rainfall later
in the day and evening. With precipitable water values increasing
to 1.75 inches or greater continuing to be fed into the region by
south to southwesterly 850 mb flow with divergence aloft being
provided by the right entrance region of an upper level
jet...the 27/00Z HREF neighborhood probability values of 10 to 15
percent for 2+ inch amounts in an hour seem pretty reasonable. 
Assuming that the GFS was too fast with the forward speed of the 
front during the period...he risk of training cells and repeat 
convection are additional concerns for excessive rainfall in 
addition to the intense rainfall rates. 

...Tennessee Valley and Surrounding Areas...

A stationary front turned warm front will track northeastward up
the Tennessee Valley through late Sunday night/early Sunday morning.
Increasing atmospheric moisture associated with the advance of an 
airmass originating off the Gulf will allow for potential training 
and backbuilding of strong storms capable of increasingly heavy 
rainfall. The surface warm front will likely slow as it approaches 
the southern Appalachians...supporting slower moving storms that 
with interactions with the terrain will be capable of flash 
flooding. With very efficient warm rain processes at
work...multiple inch per hour rainfall rates . Urban areas are at 
a higher risk for localized flash flooding with any repeating 
storms.

...Southwest US...
The risk of excessive rainfall continues to get less conducive with
time and gets shunted eastward as mid-level westerly flow
strengthens over much of the West outside of Arizona and New
Mexico. Maintained a Marginal Risk here given the lingering
moisture.

Bann


Day 3
Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER
THE TENNESSEE VALLEY...

...Southeast US...
Focus for excessive rainfall shifts eastward and is mainly focused
in the central and eastern portion of the Tennessee Valley as mid
level height falls approach from the west and precipitable water
values at or above 1.75 inches will already be in place. While the
height falls are associated with a trough that was beginning to
fill...the diffluent flow located above an instability axis should
be enough to support downpours from slow moving storms in the
afternoon and evening. Elsewhere in the Southeast US...the forcing
remains weak and nearly impossible to time with any accuracy at
this range but localized downpours from any convection that can
develop and was covered by an expansive Marginal Risk area.

...Upper Midwest to far Western Great Lakes...
The same cold front helping to focus heavy to potentially excessive
rainfall on Sunday/Sunday night will continue to make its way
eastward on Monday. The pattern becomes somewhat less favorable for
heavy to excessive rainfall but the potential for some isolated
enhanced amounts linger...especially from far northern Minnesota
into the western Great Lakes where a Marginal Risk was placed.

...Southwest US...
Steering flow in the mid and upper levels begin to weaken as 
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert)...meaning
a slight westward expansion in the Marginal risk area over 
portions of Arizona and New Mexico compared with the previous 
couple of days.


Bann
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