FOUS30 KWBC 310828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Wed Jul 31 2024

Day 1
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN
PLAINS TO THE MIDWEST AS WELL AS PORTIONS THE SOUTHWEST US AND NEW ENGLAND...

...NORTHERN PLAINS THE MID-WEST...
Convection should be on-going across portions of Iowa at the start
of the Day 1 period at 12Z...with some risk of excessive rainfall
lingering early on. Renewed convection is expected later in the day
as stronger forcing moves across the Plains later today. WPC
favored the more southerly deterministic QPF solution offered by
the HREF...and that was reflected in expanding the previously-
issued Slight Risk area a bit southward in portions of Minnesota
and Iowa. Farther south...a large anomaly of 850 to 700 mb 
moisture flux remains in place. With the HREF depicting an axis of 
1 to nearly 1.5 inches of rain possible falling over lowered flash 
flood guidance values in southeast Kentucky...felt confident enough
to agree extending the Slight Risk south and east into a region 
where the deterministic QPF from GFS/ECMWF/UKMET were meager. 
There is still a lot of model qpf spread with some typical latitude
differences with the max qpf axis.

...Northeast US...
Height falls with a wave approaching from the west will be 
spreading over portions of New York into New England during the
day...accompanied by a risk of moderate to locally heavy rainfall 
given the abundant moisture already in place. Maintained the Slight
Risk without change given its placement over an area soaked by 4 
to 8 inch amounts within the past 24 hours...making the area 
especially vulnerable to any additional rainfall and locally 
significant flash flooding remains a possibility.

...Southwest US...
Maintained the Slight Risk across portions of Arizona that was
introduced on Tuesday. Models continue to show an uptick in both 
moisture and instability compared with the past couple of days,
with satellite supporting the model indications of a weak wave 
moving in from the southeast. This combination of increased forcing
and better thermodynamic environment should support greater 
convective coverage with locally heavy rainfall rates. HREF 
probabilities are elevated as well...with 2"+ neighborhood 
probabilities of 40-60% and 1" EAS probabilities of 5-15" 
indicating at least some ensemble agreement on location/coverage 
of convection.

Bann


Day 2
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY...

The suite of numerical guidance from 31/00Z continued to support a
Slight risk area over portions of the Ohio Valley from Thursday
into Thursday night as fairly impressive forcing approaches the
region. Low level moisture should be place as the closed mid-level
low drops in from the north...helping to focus moisture flux
convergence along a quasi-stationary front in the area at the same
time that divergence aloft increases in response to the presence of
an upper level speed maximum. Given uncertainty of where the low
will be based on the model spread and where the strongest forcing
gets directed introduces plenty of uncertainty with regard to
placement of heavier rainfall. As stated earlier...the ingredients
still suggest a Slight risk area is warranted and the WPC placement
remains south of global QPF consensus...in fairly close proximity
to the UFVS machine learning ERO.

...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest during day 2...helping push the axis of above average
precipitable water values farther west into southern to central 
California and southern Nevada. This will foster an environment in
which scattered diurnal convection will be expanding into portions
of central and southern California/southern Nevada. 

Bann


Day 3
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO/TENNESSEE VALLEYS AND IN THE SOUTHWEST US...

...Ohio and Tennessee Valleys...
A closed low initially near the Great Lakes at the start of the Day
3 period will continue to sage south and eastward into the Great
Lakes region. Given plentiful moisture in place and increasing
cyclonic circulation aloft...localized downpours are in the realm
of possibility. One camp of thought in the models is that higher
amounts will be tied to the upper dynamics early on in the period
while the NAM seems to champion the idea of higher amounts along
the leading edge of the height falls closer to the better
instability. With the spread shown...went with a broad and
unfocused Marginal risk area for the time being.

...Southwest US...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
international border...another round of late day/evening showers
and thunderstorms is expected. Some northward/eastward expansion
compared with the previous couple of days is expected.

Bann
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