FOUS30 KWBC 040855
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
455 AM EDT Thu Jul 4 2024

Day 1
Valid 12Z Thu Jul 04 2024 - 12Z Fri Jul 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MISSISSIPPI VALLEY...

...Oklahoma, Missouri and into southern Indiana...
Organized convection will likely be ongoing at 12z this morning
across portions of central MO and southern IL...moving into
southern IN through the morning hours. Some west to east training
of convection is expected to be ongoing at 12z, resulting in a
flash flood risk through the morning hours. By late morning 
activity is expected to become a bit more progressive and linear in
nature, which should result in a gradual decline in the flash 
flood threat.

Additional convective development is likely by later in the
afternoon along the cold front from eastern OK into southern MO.
While the bulk of this activity should remain progressive in
nature...there may be an opportunity for some training across
southern MO into southern IL where cell motions could be more 
parallel to the frontal zone. The bulk of this second round should 
stay just south of the first round that is occurring Wednesday 
night into Thursday morning...but there may be some overlap over 
portions of MO and IL. This combined with some training potential 
suggests an isolated to scattered flash flood risk likely exists 
between ~00-09z tonight along this corridor. The flood risk is 
probably a bit lower over eastern OK...as the combination of more 
progressive cells and higher FFG suggests less of a threat. However
there is some mid and upper forcing that hangs back over this 
area, suggesting some chance for multiple rounds of convection to 
move across. Thus not confident enough to remove the Slight risk 
and so will let it ride over this area as well.

...South Dakota into Wisconsin...
A swath of heavy rainfall is likely today across portions of
eastern SD into central/southern MN, northern IA and WI. This will
be a very dynamic system with strong large scale forcing and lower
level convergence near a stationary front. Moisture and 
instability are more marginal, as being a northern stream dominant
system we do not really have a deep moisture connection from the 
sub tropics. Nonetheless, high res models do suggest instability 
could approach 2000 J/kg near the front from northern IA into 
southern MN into WI...and PWs do increase towards and even above 
1.5". Thus while rainfall efficiency will not be as high as it 
could be...still plenty of instability and moisture to drive some 
heavy rainfall rates of 1-2"/hr (as indicated by most high res 
models).

Given the strong forcing in place...likely to have rather
widespread coverage of showers and thunderstorms...although the 
exact convective evolution remains uncertain. HREF EAS probabilities
of 1" over the 24hr period are above 50%....which is indicative of
the large 1" QPF footprint that is expected. However 2" EAS 
probabilities drop into the 10-20% range suggesting much lower 
coverage of 2"+ amounts. Neighborhood probabilities of 3" are 
around 50% though...so combining these two probabilistic fields 
indicate an event where higher end (2-4" amounts are probable, but
the coverage of such amounts should stay pretty isolated in 
nature. Overall this is a solid Slight risk event...and given the 
saturated conditions over this area...the event is probably getting
close to higher end Slight risk territory. The aforementioned 
limited coverage keeps this below MDT risk levels...but with 
widespread 1-2" and isolated 2-4" amounts expected over sensitive 
areas...do expect that isolated to scattered flash flooding will 
be a concern today.

...Mid-Atlantic...
A broad Marginal risk extends from the OH Valley into the Mid- 
Atlantic. PWs are expected to be quite high today across the
region...in fact we may be looking at 95th-99th percentile PWs 
across portions of the region. Forcing to help initiate an 
organize convection is more questionable. Generally lacking a 
pronounced forcing mechanism, although do note some weak mid level 
height falls and we will have a dissipating cold front moving into 
the region. Most HREF members show at least isolated to scattered 
convective development, although there is not great spatial 
agreement amongst the models. The forecast instability and moisture
support heavy rainfall rates, but it remains unclear on if/where 
any organization to convection may drive a greater flash flood 
threat. Thus for now a broad Marginal risk should suffice, with 
environmental ingredients supporting an intense rainfall rate 
driven flash flood risk...but expected coverage remaining below 
Slight risk thresholds at this time. 

Chenard

Day 2
Valid 12Z Fri Jul 05 2024 - 12Z Sat Jul 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
SOUTHERN AND EASTERN U.S....

At least scattered convective development is likely along and ahead
of a cold front stretching from TX into New England. PWs are 
forecast at or above the climatological 90th percentile over most 
of this frontal zone...with upwards of 2000 j/kg of CAPE possible 
along the extent of the front. Enough mid level troughing and 
upper jet support to drive decent convective coverage as well. So 
looking just simply at convective coverage, instability and PWs, 
would support localized areas of heavy rainfall and flash flooding.
This assessment is also supported by the CSU machine learning ERO 
which depicts a broad Marginal risk. Not seeing much in the model 
QPF fields or environmental fields to really indicate where/if any 
more organized and higher end rainfall event could occur. Instead 
indications are that flash flood concerns generally stay isolated 
in nature...confined to where heavy rates overlap an urban or other
sensitive area...or where cell mergers result in a locally 
increased duration of higher rainfall rates. Would not be surprised
if somewhere within this broad risk sees Slight risk type
coverage/impacts...but at this lead time we do not have the 
confidence to pinpoint exactly where.

Eastern NM should also see an uptick in convective coverage 
Friday, with post-frontal upslope flow helping aid in development. 
This is also a region where a focused Slight risk may eventually 
need to be considered.

Chenard

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
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