FOUS30 KWBC 270841
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Tue Aug 27 2024

Day 1
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHWEST AND UPPER MIDWEST...

...Summary...
Compared to yesterday's Day 2 ERO, we have expanded the Marginal
Risk area to include much of the Upper to Mid MS Valley, along with
parts of the Lower MO Valley. Only minor tweaks were made to the
Marginal Risk area across the Southwest.

...Lower Missouri Valley-Mid to Upper Mississippi Valley-Upper
Great Lakes...
Upper shortwave over the Northern Plains early this morning will
continue to dampen somewhat during the day, which in term will
weaken the deep-layer Qs forcing. By the same token, as the
shortwave becomes sheared, the confluence aloft will sharpen a
compact upper level jet streak (90-100 kts) in the lee of the
flattening shortwave, thereby increasing the Qn forcing within the
right entrance region (strengthening the low-level frontogenesis).
The guidance, including the CAMs, appears to be having a tough time
resolving this transition of favorable dynamical forcing. The
ensemble of model QPFs show a bimodal distribution with the max
rainfall axes -- one to the north (central MN-northern WI-Upper
MI), behind Monday night MCS, where the DPVA/Qs forcing is most
favorable, and another farther south (northeast KS/southeast NE,
southern IA, central-northern MO, much of IL-IN), where the uptick
in right entrance region forcing (upper divergence and low-level
frontogenesis) coincides with a more moist and unstable
environment. TPWs climbing between 1.8-2.1 inches over this
southern portion, along with ML CAPEs of 1500-2500+ J/Kg, will
support locally intense shorter-term rainfall rates. Moreover, as
the front flattens (becomes more W-E oriented), more parallel to
the mean 850-300 mb flow, suspect cell training will become a
greater risk. What will likely keep this event from needing a
Slight Risk is the relatively weak low-level inflow. The mean flow
is fairly weak as well (850-300 mb wind 15-20 kts), however there
is sufficient shear to keep any organized segments forward
propagating.

Farther north, the instability won't be as robust, however the
soils will have already been primed from the MCS Monday night.
Therefore with this secondary max QPF axis that the 00Z CAMs are
depicting, another round of heavy rainfall could lead to isolated
flash flooding given the antecedent wet soils and reduced FFG.

...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon
and evening across portions of the Southwest U.S., as the moisture
plume lingers across the area and winds aloft/steering flow
weakens in the wake of the departing Northern Plains shortwave. PWs
peaking around 1.3 to 1.4 inches (1.5 to 2 standard deviations
above normal for late August), along with MLCAPEs of 500-1000 J/Kg,
will support isolated hourly rain rates of 0.75-1.00" underneath
the strongest convective cores. The 00Z HREF probabilistic
guidance depicts this, with the risk spreading a bit farther west
(into eastern AZ) compared to yesterday. Have maintained the
Marginal Risk in the Day 1 ERO given the continued localized flash
flood threat during the peak heating hours (afternoon/evening).
evening.

Hurley


Day 2
Valid 12Z Wed Aug 28 2024 - 12Z Thu Aug 29 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY, AND
PARTS OF THE MIDWEST INTO THE OHIO VALLEY AND NORTHERN MID-ATLANTIC...

...Summary...
Compared to yesterday's Day 3 ERO, the Southwest and northern-tier
Marginal Risk areas were largely unchanged. However, based on the
00Z guidance suite, the Marginal Risk across the northern Mid
Atlantic Region was expanded westward to include much of the OH
Valley and parts of the Mid MS Valley.

...Southwest...
Guidance mid-level vorticity progs continue to support a more
north-south outlook area compared to Day 1, though nonetheless
still quite similar. Instability and moisture parameters (afternoon
and evening CAPE and PW profiles) remain similar, and thus the
anticipation of more isolated/localized instances of flash flooding
than otherwise, particularly after 18Z coinciding with peak
diurnal heating.

...Upper Midwest...
Strong, deep-layer warm and moisture advection ahead of the next
vigorous mid-upper level trough (closed 500-300 mb low) and
associated surface fronts will favor more organized/widespread
elevated convection by Wed night ahead of the advancing low-level
warm front. Models are still indicating spread with the QPFs,
enough to keep the ERO risk level at Marginal for now. Later
shifts will need to assess the need to upgrade at least a portion
of the outlook area to Slight however, as the heavy rainfall signal
exists given the degree of dynamical forcing and thermodynamical
profiles. 00Z ECMWF and regional GEM in fact indicates localized
totals of 3+ inches between 00-12Z Thu.

...Eastern Portions of the Mid Mississippi Valley-Ohio Valley-
Northern Mid Atlantic...
W-E frontal boundary will sink southward on Wednesday, then remain
quasi-stationary Wed night as the mid- upper trough traverses the
central-northern Rockies and High Plains. Deep-layer moisture and
instability will be sufficient for areas of heavy rainfall along
this boundary, while the easterly storm motions (parallel to the
low-mid level fronts) would support the risk of cell training.
Outside of the 00Z Regional GEM, the guidance at this point isn't
overly robust with the QPF within the outlook area, at least not
yet, despite some uptick in right- entrance region forcing from the
upper level jet streak over northern NY-New England. Thus for now,
have kept the ERO risk at Marginal.

Hurley


Day 3
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
CENTRAL PLAINS INTO THE UPPER MIDWEST, ALONG WITH THE MID ATLANTIC REGION...

...Summary...
Few changes were made to yesterday's Day 4 ERO Marginal Risk areas
across the Central Plains to Upper Midwest and over the Mid
Atlantic.

...Eastern Portions of the Central Plains into the Upper Midwest...
Aforementioned upper trough and associated surface cold front will
continue eastward on Thu-Thu night, though with the mid-upper low
lifting north into western Ontario by 12Z Fri. This will keep the
bulk of the deep-layer forcing north of the U.S., however
sufficient low-level frontogenesis will persist ahead of the mid-
upper shortwave well south of the main mid-upper low. Much
cooler/drier post-frontal airmass will lead to a fairly tight
moisture gradient along the front, with strengthening negative PW
advection behind it. Given the anticipated swift movement of the
front (strong deep-layer shear), for now it would appear any
organized, more widespread linear convective segments ahead of the
front will too be rather progressive. Therefore, have maintained
the Marginal Risk area.

...Mid Atlantic Region...
Lingering quasi-stationary boundary across the Mid Atlantic will
couple with increasing right entrance region forcing via an upper
level jet streak traversing NY-New England. Over time, the CAD
signature becomes evident from the guidance -- i.e. cooler/more
stable low-level air and a south-southwest dip in the surface 
boundary occur along and near the Mid Atlantic Coast. However 
plenty of deep-layer instability should linger west of the 
DelMarVa, which along with the forcing, should allow for more
widespread/organized convection and a localized flash flood threat.

Hurley
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