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FOUS30 KWBC 022020
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Tue Jul 2 2024

Day 1
Valid 16Z Tue Jul 02 2024 - 12Z Wed Jul 03 2024

...THERE IS A SLIGHT RISK FROM PORTIONS OF THE UPPER MIDWEST TO THE
PLAINS...

...Upper Midwest through Central Great Plains...

19Z UPDATE: In collaboration with WFOs in the region, a Moderate
Risk has been issued, primarily over southern and central Iowa.
This is primarily driven by increased confidence in extremely high
rain rates. There continues to be relatively lower confidence in a
training configuration to convection that would sustain those rain
rates for more than an hour or two (although that cannot be ruled
out, and would lead to localized much higher totals). However, the
combination of very strong instability, with CAPE in excess of
2000 j/kg, and PWs near or in excess of 2 inches should support
rain rates peaking in the 2-3 inch per hour range in the strongest
convection. This would be enough to cause flash flooding, and the
Moderate Risk essentially covers the area where those rates are
most likely. Latest runs of the experimental WoFS model back this
up with 90th percentile hourly rates peaking in the 2-3 inch range,
and 90th percentile totals peaking above 5 inches in south-central
Iowa. This would be likely to lead to flash flooding.

PREVIOUS DISCUSSION:
Hi-res models, even the new 12Z runs, do not appear to have a great
handle on the rather dense cloud cover, and band of rain and
embedded elevated convection, that stretches from LNK-DSM-MKE.
Overall, they are far too quick to diminish this in favor of rapid
destabilization and a more organized round of convection this
afternoon. Therefore, it is a real possibility that they are
overestimating both rain rates and total rainfall on the northwest
and northern periphery of the Slight Risk area from northern Iowa
into central Wisconsin. Nevertheless, given recent reinvigoration
of some of the elevated convection, especially in southeast
Nebraska, and out of deference to the previous outlook, the Slight
Risk was largely maintained. Areas further north will generally be
more sensitive to rainfall impacts given the recent wet pattern.
However, the risk areas were reduced from central Wisconsin into
northern Michigan, where even elevated instability seems likely to
be far more limited.

Therefore that draws attention slightly further south, particularly
in southern Iowa and northern Missouri, but also adjacent portions
of northeast Kansas and western Illinois. Even though hi-res models
have struggled with ongoing clouds and rain, they have increasingly
focused the rainfall in this area. HREF EAS probabilities, which
better assess model agreement on placement of heavy rainfall, of 3
inches of rainfall are maximized in the IA-MO border region. A
limiting factor may be forward progression of convective lines and
clusters, which is suggested by most hi-res models. However, there
will be some potential for backbuilding with southwesterly low-
level inflow and an upstream reservoir of strong instability in the
central Plains. Even if convection earlier in the afternoon is more
progressive, the resulting outflow may set up a boundary more
favorably oriented for backbuilding and training, and this may be
situated in the southern portion of the current Slight Risk area in
the evening or overnight, perhaps down to Kansas City or into
central Missouri. Precipitable water values near or in excess of 2
inches and strong instability would support very high rain rates in
excess of 2 inches per hour in organized convection.

...Southeast...

The only major change to the outlook was to extend the Marginal
Risk down further into the western Florida Peninsula including the
Tampa metro area. The overall environment will be similar to areas
further north that were already in the risk area (Florida Panhandle
into south Georgia), with PWs in the 2.1 to 2.3 inch range (above
the 90th percentile) and strong instability supportive of intense
rain rates. This has already been observed this morning just south
of Savannah, GA with a slow-moving storm estimated to produce 4
inch per hour rain rates on MRMS. The key for any flash flood
potential would be a storm that is moving slow enough or any storm
mergers or brief training along sea breeze boundaries.

...Southwest...

The Marginal Risk was trimmed out of far northeast New Mexico and
the Texas Panhandle where the QPF signal on the latest guidance was
much weaker, and there may be a bit of subsidence in the wake of an
MCV evident on GOES-16 visible satellite loops. However, the
Marginal Risk was maintained over a broad portion of eastern
Arizona, and southern and central New Mexico with another day of
monsoonal convection expected. A Slight Risk was considered for
south-central New Mexico where there appears to be a boundary that
showed up briefly in enhanced west-east oriented cloud cover, and
that coincided with confluent 700mb flow on the RAP analysis. For
now, we held off, but that area will be monitored for a possible
targeted upgrade early this afternoon if convective trends warrant.

Lamers

Day 2
Valid 12Z Wed Jul 03 2024 - 12Z Thu Jul 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDWEST TO THE GREAT PLAINS...

...Midwest to Great Plains...

A cold front from a passing trough over the northern tier of states
will stall in the vicinity of Kansas and Missouri Wednesday,
providing focus for renewed thunderstorm development from eastern
Kansas and southern and central Missouri eastward into the Lower
Ohio Valley. The primary change with this outlook is to shift the
Slight Risk area southward about 60 miles and extend it east into
western Kentucky and southern Indiana to better align with the 12Z
hi-res guidance. The combination of strong instability (CAPE in
excess of 2000 j/kg) and PWs in excess of 2 inches (above the 95th
percentile) should support organized convection with rain rates
exceeding 2 inches per hour at times. Veering low-level winds and
increasingly unidirectional westerly flow could support training
and backbuilding along a generally west-east oriented boundary.

Elsewhere the Marginal Risk areas were generally maintained in the
Upper Ohio Valley and Northern Plains where low chances of heavy
rain rates and flash flooding persist.

...Southwest...

As an anomalously strong upper ridge continues to build into the
region from the Pacific coast, the overall convective pattern is
likely to be suppressed somewhat from previous days, and
concentrated increasingly east toward higher terrain and the
Continental Divide. A Marginal Risk was maintained, but was trimmed
further from the previous issuance into areas where hi-res models
indicate the greatest potential for organized convection.

Lamers/Bann

Day 3
Valid 12Z Thu Jul 04 2024 - 12Z Fri Jul 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL US...

...Upper Midwest to Mid Mississippi Valley...

Mid and upper level height falls across the Upper Midwest, in a
region with anomalously rich moisture, should result in increasing
chances of showers and thunderstorms on Thursday and Thursday
night. The rain threat will be organized by a developing cyclone
and inverted trough axis to the north. There has been a multi-run
shift over the past 24-48 hours to the south with the cyclone, and
QPF guidance has shifted south as well. It remains to be seen
whether this trend will continue or not. Given a lack of
convergence on the specific location of heavy rainfall thus far,
the Slight Risk has been maintained, just shifted to the south.
Another key uncertainty is the level of instability. Models are
only showing modest instability, generally with CAPE near or below
1000 j/kg, and PWs are not that anomalous, with the primary plume
of high PW air pushed south by a cold front in previous days.
Therefore, the threat of flash flooding in this case would likely
come from two key factors: (1) sustaining moderate to occasionally
high rain rates for longer durations (3-6 hours) and (2) the
unusually wet ground conditions across the region increasing
vulnerability. Therefore, pinpointing confidence in placement of
the low-level cyclone and associated deformation axis will be
important for any future shifts in the Slight Risk.

...Southern Plains and Mid-Mississippi Valley into the Ohio Valley
and Mid Atlantic States...

The plume of deeper and more anomalous moisture will be situated
further to the south, in the vicinity of the stalled front. PWs are
likely to generally exceed the 90th percentile along the entire
front from OK and KS eastward to the Mid-Atlantic coast. The
Marginal Risk was generally maintained in this area, and expanded
slightly in the Plains and Mid-Atlantic. No major changes were made
at this time, as corridors of greater chances of flash flooding
would be very dependent on convective evolution from the previous
day (current Day 2 period). However, where sufficient instability
exists, moisture levels will be high enough to support rain rates
approaching or exceeding 2 inches per hour in the most organized
thunderstorms and this may lead to flash flooding.

Lamers/Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt