Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
901 PM EDT Mon Jul 1 2024

Day 1
Valid 01Z Tue Jul 02 2024 - 12Z Tue Jul 02 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS
OF EASTERN NEBRASKA TO SOUTHWESTERN IOWA...

...Central/Northern Plains to Upper Midwest...

Convection has been growing upscale and will continue to do so 
into the evening across the northern half of the Plains. The most 
concerning area for flash flooding will be across parts of Nebraska
to Iowa, where the nocturnal LLJ of 50 knots will be maximized in 
a moist environment fed by both Gulf moisture and Pacific moisture 
rounding the mid-upper ridge, near a warm front. HRRR runs are 
explicitly showing 1.5-2"/hour rates, which would exceed the FFG, 
and other (albeit older) CAMs show higher total QPF. HREF 
probabilities for exceeding 3 and 6 hr FFG are greater than 50 
percent in some areas. Since the 18Z HREF is getting to be older 
available model data at this point, focused the upgraded/embedded 
Moderate Risk a bit on the southern side, where recent HRRR runs 
are focusing the best potential for some training/backbuilding. 
Refer to MPD 537 for more information on this area. Additional 
convection farther north through the night could cause additional 
flash flooding risks and the Slight Risk is maintained there in 
parts of the Dakotas to Minnesota. 

...Southeast...

A cold front slowly working its way south near the South 
Carolina/Georgia coast is pooling moisture along it for some 
ongoing convection. The risk level overall has dropped since this 
afternoon, prompting removal of the Moderate Risk but maintained a 
Slight for slow-moving storms. The Savannah, Georgia area is likely
the most vulnerable to additional localized flooding concerns. 

...Southwest...

Monsoonal moisture with PWs over the 95th percentile/2-3 standard 
deviations above normal in the Four Corners states is causing 
ongoing convection that has prompted several Flash Flood Warnings. 
Storms will persist further into the evening and a Marginal Risk 
remains in place.

Tate


Day 2 
Valid 12Z Tue Jul 02 2024 - 12Z Wed Jul 03 2024 ...A SLIGHT 
RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST TO 
CENTRAL GREAT PLAINS...

20z Update:
Still on track for a heavy rainfall event over portions of MO, IA,
WI and IL Tuesday afternoon into the overnight hours. Plenty of
synoptic forcing with a mid level trough and right entrance of an
upper jet moving across the region. Organized convective
development is likely, and with PWs over 2", heavy rainfall rates
are expected. There does appear to be some training/backbuilding
potential, with low level flow parallel and nearly equal to the
deep layer flow resulting in weak Corfidi Vectors. Do anticipate
some cold pool forward propagation off to the southeast with time,
which may end up limiting the duration of heaviest rain. Although
that cold pool could run out of momentum somewhere over northern
MO, which may enhance the flash flood risk.

We did give some consideration to a MDT risk upgrade, but opted to
remain at a higher end Slight risk. Uncertainty on the extent of
convective training, model spread, and the fact that this region is
not as saturated as areas further north...all led to maintaining
the high end Slight and not upgrading at this time. We will
continue to monitor high res model trends.

Chenard

...Previous Discussion...

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

A reinforcing shortwave trough rounding the upper trough that will
be over the Dakotas Tuesday morning crosses the Neb/SD border
through the day. Right entrance jet streak dynamics warrants
maintenance of ongoing activity over the central Plains, IA, and
central WI that then drifts south/southeast through the day that
spreads No big changes were made with this update. Concerns remain
from the KC metro and northeast over an expansive area to western
IL and up through much of WI as gulf-sourced moisture (PWs 2 to 2.5
sigma above normal) with deep layer SW flow allowing training
thunderstorm concerns. A Moderate Risk may be needed somewhere
between KC and Madison WI, but for now consider this a higher end
Slight Risk.

...Southeast...

A Marginal Risk is maintained over the Florida Panhandle and
southern GA. This will be a continuation of the convection from
Monday night ahead of a slow moving cold front. Rainfall still
expected to be lower than on Monday, but the plentiful Gulf
moisture allows for the potential for very high rainfall rates.

...Southwest...

Again maintained a Marginal over eastern AZ/western NM with a
suppressed Monsoonal flow day as troughing to the north shunts the
ridge axis farther east to the lower Miss Valley. Elevated moisture
is limited to the southern portions of those states and remnant
stratus from Monday night activity may further suppress Tuesday
activity.

Jackson


Day 3
Valid 12Z Wed Jul 03 2024 - 12Z Thu Jul 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS AND MID-MISSISSIPPI VALLEY...

20z Update:
Only minor changes made with this update. Still on track for a
heavy rainfall threat over portions of eastern KS into much of MO
and into southern IL. A stalling front and PWs over 2" should set
the stage for potential flash flood issues. Forcing is a bit weaker
to start the period, which raises some questions to the coverage
and organization of convection. By Wednesday night we should see
better forcing approach from the west and a potential uptick in low
level moisture transport into the boundary. The ingredients appear
to be in place for upscale convective growth Wednesday night over
this corridor with some training potential. Thus this is a solid
higher end Slight risk with at least isolated to scattered flash
flooding appearing probable. However still some questions as to the
coverage and magnitude of this event, and current model QPFs are
not really supportive of any higher risk at this time. So will
maintain a Slight and continue to monitor.

Chenard

...Previous Discussion...

...Midwest to Great Plains...

A cold front from a passing trough over the northern tier of states
stalls over KS/MO by Wednesday morning, providing continued focus
for heavy thunderstorm development from eastern KS through southern
IL where a Slight Risk is maintained.

The frontal zone extends east over the rest of the Midwest to the
eastern Great Lakes where a Marginal Risk is maintained with some
areas of the Northeast trimmed with the 00Z QPF consensus being
farther west.

Over the northern Plains, the next trough axis enters the northern
High Plains Wednesday night with activity over the Dakotas
warranting a Marginal Risk.


...Southwest...

A trough axis from the next reinforcing wave allows anomalous
moisture over AZ/NM to spread northeast to the southern High Plains
on Wednesday which warrants maintenance of the Marginal Risk with
some trimming based on the 00Z consensus.


Jackson


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