FOUS30 KWBC 150820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Mon Jul 15 2024

Day 1
Valid 12Z Mon Jul 15 2024 - 12Z Tue Jul 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST...

...Mid Mississippi Valley to the Northeast...
Ongoing convection over the northern Plains is forecast to 
weaken as it drops southeast across the upper into the mid 
Mississippi Valley later this morning. However, while differing on 
the details, most of the hi-res guidance shows redevelopment 
during the afternoon as mid-level energy sliding southeast into the
region begins to interact with deepening moisture (PWs ~2.25 
inches) and increasing instability supported by significant 
southwesterly low-level inflow. Confidence in the details is 
limited by the notable spread in the guidance in how this 
convection will evolve. However, despite differences in timing, 
most hi-res guidance members show a period of training convection 
along a slow-moving boundary, producing locally heavy amounts. A 
Slight Risk was maintained from eastern Iowa to northern Indiana 
and southwest Michigan. This reflects the area where the 00Z HREF 
continues to show higher neighborhood probabilities for 
accumulations of 2 inches or more. Based on the trends of the 
deterministic runs, believe the greater threat for heavy 
accumulations and flash flooding concerns will center in the 
western portion of the Slight Risk area, in the vicinity of 
southeastern Iowa into central Illinois, where the models show the 
stronger signal for training storms.

Further east, the shortwave associated with ongoing convection
moving through the southern Great Lakes is forecast to track
northeast, through the eastern Great Lakes into the St Lawrence 
Valley. Storms, redeveloping and intensifying with the return 
daytime heating, should be overall fast-moving. However, brief 
heavy downpours may pose isolated runoff concerns, especially 
across urbanized areas. 

...Southwest...
Monsoon moisture will fuel another day of afternoon showers and
thunderstorms, with isolated flash flooding remaining a concern
from southeastern Arizona and southwestern New Mexico into the
Mogollon Rim. Models have been underplaying the convection
development further to the west into southern California. With
little change in pattern and environment expected, a Marginal Risk
was also added across the Peninsular into the southern Transverse
ranges.

Pereira

Day 2 
Valid 12Z Tue Jul 16 2024 - 12Z Wed Jul 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID MISSISSIPPI AND OHIO VALLEYS...

...Central Plains to the Northeast...
As a broad upper trough begins to amplify over the Great Lakes, the
associated surface boundary and accompanying moisture pool will
become a focus for showers and storms and the potential for heavy
rainfall amounts. Southwesterly inflow into the front will support
PWs climbing upwards of 2 inches from the mid Mississippi Valley
eastward into the Ohio Valley. This is also where the guidance
shows the best mid-to-upper level forcing, raising the potential
for west-to-east training storms and heavy accumulations. The
general model consensus shows the greater threat has shifted a 
little further south and east, but is overall close to the previous
axis. Therefore, a Slight Risk was maintained from central 
Missouri eastward to far western Pennsylvania. Some of the 
deterministic guidance do show amounts that would raise Moderate 
Risk concerns. However, given the model spread, opted not to 
include any upgrades at this point.

While the model consensus indicates the greater threat for
widespread moderate to heavy amounts is further east, there is a
notable signal for locally heavy amounts developing over the
central High Plains, especially over eastern Colorado and 
western Kansas. Mid-level energy spilling off the top of the ridge
and interacting with upslope flow and moisture pooling along the
western edge of the surface front will likely support storms 
developing along the high terrain before moving eastward. While 
differing in the details, most models show at least locally heavy 
rainfall amounts, raising at least isolated flash flooding 
concerns across the region.

...Southwest...
Greater coverage of isolated to scattered diurnal convection is
expected as the upper high currently in place weakens. This will 
allow more showers and storms to develop further to the north, 
extending the threat for isolated flash flooding across a greater 
portion of northern Arizona and New Mexico.

Pereira

Day 3
Valid 12Z Wed Jul 17 2024 - 12Z Thu Jul 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
COLORADO AND NEW MEXICO...

...Southwest to the central and southern Rockies... 
A Slight Risk was introduced from south-central Colorado to central
New Mexico, where moist upslope flow will support increasing
coverage of showers and storms and locally heavy amounts. This will
raise the potential for flash flooding, especially across
vulnerable areas. Guidance indicates a notable increase in low
level moisture transport, raising PW anomalies to 1.5 standard
deviations above normal across central New Mexico. This moisture
along with weak flow aloft and daytime heating, is expected to 
support slow-moving storms capable of producing heavy rainfall 
rates. Vulnerable areas, including burn scars, urbanized areas, and
areas of complex terrain will be most susceptible to flash 
flooding.

Elsewhere, the potential for diurnal convection producing at least
isolated concerns for heavy rainfall and flash flooding will
continue to expand across the Southwest.

...Southern Plains to the Northeast...
Widespread coverage of moderate to locally heavy amounts, 
extending from Oklahoma and northern Texas all the way to the Mid 
Atlantic and southern New England, can be expected as a cold front
continues to drop slowly south through the central and eastern 
U.S. this period. Model spread contributed to the decision to 
maintain a broad Marginal Risk across this region for now. However,
there are some model signals that an upgrade to a Slight Risk may 
be required across some areas at some point. This includes areas 
from eastern Oklahoma through northern Arkansas, where some of the
deterministic models show heavy amounts developing as mid-level 
energy moving through the base of the trough interacts with a 
deeper moisture pool (PWs ~2 inches) along the front. Other areas 
include portions of the Ohio Valley and the Mid Atlantic, where 
there are signals for at least locally heavy amounts as well.

Pereira
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