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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1555Z Jul 15, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Mon Jul 15 2024

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

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

...16Z Update Summary...

The SLGT risk across the Midwest was expanded in all directions
with a greater emphasis to the north, south, and east where the
forecast track of the next convective complex will likely cause
some enhanced flash flood issues during its progression. A MRGL
risk was added across portions of Southern New England for isolated
flash flood within a zone of enhanced convergence. The MRGL risk
across the Northeast was expanded a bit further east as well to
reflect some of the trends within the QPF field on recent CAMs as
they have a better handle on the approaching shortwave and remnant
MCV currently pushing through Southeast Ontario. The MRGL risk
across the Southwest was linked and expanded somewhat to encompass
the higher instability reflection across the region where isolated
flash flood concerns will be plausible.

...Midwest through the Ohio Valley...

Area convection within the Upper Mississippi Valley is in the
process of weakening with some of the remnant energy expected to
pivot southeast within the mean flow aloft. A cold front at the
surface is analyzed over Southwest MN with a surface low expected
to motion to the northeast into the Northern Great Lakes by the
evening. The cold front will slide to the southeast allowing for
increasing convergence within the confines of a building
moist/unstable axis present between Eastern IA and points east
encompassing most of Southern WI through Northern IL. These areas
have been impacted significantly leading into the upcoming
convective pattern with some remnant flooding still located within
portions of Rockford and Chicago after the succession of 3 MCS's
the past 48 hrs. The priming of the soils in conjunction with the
incoming wave of convection is concerning as the threat will lean
on the higher-end of potential impacts given the current situation.

Convection will fire late this afternoon and quickly grow upscale
as it moves from Eastern IA into Southern WI and Northern IL, a
similar path to the previous complex that impacted the area last
night. Those areas that were hit previously will maintain the SLGT
risk from prior forecast with a higher end wording on locally
significant impacts possible across the corridor from Rockford over
into Chicago and the surrounding suburbs. The cold front will
propagate south with the assistance from cold pool generation from
the expected complex. After 00z, increasing LLJ presence will play
a significant role in enhancing the rainfall potential within the
confines of the boundary as the flow shifts parallel to the front
allowing for convective training and backbuilding to occur on the
upwind flank of the complex. This has been a consistent signature
within the latest CAMs with the HREF blended mean QPF footprint
depicting the potential pretty well across the IA/IL border south
of Davenport to points east into western IN. This is a relevant
signature given the synoptic and mesoscale evolution anticipated
with an enhanced probability field accompanying within that zone of
interest.

The two primary areas will lie within that northern and southwest
flank of the complex. Neighborhood probabilities for at least 2"
are very high within both corridors with a solid 40-70% outline
within both areas. The 1" EAS signatures were also more robust in
those zones with some lighter 2" signatures in the same region.
Considering those variables, there was enough confidence to
maintain the SLGT risk and including some higher-end SLGT wording
within those two corridors above. The SLGT was also expanded to the
east over portions of Southern MI through Northern and Central IN
as the complex will continue propagating eastward overnight with
locally heavy rainfall in-of those areas downstream. There is a
potential for a targeted upgrade within portions of Northern IL if
the setup repeats or has a higher impact across the areas where
they have been hit recently leading to greater sensitivities to
flood concerns.

...Southern New England...

A small, but fairly substantial mid-level perturbation will advect
eastward into Southern New England from the west, entering an area
of modest low-level convergence and sufficient surface buoyancy
located within parts of Northern CT up through Central MA later
this afternoon. A fairly substantial boundary layer moisture
presence is reflected within the current observed PWATs and surface
dew points located across all of the area. Diurnal destabilization
and the approach of the mid-level reflection will allow for the
development of a line of convection across the above areas, riding
east-northeast within the confines of a sharp theta-E gradient
aligned over basically the I-90 corridor. Considering the
environment and some of the probability signals being a bit robust
over the highlighted area. In coordination with the local BOX WFO,
a small MRGL risk was added over portions of Southern New England,
not including greater Boston or Southeast MA.

Kleebauer

...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.
Considering a somewhat decent signal with the SBCAPE fields off the
recent CAMs and PWAT anomalies within the interior of SoCal and
Southern NV, have expanded the MRGL risk and connected the
separated risk areas to become a more uniform MRGL over the region.

Pereira/Kleebauer

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

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