531
FOUS30 KWBC 152011
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Mon Jul 15 2024

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

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR ACROSS
PORTIONS OF NORTHERN ILLINOIS, INCLUDING THE CHICAGO METRO...

..1810Z Update...

In coordination with the local Chicago WFO, a targeted upgrade to a
Moderate Risk was made to encompass much of Northern IL, including
the Rockford and Chicago metro areas. Previous 48 hrs have been
impactful across the aforementioned area with locally 5-10" of
rainfall from a succession of three MCS as they moved through the
area. Despite the overall progressive nature of the expected
complex later today, the antecedent conditions coupled with rates
of 1-3"/hr over very saturated soils and flood prone urban
corridors were enough to warrant a targeted upgrade to the MDT risk
across that area.

Kleebauer

...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 MUCH OF KANSAS
THROUGH PORTIONS OF THE MID MISSISSIPPI AND OHIO VALLEYS...

...20z Update Summary...

The previous SLGT risk was expanded west to include the rest of the
southern half of Missouri all the back into the High Plains of
Western Kansas. A high-end SLGT is forecast over the Ohio Valley
with the primary area of focus located from Southeast MO across
Southern IN/IL/OH and adjacent Northern Kentucky. MRGL risks over
the Southwest and Northeast U.S were adjusted slightly based on the
latest QPF and instability trends, along with some local
collaboration with impacted WFOs.

Kleebauer

...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.

Recent HREF neighborhood probabilities are officially in range of
the full threat and are depicting some elevated signals for heavy
rain totals positioned across much of the Ohio Valley within the
corridor extending from Southern IL/IN all the way into Southern OH
where the >3" probability is solidly within 40-70% across the
entire aforementioned area of focus. The >5" probability was also
on the higher side considering the setup with 25-40% probs nestled
across Southern IL/IN into Northeast KY. This is likely due to the
increased LLJ interaction with the front as it bisects the area and
runs parallel to the boundary. Mean wind between 925-700mb is
aligned out of the west-southwest with small mid-level
perturbations ejecting eastward across the area. The combination of
all these variables will create an environment conducive for
thunderstorm development and training regime that would exacerbate
the flood threat in any given area along and near the front. This
area was solidly in the SLGT risk prior, but the area was extended
a bit further south to account for a typical northern bias and
likely southern surges of the front due to outflow generation from
the convection nearby.

Across the Central Plains, the front will progress south into KS by
tomorrow afternoon while a shortwave out of the Rockies will dive
southeastward around the eastern flank of a ridge west of the
Continental Divide. The interaction between the front and the
shortwave trough will allow for convective initiation across the
Southeast CO Front Range over into Southwestern KS where the best
instability and deep layered moisture will be co-located. Guidance
has ramped up the coverage and expected intensity of the
thunderstorm development in-of the above area with correlating
higher probabilities in both the 3" (40-70%) and 5" (25-35%)
neighborhood probs with rainfall rates projected between 2-3"/hr in
the strongest cells. There was enough of a signal and agreement
within the deterministic and ensemble QPF distribution to upgrade
that area with a SLGT and link with the threat further east.

Pereira/Kleebauer

...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. Isolated flash flood
threat will be plausible within the complex terrain of any of the
highlighted regions within the risk, as well as targeted areas of
focus in the copious amounts of burn scars present within northern
and central NM and AZ.

Pereira/Kleebauer

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, THE LOWER MISSISSIPPI VALLEY, AND THE
CENTRAL APPALACHIANS...

...20Z Update Summary...

Previous SLGT risk across Colorado and New Mexico was adjusted to
encompass some higher threat areas within south-central NM and to
account for recent QPF trends within the ensemble bias corrected
output. A SLGT risk was introduced across portions of the Lower
Mississippi Valley, as well as portions of the Central Appalachians.
More information included in the respective sub-headings below....

Kleebauer

...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.

Pereira

...Lower Mississippi Valley...

Frontal progression will slow considerably by mid-week with the
front bisecting parts of the Southern Plains, extending east
through the Lower Mississippi and Tennessee Valleys. The upstream
wave from the prior period over the Central Plains will nose
eastward through the front half of the period with sights on the
Lower Mississippi Valley by the second half of Wednesday. The
coupled shortwave approach, diurnal destabilization, and boundary
layer convergence will initiate a round of scattered to widespread
heavy thunderstorms in-of Eastern OK through AR into Western TN.
The signal for significant rainfall in those areas has grown within
the ensemble QPF and probability fields with the conservative NBM
even depicting 2-3" of QPF at range. NAEFS standardized anomaly
output indicates a broad area of +1 to +2 deviations above normal
PWATs with an areal average of 2+" PWATs located within proximity
of the front across the Ozarks into much of Western TN. There was
enough consensus in the expected synoptic evolution of the pattern
to add a SLGT risk over the above region with an adjustment
possible within the neighboring confines of the outlined area.

...Central Appalachians...

Approaching cold front will slowly meander into the western flank
of the Appalachians by Wednesday afternoon with pooling moisture
anchored within proxy of the front. To the north, upper jet max
across Ontario will be strengthening through the period with much
of the Central Mid Atlantic and Ohio Valley focused under the
right entrance region (RER) of the jet. This large scale ascent
pattern will help initiate more scattered to widespread convective
coverage across the area with a mesoscale focus in-line with the
terrain located over WV down into Eastern KY. The convective regime
within an above normal moist environment characterized by PWATs
exceeding +2 standard deviations over the terrain will likely cause
some flash flood concerns, some potentially significant if the
pattern unfolds verbatim. The signal for >1" QPF has grown over the
past succession of model output which is lending credence to the
threat. There was enough signal to warrant the upgrade to a SLGT
risk located over the terrain, specially the Central Appalachians
from far Southwest PA down through WV and parts of Eastern KY.

Kleebauer

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