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FOUS30 KWBC 032000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Wed Jul 3 2024

Day 1
Valid 16Z Wed Jul 03 2024 - 12Z Thu Jul 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS EASTWARD TO THE OHIO VALLEY|...

...16Z Update...

Previous SLGT risk forecast remains on target with an elevated
flash flood risk positioned across southern KS through southern MO
with an emphasis over south-central and southeastern MO. Remnant
outflow from nocturnal convection is currently analyzed across
southern MO and will be the focal point for afternoon and evening
convection with heavy rainfall likely given the antecedent
environment. PWATs over 2" are aligned along and ahead of the
quasi-stationary front across KS into MO, as well as within the
confines of the aforementioned lingering outflow. 12z HREF
neighborhood probabilities are favorable for at least 3" of
rainfall across much of south-central MO to the KS/MO state line
with values settling between 45-70% within that corridor. This
correlates well with the elevated probabilities for at least 2"/hr
rain rates suggested within the latest hi-res ensemble suite
(25-40%). The above area will run on the higher end of the SLGT
risk threshold with a non-zero chance at a targeted upgrade later
pending how convection evolves. For now, the signals are strong
enough for the current forecast, but a bit under what we would want
to see for a higher forecast risk area.

The SLGT risk was extended through portions of the Ohio Valley as
the quasi-stationary front will be a target for convective
development later this evening within a secondary tongue of
elevated theta-E`s being advected northeast into southern OH. CAMs
are leaning towards the possibility of scattered heavy rain
signatures within areal convective development. The signal for at
least 2" of precip within that area of OH has risen since the last
update, indicative of the evolving pattern with favorable
thermodynamics overlaid across that portion of the Ohio Valley.
Rates between 1-2"/hr are plausible within this corridor, enough to
be a signal for 1hr FFG exceedance within the latest hi-res
ensemble. This was enough to necessitate the extension through
portions of southern OH and adjacent KY.

There are no changes to the MRGL risk area across the Southwest as
a monsoonal setup is still forecasted to evolve later this
afternoon with isolated flash flooding possible, especially in
complex terrain and burn scars present.

Kleebauer

...Previous Discussion...

...Mid Mississippi Valley and Great Plains...

A cold front from a passing trough over the northern tier of states
will stall in the vicinity of Kansas and Missouri later today,
providing focus for renewed thunderstorm development from eastern
Kansas and southern and central Missouri eastward into the Lower
Ohio Valley. Except for a few adjustments based on the latest suite
of global and ensemble guidance from the 03/00Z model runs...there
Slight Risk area was largely unchanged from the previous issuance.
The guidance still depicts strong instability (CAPE in excess of
2000 j/kg) and PWs in excess of 2 inches (above the 95th
percentile) combining to support organized convection with rain
rates exceeding 2 inches per hour at times combining with the
potential for cell training or backbuilding.

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...

Continued to trim the Marginal risk area as mid- and upper-level
heights continue to build into the region....helping to suppress
late day and evening convection even further.

Bann


Day 2
Valid 12Z Thu Jul 04 2024 - 12Z Fri Jul 05 2024

...20Z Update...

The SLGT risk across the Upper Mississippi Valley through the Upper
Midwest was maintained with some minor modifications to the risk
area. Additional SLGT risks were introduced across the Mid
Mississippi Valley and points southwest into Oklahoma. An
additional SLGT was introduced over East and Southeast
Pennsylvania. The two additional SLGT risk discussions and an
update to the previous SLGT over the north are written below...

...Upper Midwest...

A maturing surface cyclone will be moving east out of South Dakota
with a path through southern MN into WI by the end of the forecast
period. A tongue of elevated theta-E`s will be advected on the
eastern flank of the surface low, eventually wrapping into the axis
of deformation across south-central MN and western WI. Instability
fields will be modest with MUCAPE between 1000-1500 J/kg located
with the deformation axis leading to swath of heavy stratiform rain
with embedded convective signals thanks to the large scale forcing
and improved thermodynamical environment in place. This is not a
textbook flash flood threat that usually encompassed organized
convection, but the premise of heavy rainfall within the setup
cannot be understated due to the already saturated soils located
over the Upper Mississippi Valley after repeating pattern of heavy
rain events recently. General rates will be between 0.25-0.5" on
average, but the convective clusters within the band of precip will
be capable of rates between 1-2"/hr as noted within the latest HREF
probability fields. Ensemble QPF has been relatively high over a
succession of runs, especially in the corridor extending from
northeast SD across south-central MN, including the MSP metro. HREF
neighborhood probabilities for at least 3" are running between
60-80% within the above zone, correlating well with the path of the
expected closed 7H reflection as the cyclone matures with the
relevant height fields aloft closing off during the storms
maturation. The current SLGT risk was sharpened to align with the
passage of the 7H low coordinating with the heavy precip
probabilities. Some heavy precip due to convection on the southeast
flank of the low is possible across portions of northern IA into
southwest WI, but the best heavy rain axis for flash flooding will
remain further north in that deformation zone.

...Oklahoma into the Mid Mississippi Valley...

Multiple mid-level perturbations will eject east of KS during the
period with waves of convection and accompanying heavy rainfall
situated across south-central MO. The first enhancement will occur
in the morning from a remnant complex making its way to the east
through the same region that was impacted 24 hrs prior. The FFG
indices currently are fairly low and will only stand to enhance
flash flood concerns when the convective cluster arrives. The
frontal boundary situated overhead will continue to be a focal
point for training convective potential leading to locally enhanced
totals between 2-4" possible during the first wave of rainfall. A
small break will occur through the afternoon with some isolated to
scattered showers and storms plausible with diurnal heating.
Another mid-level shortwave will exit the Central Plains with more
convection developing in-of east KS, propagating eastward into MO
by the back half of the forecast period. That`s when signals for
heavy rain return across the same areas that were impacted earlier
in the period leading to another threat for flash flood concerns.
Latest probability fields for at least 2" remain high (50-80%)
across a good portion of central and southern MO extending back
into Oklahoma.

The southern extent into eastern OK is due to the arrival of the
cold front as it begins pushing southeast through the Southern
Plains with ascent focused along and ahead of the frontal boundary.
Environment will be ripe for enhanced convective cores with heavy
rain rates between 1-2"/hr increasingly likely across that portion
of OK. Some of the recent CAMs are bullish with local totals
exceeding 4" in spots with the area from the Red River up through
Tulsa and surrounds as the main targets for the threat of heavier
rainfall. FFG`s in this area are higher than further north, however
the projected rainfall rates and suitable environment were
notable leading to the SLGT risk added across eastern OK and
adjacent northwest AR.

...Eastern Pennsylvania...

Frontal boundary over the Ohio Valley will press eastward into
PA with a very unstable airmass advecting northeast across the Mid
Atlantic tomorrow afternoon. By the mid-afternoon time frame, areal
PWATs will be approaching 2.25-2.5", a full 3 standard deviations
above normal with increasing instability centered from Central MD
up through much of PA. A round of thunderstorms will develop across
the Lower Susquehanna Valley and propagate eastward along and north
of the Mason Dixon by late-afternoon. Considering the available
environment, the threat for locally heavy rainfall is very much in
play with rates likely to hit between 1-2"/hr within the stronger
cores that develop. Forward motion will be modest, but the urban
threat within portions of Southeast PA has an opportunity to
overcome the forward speeds of the convection. There`s some
discrepancy in guidance on where the eastern extent of the heaviest
rain will occur as the ridge axis to the east will provide some
subsidence on its western flank leading to cells likely dying as
they closer to NJ. With the holiday occurring tomorrow, many will
be out and about and the borderline MRGL/SLGT for the region was
contemplated, but after coordination with surrounding offices, have
went with a targeted SLGT risk across Southeast PA to cover the
more urbanized areas from Chester County on north and east where
the parameters for convection are highest late tomorrow afternoon
and early evening. If guidance backs off on the threat, it is
possible this risk gets downgraded, but wanted to make sure
messaging is covered for the threat considering the circumstance of
it being the 4th of July.

Kleebauer


Day 3
Valid 12Z Fri Jul 05 2024 - 12Z Sat Jul 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN AND EASTERN UNITED STATES...

...20Z Update...

The main change to the period was the extension of the MRGL risk
over the southern and eastern US extended further north on the
eastern flank to include more of the Appalachians of WV, and
further west on the southwest flank to include more of the Concho
Valley and North TX. This is coincident with the trends in the
ensemble QPF forecast that indicates scattered convective
signatures within a moist, unstable environment that will likely
cause some isolated flash flood concerns in both areas. The frontal
proxy is a big factor for points of emphasis on where thunderstorms
development is most likely and where some convective anchoring
would occur allowing for a heightened threat for localized flood
concerns. The area across the Deep South is still the primary focus
for heavy rain chances, but area FFG`s are very high as the area
has been void of significant rain chances for much of the past
month. A few cells may cause some havoc over parts of the South
leading to flash flood concerns. If any organized convective
signals become apparent, or if the setup is conducive for
significant rainfall exceeding 5" in any spot when in the range of
CAMs, a targeted upgrade may be necessary. For now, continuity was
the best route for the forecast.

Changes to the northern and western MRGL were minimal and only
reflective from the current QPF footprint brought about by the
latest ensemble data and ML output. This matches fairly well with
the latest First Guess Fields.

Kleebauer

...Previous Discussion...

Corridors of higher chances of heavy rainfall will continue to be
aligned with a quasi-stationary front extending from the southern
Plains into the Southeast US on Friday where higher values of
precipitable water should be pooling in proximity to sufficient
instability. With weaker mesoscale forcing in the Southeast US...exactly
where convection develops is less certain so followed the better
clustering shown by spaghetti plots of QPF from the GEFS/SREF as
well as NCEP global runs and the ECMWF. Over the portions of the
Southwest...a renewed risk of locally heavy rainfall develops late
in the period as low level winds draw Gulf moisture upslope and is
aided by some isentropic lift as the low rides up and over the
tail end of the stalled front.

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