319
FOUS30 KWBC 171609
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1209 PM EDT Wed Jul 17 2024

Day 1
Valid 16Z Wed Jul 17 2024 - 12Z Thu Jul 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES, HIGH PLAINS, IN AND NEAR ARKANSAS, AND THE
MID- ATLANTIC STATES...

East of the Rockies overview...
A mid-to-upper level trough centered over the Great Lakes will
continue to amplify, pushing a cold front and accompanying pool of
deep moisture further south and east across the southern Plains,
Tennessee and Ohio valleys, the Mid-Atlantic, and the Northeast.
While locally heavy amounts producing at least isolated concerns
cannot be ruled out throughout this entire region, models continue
to signal more organized and widespread concerns for some areas.


In and near Arkansas...
The mesoscale convective vortex (MCV) across central OK is
forecast to move east and interact with a deep moisture pool (PWs
~2 inches) along the boundary. It brings with it effective bulk
shear of 25-45 knots, which will help organize thunderstorm
complexes. The flow at 850 hPa is much lighter, 15 kts or less,
which when combined with the effective bulk shear indicates a warm
core low environment that fosters heavy rainfall from highly
efficient convection, with hourly totals up to 3" and local
amounts of at least 6". ML CAPE is already 2500+ J/kg, and could
rise to 4000 J/kg due to daytime heating. Although storms are on
the wane near Memphis presently, organized convection is expected
to redevelop in the afternoon, with some models showing slow-
moving, heavy rainfall producing storms drifting across
southeastern Oklahoma and northeastern Texas, with some additional
development further to east across Arkansas into northern
Mississippi and far western Tennessee. The 12z HREF probabilities
show a non-zero chance of 8", with a couple members of the guidance
explicitly forecasting such amounts. Convective progression and
guidance trends led to some southward shift in the risk areas.
Localized Moderate Risk impacts cannot be ruled out.


...Mid-Atlantic States...
A Slight Risk was expanded slightly eastward across portions of
the Mid-Atlantic more into NJ and DE per the 12z suite of mesoscale
guidance, but continues to include the Philadelphia/ Baltimore/
Washington DC megalopolis. As the upper trough shifts east,
increasing southerly flow will support an uptick in moisture along
the Mid- Atlantic and Northeast seaboards with precipitable water
values rising into the 2-2.25" range. Effective bulk shear near
this boundary is 25-35 kts -- about 10 kts above the magnitude of
the 850 hPa inflow -- and ML CAPE is starting out in the 500-1000
J/kg. Daytime heating should allow ML CAPE to rise to 2000 J/kg and
thunderstorms are expected to develop along nearby topography once
CIN erodes very early this afternoon and focus closer to the front
this evening.  Hourly rain totals to 3" with local amounts to 6"
are possible where storms merge, train, or where repeated
convective rounds occur.  This would be most problematic in urban
areas and could impact less urban areas as well despite developing
drought.  Given the potential, localized Moderate Risk impacts
cannot be ruled out.


...Southwest to the Southern and Central Rockies and High Plains...
An upper high will remain centered over the Four Corners allowing
monsoonal moisture to funnel across the Southwest, fueling diurnal
convection and isolated flash flooding across the Southwest.
Precipitable water values barely eclipse 1.5" and there`s no
obvious easterly wave in the low to mid-levels of the troposphere.
The above factors support the maintenance of the Marginal Risk
level.

Further to the east, increasing low level easterly flow will
heighten the threat for flooding across portions of the southern
Plains into the Rockies. As the previously noted front in the
Plains slides into the region, post-frontal easterly flow will
support PW anomalies climbing to 1-2 standard deviations across
northern New Mexico and far southern Colorado, with values in the
High Plains locally exceeding 1.25". With ML CAPE likely to rise
into the 1500-2000 J/kg range, developing storms should be capable
of producing heavy amounts, with hourly totals to 2.5" and local
amounts to 4" anticipated.  A Slight Risk was maintained over the
high terrain and extended further to the southeast into the High
Plains to cover the heavy rain/flash flooding potential. Impacts
would be more significant in and down elevation of burn scars.

Roth/Pereira


Day 2
Valid 12Z Thu Jul 18 2024 - 12Z Fri Jul 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES, SOUTHEASTERN VIRGINIA, AND EASTERN NORTH
CAROLINA...

...Southern Mid-Atlantic and the Southeast to the Southern
Plains...
The previously described front will extend from the southern Mid-
Atlantic back into the southern Plains, remaining nearly stationary
through the period. This boundary will remain a focus for deep
moisture and storm development. Models continue to indicate the
greatest concern for heavy rainfall and flash flooding will center
over southeastern Virginia and eastern North Carolina. Guidance
shows PWs around 2-2.25 inches (~2 standard deviations above
normal across this region). This moisture, along with mid-level
energy and upper jet support, is expected to generate numerous
showers and storms across the region with heavy amounts likely.

...Southwest to the Southern Rockies...
The better low level moisture flux over the southern High Plains
shifts a little further south this period, extending the threat for
scattered flash flooding further south into the southern New Mexico
mountains. Otherwise, the overall pattern and available moisture is
expected to remain largely unchanged, resulting in similar threats
for heavy rainfall and flash flooding from Day 1 into Day 2 across
the region.

Pereira

Day 3
Valid 12Z Fri Jul 19 2024 - 12Z Sat Jul 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES AND ...

...Carolinas to the Lower Mississippi Valley...
A front will linger across the region, focusing deep moisture and
more unsettled weather. For a second day, the greater threat for
organized heavy rainfall is expected to focus along the eastern
extent of the boundary. After setting a bit further south across
the eastern Carolinas, guidance shows a wave developing along the
front bringing deeper moisture back to the north. Some portions of
eastern North Carolina could see back-to-back days of heavy rain,
raising flash flooding concerns across the region.

...Southwest to the Southern Rockies...
Moisture sufficient for diurnal convection, raising at least
isolated flash flooding concerns, will remain in place. Therefore
a Marginal Risk was maintained, covering much of the same regions
as the Days 1 and 2. A Slight Risk was added to portions of
northern and central New Mexico, similar to the previous days as
well. Guidance shows another front sliding south, adding additional
moisture to the region.

Pereira


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