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

Day 1
Valid 01Z Thu Jul 18 2024 - 12Z Thu Jul 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST U.S. ACROSS SOUTHERN PLAINS INTO PARTS OF THE MID-
ATLANTIC STATES...

East of the Rockies overview...
Main changes at 18/01Z involved trimming areal coverage of the
Marginal risk areas where a cold front has already cleared the
region. This primarily involves the area from New England southward
into Pennsylvania to northern Arkansas. Instability has been
consumed by a line of scattered to broken thunderstorms this
afternoon and early evening from New York southward towards
Virginia...allowing the Slight risk area to be narrowed/more
focused. The one exception is over southern and southwest Virginia
where satellite imagery has been showing an expanding area of
cooling cloud tops for several hours. The flow in the area was
confluent...helping the sustain the rainfall rates. 

Rockies westward overview...
Still too early to make too many changes in the Southwest US...but
radar trends still supports the Slight risk in New Mexico for a
few more hours. Farther north...focused the Marginal Risk area over
the western high plains to be east of the Rockies front
range/Laramie range and generally south of the Cheyenne ridge.

Bann



...Pewvious Discussion...
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, THE EASTERN NORTH CAROLINA/VIRGINIA BORDER, &
THE ARKLATEX...

...Southern Mid-Atlantic & South...
A stationary front will extend from the southern Mid- Atlantic
back into the southern Plains. This boundary will remain a focus
for deep moisture and storm development and in places there should
be enough effective bulk shear to organize convection. Models
continue to indicate the greatest concern for heavy rainfall and
flash flooding will center over southeastern Virginia and eastern
North Carolina, though there is an increasing signal for heavy
rainfall near what appears to be the combination of an MCV and an
upper low near the northern LA/TX border within the 12z guidance
(HREF, NAM, and ECMWF). Guidance shows PWs around 2-2.25 inches (~2
standard deviations above normal across this region) with MU CAPE
as high as 2000-4000 J/kg. The available ingredients suggest hourly
rain totals as high as 3" with local amounts to 6" being possible.
A new Slight Risk area was added to the northern LA/TX border to
account for the flash flood potential.


...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
and instability is expected to remain largely unchanged, resulting
in similar threats for heavy rainfall and flash flooding across
the region. Hourly rain totals to 2.5" with local amounts to 4" are
the expectation.

Roth/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 another 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. If the overlap
between the two days increases within the model guidance, an
upgrade to a Moderate Risk may prove useful.


...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 previous days. A Slight Risk remains across 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, and instability is expected to build due to
daytime heating.  Hourly rain totals to 2.5" with local amounts to 4" are
the expectation.


...In and near Nebraska...
A progressive, incoming front will lure precipitable water values
over 1.5", increase the low-level inflow to 25-30 kts, and lead to
MU CAPE of 2000+ J/kg, all of which befit organized convection.
Given the above, hourly rain totals to 2" with local amounts to 4"
are possible. As the front is progressive, any flash flooding
should be limited in scope, so raised a Marginal Risk to cover the
possibility.


Roth/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