Excessive Rainfall Discussion

[Abbreviations and acronyms used in this product]
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Thu Jul 18 2024

Day 1
Valid 16Z Thu Jul 18 2024 - 12Z Fri Jul 19 2024


...Southern Mid-Atlantic & South...

Convection is expected to bring up in coverage and intensity 
around 17-18Z for southeast Virginia and eastern North Carolina. 
Refer to Mesoscale Precipitation Discussion #648 for further 
details on the flooding potential. Very minor adjustments were made
to the Marginal and Slight Risks across the Mid-Atlantic region.


A cold front will advance southeastward into the Mid-Atlantic
region today while the western portion slows across the southern
Plains. Along and ahead of this boundary will remain a focus for
deep moisture (PWATs > 2 inches in some places of the Mid-
Atlantic, Southeast and Gulf Coast states. The greatest threat for
heavy rainfall appears to be with and ahead of the front along
pre-frontal convergence axes from southeastern Virginia into
eastern North Carolina. In addition to the anomalous moisture and
low level convergent axes, upper levels will be favorable within
the right entrance region of a jet positioned east of an upper
trough axis advancing eastward through the Great Lakes, with an
area of upper level diffluence forecast over the eastern Mid-
Atlantic region due to the presence of a ridge off of the Southeast
coast. For the 24 hour period ending 12Z Friday, 00Z HREF guidance
supports EAS probabilities for 3 inches of 30 percent between
Albemarle and Pamlico Sounds along with standard probabilities for
8 inches of 30-40 percent in the same location. While this is
typically a less flash-flood prone region, these probability values
are not super common and could be highlighting the potential for
locally significant rainfall totals somewhere from southeastern
Virginia into eastern North Carolina.

...Eastern Texas into Western Louisiana...

16Z update... The Marginal Risk was expanded westward to cover
portions of the Texas Hill Country were there is an active Flood


A second and relatively small Slight Risk was maintained for
locations near the Arklatex and southward along the Sabine River to
account for locally greater potential for heavy rain. Upper level
diffluence will be maximized here and greater coverage of
thunderstorms are during the afternoon/overnight near/south of an
850 mb low located near the Arklatex. Individual members of the 00Z
HREF and RRFS indicate localized QPF maxima of 4 to 6+ inches.

...Southwest to the Southern Rockies...

16Z update... The latest guidance has shown small reductions in 
QPF amounts across portions of Colorado that lead to a very small
trimming of the Marginal's northern boundary in southwest and
central Colorado. The best potential for excessive rainfall 
persists for central New Mexico and south-central Colorado.


Another day of monsoonal convection is expected for 
the southern Rockies into the Southwest as the center of the mid-
level ridge shifts slightly west from the Four Corners region 
through Friday morning. The overall pattern, available moisture and
instability are expected to remain largely unchanged from 
Wednesday, resulting in similar threats for heavy rainfall and 
flash flooding across the region. Rainfall accumulations of 1-2 
inches in less than an hour and localized maxima of 2-3 inches are 
expected with a focus across the Rockies where low level moisture 
flux will be maximized.


Day 2
Valid 12Z Fri Jul 19 2024 - 12Z Sat Jul 20 2024


...Carolinas to the Lower Mississippi Valley...
A front will linger across the region, focusing deep moisture and
another day of unsettled weather. The greatest threat for
organized heavy rainfall is expected to focus along the eastern
extent of the boundary ahead of a mid-level shortwave forecast to
track northeastward from Mississippi/Alabama. After settling 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, though it appears the main axis of rainfall will be
generally south for Friday to Saturday compared to the Thursday to
Friday period.

...Southwest to the Southern Rockies...
No significant changes in moisture are expected for Friday into
Saturday compared to the previous day. The mid-level ridge will
begin to show better movement toward the Great Basin but diurnal
convection is expected to again raise at least isolated flash
flooding concerns for many of the same areas as the Thursday to
Friday period. Therefore, the Slight and Marginal Risks remain
similar to the previous day with potential for rainfall
accumulations of 1-2 inches in less than an hour and localized
maxima of 2-3 inches.

...Eastern Nebraska and Adjacent Locations...
A progressive front is expected to bring precipitable water values
over 1.5 inches across eastern Nebraska Friday afternoon/evening
with 850 mb winds of 20-30 kt ahead of a developing low. Sufficient
instability, shear and moisture will be in place within the region
of maximized low level moisture transport to support areas of
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 and a Marginal Risk
was maintained from continuity to cover the possibility.


Day 3
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024


...Mid-Atlantic to Central Gulf Coast...
A slow moving front will continue to extend from the Mid-Atlantic
coast into portions of the Tennessee Valley with anomalous moisture
(1 to 2 standardized anomalies of precipitable water) in place in
the vicinity of the front to the Gulf and Southeast coasts.
Slightly stronger southwesterly low level winds are expected across
the Carolinas which could support areas of training and/or slow
movement of thunderstorm clusters containing localized rainfall
rates of 2-3 in/hr. Southwesterly flow will be in place to the east
of a longwave trough axis centered near the Mississippi River and
upper level jet support will likely focus convective activity over
southern Virginia into North Carolina ahead of a shortwave to move
across the Mid- Atlantic states during the day on Saturday. A
Marginal Risk remains in place where the highest probabilities for
2+ inches of rain exist in the latest ensemble guidance. There
wasn't enough agreement in the latest deterministic guidance for
anything higher than a Marginal Risk at this time, but upgrades may
be warranted with future forecast cycles.

...Central Plains to Texas Panhandle...
While some minor differences exist in the models, there is broad
agreement that the central to southern Plains will be situated in
northwest flow aloft with a possible closed mid-level center in the
vicinity of Iowa. A weak frontal boundary may exist across northern
Texas but the focus for potentially excessive rainfall will exist
ahead of an 850 mb low/trough extending from near the Mid-Missouri
River Valley into the Texas Panhandle with weakly anomalous
moisture from northwestern Texas into the Missouri River Valley.
Current thinking is for some 2-4 inch totals, which may fall over a
short period of time supporting localized areas of flash flooding.

...Central/Southern Rockies into the Desert Southwest...
Mid-level ridging is expected to migrate westward into the Great
Basin from Saturday to Sunday morning. The translation of the ridge
and possible shortwave trough advancing southeast across the
southern Plains will increase the easterly component of the low to
mid-level flow across New Mexico into eastern Arizona. Moisture is
forecast to be seasonable to above average, with the best
potential for above average moisture setting up from West Texas
into southern New Mexico and southern Arizona. In terms of upper
level forcing and mean steering flow however, nothing stands out as
anything greater than a Marginal Risk of flash flooding at this
time with typical monsoonal convection.


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

Last Updated: 1159 AM EDT THU JUL 18 2024