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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0006Z Jul 10, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
806 PM EDT Tue Jul 9 2024

Day 1
Valid 01Z Wed Jul 10 2024 - 12Z Wed Jul 10 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST, MID MISSISSIPPI AND OHIO VALLEYS, AS WELL AS FOR THE
SOUTHEASTERN LOUISIANA...

...01Z Update...
Main update associated with Beryl was to trim back the Marginal and
Slight risk areas where Beryl has moved through and rain has ended
or taoered off. In the broadest sense, that is along and west of
the Mississippi River. Farther east...hi res guidance and late
afternoon satellite/radar imagery has been showing things
simmering down to removed the Marginal risk areas in the Southeast
U.S....except maintained the Marginal over the portion of the
Florida peninsula east of Lake Okeechobee. Thinking was that
convection in parts of Texas and the Southern Rockies will
eventually wind down with loss of daytime heating but that it was
too early to remove the Marginal risk areas there.

Bann


...16z Update Summary...

Two primary updates to the previous D1 to reflect changes in
guidance and to update on the recent radar reflection going
forward. The SLGT risk across the center of the country into MI was
expanded to the east over southern IN and further northeast across
much of southern MI. The second update was across the Central Gulf
Coast where the previous SLGT risk was scaled back in size to
mainly encompass the New Orleans metro and surrounding locales
given the recent radar trends and CAMs output through the period.
More details were added to the respective sub-headings below....

Elsewhere over the CONUS, the other notable change exists across FL
where the MRGL was cut over much of the western coast and over far
southern FL due to the setup for flash flood potential being
primarily confined to the FL Panhandle, FL Big Bend, and the
eastern coast given the flow pattern conducive for localized
enhancement from the Atlantic sea breeze. Spotty max totals over 3"
are forecast within CAMs, but the placement is very variable, so
wanted to maintain some coverage to account for the threat, but
local uncertainty. The setup across New England, Southern
Appalachians, and portions of the Southwestern US are all fairly
similar with regards to the threat being localized for flash
flooding within the respective terrain. The best threat remains
over the burn scars in NM where any convective impact over the
affected areas will be susceptible to flashy returns. The other
areas within New England and the Southern Appalachians lie within
the lower-end of MRGL, but non-zero considering the environment and
terrain focus.

Kleebauer

...Mississippi Valley and Midwest...

..16Z Update..

Increasing signals for heavy rainfall from the remnants of Beryl
will impact some of the urbanized areas extending through the
Detroit Metro with a greater convective concern located over
southern IN. Southern IN will be within the best instability axis
in the Ohio Valley with several small mid-level perturbations
advecting overhead this morning through the evening around the
broad periphery of Beryl's circulation. Recent HREF mean QPF across
the area has really ramped up with storms already impacting the
southern fringe of the state with locally heavy rain as rates top
1.5-2"/hr in the recent radar QPE output. A second round is
expected this evening in wake of better instability leading to a
multi-wave threat of convection within portions of the area.
Considering the priming this morning and eventual impact later, the
threat for flash flooding has increased enough to warrant the SLGT
expansion. Across southern MI, the forward speed of Beryl's
remnants will allow for a solid 925-700mb moisture flux through the
region with sights on the Detroit metro by the end of the period.
HREF neighborhood probabilities for at least 3" have shot up to
40-60% across portions of Southeast MI, including Detroit with
signals over Southwest MI growing to 80-90% for at least 3" and
25-40% for potential of 5". This was a significant enough signal to
warrant the SLGT expansion across Southern MI through the Detroit
Metro with agreement from the local Detroit WFO.

Kleebauer

..Previous Discussion..

Tropical moisture associated with Beryl will spread a swath of
heavy rain across the Mississippi Valley and into the Midwest as a
cold front advances east through the region. The last few runs of
guidance have trended toward a more narrow axis setting up right
along the front and QPF values increasing further northeast across
Illinois/Indiana and southern Michigan. A sizable swath of 3+
inches continues to span from the Missouri Ozarks to northwest
Indiana with local maximums upwards of 8 inches.

A Slight Risk extends from central Arkansas to southern Michigan.
A Marginal Risk stretches from the central Arkansas northward to
southern Michigan and central Ohio.

Campbell

...Central Gulf Coast and Southeast...

..16Z Update..

Recent trends in the enhanced QPF focus across the Central Gulf
coast have led to the New Orleans metro and surrounding locales
over Southeast LA as the focal point for heavy rainfall today into
the evening. Considering very high FFGs located over the coastal
portions of LA/MS/AL, the threat will be mainly an urban flood
threat with the New Orleans metro easily being the target for the
current setup. HREF probabilities for at least 2"/3-hrs is highest
across Southeast LA with values mainly between 25-40% over a span
of several hrs beginning now through 06z with the spatial coverage
centered right over NoLA and surrounding Parishes. This allowed the
previous SLGT to shrink in size to mainly encompass the New Orleans
metro and Parishes just off to the south and southwest.

Kleebauer

..Previous Discussion..

Deep, onshore flow will continue to drive convection to form along
the Gulf Coast, with the potential for heavy down pours. Areal
average rainfall is expected to be in the 1 to 2 inch range however
the CAM guidance suggests very localized maximums of 3 to 5 inches
possible. A Slight Risk is in effect for portions of the Gulf Coast
from eastern Louisiana to the Florida Panhandle.

Campbell

...Previous Discussions...

...Southwest...

Isolated to widely scattered convection is likely to form along
the mountains once during the afternoon and evening, which could
very well move over flood-sensitive areas. A Marginal Risk remains
in effect for portions of Arizona, New Mexico, southern Colorado
and extreme western Texas.

...Rio Grande of Texas...

Convection that forms along the mountains of Mexico will have
enough of an eastward push that some may cross into the Del Rio and
Eagle Pass areas. Isolated instances of flash flooding may develop,
therefore a Marginal Risk remains in effect.

...Southern Appalachians...

Storms are likely to form in the deep tropical moisture indirectly
associated with Beryl with the mountains acting as the primary
forcing. Numerous flood sensitive areas in western NC may support
flash flooding with any slow moving or stationary storms. A
Marginal Risk area remains in effect.

...Northeast...

Deep tropical moisture pumping north well ahead of Beryl will make
for anomalous PWATs across New England. The Appalachians of this
region may also act as forcing for widely scattered thunderstorms
capable of heavy rainfall. A Marginal Risk area is in effect for
across this region.

Campbell


Day 2
Valid 12Z Wed Jul 10 2024 - 12Z Thu Jul 11 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
UPSTATE NEW YORK THROUGH MUCH OF VERMONT AND NORTHERN NEW
HAMPSHIRE...

...20Z Update...

A Moderate Risk was introduced across portions of Upstate New York
through much of Vermont and Northern New Hampshire. More on this
setup within the the "Northeast" sub-heading below...

Elsewhere, changes were minimal within the SLGT risk over south-
central NM as locally heavy rainfall over the Sacramento Mountains
is increasingly likely during tomorrow afternoon. This will
interact with a highly sensitive area due to multiple burn scars
within the terrain. A higher probability for impact reflects the
necessity for a maintenance of the SLGT risk in place. The stalled
front over the Central Gulf Coast will lead to a focal point for
training convection as thunderstorms continue to fire within
proximity of the boundary. Very high FFGs within the encompassing
MRGL risk will limit flash flooding to very isolated and mainly
within larger urban zones like New Orleans and towns based in the
southeastern Parishes. Guidance remained consistent on the
potential within that area of the CONUS, so maintained continuity
for the time being, but could see this downgraded if the threat
shifts further south, off the coast.

...Northeast...

Remnant moisture and mid-level energy from Beryl will translate
northeastward out the Ohio Valley and Great Lakes creating a
heightened threat for widespread heavy rain and flash flooding
across Southeast MI through NY state and Central/Northern New
England. At the surface, a warm front will bisect much of NY
through Central New England, marked very well by a sharp theta-E
gradient within the latest deterministic suite. This will be a
focal point for convection prior to the main circulation moving
overhead, as well as the axis of where the highest PWATs will be
confined during the unfolding of the synoptic scale event. Low-
level instability will be highest within the confines of the warm
front and points south with the best upper level forcing likely
along and north of the boundary creating a dynamic scenario with a
zone of highest heavy rain potential where all three
characteristics overlap. The signal is becoming more pronounced
after the latest 12z suite with the highest heavy rain prospects
lying within the Adirondacks and points east into north-central VT
and NH, including the Green and White Mountains in the respective
states.

12z HREF EAS probabilities were much more aggressive in the signals
for at least 2" and 3" across the aforementioned areas with a
50-80% probability for at least 2" within the Adirondacks to just
south of the Champlain Valley, a strong signal for higher totals
given the necessary overlap of CAMs to exhibit such a larger
probability. Historically, when an EAS signal is above 70%, the
expectation for widespread coverage of that value of rain or more
is very certain and regardless a higher risk consideration if
the areal FFG indices allow. HREF EAS for 3" was not as strong, but
still manages a large coverage of 20-35% with the highest potential
across the Adirondacks and northern Mohawk Valley. This is right
within the inflection of where the warm front is forecast to
reside, creating a zone of higher confidence for heavy rainfall.
The main prospects for flash flooding will occur between 18z
Wednesday until about 06z across NY state and till the end of the
period for points further north and east.

Heavy rain will be possible all the way into ME where the elevated
PWAT anomalies between +2 to +3 standard deviations will be
recognized leading to a higher end SLGT risk residing from
Southeast MI all the way into western and central ME. The area of
highest potential is not characterized by a Moderate Risk upgrade
located across much of Upstate NY and Vermont, as well as Northern
New Hampshire with emphasis on the White Mountains where complex
terrain and topographic ascent will create a potential QPF maxima
away from the primary zone. Totals of 2-4" are anticipated with
local maxima to 6" plausible within the above areas in the MDT
risk. 1-3" will be possible as far south as central and
northeastern PA up through the NY Capital District due to
convection developing along and ahead of the trailing cold front
moving through the area tomorrow afternoon and evening.

Kleebauer

...Previous Discussion...

...Central Rockies and portions of the Southwest...

Continuation of the monsoonal moisture and diurnal heating will
keep convection possible across portions of the Central and
Southern Rockies and southwest New Mexico during this period.
Higher QPF is expected to concentrate in the vicinity of the
Sacramento Mountains where there have been a few recent wildfires.
The burn scar complex (Blue-2, South Fork, Salt and McBride) is
very sensitive to anything more than a gentle, light rain. QPF is
forecast to range from 0.25 to 1 inch across portions of central
New Mexico which elevates the threat for flash flooding and debris
flows. A Slight Risk was raised for this part of the state given
the aforementioned sensitivity.

...Central Gulf Coast...

A stalled frontal boundary along the Gulf Coast may continue to
provide focus for convection capable or producing heavy rainfall. A
Marginal Risk area remains in effect.

Campbell


Day 3
Valid 12Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST THROUGH THE MID ATLANTIC, AS WELL AS THE SOUTHWEST
U.S...

...20Z Update...

The SLGT risk from previous forecast was downgraded due to
significant changes from previous deterministic output as moisture
and ascent from the remnants of Beryl will be exiting quickly out
of New England through Thursday morning. More of the change in the
"East Coast" sub-heading below...

Elsewhere, monsoonal activity will be focused across portions of AZ
into NM where a continuation of flash flood prospects within area
burn scars and smaller urban corridors will maintain some chance
for flooding over the course of the period. Local totals of up to
1" signal a lower-end threat, however it remains within the MRGL
risk threshold leading to little change in the previous forecast.


...East Coast...

Remnant moisture and ascent from Beryl will continue to push out
of New England with the main threat confined to the morning hours
of Thursday across Northern New England. Additional totals of up to
1-1.5" over the northern tier will lead to some local flash flood
potential before the event subsides, so wanted to cover bases for
the threat. The SLGT risk from prior was not conducive given the
current QPF forecast and trends within the ensemble fields to limit
the threat closer to a MRGL risk.

Further south, a cold front trailing the primary low across Quebec
will be the focus for convective potential with the highest risk
of flash flooding aligning within the Delmarva coastal plain down
through the eastern Carolinas. An approaching upper level
disturbance of the Atlantic will round the western fringe of the
ridge out in the western Atlantic leading to an advection of
enhanced upper forcing and moisture as it moves closer to the
Carolina coast. Surface convergence pattern along and ahead of the
slow- moving frontal boundary will create a period of enhanced
rainfall after the diurnal destabilization window reaches peak and
ignites a widespread area of thunderstorms within an above average
PWAT environment (+2 standard deviations). The slow forward
propagation of cells will create instances of training storms
capable of totals reaching 2-4" within the area mentioned above in
the Mid Atlantic. There is an opportunity for an upgrade across the
VA Tidewater down through eastern NC if the signal remains
consistent or enhances further as the combo of ascent from the
front and the upper level disturbance will create a formidable
window of opportunity within that region.

Kleebauer

...Previous Discussion...

...Southwest...

The monsoonal pattern of diurnal convection will persist thus
maintaining an elevated threat for localized flash flooding. A
Marginal Risk remains in effect for portions of eastern Arizona and
much of New Mexico.

Campbell


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