Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Fri Aug 2 2024
Day 1
Valid 16Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST, OHIO VALLEY, AND NORTHEAST, ALONG WITH THE EASTERN
TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS...
...16Z Update...
Upper energy across Western PA will pivot eastward through the
afternoon with large scale ascent focusing more into Central and
Eastern PA into portions of Southern NY state near the Lower Hudson
Valley by this evening. The departure of the focused ascent across
Western PA will lead to a cull, of sorts with regards to the
convective coverage that has been plaguing the area all morning. In
coordination with the local Pittsburgh WFO, have removed the
previous SLGT risk located across much of Western PA, but a non-
zero threat of a few stronger cells could cause some localized
flash flood concerns within the aforementioned zone.
Elsewhere, the forecast remains on track with the broad forcing
shifting eastward into the Northern Mid Atlantic allowing for
upscale growth of convection from the Susquehanna Valley to points
east as the southwesterly regime ahead of the mean trough in the
Ohio Valley pushes the main vorticity maxima downstream. Latest 12z
HREF blended mean QPF footprint continues to depict the heaviest
totals (>1.5") across Eastern PA into Southern NY where the
greatest large scale forcing will coincide with the deeper moisture
returns that have advected northeast through the morning, and will
continue into the afternoon and evening. 12z soundings out of KPIT
this morning indicated PWATs greater than 2" which is the
environment that is forecast to advect downstream in the area of
interest, placing much of the Mid Atlantic in 1-2 deviations above
normal PWAT anomalies based off the latest ECMWF ENS and NAEFS
output. This is a sufficient environment for locally heavy rainfall
and will allow for a heightened threat of flash flooding within any
slower moving cores and/or areas with a larger urban footprint. The
SLGT was maintained through Central and Eastern PA into part of the
I-95 corridor southwest of NYC down into Northeastern MD.
Across the Southern Appalachians, a mid-level speed max will rotate
around the base of the trough to the north with the nose of the jet
core entering into the eastern half of TN and Western NC by the
afternoon. Decent surface buoyancy on the order of 1000-2000 J/kg
SBCAPE will be located across the I-75 corridor with a max of
2000-2500 J/kg located over the terrain in Western NC. The
combination of buoyancy and increased large scale ascent will aid
in scattered to widespread convection later this afternoon with
neighborhood probabilities for at least 3" still hovering around
20-30% for a broad area encompassing the above region. Considering
the lower FFGs across the area thanks to a multi-day convective
episode that has already led to flooding in areas highlighted, the
threat will remain on the higher side for flash flooding concerns
despite a relatively lower magnitude of rainfall compared to
recent days. Thus, the SLGT risk was maintained for continuity.
The area across the Ohio Valley will continue to see scattered
convective development under the periphery of the upper low,
trough center with a slower storm motion and higher potential for
training. The signal is still fairly robust for flash flood
concerns on the southern side of Lake Erie out into Northwestern OH
and far Southeast MI with the latest HREF probabilities for 3hr
FFG exceedance running between 25-45% with the highest located
under the expected path of the upper low as it pivots eastward out
of IN. This is more than sufficient for a continuation of the
previous SLGT risk with only minor modifications from the previous
forecast.
Lastly, the MRGL risk was extended to the north across Southwestern
UT for increasing signals of isolated heavy rainfall within the
Zion National Park region which includes several flood prone slot
canyons in the vicinity. In coordination with the Salt Lake City
WFO, the MRGL was added to encompass the more sensitive areas in
that region.
Kleebauer
...Previous Discussion(s)...
...Eastern U.S....
Slowly advancing upper trough and surface front will focus broad-
scale ascent for scattered-numerous convective elements across
much of the eastern CONUS. Two areas of particular concern include
a W-E corridor from SE MI and the OH Valley eastward to NJ and
southern NY, where the deep-layer forcing (DPVA) ahead of the
opening mid-level low will be a bit more robust. The combination of
favorable deep-layer instability, especially during peak heating
hours (MLCAPEs 1000-2000+ J/Kg) along with TPW values of 1.75-2.00+
inches will make hourly rainfall rates of 1-2+ inches likely
underneath the strongest cells, with potentially higher hourly rates
(2-3") farther east toward eastern PA, NJ, and southern NY given
the closer proximity to the max TPW and low-level moisture
transport axes.
Farther south, the guidance is also favoring eastern portions of
the TN Valley into the southern Appalachians of northern GA-western
NC for more widespread convection and an enhanced risk of flash
flooding (i.e. Slight). This given the upslope low-level component
along with a more vigorous vort lobe traversing the area.
Elsewhere, any flash flooding is expected to be more
isolated/localized, thus the Marginal Risk areas.
...West...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
International Border (precipitable water values up to 1.75"),
another round of late day/evening showers and thunderstorms is
expected. Continuity was generally maintained, though we did expand
the Marginal Risk across the Sierra Nevada, Transverse Ranges, and
adjacent foothills. Hourly rain totals to 2" should be possible.
Locally heavy rainfall would be most problematic in any area burn
scars and dry washes/arroyos.
Hurley
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Fri Aug 2 2024
Day 2
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN
FLORIDA PENINSULA, FLORIDA KEYS, AND PORTIONS OF THE SOUTHEAST AND
SOUTHERN MID ATLANTIC...
...20Z Update...
Minor adjustments were made to the MRGL risk areas across the
Southwest, West, and Upper Midwest. The general consensus for
those areas remain a more isolated threat for convective flash
flooding with each area exhibiting modest instability and focused
large scale ascent thanks to shortwaves rippling through the outer
edge of a broad ridge centered over the Interior West. The
Southwestern U.S will remain the better risk of flash flooding due
to the remnant burn scars and more sensitive locales encompassing
parts of NM/AZ and UT. The MRGL risk was expanded into portions of
the Central and Southern High Plains as a stronger mid-level
perturbation continues to pivot south out of the Front Range with a
redevelopment of convection expected Saturday afternoon in-of
Western KS into the OK and TX Panhandles. Enough forcing and
instability have allowed for more emphasis on the potential
compared to previous forecasts, so wanted to include the area of
interest given some modest probabilistic depictions via the HREF
and upper quartile NBM output.
The MRGL risk was maintained across the Central Mid Atlantic and
Northeast, however there is an opportunity for a targeted upgrade
in future updates pending the outcome of today's convective
impacts. Latest UFVS First Guess Fields do place a SLGT risk across
the Lower Susquehanna Valley through much of Eastern PA. This is
likely due to the total QPF output and signal for heavy rainfall
Saturday afternoon and evening along the frontal convergence zone
in place from NoVA through the Lower Hudson Valley. Area FFGs are
running higher in those areas compared to normal, but the priming
from today's precip could bring those down to more reachable
levels, thus implying a better chance for an upgrade. This will be
reassessed in later updates so long as guidance remains at least
continuity or increases their heavy rainfall footprint.
The SLGT risks are updated below within their respective sub-
headings...
...South Florida...
Tropical wave over Eastern Cuba will continue to become better
organized over the Florida Straits the next 24 hours with a
stronger core allowing a more defined circulation with heavy rain
encompassing the circulation and general surrounds. Latest National
Hurricane Center (NHC) forecast has labeled the convective cluster
as AL4 alluding to an expected tropical disturbance sometime in
the next 24-36 hours. This has prompted a greater emphasis on heavy
rain prospects within the Southern FL Peninsula down through the
Florida Keys who will see the initial brunt of the wave beginning
Saturday morning. A persistent onshore flow across the Southeast FL
coast will lead to increasing frictional convergence with outer
banding feeding into the storms eastern flank of the circulation.
This is classic with evolving tropical disturbances and a sneaky
way for areas well away from the storm center to see heavy rainfall
in the form of repeated bands and training that would allow for
flash flood threats to arise. Ensemble guidance, both hi-res and
global resolutions signal a stronger case for more widespread heavy
rain between 2-5" expected along the population centers based from
Miami up to just south of West Palm Beach. This area is notorious
for flash flooding due to the focused lower FFGs within a sprawling
urban footprint that runs up I-95. The threat for heavy rain is
also present within the Florida Keys with the combination of the
center of the wave moving overhead, as well as trailing bands
remaining persistent in-of the entire island chain. Totals of 3-6"
with locally higher are forecast across the Keys with the highest
totals located near and south of Marathon down into Key West. In
coordination with the local Miami and Key West WFOs, the SLGT risk
from the previous period was expanded southeast to include all of
the Southern FL Peninsula, as well as the entire island chain of
the Florida Keys.
Further northwest into the Southwestern side of FL, the threat for
significant rainfall has increased substantially with NHC's
forecast depicting a close proximity of what is trending to be the
next tropical storm as the circulation wanders just off the
Southwest FL coast. A steady south to southwest flow around the
eastern flank of the low will create an onslaught of very heavy,
efficient tropical rainfall poised to migrate onshore around the
Southwestern Everglades up through the populated areas of Naples
and Fort Meyers. HREF neighborhood probabilities are about as high
as you could expect for a nearby tropical disturbance with >5"
signals running between 60-80% along the coast with >8" a
formidable 25-45% positioned right over the coastal areas of Lee
and Collier counties. The previous SLGT risk has plenty of merit
with even some inference of a potential upgrade given the signals
present. There is some modest uncertainty on the exact location of
the circulation as it makes progress to Southwest FL latitude as a
further westward track would mitigate some of the heavier rainfall
to just off the FL coast. The the threat is currently sufficient to
label as a high-end SLGT with an upgrade likely over the Southwest
coast if there begins to be increasing confidence in the exact
track hugging the western FL coastline. This is an evolving
situation, so stay tuned for future updates as we move closer to
the event.
...Southeast and Southern Mid Atlantic...
Increasing convergence pattern along an approaching cold front will
allow for greater convective coverage and magnitude across portions
of Southern VA down through the Central Carolinas into far Eastern
GA. A surface trough will bisect the above locations with a cold
front migrating eastward out of the Carolina Piedmont.
Destabilization from diurnal heating will present a strong
instability pattern located over the interior portions of the
Carolinas with several deterministic aligning a 3000-4000+ J/kg
SBCAPE signature from Columbia up through Southeastern VA. In
combination with PWATs running ~1 deviation above normal, this will
be sufficient for locally heavy rainfall with higher threat of
urban flooding, especially considering the mean flow being lighter
as the trough continues to lift northward. The best signature for
heavy rain lies within Central NC where 12z HREF EAS probabilities
are running between 50-80% for at least 1" of rainfall, a strong
signal for the EAS with historical precedence for at least a SLGT
risk when depicted across most areas east of the Appalachians. This
was enough consensus, along with relatively high neighborhood
probabilities for at least 2" and 3" respectively in the same
locations with low-end >5" probabilities within the Carolina
Piedmont. The SLGT risk from previous forecast was maintained with
little change in the orientation.
Kleebauer
...Previous Discussion(s)...
...Southwest...
Monsoonal convection over the Southwest (precipitable water values
up to 1.75" near the International Border) continues to support a
Marginal Risk area. Locally heavy rainfall would be most problematic
in any area burn scars and dry washes/arroyos.
...Sierra Nevada into Oregon Cascades, Blue Mountains, and parts
of the Northern Rockies to western portions of the Great Basin...
Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations
above normal per the GEFS) will advect northward through the West
on the backside of the upper ridge and ahead of the shortwave
trough off the PAC NW coast. The guidance gets TPW values up to
1-1.25", which would be most anomalous in terrain. Given these
moisture profiles, along with the diurnal heating (MLCAPEs peaking
at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa
1" will make for localized runoff issues, especially over burn
scares.
...Upper Midwest...
The guidance is generally showing a modest signal for heavy
rainfall in and near Minnesota this period. Precipitable water
values rise about 1.25" with a warm advection pattern. This led to
an introduction of a Marginal Risk here, as flash flood guidance
values are modest after recent heavy rainfall.
Hurley
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Fri Aug 2 2024
Day 3
Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
FLORIDA PENINSULA AND PARTS OF THE SOUTHEAST COASTAL PLAIN...
...20Z Update...
Only minor changes were necessary to any of the risk areas across
the CONUS with the largest change occurring over the Northern
Plains and Upper Midwest with the previous MRGL risk expanded to
the east to reflect a faster forward propagation of a shortwave
traversing the area. The main concern for the period continues to
be across Florida into the Southeastern Coastal Plain thanks to an
eventual tropical disturbance/storm migrating north and northeast
along the western FL coast before cutting across the northern half
of the Peninsula. There is still too much uncertainty on the exact
location of the general circulation of the disturbance, so wanted
to maintain continuity based on the ensemble mean QPF and some of
the better probabilities for heavy rainfall within the 75th
percentile of the NBM 24hr QPF.
The area across the Mid Atlantic and Northeast is subject to the
location of a stationary front where the primary focus for
convection will be located as this will ultimately determine the
western extent of the flash flood threat. The setup would allow for
better urban flash flood prospects with the frontal positioning
bisecting the urban corridor from DC up through Southeast New
England. The MRGL risk was maintained with some minor adjustments
based on the 12z ensemble bias corrected QPF footprint.
...Florida through the Southeast Coastal Plain....
AL4 will eventually evolve into a defined tropical cyclone with the
latest NHC forecast developing the system into our next tropical
storm sometime during D2, carrying through the D3 time frame. The
uncertainty of exact location of the low given the timing and
interaction with the approaching trough/front to the north has
limited the capabilities for discerning specifics for the Sunday
into Monday period. There is some general consensus on heavy
rainfall potential within the Western FL coastal plain, including
areas like Sarasota up through Tampa and Clearwater before the
ensemble forecast takes the low up through the north-central FL
Peninsula. If this track verifies, this would send a dearth of
heavy rainfall across interior areas with prevailing onshore flow
to the north ransacking the Southeastern U.S coastal areas in GA
into SC. Some models try to develop a Predecessor Rain Event (PRE)
across the Southeast coast thanks to the timing of the system's
approach with interaction of the coastline. This something to
monitor in future updates as these phenomena are tough to gauge at
range, but can be focused significant rain events where they occur.
Regardless if a PRE forms or not, the highly anomalous tropical
moisture flux and persistent heavy rain banding around the storm
circulation will cause locally enhanced flash flood concerns across
much of FL into the GA/SC coast. The SLGT risk was maintained given
the ensemble QPF continuity and expected NHC track with the
opportunity for future upgrades pending increased confidence on the
storms forward motion.
Kleebauer
...Previous Discussion...
...Montana into the Northern Plains and Upper Mississippi
Valley... Aforementioned axis of anomalous TPW out West during Day
2 will ultimately get absorbed in the upper level jet streak and
mid- level vort lobe that will cross MT and the Dakotas on Day 3.
Dynamical forcing along with the thermodynamical profile will favor
more widespread showers and thunderstorms, though the shear
profiles will certainly favor more progressive/forward-propagating
cell motions and thus make for more of an isolated flash flood
threat. Areas across northwest MT, including Glacier NP, would be
the most susceptible given the lower FFG (1" or less within 3 hours
for most areas). An upgrade to Slight may be needed over this
region in later outlooks, especially if we start seeing a consensus
of heavier QPF from the models.
Hurley
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
318 PM EDT Fri Aug 2 2024
Heavy rain is likely along the path of Potential TC Four, but with
plenty of uncertainty with how much falls onshore versus offshore
depending on the system's exact track. A Slight Risk lingers into
Monday across northern Florida with any feeder bands behind the
storm, and is also in place northward into southeastern Georgia and
eastern South Carolina for Monday/Day 4. A Slight Risk was
introduced for Day 5/Tuesday for much of the coastal Carolinas as
the system slowly moves northeast. These risk areas attempt to
follow the current forecast track of Potential TC 4, i.e. offshore
but near enough to land to spread heavy rain amounts that may cause
flash flooding inland. Expect further adjustments as the forecast
evolves.
Farther north, models show a potential mesoscale convective system
riding the top of the ridge into the Upper Midwest/Upper Great
Lakes on Monday. For this activity, a Marginal Risk on the Day 4
ERO stretches from the northern High Plains to the Lower Great
Lakes region with an embedded Slight Risk across parts of southeast
Minnesota and southern Wisconsin given ample moisture,
instability, and some soil sensitivity. While rainfall amounts are
more uncertain farther east, some guidance showing 1-2 inches of
rain across northern New England in an unstable environment front
seemed to warrant a Marginal Risk there. Then by Day 5/Tuesday,
moisture streams could combine in the Lower Great Lakes to
Northeast to focus rain near the front in what might be
considered a predecessor rain event (PRE). Lack of model
consistency in placement of rain amounts precluded any embedded
Slight Risk within the larger Marginal at this time.
Monsoonal moisture on the western side of the ridge should keep a
wet and unsettled pattern in place from the Four Corners region
northward into the Rockies/High Plains through much of the
extended period. Marginal Risks for excessive rain are in effect
for parts of the Southwest/Four Corners for Monday and Tuesday,
with some expansion of the risk area west on Tuesday with the
meandering moisture plume and to cover the sensitive slot canyons
in southern Utah.
The West can expect above normal temperatures into next week that
gradually reduce in magnitude. The Southwest should see the
longest-lasting heat concerns, with daytime highs exceeding 110F
likely, but this is only a few degrees above average. Meanwhile,
hazardous heat will be likely for the Central/Southern Plains and
Middle/Lower Mississippi Valley early in the week. After a cold
front passes southward through the central U.S. Tuesday-Wednesday,
excessive heat will then focus across the Southern Plains, Lower
Mississippi Valley, and Southeast. Experimental HeatRisk values are
generally major for much of this region to even extreme for the
central Gulf Coast area. The north-central U.S. should remain near
to below normal through the period as weak shortwave energies move
through.
Tate
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
318 PM EDT Fri Aug 2 2024
Heavy rain is likely along the path of Potential TC Four, but with
plenty of uncertainty with how much falls onshore versus offshore
depending on the system's exact track. A Slight Risk lingers into
Monday across northern Florida with any feeder bands behind the
storm, and is also in place northward into southeastern Georgia and
eastern South Carolina for Monday/Day 4. A Slight Risk was
introduced for Day 5/Tuesday for much of the coastal Carolinas as
the system slowly moves northeast. These risk areas attempt to
follow the current forecast track of Potential TC 4, i.e. offshore
but near enough to land to spread heavy rain amounts that may cause
flash flooding inland. Expect further adjustments as the forecast
evolves.
Farther north, models show a potential mesoscale convective system
riding the top of the ridge into the Upper Midwest/Upper Great
Lakes on Monday. For this activity, a Marginal Risk on the Day 4
ERO stretches from the northern High Plains to the Lower Great
Lakes region with an embedded Slight Risk across parts of southeast
Minnesota and southern Wisconsin given ample moisture,
instability, and some soil sensitivity. While rainfall amounts are
more uncertain farther east, some guidance showing 1-2 inches of
rain across northern New England in an unstable environment front
seemed to warrant a Marginal Risk there. Then by Day 5/Tuesday,
moisture streams could combine in the Lower Great Lakes to
Northeast to focus rain near the front in what might be
considered a predecessor rain event (PRE). Lack of model
consistency in placement of rain amounts precluded any embedded
Slight Risk within the larger Marginal at this time.
Monsoonal moisture on the western side of the ridge should keep a
wet and unsettled pattern in place from the Four Corners region
northward into the Rockies/High Plains through much of the
extended period. Marginal Risks for excessive rain are in effect
for parts of the Southwest/Four Corners for Monday and Tuesday,
with some expansion of the risk area west on Tuesday with the
meandering moisture plume and to cover the sensitive slot canyons
in southern Utah.
The West can expect above normal temperatures into next week that
gradually reduce in magnitude. The Southwest should see the
longest-lasting heat concerns, with daytime highs exceeding 110F
likely, but this is only a few degrees above average. Meanwhile,
hazardous heat will be likely for the Central/Southern Plains and
Middle/Lower Mississippi Valley early in the week. After a cold
front passes southward through the central U.S. Tuesday-Wednesday,
excessive heat will then focus across the Southern Plains, Lower
Mississippi Valley, and Southeast. Experimental HeatRisk values are
generally major for much of this region to even extreme for the
central Gulf Coast area. The north-central U.S. should remain near
to below normal through the period as weak shortwave energies move
through.
Tate
