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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0051Z Jul 12, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
850 PM EDT Thu Jul 11 2024

Day 1
Valid 01Z Fri Jul 12 2024 - 12Z Fri Jul 12 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHEAST
VIRGINIA AND EASTERN CAROLINAS...

...01Z Update Summary...
Given support from the 18Z HREF, HRRR and NAM for convective
redevelopment later tonight in the eastern US...maintained the
Slight Risk from the Carolinas into the southern Delmarva. Also
maintained the Marginal risk area over parts of the desert
southwest. Elsewhere...convection has shown a downward trend with
the loss of daytime heating so felt confident enough to remove the
Marginal risk ares over southeast Florida and parts of the Midwest.

Bann

...16Z Update Summary...

SLGT risk across the Carolinas into Southern Virginia was expanded
to the west to align with the current Flood Watch collaborations
with some of the local WFOs. A new MRGL risk was added to the
Southeast FL coast to account for a local sea breeze enhancement
within the urban corridor extending from just north of Fort
Lauderdale down through the Miami Metro. The remainder of the MRGL
risk areas across Maine, the Midwest, and Southwestern U.S were
maintained given spatial continuity in scattered QPF signatures
within their respective synoptic/mesoscale evolutions.

Kleebauer

...Coastal Mid Atlantic...

...16Z Update...

A sheared axis of heavy rainfall is currently bisecting a portion
of the Eastern Carolinas with local rainfall rates exceeding 2"/hr
in some of the pulse cells along the stalled frontal boundary.
Recent radar/satellite trends indicate a formidable pulse
convective environment with totals along and east of NC11 and US13
producing totals of 3-6" in some portions of the above zone. The
axis of precip will be reluctant to move as the cells will remain
anchored to the boundary over the course of the afternoon which
could lead to some totals exceeding 8" within the course of impact.
There will be a decay in the current evolution after 21z, but not
before a thin band of heavy rain will induce flash flooding
concerns for several more hours. The SLGT risk was maintained with
a higher end SLGT located within that small corridor where rainfall
will continue.

Energy from the leading edge of a disturbance approaching from the
southeast off the Atlantic will redevelop multiple bands of
convective rainfall later this evening through tomorrow morning
with a general expanse to the northwest overnight. Some of those
cells could produce enhanced rainfall with rates exceeding 2"/hr,
especially over Southern and Southeastern VA down into Eastern NC.
The current HREF QPF footprint outlined the secondary convective
bands very well with a QPF maximum situated over portions of
Southern VA with the northern extent reaching Southern MD,
extending down into to the NC/VA border. As a result, the SLGT was
also expanded a bit to the north with the accompanying MRGL also
being spread further northwest to account for the trends.

Kleebauer

...Previous Discussion...

Stalled frontal boundary will bisect portions of the Eastern
Seaboard with elevated PWATs mirroring the alignment of the front
to points east. Local environment will be primed for convective
potential, but will have some assistance in upper level support as
a meandering wave off the Southeast coast lifts northwest on the
western fringe of the ridge in the Atlantic, acting as a beneficial
forcing mechanism to enhance regional rainfall potential. The best
chance will lie from the Southern Delmarva through the Virginia
Tidewater down into the Eastern Carolinas. Latest HREF
probabilities are fairly aggressive in their signature for local
totals between 2-5" as neighborhood probs for 2-3" totals are
running between 60-90% with an area of 5" probabilities between
50-70% across Eastern North Carolina. This area has been very dry
as of late, and is well reflected within the FFG indices present
for 1/3/6 hour markers. Isolated to scattered urbanized flooding
and/or flash ponding possible for 2-4 inches/hour rates as per the
latest HREF mean hourly QPF output. A Slight Risk was raised for
the North Carolina coast and a very small potion of southeast
Virginia and northeast South Carolina. A Marginal Risk spans from
South Carolina to southern New Jersey.

Kleebauer/Campbell

...Northern Maine...

...16Z Update...

Remnant moisture and trailing mid-level energy behind Beryl's
circulation will keep the threat of convection across Northern and
Central ME for at least the next 3-6 hrs before fully escaping into
the Canadian Maritimes. Probability fields are not as enthused as
yesterday for significant totals, but antecedent conditions along
with any convective focus could cause some localized flood concerns
in-of the terrain up to the Canadian border near Caribou. The
threat was sufficient enough to maintain continuity, especially
with the elevated moisture presence still lingering across the
northern tier.

Kleebauer

...Previous Discussion...

Lingering convective threat across Northern Maine could induce
some local flash flood concerns, mainly for northern and Downeast
Maine. The latest guidance continues to signal the highest QPF across
Downeast Maine where the highest PWAT anomalies remain leading to
some deterministic output indicating 1-2" of rainfall within the
first 3-6 hours. The Marginal Risk was kept for this issuance, but
may be considered for removal is convection dissipates sooner than
forecast.

...Midwest...

...16Z Update...

There are no changes necessary to the previous MRGL risk across the
Midwest as scattered thunderstorms within a much lower FFG
footprint across the region thanks to impacts from Beryl will
create a localized threat for flash flooding as a shortwave trough
traverses overhead.

Kleebauer

...Previous Discussion...

Trailing shortwave trough pivoting southeast around the backside of
the mean trough over the Great Lakes and Ohio Valley will
strengthen as it moves across Iowa into Illinois by tomorrow
afternoon with large scale forcing increasing across the Central
Midwest and adjacent Mid-Mississippi Valley. Convection is expected
to fire up across Illinois, Missouri and Indiana and within these
storms some will be stronger and capable to producing hourly rates
of 1 to 2 inches/hour. Much of the region was soaked by Beryl as is
passed through and soils have not fully recovered. Antecedent
moisture in the top layer of the soils is well-defined with NASA
SPoRT moisture percentiles relaying the 0-10cm layer running
between 75-90% leading to higher runoff capabilities. Given the
sensitive nature of soils and the potential for a few inches, a
Marginal Risk for excessive rainfall was maintained for this
period.

Kleebauer/Campbell

...Southwest...

...16Z Update...

No changes to the previous forecast as the Monsoonal pattern and
forecasted QPF within zones of higher impact (Burn scars) remains
on track.

Kleebauer

...Previous Discussion...

The persistent monsoonal pattern will continue to support highly
isolated convection with the primary areas of interest being the
burn scars in New Mexico and small urban threat of towns within
complex terrain across Southeast Arizona through much of New
Mexico. A Marginal Risk remains in effect for southeast Arizona and
a majority of New Mexico.

Campbell

...Southeast Florida...

Sheared axis of convection is currently positioned off the east
coast of FL with accompanying energy slowly migrating southwest
towards the Southeast coast of FL. Sea breeze will propagate
inland, but will be met with the west coast sea breeze to initiate
a strong convergence signal in-of of the urban corridor along the
Southeast FL coast. This will include areas like Fort Lauderdale
down to the Miami Metro with some guidance fairly bullish in their
QPF representation. Totals of 2-4" with locally upwards of 6" will
be plausible as the environment remains conducive for heavy
rainfall given the forecast sounding signatures of tall, skinny
CAPE, SBCAPE around 2500-3000 J/kg, and a steering pattern focused
parallel to the coastline. This type of setup has historical
precedence in being a sneakier flash flood setup and the addition
of a MRGL risk was in agreement with the Miami WFO.

Kleebauer


Day 2
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND THE CAROLINAS...

...20Z Update...

Previous SLGT risk was expanded further north and west to encompass
the I-95 corridor from DC to NYC given the latest trends in
proposed QPF. The overall synoptic evolution remains fairly similar
from previous discussion, however the expansion of the convective
pattern towards the north and west was evident within the latest
ensemble QPF output. 12z HREF probability fields were also more
pronounced with the neighborhood 3" probability range between
40-60% along and west of I-95 from Philadelphia down through
Central NC. 60-90% is located to the east of I-95 with the highest
probabilities centered over the Northern Delmarva, south-central
VA, and Eastern NC, most of these areas expected to see multi-day
impacts from heavy convection from the pattern evolution. Hourly
will be highest across the Carolinas up to the Virginia Tidewater
as evidenced by the 2"/hr probability fields with a degrading
signal north of US50.

Environmental factors are more than capable to allow for a higher
end evolution, so the current forecast SLGT is bordering the
higher-end of the thresholds with a non-zero opportunity for a
further upgrade, especially south and east of the I-95 corridor.
With flow becoming parallel to the front, there's a greater
opportunity for training convection within the confines of the
boundary. This is likely the case for the higher neighborhood and
EAS probabilities in-of the Mid Atlantic down into the Carolinas.
Tropical moisture involvement and those environmental distinctions
alone have allowed for a greater emphasis on the potential, and
could be exacerbated if the population centers from Richmond to
Philadelphia get involved more than currently forecast.

Kleebauer

...Previous Discussion...

A second round of slow moving convection is expected near the
stalled frontal boundary. The greater impacts of scattered
instances of flash ponding and flooding be primarily be focused
over urbanized areas and areas that see repeat heavy rains from
the period prior. The ridge will assist the convection spreading
rains further inland. The Eastern Carolinas will remain the
primary target for heavier rainfall with some areas in the two day
period potentially receiving over 6 inches from the evolving
event.

Campbell


Day 3
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024

...THERE ARE MARGINAL RISKS IN PORTIONS OF SOUTHERN NEW ENGLAND,
COASTAL SOUTHERN MID ATLANTIC, NORTHERN MIDWEST, AND THE
SOUTHWESTERN U.S...

...20Z Update Summary...

The previous MRGL risk areas across the Upper Midwest and
Southwest U.S were generally maintained outside a few modifications
to account for the latest QPF trends and First Guess field
alignment(s). An additional set of Marginal Risk areas were added
to the Hampton Roads area down through Eastern NC as well as across
the eastern sections of Southern New England.

Kleebauer

...Southwest...

...20Z Update...

The threat across the Southwest will be relegated to
a typical Monsoon based isolated to scattered local impacts with
the main areas of interest occurring within slot canyons, burn
scars, and smaller urban corridors within AZ and NM. Precip
forecast is relatively benign, but there are pockets of higher QPF
littered within the regional domain which is classic for this time
of year with the Monsoonal component. The MRGL risk was more than
sufficient given the setup.

Kleebauer

...Previous Discussion...

There will be a resurgence of monsoonal flow back into the
Southwest. Diurnal heating will help fire up convection during the
afternoon and evening hours with the potential for higher rainfall
fall rates. Maintained the Marginal Risk area for much of Arizona,
northwest New Mexico and southwest Colorado for this period.

Campbell

...North Dakota and Minnesota...

...20Z Update...

Scattered convective signals remain across northeastern ND through
northern MN with a growing emphasis on potential further southeast
into WI. The latter portion of the areas mentioned is still a bit
light for a potential expansion of the MRGL risk, but the First
Guess Fields have introduced a MRGL risk signal into the WI area as
some organized convective prospects could add the area into the mix
in future updates. Spread among guidance for exactly where the
heaviest convection will develop is still an issue and is one of
the reasons for a lack of a higher risk area. That and
climatologically the area over northern MN is less prone to flash
flooding outside a significant, more organized synoptic scale
setup. For this reason, the MRGL was preferred with some time for
interrogation of the convective threat the next set of forecasts,
this time with the introduction of CAMs.

Kleebauer

...Previous Discussion...

A shortwave trough will be tracking east across Canada and will
send a surface cold front south-southeast through the Canadian
Prairie and into the northern tier U.S.. Showers and thunderstorms
are expected to form along and ahead of this feature which will
predominately focus over North Dakota and northern Minnesota
during this period. This part of the country has been above/well-
above normal on moisture for the past few months and remain
sensitive to additional rainfall. A Marginal Risk is in effect for
eastern North Dakota and northern Minnesota.

Campbell

...Southeast Virginia and the Carolinas...

The back end of the precip threat from D2 will spill over into D3
across the Eastern Carolinas up into the Hampton Roads/VA Beach
area on Saturday morning. The front that has been a focal point
over the previous periods will finally begin pressing off the
Atlantic coast, but very slowly at latitude leaving some remnant
moisture and convection lingering across areas that will have seen
significant precip prior periods. An additional inch or two is
still plausible across the impacted area, more than enough to cause
some issues if it occurs over a spot that has seen copious amounts
of rain earlier in the week. A MRGL risk was introduced, mainly to
cover for the continued threat from D2 with an eventual end
sometime late Saturday morning or early afternoon.

...Southern New England...

Sheared mid-level vorticity will bisect Southern New England,
interacting with the northern extent of the front that has lingered
for several days. Elevated PWATs in vicinity of the front, coupled
with the focused ascent pattern will generate one last period of
thunderstorm activity over Eastern LI up through the eastern half
of Southern New England. Precip totals around 1-2" with a max of 3"
will be plausible on Saturday morning and afternoon before the
threat kicks out of the area and we end the overall threat. An
introduction of a MRGL risk was made over the above area with
agreement from the CSU First Guess Fields and QPF footprint off
general ensemble forecasts.

Kleebauer

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