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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1954Z Jul 12, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Fri Jul 12 2024

Day 1
Valid 16Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024

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

...16Z Update...

Some minor adjustments were made to the respective SLGT risk across
the Mid Atlantic and the MRGL risk over Missouri as there has been
little variability within the recent guidance compared to
overnight. There was no change to the MRGL risk across the Upper TX
coast, including the Houston metro into the Piney Woods area to the
north.

Heavy rain axis is positioned over the Delmarva down into
central and southern VA through portions of NC. Diurnal
destabilization factors will contribute to an invigoration of
convection across Eastern NC up through VA this afternoon with
locally heavy rainfall rates of 1-2"/hr increasingly likely after
cell initiation. Deep moisture presence within the profile is
evident when assessing some of the regional 12z RAOB soundings, as
well as hi-res forecast soundings within the CAMs suite. The
presence of the stalled front will maintain a focus for convective
development and training potential as the flow begins aligning
parallel to the boundary leading to repeated impacts across the
above areas. A high end SLGT risk is most prominent within NC up
through the I-64 corridor with the secondary area of focus across
the northern Delmarva into Southeastern PA with guidance suggesting
a more organized convective cluster traversing the I-95 corridor
from NoVA up the I-95 corridor and points east during the overnight
period. This is likely due to a more consolidated vorticity maxima
that will originate from convection over Southern VA and propagate
northeast within the mid-level southwesterly flow. There is still a
non-zero chance for a targeted upgrade within the Northern Delmarva
given the signal, however the current forward speed of any
organized convection is less favorable for a more significant
impact, although if the convective cluster exits into the
Philadelphia metro area, that would be more conducive for higher
impacts given the urbanization factors that would be most
susceptible for flash flooding. Will be monitoring closely.

MCV over Southern MO will pivot east, aligned within the persistent
westerly flow based over the Mid-Mississippi Valley. Modest
instability within the focal area of ascent will likely yield some
scattered cell clusters capable of heavy rates exceeding 1"/hr,
locally as high as 2"/hr in the strongest cells. East and Southeast
MO is a bit more susceptible for impacts given the still moist
soils after the impacts from Beryl's remnants. This creates a more
conducive environment for localized flooding and within the lower
end of the MRGL threshold necessary for the current risk area. The
MRGL was maintained due to those factors, but adjusted on the
western flank due to the end of the convective threat, or at least
the flash flood prospects with the focal point of the MCV shifting
eastward.

Isolated bouts of heavy rainfall will impact the Upper TX coast
through the Piney Woods area leading to a low-end MRGL risk for
flash flooding in a spot that is still recovering from Beryl.
Grounds will remain very wet and the combo of elevated top soil
moisture and urbanization factors will allow for a spotty flash
flood threat through the afternoon, ending around sunset due to the
loss of diurnal heating.

Kleebauer

...Previous Discussion...

...Mid-Atlantic...

No major changes were made with this update, but certainly the
active pattern typical of July continues. A strong front featuring
an impressive moisture gradient is set up along the eastern slopes
of the Appalachians this morning. Meanwhile, a shortwave in the
upper levels will lift up the Ohio Valley and into the Northeast
today through tonight. On the warm side of the surface front, PWATs
are near their maximum for this time of year, with amounts above
2.25 inches over southeast Virginia and the Carolinas. Instability
is somewhat subdued over land, but values quickly spike just off
the coast to over 3,000 J/kg, with a strong southeasterly wind
helping advect it northwestward. Rates with the stronger cells this
morning are routinely exceeding 1 inch per hour.

As the band moves northwestward, it will eventually run into the
front near the I-95 corridor, resulting in a slowing and eventual
stalling of the band. With the upper level shortwave shifting
northeastward, expect multiple rounds of storms training
northeastward along the front today into tonight. The Chesapeake
and Delaware Bay breezes may help to focus that convection by
locally adding forcing, with HREF probabilities over the far
northern Delmarva of 30% for 8 inches of rain in 24 hours as well
as a 30% chance of reaching 100 year ARIs in the 40 km neighborhood
probabilities. For the ERO, the area remains in a high-end Slight,
but should those 8 inch probabilities realize, expect flash
flooding. The guidance suggests overnight tonight is the best time
for the heaviest rain for northeastern MD, northern DE, far
southeastern PA, and southwest NJ where the highest threat remains.
It's possible a targeted MDT may be needed for this region, as it's
the one part of MD not in a drought as noted in the latest drought
monitor. The DC-Baltimore I-95 corridor's forecast rainfall totals
have also increased with a small northwestward adjustment to the
axis of heaviest rainfall in the forecast, so urban and small
stream concerns have also increased if the band over eastern MD
stalls there later today into tonight. There remains a high
likelihood of a sharp gradient of rainfall totals on the north and
western side of the front due to the rapid drop off of atmospheric
moisture on the dry side of the front. Thus, much of western MD,
eastern WV, and western VA will see very little if any rainfall
from this event.

For the Carolinas, many areas can expect 2 separate rounds of rain
as the front pushes west today, then retreats back to the east
overnight. This should overall reduce the flooding threat, though
some areas along the Pamlico and Albemarle Sounds have seen some
heavy rain in recent days which may locally increase the flash
flooding threat.

...Southern Missouri...

A Marginal Risk area was introduced with this update for continued
convection through the morning associated with the current
convection over southeastern Kansas. The storms are very slow-
moving, but some of the guidance does speed it up through the
morning with increasing westerly flow. Much of southeastern
Missouri was recently hard hit with Beryl's remnants, and therefore
have lower FFGs and therefore somewhat more favorable antecedent
conditions.

...Southeast Texas...

Also due to the remnants of Beryl, the potential for typical daily
afternoon convection over this hard hit area may cause isolated
flash flooding in the possible event any stronger cells remain
nearly stationary. A small Marginal Risk was introduced for this
reason.

Wegman

Day 2
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
DELMARVA, CENTRAL AND SOUTHERN NEW JERSEY, ALONG WITH THE
PHILADELPHIA METRO AREA...

...20Z Update Summary...

SLGT risk across Southern New England was removed with a new SLGT
risk in effect across much of Central and Southern NJ down through
the Northern Delmarva, including the Philadelphia metro area. A
MRGl risk remains in place from Southern New England down into the
Eastern Carolinas. Previous MRGL risk over the Upper Midwest was
expanded to the Southeast to include more of WI, including the
local areas around Green Bay to Milwaukee. There were little to no
changes necessary for the MRGL risk across the Southwest U.S, and
the Upper TX coast into the Piney Woods area. 

...Mid Atlantic...

The previous SLGT risk across Southern New England was removed, but
a new SLGT risk is in effect across the Northern Delmarva, Central
and Southern NJ, along with the Philadelphia metro area. Recent
trends within most, if not all major deterministic, including the
CAMs have shifted the axis of heaviest rainfall across the Central
Mid Atlantic with the northern extent of the frontal boundary now
expected to be further east with the best surface convergence
signature located over the eastern half of Long Island to just off
the Southern New England coast. Further south, a consolidated mid-
level vorticity maxima is expected to eject northeast during the
end of the D1 period, moving overhead of the Philadelphia metro
area and surrounds by tomorrow morning. Surface front is forecast
to be aligned along and just to the east of the I-95 corridor from
Baltimore up through NYC leading to a focused surface convergence
axis within the zone above. Forecast soundings from multiple CAMs
show a well aligned 925-700mb steering flow that would lead to the
boundary layer moisture field running parallel to the surface
front in place. Textbook signature of the parallel flow with
increased mid and upper forcing will enhance a regional QPF maxima
in-of the aforementioned area before being displaced to the
northeast by the early to mid-afternoon time frame.

The best signatures within the HREF probability fields have
shifted focus from once over Southern New England to further south
within the Central Mid Atlantic as of the latest 12z suite.
Neighborhood probabilities for at least 2" are upwards of 50-80%
within the Northern Delmarva up to north-central NJ with a
bullseye for at least 3" located over the Northern Delmarva to
about the Philadelphia metro area. EAS probability fields also show
a better defined corridor for heavier precip focus within the above
area with 30-40% signatures showing up for at least 1" of rainfall
in-of the targeted locations. This is coincident with a better than
MRGL risk forecast, especially considering the timing of the
precip falling mainly within a 6 hr window between 12-18z Saturday.
With coordination from the local Philadelphia WFO, have upgraded
much of Central and Southern NJ down into Northeast MD and Northern
DE within a SLGT risk of excessive rainfall.


Marginal Risks

...Upper Midwest...

Shortwave trough moving into the Northern Midwest will provide
sufficient large scale forcing within an axis of marginally
favorable moisture and instability to create a multi-round segment
of thunderstorms across the northern half of MN down through WI
through much of Saturday into Sunday morning. Progressive nature of
the storms will limit significant flash flood potential, although a
few cells may backbuild across WI during the second period of
convection as a nocturnal MCS is forecast to develop late Saturday
evening and moving southeast from western Lake Superior to near or
overhead of the Milwaukee metro area. A large extent of >50%
neighborhood probabilities for at least 1" and 2" can be found from
the western portion of the Arrowhead down into Southeast WI,
correlating well with a reasonably high EAS probability (30-50%)
for at least 1" as of the 12z HREF. This is sufficient for a MRGL
risk area with a broader scope of impact, especially with some
deterministic output being quite bullish for the prospects of an
MCS developing upwind of Lake Michigan with chance at embedded
1-1.5"/hr rainfall rates in its path. Considering the enhanced QPF
footprint off the ensemble means and First Guess fields being
focused further southeast with the MRGL risk coverage, have opted
to expand the previous risk area to match the latest trends.

...Southeast Texas...

Another afternoon of diurnally driven convection in-of the Houston
metro and points to the north and east where saturated soils and
urbanization characteristics will entice another low-end threat for
flash flooding in the heavier cell cores. The threat is on the
lower end of the risk threshold, but a overachieving cells cannot
be ruled out considering a favorable thermodynamic environment and
wet antecedent conditions. The previous MRGL risk was maintained.

...Southwest...

Another round of Monsoonal activity is expected across much of AZ
into Northwest NM. The primary concerns are for locally heavy rain
to impact remnant burn scars, slot canyons, and smaller urban
corridors within the terrain. Best threat will lie north of I-10
around the Mogollon Rim into the Sangre de Cristos in NM. Totals
generally less than 1" with a few stronger cores capable of 1-2" if
persistent.

Kleebauer

Day 3
Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
GREAT LAKES AS WELL AS FOR ARIZONA AND PORTIONS OF ADJACENT UTAH,
NEW MEXICO, AND COLORADO...

...20Z Update...

Only very minor adjustments were made to the previous forecast MRGL
risk areas across the Upper Midwest, Great Lakes, and Southwest U.S
for the period. Latest ensemble output was very much in line with
the previous forecast with only some minor fluctuations in the
position of relevant QPF maxima considering the less organized
nature of the convective potential within each area. Best areas of
interest for an upgrade in future updates would be over the Great
Lakes if future guidance indicates a more organized complex of
thunderstorms that could be capable of a more prolific, local QPF
footprint, along with the portions of AZ and Southern UT
considering an increasing Monsoonal signature over heavily prone
regions of slot canyons and complex terrain. Continuity was
preferred given the above, but check back for updates in upcoming
forecasts for those potential upgrades.

Kleebauer

...Previous Discussion...

...Great Lakes...

A series of weak shortwave troughs will move across the Great Lakes
and southwesterly flow of Gulf moisture advects into the region.
Expect widely scattered showers to impact the area, resulting in
isolated flash flooding.

...Four Corners...

Increasing moisture with the monsoon will advect northward into the
Mogollon Rim of Arizona. Meanwhile storms that develop over the
mountains of Mexico may advect northwestward across the border near
Nogales, where there's good agreement on over an inch of new
rainfall. While coordination with the associated offices resulted
in agreement to remain a Marginal, it's likely a Slight Risk will
be needed for portions of Arizona with future updates for Sunday
afternoon.

Wegman


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