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Excessive Rainfall Discussion
(Latest Discussion - Issued 2007Z Jul 22, 2024)
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Mon Jul 22 2024

Day 1
Valid 16Z Mon Jul 22 2024 - 12Z Tue Jul 23 2024


...16z Update...

Split the Mid Atlantic Slight Risk with this forecast update,
mainly to exclude the DMV region (due to the combination of lower
QPF signal and relatively high FFGs with drought conditions in
place). Also removed northern LA from the Slight (for similar
reasons). The northern Mid Atlantic Slight and Marginal were
extended a bit northward into NY State (where FFGs are quite a bit
lower). In addition, the Marginal was also extended a bit eastward
along the south coast of New England, Cape Cod, and adjacent
islands (where a Marginal Risk was already in effect for tomorrow,
as the event may start for those areas prior to 12z). Overall these
changes were not very significant, and adjustments elsewhere were
minor (based on observational trends and the new 12z HREF).


...Previous Discussion...

...Southern Plains...

A quasi-stationary front will bisect much of TX during the period
with a succession of mid-level disturbances entering the region
beginning later this morning, carrying through the rest of the
period as they pivot south-southeast around the western flank of a
mean trough to the north. A deep moisture presence will be situated
across Southwest TX through points east with a stronger low-level
convergence pattern and favorable anomalies tied to the lingering
front. Scattered convection will form across the Davis Mountains
and Stockton Plateau in the early afternoon time frame with cells
across the Edwards Plateau initiating not long after. Considering
the moist anomalies and relatively slow storm motions off the
terrain, some cells will be able to produce locally enhanced
rainfall with rates between 1-2"/hr on average with some of the
stronger cores capable of breaching 2"/hr anywhere across the
aforementioned area(s). Numerous multi-cell clusters will develop
by mid-afternoon with outflow generation likely considering the
higher DCAPE environment forecast across the western half of TX.
Outflow propagation will make progress to the east with more cell
initiation forming in the favorable environment away from the cold

Storms should develop initially over Hill Country to the
I-35 corridor, but more organized convective clusters will enter
the picture by early evening leading to more widespread heavy rain
chances and flash flood concerns as rates can sufficiently hit
2-3"/hr as reflected in the HREF hourly rate probability fields.
There's a very high spatial coverage of higher probabilities for at
least 1" of rainfall in the HREF EAS fields (30-50%) with the
neighborhood probabilities for at least 3" and 5" depicting a
formidable areal extent of 60-80% and 25-40% respectively for each
threshold. There's even some low-end >8" probabilities reflected in
the HREF output as well, indicative of a locally significant impact
potential across portions of Western and Central TX. The best
probabilities lie within the Edwards Plateau through Hill Country
with a small bullseye showing up in the means to the I-35 corridor.
This is well within the bounds of a SLGT risk with higher end SLGT
risk wording necessary for the current forecast. There is a non-
zero chance for an upgrade somewhere across the above region, but
there is less of a true, organized heavy rain risk to pinpoint, but
the area that will be impacted will see those locally significant
flash flood concerns arise.

...Mid Atlantic and Carolinas...

Persistent troughing to the northwest across the Great Lakes will
edge eastward with southwesterly flow and increasing diffluence for
much of the Mid Atlantic and Carolinas, moving into the Northeast
U.S by the back end of the period. At the surface, a lingering
quasi-stationary front will settle over the VA Tidewater down
through the Carolina's and points west with a stronger low-level
convergence footprint tied to the boundary. PWAT anomalies of 1-2
deviations above normal will spread north as we see a bit more
backing of the flow within the Mid Atlantic with the strongest ties
across the Southern Mid Atlantic thanks to a surface wave riding up
the front once again, reaching the VA/NC coastal areas by the late
afternoon hours.

The area(s) of highest interest are located across the VA Tidewater
and adjacent Northeast NC, the eastern side of the Carolina
Piedmont, and up into the Susquehanna Valley of PA where the HREF
neighborhood probability fields are all lit up for the potential
of seeing >3" in spots with even a strong >5" signature located
along the I-64 corridor in Southeast VA (50-60%). The increased
upper forcing due to better aligned mid-level diffluence and some
right-entrance region of a developing upper jet streak just off to
the north will allow for a blossoming of scattered convection
across portions of the Mid Atlantic through Central PA. The primary
initiation points will likely be tied to the terrain at first, but
as cells propagate off the terrain, they will experience some
modestly favorable shear that would help sustain updrafts that
could lead to stronger cell cores and locally heavy rainfall. PWAT
anomalies are most favorable for heavy rain along and east of the
I-99 and US-15 corridors leading to a lot of CAMs members
ratchetingup the potential with isolated pockets of 2-4+" totals
within the deterministic output, and reflected in the HREF blended
mean data set showing the expanse of higher totals and increasing
flash flood concerns. The threat will also reside within the urban
corridor from DC to Philadelphia, but there is some disagreement
within the CAMs on the potential due to a small cull in the precip
field with initiation away from the major metro areas. The UFVS ML
First Guess Field is still insistent on a widespread SLGT risk
through the urban areas extending all the way up to the PA/NY
border. The QPF footprint within ML output is also insistent on
potential maxima within the metro areas, so the best course of
action was have that area within the new SLGT risk proposal with
areas to the north into PA outlined as well given the better
signals and agreement in the First Guess Fields.

Further south into Southeast VA and the Carolinas, the threat is
more defined thanks to the presence of the stationary front leading
to ample low-level convergence potential coinciding with fairly
stout theta-E indices located along and south of I-64. The highest
threat will reside over the Hampton Roads area up to Williamsburg
where several days of heavy rainfall have degraded the FFG indices
considerably leading into today. This signal of degraded indices is
documented down through the NC Piedmont all the way into Columbia,
SC where back-to-back days of significant convective impact have
led to flash flooding reports all across southern and central SC.
The threat for today extends into those areas again thanks to weak
mid-level perturbations ejecting northeast within the mean flow
correlating with the stationary front to provide another focused
area of heavy thunderstorms from basically the GA/SC border and
points northeast. As a result, the SLGT risk addition was also
extended to include part of the Delmarva down through Southeast VA
into portions of the Central and Eastern Carolinas.

...Southwest and Southern Rockies...

Continued Monsoonal convective pattern will impact the Desert
Southwest this period with focus along the terrain in AZ and NM,
including the hotter spots of the Mogollon Rim over into the Sangre
de Cristos and the Sacramento Mountain chains. Probabilities
continue to be modest for 1-2" maxima with some higher end
potential in stronger cores that linger within the terrain. The
continued mid and upper ridging in place has led to slower storm
motions with some training concerns under the weak mean steering
flow. Considering the onslaught of scattered to widespread
convection for the past several periods leading to many Flash Flood
Warning issuances by the WFOs across the Southwest, another SLGT
risk was maintained within the Southern Rockies where the impact
potential is highest thanks to remnant burn scars and persistent
flooding over the past week. A MRGL extended to much of the
Southwestern U.S with the Mogollon Rim as the secondary candidate
for higher totals and impacts for localized flash flooding
concerns. The threat probabilistically is still on the middle to
high end of MRGL for the aforementioned area, but a short term
upgrade is plausible if coverage ends up more sufficient than
currently forecast.

...Upper Midwest...

Shortwave trough across Manitoba will pivot southeast into the
Arrowhead of MN with increasing large scale ascent and accompanying
positive vorticity advection (PVA) within the confines of the area.
Relatively steep mid-level lapse rates within a corridor of modest
instability will result in scattered thunderstorm initiation once
the disturbance crosses the border into the U.S. Progressive storm
motions will limit the threat of flash flooding to more isolated
signals, however some heavier cores with rates between 1-2"/hr will
be plausible given some of the low-end probabilities within the
latest HREF output. Totals are generally within the 0.5-1" in areal
coverage, but the matched mean does indicate a few cells capable of
dropping 2-3" near the northern shores of Lake Superior, including
near DLH where urban flooding is more probable. A MRGL risk was
introduced in coordination with the local Duluth WFO for low-end
flash flooding concerns, but enough to warrant the targeted risk.


Day 2
Valid 12Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024


...20z Update...

New hi-res guidance (12z HREF) facilitated minor changes along the
borders of the inherited Slight Risk area, as well as the boundries
of the more sprawling Marginal Risk areas. The most significant
shift in the Slight Risk was an expansion eastward into southwest
LA, where some of the highest probabilities for localized 5"
exceedance are located (per 40-km HREF neighborhood probabilities
of 30-50%). Overall meteorological thinking is unchanged from below


...Previous Discussion...


Quasi-stationary front will push north and eventually wash out
across the Southern Plains as surface ridging across the Gulf
begins flexing more northwest allowing for deeper moisture presence
to advect into Southeast TX. Scattered convection will develop from
the Lower Trans Pecos through all of Central TX by the afternoon
due to diurnal destabilization with lingering outflow boundaries
from previous periods convection becoming a potential focal point
for heavy rainfall and training over much of the area. Areal QPF
averages are between 0.5-1" across West-Central and Central TX with
some of the upper quartile outcomes closer to 3-4" within different
deterministic forecasts. This seems to be towards the upper
threshold of the potential across the region, but after what is
expected today, there's expected to be a lingering flash flood
threat after much of the area becoming primed from previous
rainfall. This was sufficient for a maintenance of the previous
SLGT risk.

Further to the southeast, the additional surge of low-level
moisture out of the Gulf will be accompanied by a weak mid-level
disturbance that will advect north out of the Bay of Campeche,
already causing some convective flare ups down that way when
assessing the latest IR satellite. Ensemble means are becoming more
bullish on the threat of heavier rainfall tied to the Middle and
Upper TX coast with some deterministic outputs pushing 3-5" between
Corpus to the far Upper TX coast, including coastal Houston.
The threat is gaining favorable within the ensemble probability
fields as well with the NBM now depicting a 25-40% chance of >2"
within the zone encompassing Matagorda up towards Port Arthur.
Considering this is working off the mean QPF of the blend, that is
impressive at 2-day leads. The extension of the SLGT was made to
encompass that area of the coast from CRP up to the Southwestern
corner of LA.

...Southwest and Southern Rockies...

Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of NV leading to more coverage of the
MRGL risk across those areas. QPF means are generally light, but
some of the higher end outputs within the deterministic suite are
upwards of 1-2", especially across the Mogollon Rim. This is a
classic Monsoonal setup with the favored terrain being the primary
focus for the period. The MRGL risk inherited was maintained with
full continuity as ensemble mean QPF did not change much in terms
of magnitude and precip placement from previous forecast.


Shortwave trough over the Northern Great Lakes will dip further
southeast with trailing mid-level vorticity pivoting around the
base of the trough situated over Ontario. Scattered cells in the
more will move southeast, but lose fervor as they enter into MI. A
secondary pulse of convection is expected later Tuesday with a more
organized area of thunderstorms expected to form upstream near Lake
Superior, dropping southeast through WI and the western portions of
the UP. The storms will remain on the progressive side, but the
threat of rates up to 2"/hr could cause some isolated flash flood
concerns in more urbanized zones over Central and Eastern WI. This
includes places like Green Bay, Northern Milwaukee, Oshkosh, and
Sheboygan. The previous MRGL risk was maintained with some minor
adjustments on the western flank of the risk area to account for
QPF shifts in the ensemble means.

...Southeast through the Northeast U.S...

Scattered thunderstorms will continue across the Southeast all the
way up into the Northeastern U.S thanks to persistent synoptic
scale forcing and a relatively modest thermodynamic environment in
place. Smaller mid-level perturbations will be entrenched in the
mean flow and will help trigger some smaller, organized cell
clusters capable of isolated flash flooding basically extending
from the Deep South into the Mid Atlantic. Any cell generation
in the deep, moist environment will be capable of flash flooding
concerns with the highest threat likely within the Carolina's due
to the antecedent wet conditions in place over much of the eastern
2/3's of the region. A broad MRGL risk is in place extending from
the Southeast through the portions of the Northeastern U.S given
the threat.


Day 3
Valid 12Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024


...20z Update...

Little change needed for this update, as the new model guidance
continues to produce a strong signal for high QPF along the Middle
and Upper TX coast. An ensemble bias-corrected blend suggests
localized totals of 3-5" across this region, and the new WPC QPF
reflects areal average totals just shy of these amounts (2-4").
While the 12z GEFS (and resulting GFS) signal is clustered on the
Upper TX coast into southwest LA, the 12z ECENS (and resulting
ECMWF) has shifted primarily over the Middle TX coast. This
suggests that one of these solutions may eventually win out,
necessitating a targeted Moderate Risk upgrade (especially
considering the potential for heavy rainfall that may pan out on
Tuesday as well). For the time being, the inherited Slight Risk
(suggesting higher-end, 25%+ probabilities) is appropriate and
encompasses both ensemble systems (as well as a variety of other
deterministic solutions, many of which are clustered in the


...Previous Discussion...

...Texas and Louisiana...

Disturbance off the Gulf will continue to press north around the
western flank of the ridge located within the Gulf. There's some
discrepancy within the deterministic on the exact placement on the
core of the mid-level energy as it enters out of the coastal waters
into the U.S. Despite the lack of consensus on the where for the
energy, the deep moist advective pattern across Southeast TX into
portions of the Central Gulf Coast is not so much a question with
only where the general precip maximum will focus. There's growing
consensus that significant rainfall on the order of 2-5" with
locally higher will impact somewhere within the Middle and Upper TX
coast, over into Southwestern LA. ML output from the ECMWF AIFS and
GFS Graphcast are closely aligned to the Upper TX coast which makes
a bit more sense synoptically given the favored mid-level ridge
positioning and western weakness aimed for the Upper coastal areas.
The previous SLGT risk was not changed much due to the modest
uncertainty in the precip maximum with the setup, however the
threat is more on the higher end of the SLGT threshold due to the
potential for significant rainfall in any area of expected impact
with the highest impacts focused where the mid-level disturbance
makes its presence closely known. This is a period to monitor as an
upgrade to a higher risk is plausible considering the upper
quartile output of ensemble QPF and the tropical connection.

...Southern Rockies and Southwest...

Monsoonal convection will encompass much of the Southwest U.S with
primary coverage in the Great Basin as the mid-level ridge pattern
shifts focus to the west of the Four Corners. Modest moisture
anomalies and relatively formidable instability across much of the
region will allow for scattered thunderstorms with isolated heavy
rain cores that could spell issues if they fall along complex
terrain, burn scars, urban footprints, and slot canyons. A MRGL
risk is in effect across much of the climatologically favored
areas in the Southwestern Monsoon.

...Great Lakes and Ohio Valley...

Surface low will shift eastward over northern MI, eventually
pushing northeast into neighboring Ontario with a cold front
trailing the surface reflection. The combination of marginally
better upper forcing with increasing surface convergence in-of the
cold front moving eastward will create a period of scattered
showers and thunderstorm along and ahead of the front. Some
isolated pockets of heavier rainfall within the urban corridors
spanning from Chicago up through Southern MI will have the
opportunity for flash flooding as the system progresses through the
area. The signal is fairly weak overall and could very well fade
in later updates. Considering the synoptic pattern expected and
some time to delve into greater detail, maintained much of the
continuity in the forecast with some minor adjustments on the
northern flank of the risk area.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: