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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0053Z Jul 17, 2024)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue Jul 16 2024

Day 1
Valid 01Z Wed Jul 17 2024 - 12Z Wed Jul 17 2024


...01Z Update...
Was able to trim the Slight Risk area out of much of the Ohio
Valley as convection in that part of the country weakened with the
loss of daytime heating. The Slight risk area remained in place
from parts of Arkansas and southern Missouri westward to the
Central High Plains as additional convection rolls off the higher
terrain and propagates along and immediately north of a quasi-
stationary boundary...with increased chances of heavy rainfall as
storms encounter a strengthening low level jet. Still lingering
convection in northern New England and the Southwest US that should
gradually weaken (in both areas) later this evening.


...16Z Update...

Adjustments this forecast cycle were to expand the Slight Risk area
over the Central Plains farther south to include more of the
northern TX Panhandle, northern OK, and northern AR. Latest
guidance has shown the nearby stationary front inching a little
more south compared to overnight guidance, which would allow for
segments of storms tonight being farther south than initially
progged over northern OK and even as far west as the TX Panhandle.
Closer to northern AR, strong surface based heating that further
destabilizes the atmosphere ahead of the front may lead to strong
storms by itself given the 89F convective temp via the 12Z LZK
sounding. Then there is the concern for outflows from thunderstorm
activity over northern OK that may approach and act as another
trigger tonight over the Ozarks.

In the Northeast, the soils are more primed in parts of NY's North
Country and around the Finger Lakes following yesterday's stormy
afternoon. Another MCV approaching from the west may lead to a near
carbon-copy setup for today, but storm motions should remain
progressive enough to limit the areal extent of flash flooding to
be more localized. One wildcard to factor in would be areas where
more extensive tree damage occurred that could be more prone to
flash flooding. The 12Z HREF also showed moderate-to-high
probabilities (50-70%) for 3-hr QPF surpassing 3-hr FFGs just east
of Lake Ontario and along the Tug Hill. Still, the progressive
nature of the storms kept any Slight Risk from being issued for the
time being. One adjustment was to expand the Marginal Risk across
most of northern New England as the soils (particularly in northern
ME) are a little more saturated (NASA SPoRT-LIS sows >80% soil
moisture percentiles). Given their added sensitivities and PWATs
that are topping 1.5" (>90th climatological percentile according ot
ECMWF SATs), the Marginal Risk extension was decided upon this
forecast cycle.


---Previous Discussion---

...Mid-Mississippi and Ohio Valleys...
Strengthening southwesterly flow ahead a shortwave moving across
Kansas will support deepening moisture ahead of a cold front
sliding southeast into the mid Mississippi Valley this morning and
the lower Ohio Valley later today. Persistent inflow into this
slow-moving boundary along with favorable forcing aloft is
expected to support periods of training storms, with areas of heavy
rainfall likely, especially from central and southern Missouri
eastward into the lower Ohio Valley. Excessive rainfall is
especially a concern from central and southern Missouri through
southern Illinois to the western Indiana and Kentucky border, where
the 00Z HREF is showing high neighborhood probabilities for
accumulations of 3 inches or more through the period.

...Central Plains...
Moist upslope flow behind a cold front settling south through the
central Plains will support storms developing over the high terrain
before these storms spread east into the Plains during the
evening. Merging storms along with increasing organization ahead of
a shortwave digging southeast through the Plains is expected to
support locally heavy amounts from southeastern Colorado through
southwestern Kansas and northwestern Oklahoma, where the HREF is
also showing some notable probabilities for 3 inches or more.

...Upper Ohio Valley to the Northeast...
Ample moisture ahead of an amplifying upper trough over the Great
Lakes and its associated cold front will fuel additional storms
later today. Overall, storms that develop are expected to be
progressive in nature. However, intense rainfall rates may produce
isolated runoff concerns.

An upper high sitting over the Four Corners will allow moisture to
funnel across the region, with an expansion of convective activity
expected today, bringing the risk for isolated heavy amounts and
flash flooding to a greater portion of Arizona and New Mexico.


Day 2
Valid 12Z Wed Jul 17 2024 - 12Z Thu Jul 18 2024


...2030Z Update...

...Northern Mid-Atlantic I-95 Corridor...

As daytime heating unfolds, the atmosphere will rapidly destabilize
as high temperatures rise into the mid 90s and dew points average
around 70F. Latest CAMs guidance shows thunderstorms firing
initially along a pre-frontal trough Wednesday afternoon with
potential redevelopment Wednesday evening as the cold front
traverses the region. PWs are expected to range between 2.0-2.2",
which is between the 90-97.5th climatological percentile for the
time of year. Soundings in the region also show no shortage of
MUCAPE (at least 2,000 J/kg) and hodographs that are not only
supportive of organized clusters of storms, but potentially capable
of containing organized mesocyclones (which are more efficient
rainfall producers). The storms will have a rather progressive
motion and soils can most definitely use the rain. However, the
potential for >2"/hr rainfall rates along a highly urbanized
corridor that contains no shortage of hydrophobic surfaces is a
recipe for potential flash flooding. The potential for additional
storms in the evening in wake of the initial pre-frontal storms may
also make some soils more sensitive for the final round of storms
along the approaching cold front. Given these reasons, and in
collaboration with LWX and PHI, a Slight Risk was introduced for

Otherwise, did adjust the Slight Risk in the Lower Mississippi
Valley and ArkLaTex a little farther south given the slightly
farther south progression with the cold front. The rationale behind
the threat area remains unchanged. Made just minor adjustments to
the Marginal and Slight in the Southwest to account for recent QPF
changes and 12Z HREF guidance.


---Previous Discussion---

....Southwest to the Central to southern Rockies and High
As the previously noted front continues to slide to the
south through the Plains, increasing moisture afforded by low-
level easterly flow will support an increasing threat for locally
heavy rainfall and flash flooding concerns. Ample moisture along
with increasing lift will support storms capable of producing heavy
rainfall rates, that will develop and drop south across the
region. The greater threat continues to center from the Colorado
Sangre de Cristos into the northern and central New Mexico ranges,
where models show some of the better low level inflow and greater
PW anomalies.

Elsewhere, a more typical monsoon pattern will continue, with
diurnal convection generating isolated heavy rainfall and flash
flooding concerns across much of the Southwest.

...Southern Plains to the Northeast...
An upper trough will continue to amplify over the Great Lakes,
pushing a cold front that will extend from the Northeast to the
southern Plains this period further south and east. While locally
heavy rainfall and an isolated threat for heavy rainfall cannot be
ruled out for any area along the front, deeper moisture and
stronger forcing are expected to elevate the threat for some areas.
This includes portions of the Red River and Arkansas basins into
the Mid-South. Deepening moisture and increasing ascent ahead of a
shortwave approaching from the northeast is expected to support
organized heavy rainfall across the region. With plenty of typical
difference in the details, several of the overnight deterministic
models indicate locally heavy amounts of 2-3 inches within the
Slight Risk area.


Day 3
Valid 12Z Thu Jul 18 2024 - 12Z Fri Jul 19 2024


...2030Z Update...

Little in the way of adjustments were made given guidance has
generally remained steadfast in support of the ongoing risk areas.
Only made some subtle adjustments based on the latest Day 3 QPF.
While QPF is not as high in parts of the Southern Rockies as on Day
2 at the moment, soils will likely be more sensitive following yet
another day of heavy showers and storms throughout the region on
Wednesday. For that reason, I kept the Slight Risk that was
inherited from the previous forecast. Otherwise, no additions or
subtractions of threat areas for this forecast cycle.


---Previous Discussion---

...Southern Mid-Atlantic and the Southeast to the lower Mississippi
The previously noted cold front will continue to settle further
south and east, bringing heavy rain and the threat for flash
flooding into portions of the southern Mid Atlantic, the Southeast,
and the lower Mississippi Valley. A Slight Risk was maintained
across portions of southeastern Virginia and eastern North
Carolina, where PWs are forecast to increase to ~2.25 inches as the
front settles and begins to stall across the region. This moisture
along with some mid-to-upper level support is expected to fuel
heavy amounts across the region.

Elsewhere, models suggest less organized storms with more isolated
threats for flash flooding further west along the front into the
lower Mississippi Valley.

...Southwest to the central and southern Rockies...
With no significant change to overall pattern expected and plenty
of moisture remaining in place, maintained much of the same outlook
areas from Day 2 into Day 3, including a Slight Risk centered over
north-central New Mexico extending into far south-central Colorado.


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