275
FOUS30 KWBC 280100
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Mon May 27 2024

Day 1
Valid 01Z Tue May 28 2024 - 12Z Tue May 28 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE NORTHERN AND CENTRAL MID-ATLANTIC STATES...

Will maintain a small Slight risk across portions of northeast MD
into eastern PA and southern NY to account for convection over the
next few hours. These cells are moving into areas that saw 1-2" of
rain earlier today, and so even though they should remain quick
moving, a few flash flood issues will be possible.

Clusters of convection will continue to pose an isolated flash
flood risk over portions of southern AL/GA and into the FL
Panhandle. This activity may develop into a southward sinking
squall line tonight. Increasing cell motions off to the south and
high FFG should keep the risk localized in nature.

Isolated flash flooding remains possible over portions of
southern TX this evening. Convection moving out of Mexico will
bring some risk to areas near the Rio Grande over the next few
hours. Elevated convection is expected to develop towards morning
over portions of southern OK and north TX as the low level jet and
moisture convergence increases. Most guidance would suggest any
more robust development waits until after 12z. However we have
seen elevated convection like this get going a few hours earlier
than expected in some past events. So to cover this potential will
have a Marginal risk from north TX into OK. Still think the greater
flash flood risk near the red River ends up after 12z, but can not
rule out some impacts anytime after ~10z.

Chenard


Day 2
Valid 12Z Tue May 28 2024 - 12Z Wed May 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
TEXAS AND SOUTHERN OKLAHOMA...

20Z Update...
Main focus for excessive rainfall shifts from the eastern U.S. on
Day 1 back to the Southern Plains on Day 2 as air with higher
Thetae gets drawn back northward during the day as low level flow
backs and strengthen ahead of an approaching mid-level shortwave.
Model QPF has been increasing but placement has not varied much
over time. Except for a minor adjustment to the southern boundary
of the area based on latest guidance. There is also a low-end risk
of flooding in parts of the Northwest given potential for 0.5+ inch
per hour convective rates late Tuesday and Tuesday evening in
complex terrain driven/focused by the passage of amid- and upper-
level system with a cold front at the surface.

Bann

...0830Z Excessive Rainfall Discussion..,

...Southern Plains...

Extensive surface ridging to the north will aid in the advection of
rich Gulf air to the northwest across much of TX into southern OK
with the axis of higher theta-E`s really denoting the wedge of
elevated instability over the southern plains. A weak mid-level
ridge will develop across west TX and parts of the southwest US
which will assist in the expected steering pattern of convection
across the region. Shortwave propagating across the northern
periphery of the ridge will help initiate convection across
portions of northwest TX into southern OK with a secondary wave of
development over west TX thanks to anticipated diurnal
destabilization to the east of a dryline positioned across the
Caprock down through the Texas Big Bend. Ample instability in place
given by the superfluous MUCAPE forecast over across the
aforementioned areas will lead to an enhanced convective
environment within a moisture rich environment thanks to the
poleward advancement of a Gulf- centric airmass. In fact, recent
ensemble guidance is in agreement on a broad area of 2-3 deviations
above normal axis of PWATs situated over the Concho Valley down
through central TX which will be important in the forecast when it
comes to the expected heavy rain threat.

Convective initiation will blossom and eventually congeal with
areal cold pool mergers moving southward due to the previously
mentioned storm relative steering pattern dictated by the mid-level
ridge to the west. Area thunderstorms are forecast to ride along a
tight theta-E gradient bisecting northern and central TX with
expected forward propagation to the south and southeast through the
TX Hill Country, eventually into the I-35 corridor between Waco to
San Antonio. QPF maximums of over 4" are becoming more common
within the latest guidance with the ensemble bias corrected QPF
driving upwards, closer to 5" in spots with the Hill country to the
northeast of the Edwards Plateau. Considering the environment in
place, rainfall rates exceeding 2-3"/hr is very likely within some
of the stronger cells with mature mesocyclones. A general 2-4" QPF
swath is in place over southern OK down through central TX with an
areal extent of at least 1" going back close to the dryline
positioning in west TX, and as far east as portions of the
ArklaTex.

Considering the magnitude of rainfall in place, the threat for
flash flooding, including some considerable flash threats a SLGT
risk was maintained from previous forecast with a higher-end
wording for flash flooding occurring over northern and central TX
with a relative bullseye across Hill Country. There is still some
discrepancy on the exact placement of the heaviest precip axis with
the global deterministic keying on different areas. This leads to a
slight pause on any further upgrades, however, the next run of CAMs
at 12z will be out through the full time frame of interest with
the accompanying hi-res ensemble suite adding in references of
probability and mean QPF fields. This will enhance the area of
focus more, likely leading to an upgrade of a MDT in the next
succession of updates across parts of TX, perhaps as far north as
the Red River.

...Northwest...

Surface low generation with a trailing cold front will be
positioned over the Pacific Northwest come Tuesday with attendant
height falls occurring to the west thanks to an incoming, strong
shortwave trough moving into the region. Increased upper level
ascent focused within a broad axis of diffluence on the lead side
of the mean trough will take aim over the interior Pacific
Northwest with an area of convection forming in-of the northern ID
mountains, likely along due to weak instability ahead of the
surface cold front and ample large scale ascent over the terrain.
Scattered thunderstorms capable of rainfall rates bordering
0.5-1"/hr will allow for localized flooding concerns within the
complex terrain over central ID up through the western portions of
MT. Total precip forecast is relatively modest with guidance
settling between 0.75-1.5" over the aforementioned area, but that
is just enough to cause some low-end flood concerns within the
rugged terrain and adjacent valleys north of the Snake River.
A MRGL risk was added to the above areas to account for the low-end
threat with the evolving dynamical setup.

Kleebauer


Day 3
Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE SOUTHERN AND CENTRAL PLAINS, LOWER MISSISSIPPI VALLEY, MONTANA,
AND NORTHWEST NORTH DAKOTA...

20Z Update...
Synoptic scale becomes less favorable for excessive rainfall in
both the Southern Plains and the Pacific Northwest although the
risk is not expected to completely end on Day 3. See details below
about expected evolution.

Ban


0830Z Excessive Rainfall Discussion...

...Southern and Central Plains through the Lower Mississippi...

Mid-level ridge axis across the Southern Plains will slide
eastward over the course of Wednesday, but more convection will
spawn upstream under the broad expansion of relatively prominent
axis of theta-E`s that will advect as far northwest as the Front
Range of the Rockies on the western flank of the surface ridge
encompassing much of the eastern CONUS. More shortwaves will round
the northern periphery of the mid-level ridge, helping to spawn
another round of convection Wednesday afternoon and evening, moving
south to southeast with the mean steering flow and expected cold
pool propagation. This round of convection is not currently
forecasted to be as robust due to lower bounds of instability
compared to the previous D2 offering. Still, the threat for more
flash flooding within zones expected to see impact during the
previous period, as well as an extension of the flash flood threat
further north and east has lead to a broad MRGL risk area extending
through portions of the southern and central plains, all the way
into the Lower Mississippi Valley as the steering pattern places
areas downstream into a better chance for organized convection
moving overhead. Ensemble guidance favors heavier precip further to
the east over the ArklaTex and portions of south-central TX within
the eastern flank of Hill Country through the I-35 corridor.
Considering the threat being conditional on what happens the day
prior, and QPF magnitudes less than appealing for a higher end risk
(1-2" totals), decided against any upgrade, but will mention the
likelihood of at least a SLGT is very high as we move closer and
have better definition on where the heaviest precip axis will lie.
For now, the inherited MRGL risk was maintained with some minor
modifications along the edges of the risk area.

...Montana and North Dakota...

Diffluent pattern across the Northwest will allow for a continued
threat of convection within the favored ascent pattern focused
within a formidable jet coupling situated over northern MT back to
WA state. The enhanced upper level dynamics in conjunction with
sufficient surface forcing will promote a period of scattered
strong to even potentially severe thunderstorms to propagate
northeast out of western MT and southeast ID through central and
eastern MT with the northeast expansion through northwest ND by the
end of the forecast period. QPF forecasts are sufficient for flash
flood concerns within the complex terrain of central MT, especially
with the environment capable for rainfall rates breaching 1"/hr at
times within the stronger cell cores. The previous MRGL risk was
maintained, but would not be surprised for a targeted SLGT risk
introduction in later updates as we move closer to the event time
and gain more confidence in the relative magnitude and placement of
the convective heavy rain potential.

Kleebauer


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt