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FOUS30 KWBC 290117
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
917 PM EDT Tue May 28 2024

Day 1
Valid 01Z Wed May 29 2024 - 12Z Wed May 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS...

...01Z Update...

...Central High Plains through Texas...

Semi-organized activity that developed along the dryline in west
Texas has shifted east along with clusters of thunderstorms over
the high Plains of CO/KS will continue to shift east overnight.
Lack of instability over North Texas warranted removal of the
Moderate Risk that extended over the Dallas-Ft Worth Metroplex.
Despite vulnerability from heavy tstms this morning, confidence is
great enough to limit the overnight risk to a Slight. CAMs continue
to suggest overnight activity lasting past 12Z which may warrant a
Slight Risk for the new Day 1 for after 12Z Wed.

Organized activity over south-central Texas will continue shifting
east and southeast overnight with enough progression to limit
overall excessive rain risk. The Marginal Risk was expanded toward
the lower Rio Grande Valley given the activity currently along and
west of the Rio Grande.


...Northern Idaho and Western Montana...

Trough axis over the PacNW will shift inland overnight with
scattered thunderstorms continuing to develop ahead of the
associated cold front from along the OR/ID border into western MT.
Stretched the Marginal Risk a bit southwest to northeast given
radar trends with cells along the OR/ID border producing 1"/hr
rainfall estimates with some orientation in the direction of
motion. 0.75" PW is about 1.5 sigma above normal which combined
with MUCAPE of up to 1000 J/kg should allow continued redevelopment
through this evening.

Overnight activity should be largely west of areas over North and
East Texas that had heavy rain earlier today. That said, the CAMs
today have struggled with this western Texas activity so far, so a
broad Slight Risk is still warranted from western Oklahoma down
into south-central Texas.


...Southeast Florida...

Isolated activity between Lake Okeechobee and the Miami metro
should continue to wane through this evening, warranting removal of
the Marginal Risk.


Jackson


Day 2
Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024

... THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN AND CENTRAL PLAINS, LOWER MISSISSIPPI VALLEY,
NORTHERN UPPER HIGH PLAINS, AS WELL AS PORTIONS OF THE MID-ATLANTIC
TO SOUTHERN NEW ENGLAND...

...2030Z Update...

A new Marginal Risk area was introduced from central Ohio east to
the southern New England coast, including much of the Mid-Atlantic
and the DC to Boston I-95 corridor. Guidance shows several areas
embedded within the Marginal where training cells may develop.
However, none show particularly strong storms as part of the
training. Given the sensitivity of much of the Marginal Risk area,
and poor CAMs performance given decent instability expected
Thursday afternoon and evening, think isolated flash flooding can
be expected should training occur over any particularly sensitive
area.

No significant changes were needed to the inherited Marginal Risk
areas. An expansion east to include more of the lower Mississippi
Valley was added due to the latest model runs suggesting the
heaviest rainfall totals may be centered around southern
Arkansas/Arklatex region. Even there, 1 inch amounts appear to be
spotty Thursday, and therefore given the high variability among the
guidance, confidence still is not there for any upgrades. That
said, this area could be considered a "high-end Marginal", in that
should future runs remain consistent, a Slight may need to be
considered given favorable antecedent conditions/low FFGs in the
area.

The Marginal over the northern High Plains remains largely
unchanged, albeit with some east/southeast expansion based on
latest guidance trends.

Wegman

...Previous 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. Previous period convection
will play a significant role in the prospects for more widespread
thunderstorm coverage over the southern plains as remnant outflows
and eventual positioning of the frontal boundary across TX will
dictate where there could be more focused convection leading to
better training and heavy rainfall potential. As of now, the
coverage is relatively sporadic among most numerical guidance with
a few outliers indicating some areas within east TX seeing
sufficient rainfall that could allow for a targeted upgrade from
the current MRGL risk. As of now, maintained continuity with enough
of a general instability pattern situated from the Colorado Front
Range down through the Lower Mississippi Valley to constitute some
isolated flash flood threats with the best prospects likely closer
to the elevated theta-E environment near the Gulf coast over into
south-central TX near where the frontal boundary may lie.

...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 as recent ensemble means generally hover between
0.75-1.5" in the hardest hit locales. Considering the complex
terrain of central MT, flash flood concerns are heightened,
especially within the environment capable for rainfall rates
breaching 1"/hr at times within the stronger cell cores, as
indicated by the latest HREF probability fields (30-50% for
>1"/hr). Recent NAEFS has come up with the regional PWAT anomalies
situated over northeast MT into northwest ND with an area of 2-3
deviations above normal being depicted along the border. The
evolution of the upper level pattern is formidable with the trough
axis tilting negative during the period in question. It is
plausible that guidance is underplaying the threat and there is an
opportunity for an upgrade in future forecast updates, likely
somewhere in eastern MT. For now, the previous MRGL risk was
maintained with little deviation from the previous forecast.

Kleebauer


Day 3
Valid 12Z Thu May 30 2024 - 12Z Fri May 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL AND SOUTHERN PLAINS...

...2030Z Update...

There is a much more consistent signal for convective heavy
rainfall across the central and southern Plains Friday as compared
with Thursday. However, the placement of the heavy rain within the
large Slight remains in poor agreement. A higher-end Slight is
considered in effect for portions of North Texas, generally where
the current Day 1 Moderate is in effect. While this area appears as
likely to pick up heavy rainfall on Friday as much of the rest of
the Slight, given the low FFGs around the Dallas-Fort Worth
Metroplex, that area is under a higher threat due to the more
favorable antecedent conditions for flash flooding.

No significant changes were needed given the above described
meteorological setup.

Wegman

...Previous Discussion...

Longwave pattern will remain highly amplified with a strong mid-
level ridge positioned along and east of the Mississippi River with
a quasi-zonal pattern situated over the plains back to the western
US. A negatively tilted trough over the northern Rockies will
pivot into Canada with a few mid-level vorticity maxima traversing
over the northern plains off the southern base of the shortwave
trough. The combination of relative vorticity advection to the
north and several mid-level perturbations ejecting eastward out of
the zonal pattern to the south will create a large area of favored
ascent within the central and southern plains on Thursday
afternoon through the end of the period. A large surface ridge over
the eastern CONUS will expand back west of the Mississippi with
the backside of the ridge allowing for a funnel of elevated
theta-E`s to migrate poleward into the plains. The combination of
the enhanced instability fields and incoming shortwave advection
will generate a prominent areal extent of convection from the Front
Range all the way across towards the Mid and Lower Mississippi
Valleys. Congealing cold pools across the plains will likely
develop at least one MCS in-of the central plains leading to
widespread heavy rainfall and increasing concerns of flash
flooding. Other areas within the confines of the central and
southern plains will have other opportunities just based of general
convective evolution within a favorable environment. Complexities
arise when trying to decipher where the best cold pool convergence
will reside, which is typical in setups like these where there is
no one, true disturbance to key on. Regardless, ensemble means are
downright bullish with quite an expanse of 1-3" totals spanning
central and north TX up through the central plains as far north as
eastern ND. A very large SLGT risk was inherited and maintained for
now with an opportunity for an upgrade if there is a general
consensus on exactly where the heaviest QPF footprint will reside.

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