084
FOUS30 KWBC 282019
QPFERD

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

Day 1
Valid 16Z Tue May 28 2024 - 12Z Wed May 29 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTH TEXAS...

...16Z Update...

...Southern Plains...

The Moderate Risk area was adjusted largely based on impacts from
this morning`s convection across the Dallas-Ft. Worth Metroplex.
CAMs guidance is not overly excited about this afternoon`s
convection. Once the convection gets going, which is widely agreed
upon, it quickly becomes fast moving. Only a brief opportunity for
training appears when the storms first form across the eastern
Texas Panhandle and western Oklahoma this afternoon.

The Moderate Risk was trimmed on the southern end of the inherited
area as that region was completely missed by this morning`s
convection. Instead, it was extended east, in coordination with
FWD/Dallas-Fort Worth forecast office, to include the Metroplex.
With ongoing flash flooding from this morning`s convection in the
area, should additional storms, if fast-moving, move over the
Metroplex again this evening, then flash flooding is likely.

The current slow moving convection along an instability gradient
over northwest Oklahoma near Woodward suggests that should the
second round of storms this afternoon form as expected, they could
cause flash flooding in that otherwise dry area. Thus, while this
first round appears unlikely to cause significant issues (though
isolated flash flooding can`t be ruled out), the Slight was
extended north to include that region.

A higher-end Slight was expanded to include much of east Texas with
the current activity potentially paving the way for any late night
convection that may try to make its way that far east...though at
the moment the current activity is by far the greater threat.

...Southeast Florida...

A small Marginal risk area was added to southeast Florida for this
afternoon as convection may develop along the sea-breeze front and
may remain stationary due to opposing flow aloft.

...Northern Idaho and western Montana...

No changes were made with this update as the axis of heaviest
rainfall expected there from late this afternoon through tonight
remains in that same area.

Wegman

...Previous Discussion...

...Southern Plains...

Current UA analysis across the west shows a developing mid-level
ridge over the Four Corners that will shift eastward through the
period, eventually residing over west TX by midday. Aloft, a few
mid-level perturbations are currently making progress over the
southern plains with convective initiation ongoing just over the
Red River in southwest OK. This will be a theme to the period as
these smaller shortwaves will ripple through the northern periphery
of the ridge and eject over the plains allowing convective flare
ups to occur south of I-40 through the morning. By the afternoon, a
stronger shortwave will traverse the southern plains with rapid
convective development in-of a dryline situated from southwest TX
up through the Panhandle. Ahead of the dryline to the east, a
stationary boundary will delineate the axis of greatest
instability available come the second half of the period with the
theta-E ridge from north TX down through the Gulf Coast pretty much
outlining the stationary front bisecting the state. Convective
initiation to the west will allow for locally heavy rainfall and
severe weather to ransack areas of west TX up into the Panhandle,
but eventual cold pool propagation from area cells will congeal
allowing convection to grow upscale and slide to the southeast
along the periphery of the sharp theta-E gradient laid out by the
stationary front.

A more widespread heavy rainfall threat will occur over portions of
north TX down through the central TX Hill Country within the path
of the eventual complex. Despite some discrepancy in hi-res with
the overall evolution of the convective pattern, there was enough
agreement in the threat of widespread totals exceeding 2" with
several CAMs indicating upwards of 5-7" within the core of the MCS
path over central TX. HREF probability fields were yielding some
impressive signatures for at least 3" of rainfall with neighborhood
probs around 50-70% located from an area just south of Wichita
Falls down through the Hill Country southwest of the DFW metro.
Spotty 10-15% probabilities for upwards of 5" were also positioned
in that same corridor, a reflection of the potential with an
environment conducive for significant rainfall given the elevated
PWATs (+2 anomaly) and available instability (3000-4000 J/kg of
MLCAPE) that are necessary for enhanced rainfall rates.

The area across central TX has been the hot spot for recent
rainfall with local FFG`s much lower than climo for all 1/3/6 hour
indices, another lean towards a higher impact potential due to soil
saturation and limited recovery. The DFW metro is also an area of
interest given the impervious surfaces within the city limits and
formidable coverage expanding the outskirts of the urban corridor.
The second round of convection expected in the period is still
looking to be just to the west of the metro and as a result will be
on the cusp of more significant impact, but we`ll be monitoring the
anticipated evolution of the complex in the evening, as well as the
morning convection expected to dip south from the Red River and
maneuver over the metroplex. Pending the impact from the initial
wave of thunderstorms in the morning, a greater emphasis may be
placed on locations closer to DFW, but for now, will maintain the
SLGT risk from previous forecast over the population center. As for
the area to the west and southwest, a Moderate Risk was introduced
for portions of north TX down through parts of the Concho Valley
into the northern and eastern Hill Country with an eastward
advancement into the I-35 corridor south the DFW metro. Some
adjustments to the MDT are plausible pending convective behavior
in the morning, but ensemble means and associated probabilities
highlight the area encompassing the upgraded risk area.

...Northwest...

Surface low generation with a trailing cold front will be
positioned over the Pacific Northwest today with attendant height
falls occurring to the west thanks to an incoming, strong
shortwave trough well documented on the latest WV satellite.
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 northeast OR and the central ID mountains, likely 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. Latest HREF probability fields
indicate a small opportunity for some convective cores to breach
the 1"/hr threshold, a significant rate for the area in question as
the terrain complexities exacerbate any heavy rainfall threats,
posing a heightened risk of flash flooding. Total precip forecast
is relatively modest with guidance settling between 0.75-1.5" over
the aforementioned area with the latest HREFpmm signaling multiple
maxes within the confines of the Salmon River Mountains up through
the western chains of MT. A MRGL risk was maintained from previous
forecast.

Kleebauer


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