755
FOUS30 KWBC 302019
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Thu May 30 2024

Day 1
Valid 16Z Thu May 30 2024 - 12Z Fri May 31 2024

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

...16Z Update...

No big changes were made to the inherited giant Slight Risk area
over much of the southern Plains. The Slight was expanded east into
portions of central/southern Mississippi with this update.

A nearly stationary cold front draped across southwestern
Mississippi has resulted in convective development along it, which
has been locally producing rainfall rates of 3 inches per hour near
the Mississippi River south of Vicksburg, and a second area of
storms near the northeasternmost point of Louisiana. With
continued inflow into the front, the storms have been maintaining
their intensity, and their slow movement has resulted in locally
impressive rainfall totals. Fortunately for now the storms are over
a relatively rural area, and their coverage is small enough that
the dramatic rainfall amounts are over highly localized areas.
Since the storms have been maintaining themselves and are showing
no signs of abatement, the Slight was extended.

The other big area of focus is over much of northern Texas over the
next few hours. Multiple areas of storms are moving across the
region, and have a history of producing local rainfall rates to 2
inches per hour. As they move east and move into the Dallas-Fort
Worth Metroplex over the next several hours, the locally reduced
FFGs raise the flash flood risk, as convective coverage continues
to increase. The area is under a "higher end" Slight due to the
continued typical uncertainty as to convective evolution. This is
not just for where the heaviest rainfall will be over the next
several hours as the storms move across north Texas, but a
surprisingly well agreed upon development of a second larger but
similarly progressive line of storms to track across much of the
rest of north Texas and extending into central Texas tonight. Areas
that are hit with multiple rounds of storms between the 2 rounds
today and tonight have a higher flash flooding threat.

Finally further north into Kansas, storms taking advantage of the
same Gulf moisture plume as further south into Texas will continue
pushing east through the day. The concern for flooding comes more
towards tonight, as the storms are expected to slow their forward
progression with the potential for even some retrograding (westward
movement) into northeastern Kansas. This may impact the Kansas City
and/or Topeka areas. With plenty of uncertainty as to how that will
evolve the Slight remains unchanged, but is also locally considered
a higher-end Slight given the unusual agreement on a slowing or
stoppage of storm motion in that area among the 12Z CAMs.

Wegman

...Previous Discussion...

Two primary areas of convection remain across the Central and
Southern Plains with the latter currently progressing southeast
through the central Panhandle with eyes for north TX later this
morning. The premise for the D1 period will be widespread
convective development across TX up through the Front Range due to
a plethora of mid-level perturbations ejecting eastward out of the
Rockies and initiating a north-south wall of thunderstorms within a
defined theta-E ridge axis. Current QPF forecast based off the
latest HREF and associated CAMs has two main areas of focus for
the period. The first being a conglomeration of convective
development over west TX that will surge eastward with favorable
mid-level ascent thanks to a stronger shortwave pivoting through
the southern plains by this afternoon and beyond. A broad footprint
of 2+" of precip is forecast off pretty much all deterministic
within the confines of the northern Concho Valley up through north
TX, spreading east over the northern Hill Country out to the I-35
corridor from DFW down through Killeen. This has been the focal
point for an area of enhanced flash flood potential given the broad
swath of QPF with embedded totals breaching 3-4+" within the
deterministic and bias corrected ensemble. The area from the Red
River, south will be the best area for heightened flash flood
impacts with the forecasted QPF, as well as the signal for rainfall
rates to exceed 2"/hr at times as depicted within the probabilistic
fields indicating upwards of 60% neighborhood probability for that
to occur. This entire setup is breeding grounds for a matured MCS
that will likely slide eastward with extension that can cover
impacts from the metroplex down through central TX in the overnight
hours. As a result, this area is well within the standards for a
SLGT risk maintenance, as well as wording for a "higher end" SLGT
risk potential with an outside opportunity for a targeted MDT in
later updates, pending convective evolution.

Further north, several shortwaves will traverse the central plains,
interacting with the favorable environment ahead of the primary
ascent pattern thanks to a poleward expanse of unstable air brought
about by the backside of a sprawling surface ridge located east of
the Mississippi. Guidance is pin-pointing an area in-of central and
eastern NE down through KS as the target for a secondary QPF maxima
brought on by a potential MCS developing and moving over the above
areas. HREF probabilities are also fairly high for the area around
eastern NE with the EAS signals for at least 2" as high as the
signals across the southern plains. A few CAMs and global output
indicate as much as 4-5" of rainfall possible somewhere within the
confines of the MCS initiation point which would lead to some
flooding concerns, even in areas with higher FFG`s compared to
those over north TX. A SLGT risk remains steady over the central
plains, mainly east of the 100W parallel.

A few other areas note for local impacts include; west TX Lower
Trans-Pecos up through the eastern Permian Basin for another round
of thunderstorms that would impact an area that has seen multiple
rounds of heavy rain and flooding making this an isolated target of
opportunity. Isolated to scattered thunderstorms across OK will
have plenty of instability and elevated moisture to contribute to
local impacts, especially in any organized clusters, or if the
Central Plains MCS pushes south as some CAMs indicate. That would
lead to more widespread flood potential, thus why the area remains
solidly in the SLGT risk. The last area is well to the north within
the northern Midwest as a strong upper trough continues to provide
large scale ascent within the northern plains. Recent ML output
has been targeting the area over southern and east-central MN over
into western WI for a few days and synoptically, it makes sense
given the area situated in the RER of a prominent upper jet
bisecting the northern plains in Manitoba. Precip amounts are
generally between 0.75-1.5", but for the urban corridors within the
Twin City metro, that could be enough to spur some localized flash
flood concerns in stronger echoes.

Kleebauer


Day 2
Valid 12Z Fri May 31 2024 - 12Z Sat Jun 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS THROUGH THE ARKLATEX, OZARKS, AND LOWER MISSISSIPPI
VALLEY AND DELTA...

...2030Z Update...

As with many convective events, changes to the ERO are largely
dependent on the behavior of the previous day`s convection. In this
case, the trends in the guidance suggest that the convection will
be moving a bit faster, and therefore get further east through the
day than previous runs. Thus, the Slight was shifted eastward with
this update. The good news is that faster overall movement has
dramatically limited the amount of overlap from today/Day 1. Thus,
the threat of an eventual Moderate upgrade has diminished a bit.
However, just because Arkansas and the Lower Mississippi River
valley haven`t been hit quite as hard with heavy rain than areas
further west...there is some vulnerability there as well. Thus, a
higher end Slight remains for most of Arkansas, the eastern half of
northern Louisiana, and the western 3 columns of counties in
Mississippi (not shown). A few of the CAMs` insistence of multiple
developing rounds of storms from Little Rock, AR north and east,
for example, most certainly keeps open the possibility for an
eventual Moderate, as well as any storms occurring over the few
hard hit areas of Mississippi and Louisiana that were hit hard
today.

For now, the Slight was expanded to include the New Orleans area
with the likelihood of convection around after today`s flash
flooding, and also expanded up the east side of the Mississippi to
the Ohio River confluence, which has been hit with heavy rains
recently as well.

Across Texas, much of the eastern part of the state remains in a
Slight for Friday as a combination of wet antecedent conditions
from both previous rainfall and expected convection through
tonight, and the possibility of continued training lines of storms,
albeit fast-moving, moving over the center of the state from
roughly Abilene southeast through the Houston Metro. Associated
convection should be a first half of the day event, and can be
trimmed through the afternoon.

...Colorado High Plains and Adjacent Areas...

A Marginal Risk area was introduced with this update for dry line
convection that may develop over this area. Despite very dry
antecedent conditions, heavy convection falling over dry hardened
soils may cause isolated flash flooding again on Friday. Any areas
hit today, including the area around the western tip of the
Oklahoma Panhandle, may see renewed flash flooding again Friday
should convection move over the same areas.

Wegman

...Previous Discussion...

Active convective pattern from the previous period will shift to
the east with a strengthening shortwave trough maneuvering to the
east through the Mississippi Valley during the forecast period.
Recent trends within guidance have signaled the area from east TX
over through the Lower Mississippi Valley as the focal point for
heaviest precip, correlating well with the favorable upper pattern,
as well as the best deep layer moisture and instability being
shifted eastward thanks to a surface low forecast to progress
northeastward out of the Southern Plains with a cold front trailing
the primary low. Environment will stabilize in wake of the surface
reflection passing to the east, finally putting an end to the
onslaught of convective activity that has plagued the state of TX
for the past several days. Organized convection in the beginning of
the period will be the area(s) to monitor as guidance is not
completely in-line for where the convective clusters in the
Southern Plains will head. Recent ensemble bias corrected data, as
well as the NBM and ML output have situated the QPF maxima
generally over the Ozarks down through AR/LA with totals breaching
3" even within the means. The probability matched mean, which
incorporates some highlighted maxima embedded within the means has
precip totals getting closer to 4-5" within portions of the Lower
Mississippi Valley, relevant to the expected synoptic evolution and
accompanying instability axis residing in a targeted warm-sector
downstream of the approaching cold front to the west and a
stationary boundary bisecting portions of the Deep South into the
Southeastern US. This would certainly benefit the area in question
with the best thermodynamic potential, and it correlates well with
the focused mid-level ascent provided by the strengthening
shortwave trough pivoting into the region. As a result, the SLGT
risk from previous forecast was maintained for continuity and
sufficient agreement among the ensemble suite.

There is an opportunity for greater impacts across southeast TX as
some CAMs deliver the central TX MCS from the previous period over
into the Gulf Coast as the complex progression moves into the area
that has been hit with multiple heavy rain events in previous
weeks, including yesterday. ML output does show a signal for that
area, so the SLGT risk was extended a bit to the south and
southwest to cover for the potential.

Kleebauer


Day 3
Valid 12Z Sat Jun 01 2024 - 12Z Sun Jun 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHEAST AND LOWER MISSISSIPPI VALLEY...

...2030Z Update...

A weakening shortwave lifting northeast towards the Appalachians
Saturday will provide the forcing for an otherwise moist and
unstable Gulf air mass to continue thunderstorms once again
Saturday afternoon and evening from northern Louisiana through
central Alabama. Guidance continues to indicate the potential for
isolated 3 to 5 inch storm total rainfall in the Slight risk area.

The western half of the Slight Risk area has been hit recently with
heavy rainfall, and may see some activity before the start of the
Day 3 period. This will lower the threshold for potential flash
flooding in the FFGs. Meanwhile, further east into Alabama, there
is better signal for heavy rain, likely in the form of multiple
rounds. Thus, while FFGs are higher, the higher likelihood for the
higher amounts brings this area into the Slight risk category.

Wegman

...Previous Discussion...

The period beyond D2 is contingent on the anticipated convective
evolution downstream and the positioning of the strong mid-level
shortwave ejecting through the Tennessee Valley by the second half
of the period. Latest trends favor a widespread axis of convective
potential with the best instability fields located over the
Southeast U.S, and best upper forcing targeting the Ohio Valley and
central Midwest. There will likely be a focal point in heaviest
precip as we move closer to the period of interest, but for now
maintained continuity from previous forecast with some expansion on
the northern end of the MRGL risk area, as well as some areas to
the west in the plains due to some thunderstorm concerns with
locally heavy rainfall potential. QPF maxima is currently in-of AL
and portions of the Ohio Valley with totals generally 1-2" with as
high as 3" depicted within the means. Look for the max to shift
upwards in time as the environment and pattern evolution become
more stable and hi-res deterministic can lead to a consensus on
where the threat will be most prominent. The broad MRGL will
suffice until those details become clearer.

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