483
FOUS30 KWBC 300105
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
905 PM EDT Wed May 29 2024

Day 1
Valid 01Z Thu May 30 2024 - 12Z Thu May 30 2024

... THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHEAST TEXAS AND FAR WESTERN LOUISIANA...

...Central-Southern Plains into the Lower Mississippi Valley...
...0100 UTC Update...
Minor tweaks were made to the Slight Risk area, based largely on
the latest observational and mesoanalysis trends. Expanding area of
colder cloud tops per the recent longwave IR loops, along with the
coalescence/organization of convective clusters, support some
upscale growth this evening in vicinity of the surface warm front.
Thermodynamics in this region remain quite favorable, with mixed-
layer CAPEs still a healthy 2000-3500 J/Kg and PWs of 1.75-2.00".
Kinematics will help as well, per a boost in southerly low-level
flow this evening/overnight up and over the warm front. Increasing
upwind propagation along with the 850-300 mb flow nearly
paralleling the warm front will enhance the risk for west(erly) to
east(erly) cell training and thus flash flooding, particularly
within the Slight Risk area.

Elsewhere, expanded the Marginal Risk farther downstream across
southern LA- MS, again based on the convective trends.


...Northern Mid Atlantic though Southern New England...
...0100 UTC Update...
Convective spirals ahead of a fairly robust late spring mid- upper
trough still have sufficient instability to work with (CAPEs ~500
J/Kg), owing to the steep low-level lapse rates (7-7.5 C/KM), in
producing locally heavy rainfall within the slow moving bands.
Latest HREF guidance (18Z) shows elevated probabilities of >1"/hr
and >1.5"/3hr rainfall rates, which considering the current soil
saturation per the latest FFG and NASA SPoRT analysis, localized
flash flooding will remain a concern through the rest of the
evening hours.


...Northern Upper High Plains...
No significant changes were made to the Marginal Risk area. The
wave ejecting out of the Intermountain West may stall over the
area, with the guidance trying to suggest the MT/ND border area
under a higher threat. The area is generally flood resistant, so no
changes were needed.


...Southeast Florida...
...0100 UTC Update...
No changes were made to the inherited Marginal Risk. It may have
taken a while, but convection continues to blossom south of Lake
Okeechobee this evening south of the stationary front, in an area
of more optimal thermodynamics (mixed- layer CAPEs of 2000-2500
J/Kg and PWs nearing 1.75"). As the slow- moving convection enters
the urban corridor later this evening, isolated rainfall totaling
3+ inches within a couple of hours may result in localed flash
flooding in urban and poor drainage areas.

Hurley/Wegman


Day 2
Valid 12Z 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...

...2030Z Update...

No major changes were made to the inherited risk areas. The Slight
risk was trimmed on the northwest side (central Kansas) due to
lack of instability and very dry antecedent soil conditions, but
also expanded to the south to include San Antonio and areas from
west to northeast of Houston, as the area has already been hard hit
and may be again. A higher-end Slight is considered for the area
from about Childress, TX in the west to the southeast corner of OK
in the east, and OKC in the north to DFW in the south. Expect 2
different lines of storms in this area Thursday...one during the
morning from a line that will be somewhere near the southwest
corner of Oklahoma, and races along the Red River, with a second
similar strength line again Thursday evening. This effectively
means that the day`s 24 hour totals will be effectively split half
and half with 8-12 hours in between. This should greatly limit the
flooding potential overall...but given the very wet antecedent
conditions, still rises to a "higher-end Slight" threat level.

Guidance remains in very poor agreement overall with where the
lines track, how quickly they form, or how strong they get. Thus,
the heavy rain footprint remains very big, extending from as far
south as San Antonio all the way up to Kansas City and everywhere
in between. Thus, unfortunately the Slight remains unusually large.
This typically can indicate that within the Slight there may
eventually be Moderate risk potential, but in this case it`s due to
unusually high uncertainty. Should the guidance come into better
agreement, the Slight will likely be able to be shrunk
considerably.

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 off
general convective evolution within a favorable environment.

Guidance is beginning to key on a few areas of interest that could
have greater prospects for flash flooding. The most notable is
located within north TX through OK where the same stationary front
that has caused so many issues the past few days will once again be
a potential focal point for thunderstorm formations and storm
motions anchored to the front and associated theta-E gradient.
Storms are forecast to develop over northeastern TX the prior
period and slide southeast through north TX with a correlating time
frame that extends into the D2, so the first punch will come from
adjacent periods. As that round of convection vacates, environment
will once again destabilize with the stationary front remaining
parked over the northern third of the state. Another complex is
anticipated upstream, developing from quite a prolific mid-level
shortwave that is within pretty much all guidance at this juncture.
This will set off another round of thunderstorms with anticipated
upscale growth as it plows east- southeast through the Southern
Plains. Expect a long swath of heavy QPF across the eastern TX
Panhandle all the way through much of OK and north TX within the
path of the forecasted complex. 2-4" of rainfall is increasingly
likely within a large chunk of the South Plains due to the complex
in general, as well as other convection that spawns during the day.
Environment is favorable given the anomalous PWATs running south
to north along the western fringes of the ridge with anomalies
generally 1-2 deviations above normal across TX up through the
central plains, so the prospects for deep layer moisture presence
to initiate the heavy rain threat is highly probable.

Kleebauer

Day 3
Valid 12Z Fri May 31 2024 - 12Z Sat Jun 01 2024

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

...2030Z Update...

As with Day 2/Thursday, uncertainty remains quite high as to the
convective evolution on Day 3/Friday. However, there is somewhat
better agreement on the whole Friday as compared with Thursday. The
crux of the increased confidence is better run-to-run consistency
in at least some of the guidance, indicating the Arklatex region as
the bullseye for the heaviest rainfall. The guidance is a hair bit
faster from inherited, so the Slight risk area was extended east
and trimmed on the west side accordingly. Once again, the Slight
was shifted south based on the latest guidance and typical guidance
trends leading up to the event. The Arklatex region was highlighted
for a higher-end Slight, and should guidance remain in reasonable
agreement, could be upgraded to a Moderate given the very favorable
antecedent conditions present in that area, as well as if guidance
shows an increasing trend in amounts.

Wegman

...Previous Discussion...

Our very active pattern will continue into Friday with much of the
residual convection over the southern and central plains shifting
eastward towards the Mid and Lower Mississippi Valley allowing for
a continued heavy rain footprint to extend further eastward.
Several shortwaves will continue to eject out of the southern
stream with a stronger, more amplified shortwave positioning itself
over the central plains, likely from the organized convective
pattern from the prior period. Surface low will advance east out of
the plains as well with our stationary boundary finally lifting
north as a warm front. Frontal positioning will play a significant
role in where convection will spawn and maintain a focus over a
general area that will enhance heavy rain and training prospects
that would exacerbate flash flood concerns. As of now, the limited
consensus is across the ArklaTex where a confluence of remnant of
vorticity from previous convective output and the frontal boundary
will likely be a primary focus for heavy rain in the beginning of
the period, as well as later on as diurnal destabilization occurs
and we get scattered thunderstorm activity the second half of the
period.

Another area of interest will be over central TX where a
surface low will begin ejecting out of west TX with an increasing
convergence signal occurring over the northern Hill Country, mainly
along the western extent of the prevailing frontal boundary
previously mentioned. A cluster of strong thunderstorms is forecast
to spawn in-of the region with a focus of heavy rainfall near the
initiation point and along the expected outflow boundary is gives
off. Ensemble bias corrected data is pretty potent within the area
it develops with 2-4" of rainfall likely in the vicinity of the
cluster with some deterministic going even above 5" pending the
evolution. QPF max is well defined over central TX up through the
ArklaTex and Lower Mississippi Valley thanks to the expected
pattern evolution. This was more than sufficient to maintain the
previously inherited SLGT risk with some minor extension on the
southwest edge into TX to account for some of the ensemble mean
trends in the QPF field.

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