191
FOUS30 KWBC 290822
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Wed May 29 2024

Day 1
Valid 12Z Wed May 29 2024 - 12Z Thu May 30 2024

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

...Texas...

Multiple clusters of thunderstorms continue to progress to the
southeast across TX with the latest surface analysis pin-pointing
the stationary frontal boundary closer to the Gulf coast, extending
back into south-central TX before wrapping back through west TX as
a defined moisture gradient. All the convection earlier has allowed
for the shift in the frontal boundary with lower theta-E`s
positioned to the north of the front, a far cry from where we
started 24 hrs prior. The upper level pattern will be progressing
steadily eastward with a mid-level ridge positioned over the
Continental Divide expected to shift over the plains thanks to a
strong negatively tilted shortwave trough in the Pacific Northwest
kicking the ridge downstream. The pattern evolution will bring
about a slow moderation in the moisture field across the southern
plains, but eventual southerly winds on the western fringe of the
surface high focused east of the Mississippi will help advect a
modestly unstable airmass poleward with a large theta-E ridge
positioned east of the Rockies, tilting back into the southern
plains. The stationary boundary over the south will slowly lift
back north with diurnal mixing allowing for a distinct demarcation
of the higher theta-E availability for the afternoon and evening
hours. Small mid-level perturbations will eject out of the Front
Range with another round of convection forming from WY down
through CO/NM/TX and pressing eastward along the theta-E gradient.
Local amounts of 1-3" could fall quickly within the environment
established allowing for a broad MRGL risk area to encompass much
of the central and southern plains. Remnant mid-level perturbations
from this evenings convection will play a bigger role downstream
into the Lower Mississippi Valley where a few rounds of
thunderstorms could flare up with locally heavy rainfall in-of
places like AR/LA/MS which is the reason for the eastward extension
of the MRGL risk in place.

The one area of focus will be across north TX where lower FFG`s,
especially in-of the DFW metroplex will play a role in a higher
flash flood potential given the antecedent conditions inherited
from the previous period(s). The axis of the theta-E ridge will be
positioned almost directly overhead of north TX with a tilt from
northwest to southeast, bisecting the northern Concho Valley down
into the region just southeast of DFW. There is a growing consensus
for a stronger mid-level perturbation to initiate an area of
convection upstream over southeast CO and the northern Panhandle
and grow upscale, following the theta-E gradient to the southeast
by the end of the forecast period. A modest signal for 1-2" of
rainfall with locally as high as 4" is being depicted by select
CAMs and deterministic allowing the ensembles to correlate with a
heavy precip axis extending along and south of the Red River until
you get just to the east of the metroplex. This total rainfall
accounts for not only the anticipated complex of thunderstorms, but
also any isolated convection that develops from afternoon
destabilization, so the reflection of the QPF is a culmination of
both convective modes. In either case, the lower FFG indices within
the forecasted zone of heaviest precip was the greatest factor in
the recent upgrade to a SLGT risk located over the aforementioned
area in north TX.

Other areas of note for flash flooding purposes include southeast
TX with some guidance indicating some heavier cells developing
along the stationary front which can always cause problems due to
the focal point of the boundary helping with training cells. Local
maximums of 2-4" were popping up across areas near and north of
Houston, matching the ML output for a targeted area of convective
interest given the synoptic pattern. Rates are not generously high
and the recovery of FFGs across the region will mitigate a higher
impact scenario, so the threat easily falls within the MRGL risk.
Another area of interest is across the


...Montana and Northwest North Dakota...

A fairly potent upper trough is currently analyzed over the Pacific
Northwest with an axis of diffluent flow downstream into ID/MT
causing waves of convection to form and move northeast over the
topography of the Interior Northwest. The trough will pivot
eastward over the course of the period and begin to take on a
negative tilt indicating further strengthening which will be
reflective of the evolving convective pattern downstream. A modest
jet coupling over MT by the morning, onward signals some fairly
significant large scale ascent entering into the northern High
Plains across central and eastern MT extending as far east as the
western Dakotas. A tongue of elevated theta-E`s will extend all the
way to the Canadian border within developing surface trough to the
lee of the Rockies. This will be a focal point for scattered
thunderstorms to develop later this afternoon and push
northeastward with the mean flow. Ample shear will be sufficient
for a few relevant updrafts which will help strengthen some cells
enough to produce some locally heavy rainfall within an axis of
PWATs running 1-2 deviations above normal. The best chance for 1-2"
of precip will lie within the confines of the surface trough
positioned across eastern MT and northwest ND with the core of the
jet dynamics reaching the region by the evening. Storms will be
relatively quick moving which will limit the upper end potential
for flash flooding, but there will be some cells capable for
greater impacts with the latest HREF neighborhood probs for at
least 2" running between 30-60% within the above zone. A MRGL risk
was maintained from the previous forecast.

...Mid Atlantic and Northeast...

Shortwave trough will pivot eastward from the Ohio Valley,
eventually moving through the Northeast and Mid Atlantic by midday
and beyond. Height falls present with the trough will provide a
period of steep lapse rates within a zone of modest instability
across the aforementioned area. Scattered showers and thunderstorms
will maneuver overhead from OH/WV, across PA/MD/DE, eventually
exiting to the northeast over NJ/eastern NY by later this afternoon
and evening. Local maxes around 1-1.5" are forecast within any
heavier cells over the above areas, so any areas that favor flash
flooding in valleys or heavy urbanization will be subject to the
potential. The MRGL risk from previous forecast was maintained and
remains a lower-end of the threshold given the lack of deep layer
moisture present ahead of the disturbance.

...Southeast Florida...

Seabreeze propagation inland of southeastern FL will offer a period
of locally heavy rainfall exceeding 2-3"/hr that could cause some
flash flood potential within the urban corridor extending from
Miami up through Fort Lauderdale. Latest HREF probabilities are
running pretty high for at least 3" in spots within a short window
between 18-00z, but the fields drop off to near 0% for at least 5",
so the potential is likely capped. A low-end MRGL risk was
introduced across the above area for the threat, but will likely
remain at this lower risk given the ceiling for impact.

Kleebauer


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...

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...

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