573
FOUS30 KWBC 012109
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
509 PM EDT Wed May 1 2024

Day 1
Valid 01Z Thu May 02 2024 - 12Z Thu May 02 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
CENTRAL AND EASTERN TEXAS...

...16Z Update...

...Southern Plains...

Latest CAMs have shown a growing consensus on the potential for
training cells farther west towards the Edwards Plateau closer to
the dryline. Latest RAP guidance shows an acceleration in the 850mb
LLJ over the Rio Grande this afternoon that provides strong
theta-e advection at that level. This also coincides with the
region being ideally placed beneath the left-exit region of a 250mb
subtropical jet streak, sufficient shear for sustaining
mesocyclones, and PWs >1" that are above the 90th climatological
percentile. With some portions of the Edwards Plataea and Concho
Valley having received as much as 300% of normal rainfall over the
past week, soils are a little more susceptible to runoff. Decided
to expand the Slight Risk into this region for these reasons.

In South Central TX, latest CAMs consensus shows the QPF footprint
shifting south towards the I-35 and I-10 corridors between San
Antonio and Austin, and even on west into the heart of the Hill
Country. This is largely due to the core of the 850mb LLJ being
focused more over this part of the region in recent runs. These
areas are more elevated and can further increase low-level ascent
in a synoptic scale setup that supports large scale growth of
organized thunderstorm clusters. The 12Z RAP depicts highly
saturated surface-700mb soundings north and east of San Antonio
this evening where low-mid level RH values are averaging around 90%
saturation and warm cloud layers as deep as 13,000ft. The ECMWF
SAT shows PWs approaching 1.75" that are as high as the 99.5
climatological percentile by 00Z this evening and MUCAPE >2,000
J/kg. The 12Z HREF now sports moderate chances (40-60%) for
rainfall totals >5" near San Antonio and suburbs of the city along
I-35 and I-10. With the added concern for 2-3"/hr rainfall rates
within the more urbanized I-35 corridor, where runoff will be
accentuated, chose to expand the Moderate Risk area west the
Austin/San Antonio metro areas.

...Midwest/Central Plains...

The northern flank of the Slight and Marginal Risks were trimmed
due to less available instability over southeast SD, northwest IA,
and central NE. The Slight Risk was expanded farther south into
northern KS and northwest MO where these areas are more ideally
placed at the nose of a robust 850mb LLJ that tops the 90th
climatological percentile for wind speeds over eastern KS.
Admittedly, there remains quite the dispersion in CAMs guidance in
the placement of heaviest rainfall some some guidance as far north
as east-central NE and as far south as east-central KS. Using the
position of the 850mb LLJ and the expected frontal position as
references, opted to position the Slight just north of the frontal
boundary over central Kansas. This should be where the strongest
ascent occurs as the 850mb LLJ intersects the front and where MUCAPE
>1,000 J/kg is also likely to be in place tonight.

Mullinax


---Previous Discussion---

...Southern Plains...

Latest hi-res ensemble and associated CAMs have come into better
agreement on the development of a complex across TX with origins
out in west TX near the dryline, growing upscale through the
evening with aid from a well defined LLJ and shortwave progressive
within the sub-tropical jet over the region. Given consensus now
favoring at least 2" of precip over a large expanse of central and
eastern TX with much higher embedded amounts given the convective
nature of the precip and anomalous tropospheric moisture (2-2.5
standard deviations above normal PWATs), have allowed for an
upgrade to a Moderate Risk over the aforementioned areas. This is
now the primary period of interest with a shift away from the
previous MDT that was forecast for the now D2 time frame.

The reasoning for the shift was due to the progressive nature of
the convective pattern within the latest CAMs suite, as well as
some instance within the ensemble bias corrected mean QPF
footprint. Previous forecast iterations were more robust for the
now D2 time frame, but the addition of CAMs guidance and the
ensemble have allowed for a shift in the timing of when the
heaviest precipitation would occur, as well as the general
locations. 00z HREF EAS probabilities for at least 2" of rainfall
are now upwards of 40% across a large swath of central and east TX,
a good portion now overlapping with the area that saw extreme
rainfall totals over 6" from this past weekend. The conjunction of
heavy rain over those areas impacted and forecasted rates exceeding
2"/hr will likely force easier flash flooding potential due to the
moist antecedent conditions in place. Also of note is the area
over central TX where despite missing the previous rainfall, the
flash instances are climo favored due to the soils and high runoff
potential. With coordination with the local WFO`s involved within
both TX and LA, the MDT risk was shifted to account for the abrupt
change in convective timing and impacts.


...Midwest/Central Plains...

A closed upper low is currently analyzed over Big Sky into the
southwestern Canadian Provinces. A robust mid-level vorticity
maxima will pivot around the base of the mean trough carved out by
the upper low with sights on the central plains and adjacent
Midwest allowing for rapid convective development downstream with a
corridor of heavy rain forming later this afternoon through the
remainder of the period. Guidance has trended to a favored heavy
rain footprint across northern KS through eastern NE with the
heaviest rain with a swath of 2-4" of rainfall over the course of
this evening into early Thursday AM. 00z HREF EAS probabilities for
at least 1" have become most aggressive in-of southeast NE which
matches well with the current ML output from the past several runs.
This makes sense synoptically given the best mid-level forcing
will be downstream of the approaching vort max in an axis of
diffluence perfectly aligned overhead. Rates between 1-2"/hr will
be common within the areal extent of convection leading to some
local totals between that 2-4" marker forecast. This should be
sufficient for some higher flash flood potential within larger
urbanized settings and areas that see the higher rates in question.
The progressive nature of the precip will limit the higher end
impacts, so the previous SLGT risk forecast will suffice despite
the alignment synoptically.

Kleebauer


Day 2
Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
MISSISSIPPI VALLEY EXTENDING FROM THE NORTHERN MIDWEST DOWN TOWARDS
THE GULF COAST...

...2030Z Update...

...Midwest...

Trimmed down the far northern and western flanks of the Marginal
and Slight in the Midwest where probabilistic guidance in heavier
QPF totals >1" were decreasing and instability will be lower to the
north of the surface low track through the Upper Mississippi
Valley Thursday evening. The area most likely to see the heaviest
rainfall will be closer to the triple point low, which in this case
would favor an area from southern IA on north and east through
eastern IA and into southwest WI. NAEFS shows this designated area
sporting >90th climatological percentile PWs (1.3-1.5") between 18Z
Thurs and 00Z Fri, as well as IVTs above the 97.5 climatological
percentile (500-700 kg/m/s) in that same period. 12Z HREF shows
MUCAPE between 500-1,000 J/kg will also be present. Hourly rainfall
rates up to 2"/hr are possible in the strong est storms, which
would support the potential for slash flooding in these areas.
Farther south, a negatively tilted 500-700mb axis emerging out of
the Ozarks will still be capable of producing strong-to-severe
thunderstorms that could track across much of western MO and
eastern KS. Localized rainfall rates surpassing 2"/hr cannot be
ruled out given PWs topping 1.5", MUCAPE approaching 1,500 J/kg,
and mean 1000-500mb RH values as high as 90%.

...Southern Plains and Lower Mississippi Valley...

The expansive Slight Risk in the South Central U.S. was expanded a
little farther west more into central TX and central OK where
latest CAMs guidance has slowed down the cold front approaching
from the northwest. Farther south, there remains some uncertainty
on the position and duration of the ongoing cluster of storms in
the early-mid morning hours Thursday. Should storms stick around
longer into the morning hours along and north of I-10, there could
be the need for either an additional expansion of the Slight Risk
or an upgrade to Moderate Risk should flash flooding be more
extensive and linger beyond 12Z Thursday. Recent rainfall in
western Louisiana (2-5" above normal over the past 7 days) has left
soils in the area more sensitive to 2-3"/hr rainfall rates, which
are supported with PWs just short of 2.0" and skinny MLCAPE up to
1,000 J/kg present. Overall, the Slight Risk remains on track but
the setup in eastern TX and western LA will be closely monitored
overnight.


Mullinax


---Previous Discussion---

...Midwest...

Previous forecast still has merit with little deviation from past
issuance. Thus, maintained continuity from the last discussion with
some minor changes to details within the discussion below....

Overnight convection from this evening will continue to march
northeastward as the congealed cold pools from the initial cell
mergers will advance through the central Midwest with sights on
northern IA into WI and MN on Thursday morning. The complex will
eventually peter out as it moves northward into a less favorable
environment which will put an end to the flooding threat as we move
into the afternoon. Later in afternoon and evening, a cold front
will propagate to the east out of the central plains with a
redevelopment of convection over eastern KS, IA, and northwest MO.
This will allow for the secondary peak of flash flooding potential
as thunderstorms migrate through the mid and upper Mississippi
Valley, impacting some areas that were just affected in the last 24
hrs. With the lower FFGs expected, the threshold for flooding will
be lower than climatological norms for some areas within the
expected secondary convective regime. Totals are currently within
the 1.5-3" range on guidance which is significant enough to
warrant at least the current SLGT risk.

...Southern Plains and Lower Mississippi Valley...

As stated previously in the D1 outlook, our complex of
thunderstorms in TX will migrate eastward into the Lower
Mississippi Valley with heavy rainfall likely within the western
half of LA into southern AR. The setup has now trended towards a
"front loaded" forecast where the primary rainfall in the period
that would induce flash flood concerns will occur during Thursday
morning and early afternoon before dissipating due to the loss of
sufficient upper forcing. The back end of the latest CAMs and
associated HREF signal the opportunity for upwards of 2-3" of
rainfall during that short time frame Thursday AM which will be
enough to cause problems given the overlap from the previous
weekend event that dropped copious amounts of rainfall over
southeast TX into western LA. 00z HREF blended mean has an areal
extent of 1.5-2" with probability matched mean upwards of 3" along
the TX/LA border. HREF neighborhood probability for rates exceeding
2"/hr are also upwards of 30-40% within the same areas, correlating
with the forecasted rainfall in that 2-3" zone. The setup really
comes down to the timing of the complex in TX as to whether a
reintroduction of a MDT risk will be needed after it was downgraded
and shifted one period earlier. This will be assessed in future
forecast updates as we closely monitor radar and hi-res guidance
trends over the course of today and overnight into Thursday. For
now, the SLGT risk will remain with wording of it being on the
higher end of the risk threshold.

Further north across KS and OK, the cold front to the north will
move south with a push from high pressure nosing in through the
plains in wake of the low pressure moving to the north. An area of
convection will likely form over the central plains and move south
with the boundary leading to more locally heavy rainfall in
association with the area of convection. The threat is less
aggressive compared to what is expected further south, but the
chances are still prevalent. The SLGT risk was maintained from the
previous D3 connecting the two primary areas of interest to the
north and south.

Kleebauer



Day 3
Valid 12Z Fri May 03 2024 - 12Z Sat May 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
TEXAS, THE LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, CENTRAL
PLAINS, AND THE PACIFIC NORTHWEST...

...2030Z Update...

Little in the way of change for the inherited Marginal Risks aside
from minor adjustments based on the latest QPF. There was some
consideration to adding a Marginal Risk in portions of the Great
Lakes given the >90th climatological percentile PWs present
according to the ECMWF SAT, but thunderstorm activity looks fairly
progressive and confidence in sufficient instability being present
was not high as of this forecast cycle.

Mullinax


---Previous Discussion---

...Southern Plains and Southeast...

A multitude of shortwaves will ripple through the flow out of the
subtropical jet with convective development across portions of TX
through the Lower Mississippi Valley up into the Tennessee Valley.
The convection over TX will have the best potential for flash flood
concerns due to the regional instability and alignment of a stalled
frontal boundary forecast across the central and eastern portion of
the state with the dryline positioned over west TX. QPF
distribution is scattered in nature with the maxima shifting all
over the place pending deterministic. Considering the spread, the
MRGL risk inherited was maintained, but there is potential for an
upgrade within the two area boundaries due to the features being a
focal point for training convection and surface convergence.

Further east, widely scattered thunderstorms will produce heavy
rainfall at times with the best opportunity occurring earlier in
the period when the strongest shortwave progresses through the
region(s). Ensemble bias corrected QPF is consistent with a maximum
across LA which will have gone through some significant precip
prior, so that will be another area to monitor in time for
potential upgrades. Forecast QPF is still marginal for any upgrade
potential, but CAMs will shed more light as we move closer to the
time period of interest.

...Central Plains...

A fairly robust mid-level perturbation will eject east-southeast
out of WY/CO with convection initiating over the Front Range,
eventually congealing into a MCS as it migrates eastward through
southern NE into KS. Models are in agreement of at least some
modest QPF with 1-2" scattered across the aforementioned areas and
QPF maxima running upwards of 3" at this time. These MCS setups are
notorious for localized flooding concerns, so wanted to make sure
to add a Marginal Risk area to cover for the potential. As we move
closer, the MRGL risk will likely be shifted around to account for
the trends in guidance, especially within D1 when the CAMs and HREF
provide more favor.

...Pacific Northwest...

A relatively weak IVT pulse will enter coastal OR and northwest CA
by the second half of the period with locally heavy rainfall
expected through Saturday morning. IVT index is running at an "AR
1" which signals a weaker atmospheric river event, but one that can
still produce modest rainfall totals to the coastal plain. Ensemble
trends have come up a bit from previous forecasts with QPF running
between 1.5-2.5 over the span of 12 hrs (00-12z Saturday), with a
bit more as we head into D4. As a result, kept continuity with the
MRGL risk from previous forecast package.

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

$$