366
FOUS30 KWBC 020823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Thu May 2 2024

Day 1
Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHEAST TEXAS...

...Southeast Texas...

A complex scenario currently unfolding across southeast TX within
the confines of areas that recently received very heavy rainfall
over the weekend. There are two distinct areas of interest with a
very heavy thunderstorm cluster focused east of Bryan/College
Station near Lake Livingston and surrounds. A secondary area is
currently moving through central TX Hill Country along the I-35
corridor between Austin/San Antonio. Hi-res deterministic has
struggled to gain footing on the exact evolution of these two areas
of focus with some missing the bill altogether, parlaying the
complexity of the setup. Based on current mesoanalysis, there is a
west to east SBCAPE gradient oriented from the Hill Country west of
I-35 over into southeast TX and the LA border. Rich, Gulf air has
been pulled northward within the north-south oriented 30-35kt LLJ
positioned over the eastern 1/3rd of the state. PWATs are analyzed
between 1.75-2" across the area of interest which has aided in
significant rainfall rates between 3-6"/hr at times within the
eastern most cell organization prompting totals to reach over 5"
within the areas impacted. This will only continue through the
morning as an expected cold pool is projected to push out of the
eastern thunderstorm complex and move to the southeast over the
course of the morning. Rates of over 2"/hr are almost a certainty
with the current environment with the 00z HREF signaling
neighborhood probabilities over 50% through much of the area
between Bryan/College Station down to just north of Beaumont. The
complex will eventually lose steam and rates will dwindle back to
manageable levels, but not before a swath of 3+" of precip is laid
in wake of the complex. This is not the entire story, however as
there are other questions that are having a hard time being
resolved.

There are some instances of a boundary being left behind as
remnants and helping to produce backbuilding across the same areas
that will have been hit recently. This comes in conjunction that
the energy left from the secondary complex over central TX
continuing its progression eastward as it maintains headway with
the mean steering flow. If that were to occur, additional rainfall
exceeding 1"/hr would be possible to add to the copious amounts
being dropped currently and through the morning. To make matters
more intriguing, some guidance has another complex developing out
over west TX that eventually grows upscale this evening and moves
over the same areas hit this morning. This is well documented on
the 00z HREF 3hr mean QPF fields which would exacerbate flooding
potential further. This is a significant impact scenario unfolding
due to the compounding nature of the precip over a water-logged
area after 6-12" of rain on Sunday, as well as any additional rains
within the latter part of the period.

With collaboration from impacted WFOs across southeast TX to the LA
border, a Moderate Risk was added to the new D1 with the western
extent over Brazos county, extending eastward to
Tyler/Hardin/Jasper near the LA state line. The southern periphery
was the hardest to nail down, but utilizing the HREF probability
fields and mean QPF footprint, have extended the MDT down to
Montgomery and Liberty counties with the extreme northeastern tip
of Harris county also included. A higher end SLGT is still forecast
around the periphery, including the Houston metro, but that comes
with uncertainty on the behavior of the outflow and eventual
progression of the complex, as well as magnitude as it moves
towards the Gulf coast.


...Mississippi Valley and Southern Plains...

Current radar/sat composite shows two distinct convective clusters
propagating eastward across the central plains with the northern
periphery of the first "wave" extending all the way towards the
IA/MN border with roots back to northern KS. The second "wave" is
smaller, but following in the footsteps of the first organized
cluster with majority of its footprint based over northern KS into
southern NE. Both of these organized convective regimes will move
to the east and northeast through the beginning of the period
with locally heavy rainfall capable of 0.75-1.5" over the area it
traverses. This will set the stage for places across IA and
northern MO to see some overlap from convective development in the
afternoon hrs along the leading edge of a cold front that will
motion to the east and southeast the latter portion of the period.

Pretty consistent signal amongst all CAMs and globals for a line
of heavy QPF extending from far eastern KS up through northern MO
into IA during the afternoon hrs today with a general 1.5-3" of
rain falling within the span of 3-6 hrs when it occurs. Rates are
currently progged to be within that 1-2"/hr range with the 00z HREF
neighborhood probability for at least 1"/hr between 25-50% with
the primary area located over northern MO into southern IA. 2"/hr
rates are lower, but certainly non-zero within the 10-20% range
over the same area, so that gives us a lower and upper quartile to
work with in terms of expectation. FFGs are currently running
between that 1-2"/hr marker for exceedance, so it`s within the zone
of opportunity to cause some flash flooding potential within the
rates alone. A SLGT risk was maintained from previous forecast with
minor modifications made on the flanks of the risk area.

Across the southern plains, that same cold front will interact with
area convection forming ahead of the boundary, acting as a focal
point as it moves south through OK into the Arklatex by the end of
the period with some guidance keen on a MCS developing within the
vicinity of I-40 to the Red River. There`s significant uncertainty
on the expected evolution of any convection that does develop, but
the higher probability is thunderstorms will develop and produce
locally heavy rainfall within a core of higher theta-E`s as any
weak surface capping breaks with diurnal destabilization. Totals of
1-3" will be plausible across much of OK into the Lower Mississippi
Valley near the Arklatex over into the AR/TN border near Memphis.
Best prospects reside further west into OK which has been the
beneficiary of heavy precip in recent days leading to lower FFGs
and flood concerns. This continues a very unsettled period the past
several days.

Kleebauer


Day 2
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...

...Southern Plains and Southeast...

A trio of areas extending from the southern plains through the
Southeast US will draw attention for the period. The primary area
of focus will reside in the Lower Mississippi Valley where some
guidance is now suggesting the development of convection upstream
within the axis of an approach cold front, developing upscale and
maneuvering through the ArklaTex into LA by the beginning of the
forecast period. Locally heavy rains would be a certainty if this
evolution occurred and will need to be monitored closely with
regards to timing and magnitude as a stronger complex or later
arrival time may put the area into a higher risk (Slight) given the
antecedent conditions expected upon arrival. HREF blended mean QPF
resides around 1.5-2" for the area encompassing the ArklaTex
through northern LA which could be sufficient for an upgrade.
However, the deterrent is an inordinate amount of spread in the
hi- res for where any complex would descend upon with some much
further west and others barely showing much at all. Based on the
grand ensemble blend, including global deterministic, the prospects
are higher for something to affect the area, so the MRGL risk was
maintained with wording for possible upgrades.

...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. Current QPF maxima
is within the vicinity of southern NE into northern KS, but the
expectation is for multiple pockets of higher QPF embedded within
the complex as it moves to the southeast. As of now, the system
looks to remain progressive, but will monitor closely as we
approach the event in question to see if we can hone in on any
specific area for potential upgrades, or maintain continuity.

...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 D3. As a result, kept continuity with the
MRGL risk from previous forecast package.

Kleebauer


Day 3
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR WEST AND NORTH
TEXAS INTO SOUTHWEST OKLAHOMA...

...Southern Plains...

A cold front will move south through the course of Saturday with
sights on north TX by the afternoon hours as it progresses out of
the plains. A dryline will be focused over west TX by the morning
and will begin mixing eastward through peak diurnal instability.
Aloft, a shortwave will eject northeast out of MX which will
provide areal ascent within the confines of where the dryline and
cold front will intersect. Guidance is in agreement on a rapid
development of convection in-of the western rolling plains,
northeast Permian Basin up to the Red River with heavy rainfall
likely along and east of the dryline as dew points in the upper-60s
and low-70s will exist on the moist side of the boundary. The
convergence of the front and dryline will induce a large cluster of
heavy thunderstorms capable of significant rainfall and rates
upwards of 1.5-2"/hr based on local climo and expected PWAT
anomalies of 2-2.5 deviations above normal. Current rainfall totals
are already pushing 2.5-3" on the ensemble bias corrected QPF with
likely higher totals once we factor in CAMs later into the
equation. A SLGT risk was maintained and expanded to encompass the
1"+ contours within the ensemble bias corrected forecast and WPC
Guidance which is factoring in higher localized totals within area
convection. Would not be surprised to see this be labeled a higher
end SLGT, or an even higher risk given the early signals and the
forecasted synoptic evolution.


...Central and Southern Appalachians

A cold front will approach from the west with slow progression
through the Ohio and Tennessee Valleys before aligning parallel to
the Appalachian front by mid-day Saturday. A broad ridge to the
east will provide a convergent signature within a band of elevated
PWATs along and east of the terrain in the Mid-Atlantic. Modest
instability and upper forcing along with the surface convergence
pattern will lead to scattered convection over portions of the
western Carolinas up through western Pennsylvania through most of
the period, especially the afternoon and evening Saturday. Locally
heavy rainfall will be plausible in the setup with weak steering
flow aloft allowing for meandering and/or training cells within the
confines of the complex terrain in the interior. FFG indices are
relatively low, but not significant so, so the threat currently is
fairly minor, but within the threshold for a MRGL risk that was
inherited from prior forecast packages. Another instance of
continuity with a better analysis in future forecast cycles with
the help of CAMs and associated ensemble(s).


...Northern Sierra Nevada Foothills...

A fairly robust IVT pulse will slide down the west coast with
southwesterly flow aimed orthogonally to the northern and central
Sierra`s by Saturday. Higher elevations will be greeted with more
snow, but the adjacent foothills will remain warm enough to benefit
from all rainfall. Rates will be borderline overall, but the
prospects for over 1" of precip has grown in the past succession
of runs with the ensemble bias corrected guidance now upwards of 2"
now forecast within the elevations below 8000` MSL, falling mainly
in a span of 8-12 hours. The threat is on the lower end of MRGL,
but does have merit given the signal at leads. Maintained
continuity from previous forecast and will assess whether it needs
adjusting, or removal pending future trends in guidance.

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