Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Tue Oct 03 2023

Day 1
Valid 16Z Tue Oct 03 2023 - 12Z Wed Oct 04 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TEXAS COAST...

...16Z Update...

Only some minor adjustments for the SLGT risk across the Texas
Gulf coast where primary convergence signal will be located in a
narrow corridor from eastern Houston down into Matagorda. Surface
analysis pins a weak trough over the coastal plain of TX with a
persistent onshore flow within the southeast periphery of the
surface ridge extending across the Gulf. PWAT's at 12z for the
KCRP sounding were a touch above 2" with guidance indicating
slightly higher PWAT indices to the NE around Houston (2.1-2.3")
which puts the area within the 1.5-2.0 deviations above normal
territory. This is the kind of environment that is conducive for
locally heavy rainfall with rates pushing 2-3"/hr, which is
already the case within the first wave of precip in the area. 12z
HREF mean QPF is an area of 1.75-2.25" with blended mean QPF
footprint showing a swath of 2.5-3.5" located just to the
southeast of Houston proper. Deterministic QPF is gaudy with local
5-7" totals being depicted anywhere within the SLGT risk zone
forecast. The main period of interest continues to be between
18-00z when local convective risk peaks as the core of the
moisture flux from the Gulf moves ashore as noted across all
hi-res guidance and 12z HREF probabilistic measures for 2"/3-hr
and 3"/3-hr with 60-70% and 40-50% respectively for each
indicator. Despite the area being dry over the past several
days/weeks, the urban corridor within Houston and surrounding
suburbs, as well as anticipated very heavy rainfall rates
maintains a prominent signal for enhanced flash flooding concerns
over the course of the afternoon and early evening hours. Thus,
have maintained the previous Slight Risk issuance with adjustments
made to the eastern and western extension based on 12z HREF
blended mean QPF footprint, as well as HREF probability outline
for 12-hr precip between 18z-06z of at least 40% or greater.

Locally heavy rainfall from convection over the central plains
will offer local flash flood potential, mainly within urban areas
and places where rainfall rates can reach up to 2-3"/hr over an
hour or two period. Storm motions will be fairly progressive
across the plains, so the prospects for training precipitation is
lower, but certainly non-zero given boundary behavior from outflow
generation. High FFG's are the main detriment to raising the MRGL
to a higher risk category, but pending the evolution of
convection, there could be a targeted SLGT somewhere within the
confines of central KS up through the eastern half of NE.

Dryline activity over west TX will create a localized QPF maximum
likely within the confines of the Stockton Plateau up into the
southern Permian Basin. A solid signal for heavy rainfall has been
forecast over the past few successions of hi-res deterministic and
associated ensemble guidance. This likely stems from elevated
moisture field ahead of the dryline and supercell storm modes
capable of enhanced rainfall. This threat will be weighted towards
the front end of the MRGL placement into TX with initiation likely
after 18z and ending as early as 00z when convection kicks east
and cold-pool environment helps decay convective longevity.

Kleebauer


...Texas Coast...

Southeasterly flow from the Gulf of Mexico will interact with a
coastal trough to produce showers and thunderstorms this afternoon
and early evening. Heavy rainfall is possible within the Slight
Risk area, between Houston and Corpus Christi this afternoon, when
instability will be at its peak (1000-1500J/Kg). There will be
plenty of moisture present, despite the lack of dynamic support,
with PWATs between 2-2.5" along the Texas coast. HREF neighborhood
exceedance probabilities of 2-5" are reasonably high at this time
so those were used as a proxy for this day 1 Slight Risk area.


...Great Plains and Upper Midwest...

A cold front moving across the Great Plains will act as a focus
for showers and thunderstorms today. Some locally heavy rain could
cause flash flooding, particularly over parts of eastern South
Dakota/Nebraska, but considering how dry the region is, that
threat is low for the most part. However, there's an increasing
signal for heavy rainfall to develop over portions of west Texas
south of Midland late this afternoon and evening. HREF exceedance
probabilities of over 2" are high for that area and there will be
substantial instability in place (1500-2000J/Kg) so an afternoon
upgrade may be necessary if upward trends continue.


Kebede



Day 2
Valid 12Z Wed Oct 04 2023 - 12Z Thu Oct 05 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN PLAINS AND THE LOWER/MIDDLE MISSISSIPPI VALLEY...

...2000Z Update...

The overall synoptic pattern is still on track for Wednesday into
Thursday AM with an increase in heavy rainfall potential beginning
late-Wednesday afternoon, carrying through the end of the D2
period. Models are in agreement on placement of a robust QPF axis
extending from southern MO into central TX with a bullseye of the
heaviest rainfall over eastern OK and northwest AR in due part to
an expected MCS migrating across OK and the adjacent Red River
Valley. 12z HREF mean QPF and ensmeble bias-corrected QPF across
the above areas are hefty with a widespread 2-3" footprint, and
embedded totals likely over 4" within the core of heaviest
rainfall. The root cause for the potential stems from an anomalous
airmass advecting northward from a forecasted strong LLJ located
over the southern plains. GEFS/NAEFS PWAT anomalies are
approaching 2-2.5 devaitions above normal for the setup which
shows up very well when looking at forecast bufr soundings out of
Tulsa and OUN where PWATs are surging above 1.5", something
typically found in these highly amplified patterns. The other
notion for the potential is the 12z HREF progs for 1-hr and 3-hr
rates/totals where probabilities are approaching 50-60% for a
large area encompassing north TX up through eastern OK for 2-3"/hr
rates and 3"/3-hr totals. Total forecast QPF is between 3.5-4.25"
across southeast OK where most guidance has the 24 maximum,
including the ensemble bias-corrected QPF which is very aggressive
in its presentation. After collaboration with all the local WFO's
within the SLGT, have decided to maintain the Slight Risk given
the area FFG guidance is still fairly high for the area, which is
climatologically favored to have less major flooding concerns due
to soil type and higher flood thresholds. Locally higher impact
flooding will still be possible, especially over terrain focused
in the Ozarks and towards the OK border. Urban corridors within
the SLGT are also at risk, especially within persistent heavy rain
bands that are embedded within the core of the MCS. Overall, this
is a a "higher end" SLGT risk for the region with full agreement
from the local FO's.

Kleebauer

...Southern Plains and Lower/Middle Mississippi Valley...

There continues to be reasonably high confidence of an excessive
rainfall event unfolding across eastern Oklahoma, western Arkansas
and northeastern Texas on Wednesday. Embedded shortwave energy
within an amplified upper trough will provide ample forcing for
waves of convection to propagate across Oklahoma and north Texas
beginning tonight and continuing on Wednesday. Plenty divergence
aloft and sufficient low level jet support will promote forcing
for heavy rainfall. Instability between 500-1000J/Kg and PWATs
between 1.75-2.25" should support rain rates of over 1.5" with
locally higher amounts. We're expecting between 3-5" of 24hr QPF
for much of the slight risk area with locally higher amounts
possible. An upgrade could be possible if things like the ensemble
means (GEFS and ECE) come into better agreement with respect to
the axis of heaviest precip. The GEFS favors a more northerly
solution while the ECE is farther to the south. 24hr exceedance
probabilities of over 2" remain high though for both ensemble
means. Some heavy rainfall may also develop over parts of Texas'
central Gulf Coast where they're expected to receive some
antecedent heavy rain today, which will make their soils
vulnerable to runoff.


Kebede

Day 3

The Day 3 outlook will be updated by 2030Z.


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt