048
FOUS30 KWBC 030900
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Fri May 3 2024

Day 1
Valid 12Z Fri May 03 2024 - 12Z Sat May 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GULF COAST AND SOUTHERN PLAINS...

...West Texas to Gulf Coast into Tennessee Valley...
Main concern for excessive rainfall today will be along the Gulf
coast due to one or more rounds of thunderstorms over region that
has become water- logged recently. Of most immediate concern is how
much convection that developed overnight in eastern Texas will
persist into the Day 1 period at 12Z and how much of an overlap
will there be with parts of the state that received 2 to 5 inches
of rain in the previous 24 hours and 5 to 10 inches of rain since
early this week. With cloud tops still cooling as of 08Z...the
expectation is that some areas of higher intensity rainfall will
still be occurring at 12Z with the concern for the rainfall to
exacerbate on-going flooding and for the additional rain to push
some areas into flood. However...weaker instability immediately
downstream and the progressive nature of the convection should lead
to weakening rainfall rates during the morning. The 03/00Z suite
of guidance has continued the idea from earlier runs that focus any
convection that forms later today or this evening will be shifting
either east with a mid-level trough or shifting out west near the
dryline. As a result...opted to propagate the Slight Risk area from
the Day 2 period into todays Day 1 outlook period rather than
issue a focused, short term extension of the Moderate risk after
collaboration with HGX/SHV. Maintained a Marginal Risk area farther
eastward from the Gulf coast towards the Tennessee Valley as the
mid-level trough propagates into better instability and out in West
Texas near the dryline.

...Western High Plains...
A fairly robust mid-level perturbation will eject east-southeast
out of WY/CO with convection initiating over the Front Range,
potentially developing 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.
Present indications are that the convection looks to be progressive
enough over areas with sufficiently high flash flood guidance that
a Marginal Risk area still adequately covers the risk.


...West Aspect of the Appalachians... Showers capable of producing
locally intense downpours will be moving towards parts of the
Appalachians/far eastern Ohio Valley later today. Given 3-hour
Flash Flood Guidance around 1.5 inches per hour...and some ensemble
guidance suggesting an inch of rain in the outlook area was
possible...maintained the previously issued Marginal Risk area
although spaghetti plots of QPF showed more enthusiasm for 2 inch
amounts from the ARW core.


Bann

Day 2
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
WESTERN AND SOUTHERN HIGH PLAINS...

Height falls aloft ejecting eastward from the Southern Rockies and
the presence of a dryline over West Texas will foster an
environment for thunderstorms capable of producing heavy rainfall
by later in the day on Saturday that persists into Saturday
night/early Sunday morning. Precipitable water values will have
increased to nearly 1,5 inches...some 2 to 2.5 standardized
anomalies greater than climatology...over portions of central and
west central Texas by 00Z Sunday in response to a period of
southeasterly flow off of the Gulf of Mexico. Consensus of model
guidance QPF in the range of 1 to 3 inches seems
reasonable...although the UKMET was a notable outlier.

...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. Maintained
continuity from previous forecast and will assess whether it needs
adjusting, or removal pending future trends in guidance.

Bann

Day 3
Valid 12Z Sun May 05 2024 - 12Z Mon May 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE SOUTHERN
PLAINS...

The potential for excessive rainfall makes its way eastward on
Sunday in association with a closed low aloft and a cold front at
the surface. Model QPF was not as heavy as forecast on Day 2 as
shifting low level flow results in weaker low level moisture flux
convergence along the front. Despite model QPF generally being 2
inches or less...opted for a Slight Risk given the wet antecedent
conditions over portions of eastern Texas and eastern Oklahoma.

Bann


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