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000
FOUS30 KWBC 180024
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
824 PM EDT Wed Apr 17 2024

Day 1
Valid 01Z Thu Apr 18 2024 - 12Z Thu Apr 18 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES, OHIO VALLEY AND WESTERN PARTS OF THE NORTHEAST...

...01Z Excessive Rainfall Outlook...

Progression of the cold front has allowed the removal of Michigan
and much of Ohio from the Marginal Risk area. The threat of
excessive rainfall remained over far eastern Ohio into parts of
adjoining states where the low/mid level flow continues to draw
moisture ahead of the approaching cold front. In the immediate
term...the concern is greatest where the southern part of a line
is becoming aligned with the mean low-level flow setting up the
possibility of cells training. Part of the convective line farther
north was more progressive but poised to move over areas that were
soaked with very heavy rain recently...so there is at least a low-
end threat into the overnight hours. Some convection over
northeast Mexico may make it across the international border into
parts of the middle Rio Grande Valley in Texas during the evening...but
the 18/00Z sounding from Del Rio showed an atmosphere that was
largely capped atmosphere...an idea that the HRRR has been
suggesting for a few runs and which should preclude much of an
excessive rainfall threat.

Bann


...16Z Update...

Minor adjustments were made to the southern edge of the previous
MRGL issuance across the Ohio Valley into the Mid Atlantic. This
was in conjunction with the latest trends in hi-res deterministic
and the QPF footprint within the NBM. Mid-level shortwave will
rotate around the base of the closed 5H reflection over the Great
Lakes and propagate eastward over northern IL/IN/OH before pivoting
across PA by the end of the forecast cycle. Locally heavy rainfall
from progressive convection will allow for flash flood concerns
within that area bordering Lake Erie down to about I-70 where the
best forcing will occur. This will cross over an area of lower FFGs
due to recent rainfall events causing antecedent conditions to be
primed for easier exceedance thresholds. The area of focus will be
over northeast OH into western PA where even 1 and 3-hour FFG
markers are hovering around that 1-1.5" range which is very low
and a possibility within today`s setup. 12z HREF EAS probability
field is between 20-40% for exceeding 1" total rainfall between
Cleveland to Erie, aligned well with the ML algorithms from both
the GFS and ECMWF for the targeted location for best chance of
impact. As a result, did not feel there was a need to adjust the
northern extent of the risk area as it solidifies the forecast and
maintains continuity.

Across the Deep South, conditions have been fairly tame with
regards to the previously highlighted area. 12z CAMs remained "hot"
with the QPF interpretation, but radar and observational trends
negate the former runs and the newer hourly guidance is more in-
line with what is occurring which would be well below any chance of
a MRGL risk issuance. Will continue to monitor, but the threat of
flash flooding is trending closer to 0% than it is to the bottom of
the Marginal Risk threshold (5%).

Kleebauer

...Previous Discussion...

Showers and thunderstorms tracking across the Ohio Valley may
produce periods of heavy rainfall. Much of eastern Ohio and western
Pennsylvania have Low FFG which does maintain an elevated threat
for excessive rainfall and local flooding concerns. Model consensus
has accumulations less than 2 inches across this area. Multiple
rounds of convection will track through eastern portions of
Pennsylvania and into New Jersey could reach 1 inch within areas of
low FFG indices, especially in the 3 and 6-hr FFG intervals. The
Marginal Risk area that was already in effect was maintained for
this period.


Campbell/Kleebauer


Day 2
Valid 12Z Thu Apr 18 2024 - 12Z Fri Apr 19 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHERN PLAINS AND MISSISSIPPI VALLEY...

...20Z Update...

Marginal Risk area was expanded to the northeast to include more of
the Ohio Valley as latest guidance trends more bullish with the
prospects of greater convective coverage in-of eastern MO, southern
IL, and western KY. Latest probabilities for at least 1" and 2"/hr
rates have ramped up to 40-60% and 10-20% respectively across the
Mid-Mississippi Valley with the secondary pulse of heavier rates
located within central TX. Regional instability is highest across
the southern plains, but the combination of stronger mid-level
forcing along with sufficient buoyancy is located over the Mid-
Mississippi Valley where the best prospect for flooding is
anticipated. Overall, the primary synoptic and mesoscale pattern
are unchanged with only deviations anticipated in the general
convective placement within the MRGL risk area. Relative
progressive nature of the cold front will limit higher end
potential over the Missouri Basin, but an upgrade to a Slight is
not completely out of the question, pending evolution and trends
upstream as the shortwave exits out of Nebraska.

Kleebauer

...Previous Discussion...

A surface low pressure system and associated front will develop
across the Central Plains and advance toward the Mid-Mississippi
Valley and convection will fire off as the moist return flow from
the Gulf of Mexico drawls northward into the system. Locally heavy
rain will setup over Missouri and points southwest to central
Texas. The better concentration of the heavy rain will focus across
Missouri and surrounding locations. Recent rains have lowered some
of the FFG across this region and may have an increased
sensitivity to additional rain/heavy rain. A Marginal Risk area
covers part of northeast Texas to southern Missouri and western
Kentucky.

Campbell


Day 3
Valid 12Z Fri Apr 19 2024 - 12Z Sat Apr 20 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE TEXAS BIG BEND AND WEST TEXAS...

...20Z Update...

The low pressure system will continue to have its associated
frontal boundary draped across the Midwest and Plains during
period while dryline sets up from the Oklahoma Panhandle to the Big
Bend area. Convection is expected to be along and south of the
front over the Southern/Central Plains and Lower Mississippi Valley
with the higher QPF near/east of the dryline in west-central
Texas. The consensus for the area over the Texas Big Bend up
through the Edwards Plateau has grown within this suite of
guidance with the environment primed for higher end convective
development from the Lower Trans Pecos, eastward into the Rio
Grande and adjacent Edwards Plateau. Ensemble probabilities remain
on the lower end, however they are within the 10-20% range for
totals exceeding 2" which would be sufficient for localized flood
concerns within the terrain focused over the area. Further
northeast into the Red River and southern OK, there`s some
differences in timing of the approach of the front into that area
with the convergence pattern expected towards the back end of the
forecast period with a higher potential for flash flooding in the
successive D4 time frame. As a result, have removed the MRGL, for
now and will monitor any trends over the next series of numerical
guidance output. As we intake more hi-res deterministic data in the
following 48 hours, a secondary MRGL is possible for the Red River
basin with a gap in-between as probabilities for any appreciable
convective coverage is low between the Concho Valley into north TX.
A Marginal Risk area spans the Big Bend area northeast into the
Edwards Plateau and Concho Valley.


Campbell/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