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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0023Z Nov 03, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
823 PM EDT Sat Nov 2 2024

Day 1
Valid 01Z Sun Nov 03 2024 - 12Z Sun Nov 03 2024

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


A deep, slow-moving longwave trough will move across the Desert
Southwest through tonight. Multiple shortwaves of upper level
energy will move through the jet stream rounding the base of the
trough. As those shortwaves eject over the High Plains of Texas
and Oklahoma, they will meet up with increasing Gulf moisture
tracking northward across Texas. At the surface, the trough-ridge
interface will feature a slow-moving cold front which will also add
to the forcing in the area. PWATs increase above 1.75 inches
across much of the Slight Risk area.

The combination of the increasing moisture and ample low and upper
level forcing will be multiple rounds of training thunderstorms
initiating over northwest Texas and the Panhandle, then quickly
organize into segments which will slowly track east as embedded
heavier cells track northeastward and roughly parallel to the
lines. This will support training convection which in addition to 2
inch per hour rates, could mean any particular location in the
Slight Risk area could see multiple rounds of heavy rainfall.

In contrast to the heavy rains, mitigating the flood risk has been
multiple weeks of very little rain across the middle of the
country. The dry streams and creekbeds will easily be able to
handle most of the rain that falls, but should training of multiple
storms occur, especially over any urban areas such as Oklahoma
City, then flash flooding impacts could quickly arise. Since the
storms will be slow moving, this will support widely scattered
flooding impacts, especially from Wichita Falls, TX through
Oklahoma City.

Changes to the previously issued outlook were to account for storm
motion and the 18z HREF hourly and 12 hourly probabilities for
heavy rainfall. This led to trimming of the northwest side of the
previously defined areas, but no change to the risk level across
the region as a whole.

Roth/Wegman

Day 2
Valid 12Z Sun Nov 03 2024 - 12Z Mon Nov 04 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
NORTHEASTERN OKLAHOMA AND ALONG THE KANSAS/MISSOURI BORDER...

...2030z Update...

The inherited risk areas remain mostly unchanged, save for some
minor tweaks to the northern portions of the Marginal and Slight
Risk areas. The previous discussion remains valid. 12z guidance
hasn't changed much in its presentation of the axis of QPF over the
affected regions.

Kebede

...Previous Discussion...

Storms from the overnight period Saturday night will diminish with
daytime heating and the typical daytime weakening of the LLJ.
However, throughout the day the atmosphere will recharge such that
by Sunday night the passage of another shortwave will result in a
refiring of the storms across central and eastern Oklahoma, which
will grow and train upscale into far eastern Kansas, Missouri, and
eventually Iowa and far western Illinois. The Moderate Risk was
largely unchanged, with a small northward expansion. This is
primarily the portion of the region which is expected to get some
rain tonight, which will prime the soils for the heavier rains for
most of eastern Oklahoma and points north and east by Sunday night.
This expected overlap of rainfall is what will push the flash
flooding impact and coverage potential into the Moderate Risk
category, albeit the lower end of the category. Obviously this will
largely depend on how much rain is seen in the Moderate Risk area
Saturday night before the start of the period.

Once again the LLJ off the Gulf will supply a stream of ample
moisture to the slow-moving storms, so the flooding impact
potential, especially in areas where Sunday is the first
significant rains seen in the area in weeks, will be dependent on
the upper level energy tracking along the surface front keeping
storms moving over the same areas. The ERO risk areas were
conservative on the north and east sides and more liberal on the
south and west sides due to expected prior rainfall (or lack
thereof) on Saturday. The lack of prior rainfall has gotten river
levels to very low conditions, making plenty of room for the water
from heavy rain. This will greatly diminish the flooding threat
broadly. Had there been recent rains in this area, then a more
expansive Moderate Risk may have been needed. From Day 2 onward the
flooding potential across the middle of the country will be highly
dependent on how well previous days rains were able to result in
flooding, as amounts in the highest rainfall risk areas will be
largely the same day to day, so overlap with previous' days will be
critical. Of course, urban and low-lying/flood prone areas will be
at greater risk of flash flooding through Sunday night.

Wegman

Day 3
Valid 12Z Mon Nov 04 2024 - 12Z Tue Nov 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST
TEXAS THROUGH THE CHICAGOLAND AREA...

...2030z Update...

The Marginal and Slight Risk areas were expanded on their northern
and southern peripheries to include parts of Michigan and more of
eastern Texas. PWAT anomalies in the 3-5 standard deviation range
and a substantial low level jet (50kts+) over the Upper Midwest
was enough to warrant an expansion of that Slight Risk into those
areas. An upgrade to a Moderate Risk area over portions of the
Lower Missouri Valley was considered, but ultimately not
implemented since it'll depend on how much priming occurs during
the days 1 and 2 timeframes.

Kebede

...Previous Discussion...

The slow moving cold front that will have brought rains to areas
west of the Mississippi this weekend will move a little faster to
the east Monday and Monday night. Additional lines of storms are
expected again on Monday, but mostly for areas not expected to get
too much rain this weekend. Thus, the Slight Risk remains good as
the flooding from heavy rain is offset by the dry soil conditions.
By this point we should have a good idea as to how much flooding
will be seen as both Saturday and Sunday's forecast rains will be
on repeat over the Slight Risk area again on Monday. No significant
changes were made or needed from the inherited Day 4 forecast.

Wegman


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