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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2025Z Feb 26, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Mon Feb 26 2024

Day 1
Valid 16Z Mon Feb 26 2024 - 12Z Tue Feb 27 2024

...16Z Update...

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 2
Valid 12Z Tue Feb 27 2024 - 12Z Wed Feb 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO AND MID-MISSISSIPPI VALLEYS AND THE CENTRAL
APPALACHIANS...

...2030Z Update...

Very few changes were made to the inherited Marginal Risk area.
The pattern remains one quite favorable for severe weather, but
nowhere near as much so for flash flooding. Despite PWAT values
2-3 sigma above normal, a 60 kt LLJ, and 500-1000 J/kg of
instability noted in the latest guidance (with locally higher
maxima) these ingredients favor wind-related impacts more than
flash flooding impacts. The primary factor working against flash
flooding is the incredibly fast motion of any storms that develop
in this region. This will be supported by the 60 kt LLJ and a
supporting 150 kt upper level jet. Much of the CAMs guidance that
covers the Tuesday afternoon and overnight period suggests
anywhere from 1 to 3 rounds of storms...separated by a few
hours...will move across the Ohio Valley. These storms will be
capable of heavy rainfall rates, with embedded hail with the
stronger storms. Hail in this area will cut down on overall
precipitation totals. The very rapid movement of the storms
suggests any one area may see an hour or less of rainfall with any
one storm, so it will likely take 2 or 3 rounds of rain for flash
flooding to develop. The storms are likely to grow upscale into
MCS's and lines rather quickly, so that will favor a bit more
widespread rain, though not everyone will see heavy rain due to
more widely scattered storms and hail.

Secondly, atmospheric moisture. As noted, PWAT values are 2-3
sigma above normal. However as this is the dry time of year, that
only equates to PWATS of 1 to 1.25 inches. This will therefore
limit overall precipitation totals as well as more moisture goes
into maintaining the storms with less resulting in rainfall.

Finally we have very dry soil conditions, with NASA Sport
suggesting soil moisture over the Ohio Valley near 50% saturation,
which in some areas is less than 10% of normal. The empty river
basins will be easily able to handle any and all precipitation.
Flash flooding would most likely be in any local creeks, streams,
and poor-drainage urban areas.

Taking all of this together, think it will take multiple rounds of
heavy rain (or mostly rain) moving over a poor drainage area to
result in flash flooding. Thus, instances of flooding should be
very isolated. The Marginal was considered for cancellation, but
the prevalence of urban areas, somewhat low FFGs for Ohio and
points east (1-1.5 inches per hour FFG values), and then terrain
considerations in the east may be enough to support an isolated
occurrence or two, as well as keeping with consistency. For
western areas (IL/IN) where FFGs are a little higher, the storms
are likely to be a bit stronger as they will move through closer
to peak heating Tuesday afternoon and evening.

Wegman

...Previous Discussion...

A robust mid-level shortwave will propagate southeast out of the
Pacific Northwest on Tuesday with increasing difluent flow ahead
of the mean trough that will help initiate a round of convection
within the confines of the Mid-Mississippi and Ohio Valley's.
Focused meridional flow ahead of the mean longwave trough will
advect deeper moisture and associated theta-E's poleward with a
tongue of elevated instability building within the areal theta-E
advection regime. Getting more into the hi-res time frame, we are
seeing some forecast MUCAPE values approaching 500-1000 J/kg with
locally higher lying along a projected warm front situated to the
east of a developing surface reflection in the central Midwest.
PWAT indices soar to 2-3 standard deviations above normal within a
large zone spanning from the Mid-Mississippi Valley up into the
Great Lakes by Tuesday afternoon as the enhanced low and mid-level
flow usher Gulf moisture into the outlined area. By late-Tuesday
afternoon, convective development is likely thanks to the approach
a mid-level vorticity maxima moving out ahead of the shortwave
trough as it migrates eastward. Models are keying on a band of
heavy rainfall within the proximity of a warm front, denoted
fairly well within the theta-E fields on guidance. The question
that still needs to be solved is where that boundary will lie.
Current consensus is north of I-70 with some guidance as far north
as southern MI which leads this to be the focal point of expected
convective development. Recent HREF probability fields for at
least 1"/hr have risen to 25-30% within the confines of the
IN/MI/OH border which is where most of the CAMs currently have the
warm front situated when examining the surface moisture/temp
fields. Further south will have the best deep layer moisture and
elevated PWATs, but the lack of a focused boundary like the warm
front in question will lead to more isolated flash flooding
concerns, mainly within urbanized areas, and terrain as you head
further east.

Any convective development will continue into the overnight hours
as we approach Wednesday with upscale growth of convection likely
after 06z as the shortwave trough pivots east of the Mississippi
and begins to take on a more neutral tilt by the end of the
period. Large scale forcing will be at its maximum by the end of
the forecast cycle with a line of heavy rain likely extending from
eastern OH, south into the Tennessee Valley. This will continue to
advance eastward and the focus shifts into the Appalachian front
and terrain-centric areas in the Mid-Atlantic and Ohio Valley as
we roll into D3. The Marginal Risk from the previous forecast was
maintained, but adjusted to reflect the latest QPF trends within
the convective-allowing environment being forecast. A Slight Risk
upgrade is non-zero, but current forecasted rates within any
convection keep this capped for the time-being.

Kleebauer 

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