289
FOUS30 KWBC 240751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
351 AM EDT Fri May 24 2024

Day 1
Valid 12Z Fri May 24 2024 - 12Z Sat May 25 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE ARKLATEX
TO THE ARKLAMISS...

...ArkLaTex into the Mid-South...
A broad marginal risk area continues from continuity, with a new
Slight Risk added near the LA/AR border. Precipitable water values
across these regions will remain above average, with additional
shortwaves in from the west supporting scattered convection and
locally heavy rainfall amounts. The ingredients in place support
hourly rain totals to 2.5" with local amounts to 6". However, the
00z NAM was far more bullish, though somewhat to the north, with a
maximum of 11"+. The concern is greatest in and near the Slight
Risk area, which saw heavy rainfall Wednesday and Thursday.
Considering the NAM signal, localized Moderate Risk impacts cannot
be ruled out should a long enough period of training evolve.


...Upper Mississippi Valley into the Upper Great Lakes...
The next strong area of height falls moving into the northern High Plains
will continue to push eastward through the Northern Plains and into the
Upper Mississippi Valley. The associated front on the leading edge
of the upper trough will have strong frontal convergence which
causes an axis of PW values 1 to 1.5 standard deviations above the
mean.  This will support potential for moderate to locally heavy
rainfall totals. This front does look progressive, which may limit
very heavy amounts. The marginal risk was drawn to where there may
be overlap of locally heavy rains Friday through Friday night with
heavy rains that have fallen as of late. The area saw tweaks from
continuity, but nothing major. Hourly rain totals to 2" and local
amounts to 4" are expected where cells merge or manage to train.


...Portions of the East...
Near and ahead of a front and lee trough, precipitable water values
of 1.5-1.75" pool within a region with weak to modest low-level
inflow and MU CAPE up to 2000 J/kg. Effective bulk shear of ~25 kts
is expected, which could lead to some convective organization,
particularly during the period of daytime heating. Interestingly,
the mesoscale guidance was a bit more bullish in this area than the
global guidance. But, given the ingredients, hourly rain totals to
2" with local amounts to 4" are possible where cell training or
cell mergers occur. Flash flood guidance values are rather high
partially due to soil type in the area, but also because the
otherwise frequent rainfall lately hasn`t been heavy --- 7 day
precipitation has been fairly low, roughly 25% of average and 14
day rainfall is more or less average. Synoptically, this set up
tends to lead to isolated flash flooding, particularly in urban
areas, so added a Marginal Risk area from the coastal Carolinas up
into the Eastern Shore on this update.

Roth


Day 2
Valid 12Z Sat May 25 2024 - 12Z Sun May 26 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR THE
KANSAS/MISSOURI BORDER...

The Marginal Risk remains similar to continuity, with some changes
made to the Slight Risk area. A cyclone emerging into the central
Plains will bring precipitable water values of 1.5"+ across the
region on the heels of sufficiently strong low level
inflow/effective bulk shear. The region lies within a general
instability gradient, with CAPE expected to rise to 4000+ J/kg
within an increasingly capped atmosphere across the southern
Plains. Pockets of moderate to heavy rainfall are forecast within
this region. There is increasing overlap in the QPF pattern near
the KS/MO border within the 00z guidance where three hourly flash
flood guidance is modest, so went ahead with an upgrade to a Slight
Risk for this update. However, given the ingredients, hourly rain
totals to 2.5" with local amounts to 5" are possible where
mesocyclones form and/or cells manage to train.


...Portions of the East...
Near western PA and WV, showers and thunderstorms along a
progressive front could drop locally heavy rainfall within an area
of modest flash flood guidance. Hourly rain totals to 1.5" with
local amounts to 3" are possible where short bouts of cell training
or cell mergers manage to occur. Since the flash flood threat
appears to be isolated, added a Marginal Risk. Down south in the
Carolinas, higher moisture pools near the coast as a front slowly
moves through. With MU CAPE as high as 3000 J/kg and effective bulk
shear expected to be sufficient to organize convection to some
degree, added a Marginal Risk within a regime that can lead to
isolated flash flooding during the warm season.


Roth


Day 3
Valid 12Z Sun May 26 2024 - 12Z Mon May 27 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MIDWEST AND THE MISSISSIPPI & OHIO VALLEYS...

A cyclone moving across the Midwest and Great Lakes is expected to
draw ample moisture (precipitable water values of 1.50-1.75") into
the region. The area that is expected to see the greatest residency
time with higher moisture and instability is across portions of TN
and KY. However, warm temperatures at 700 hPa appear to be a
limiting factor the farther down its front you go. The best warm
air advection ahead of the low is slated to occur from portions of
IL across OH, where the guidance is more robust in its QPF signal,
with local amounts in the 3-5" range. Considering the progression
of the warm air advection pattern and best moisture across the
region, that degree of heavy rainfall would need to fall quickly;
hourly rainfall potential to 2.5" is possible within the organized
thunderstorms, with embedded mesocyclones, could get them there
given the low- level inflow and effective bulk shear expected. Went
ahead and added a Slight Risk across the region, where flash flood
guidance values are modest.

Roth


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