805
FOUS30 KWBC 070810
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 AM EDT Tue May 7 2024

Day 1
Valid 12Z Tue May 07 2024 - 12Z Wed May 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY...

...Ohio Valley...
The strong spring system will evolve into a large closed upper low
over the Northern High Plains while a piece of embedded shortwave
energy quickly lifts through the Midwest and lower Great Lakes.
Initial decaying line of convection will move into the region later
this morning which will likely be fast-moving and have diminishing
intensity in rain rates. This will likely set up an outflow
boundary across parts of Indiana and Ohio that will then be the
focus for additional thunderstorm development in the afternoon/evening
hours once convective temperatures and maximum destabilization
occurs. Meanwhile, a zone of higher PWs is forecast to develop,
characterized by PWs of 1.5+", along the Ohio River. As storms
develop later today, they will be moving generally along/parallel
to the mean flow with some south/southeast component and the
latest hi-res guidance shows some support for training/repeating
rounds and the latest HREF neighborhood probabilities indicate
hourly totals of 1-2" appear possible. Some of this rainfall may
fall over more saturated areas of southern Ohio and northern
Kentucky. Given the environmental ingredients, possible 1-2 rounds
of convection, and some greater sensitivity in ground conditions, a
Slight Risk was introduced across parts of Indiana, Ohio, and
Kentucky.

...Upper Midwest and Great Lakes...
The impressive line of thunderstorms currently moving eastward
across the Plains and Midwest will continue to lift north/northeast
this morning into the Upper Midwest, generally working along the
leading edge of the warm frontal passage and best isentropic lift.
Moisture in the low levels will continue to rise through the
period, with an axis of PWs > 1.25" expected from Minnesota
southward into Illinois which will support isolated/embedded cores
producing heavier rain rates. The latest hi-res guidance does
suggest potential for some repeating/redeveloping convection as
the flow becomes parallel to the storm motions over Wisconsin,
which could produce isolated/streaks of higher rain totals.
Altogether, the risk for flash flooding lies in the lower end of
the range and the Marginal Risk looks good for this forecast
update.

...Northern Plains/Rockies...
The closing mid/upper level low over the Northern High Plains will
linger through the period, with embedded vort energy
pivoting/retrograding westward across the region. This should
continue the forcing/lift across the region and with the anomalous
moisture in place and orographic enhancement, an additional 0.5-1"
is expected and this could cause additional flooding concerns.

Taylor


Day 2
Valid 12Z Wed May 08 2024 - 12Z Thu May 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
MID-MS VALLEY, TENNESSEE VALLEY, AND OHIO VALLEY...


...Mid-MS Valley to Ohio Valley...
The strong closed low from today/tonight is expected to open up
into a large positive tilted trough axis stretching from the
Rockies to the Great Lakes. Embedded within that, pieces of
shortwave energy will ripple through the Plains to Ohio Valley and
be the primary focus for forcing in the mid/upper levels. At the
surface, a low pressure will deepen/organize in response to the
large scale forcing and track through the Mid-MS Valley to Ohio
Valley, pulling a warm front northward into the Ohio Valley by the
afternoon. In the warm sector, the airmass will be characterized by
dewpoints well into the 60s, perhaps into the low 70s, across the
region and along with PWs rising well above 1.5", this should be
more than sufficient moisture to work with. And with the increasing
height falls aloft and steepening lapse rates, a large area of
favorable instability is expected, reaching 1500-2000+ J/kg MUCAPE.
With daytime heating and convective temperatures reached, rounds
of strong to severe thunderstorms are expected from the Ohio Valley
initially that then build/grow along the advancing frontal
boundary southwest across the Mid-MS Valley.

Compared to earlier issuance, the northern edges of the ERO
outlines have been trimmed back some as the latest guidance
suggests the warm front may not lift as far north, stalling along
the Ohio River. This may keep the greatest threat for
training/intense thunderstorms south into KY and into TN tied to
the better instability. This is where there is the greatest overlap
of forcing, moisture, and instability and the setup for training
convection that pushes the ERO risk toward the higher-end of the
Slight Risk range. This also lines up also where the antecedent
ground conditions are wet with recent heavy rainfall events giving
lower/reduced FFGs and more vulnerability to flash flooding.

...Upper Midwest...
Compact area of deeper convection is expected to develop along an
inverted trough underneath the core of the upper low in the
Midwest that will be capable of producing localized heavy rainfall
and flash flooding. A narrow axis of higher moisture is expected,
perhaps above 1-1.25" from far southeast South Dakota through Iowa
and southern Minnesota. Given this greater moisture, broad forcing,
and steepening lapse rates, pockets of slow-moving and pivoting
showers and thunderstorms are likely. Rain rates may
approach/exceed 1-2"/hr at times and while isolated, there is at
least some flash flood risk so the Marginal Risk covering the area
looks good.


Taylor


Day 3
Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST U.S....

...Southeast...
The strong spring system over the Central U.S. will begin to
elongate/stretch in a lengthy positive tilted shortwave trough by
the start of the forecast period and sink southward toward the
region. Earlier convection from the previous day is likely to push
outflow boundaries and a cold front through the region while
another embedded shortwave trough is expected to move through in
the southern stream. Altogether, this should put the region in a
favorable region of forcing for ascent. Deeper moisture is expected
to pool along/ahead of the surface boundaries and will be
characterized by PWs greater than 1.5", which is around 2 std above
the climatological normal for this time of year. With sufficient
instability developing that will overlap with the better forcing
and moisture, rounds of thunderstorms are expected to develop along
the boundary before slipping south/southeast toward the Gulf
Coast. By this point in the storm system evolution, the flow is
likely to be more parallel to the overall storm motions and this
could favor some repeating rounds or training convection
particularly over portions of MS/AL. The environmental ingredients
will support intense rain rates over 1-2"/hr at times and this
could bring scattered instances of flash flooding and for this
update, the Slight Risk inherited looks good was largely unchanged
from the previous forecast.


...Central Appalachians to the Northeast...
Low pressure is expected to track from the lower Ohio Valley to
the eastern Great Lakes over the forecast period as shortwave
trough energy aloft and height falls over the region. A strong
southwesterly flow in the low levels will bring higher moisture,
characterized by dewpoints in the upper 50s to lower 60s and PWs
above 1", especially from the Central Appalachians and southern
areas of the NOrtheast. With the uncapped and warm/moist
environment, there should be scattered to numerous showers and
thunderstorms that develop that will be capable of producing
isolated/localized rain rates in excess of 1-1.5"/hr at times. This
could cause isolated flash flooding, particularly for some of the
areas that have seen recent heavy rainfall and more saturated
ground conditions as well as urban locations.

Taylor

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