627
FOUS30 KWBC 081548
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1148 AM EDT Wed May 8 2024

Day 1
Valid 16Z Wed May 08 2024 - 12Z Thu May 09 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO AND TENNESSEE VALLEYS...

...16Z Update...

Current radar/sat composite shows two distinct areas of impact this
morning; a west-east elongated boundary with training bouts of
convection across southern KY into northern TN, and a prolific
supercell propagating to the east-southeast across MO. The
supercell over MO is important in the overall evolution of flooding
prospects across the Ohio and Tennessee Valleys as the mesocyclone
from this cell will pivot southeast, becoming the main focal point
for congealing convection with expected upscale growth into a
significant MCS that is well-stated with the latest SPC D1 Moderate
Risk for severe. Rainfall across southern KY into northern TN has
already produced localized totals of 1-2" with another 1-3"
anticipated over the course of the morning and early afternoon
before the main precip shield makes headway into the region this
evening through the overnight time frame (For more information on
the current rainfall, see MPD #0239). The compounding factor
creates a significant threat for flash flooding potential as areal
priming from the first wave of rainfall will degrade the regional
FFG`s leading to a proned zone for further impact. This generally
lies within the confines of the remnant boundary where the current
precip is ongoing, and where the MCS will become rooted later in
the period.

Theta-E gradient is well-documented given the latest RAP surface
analysis with some modest advection anticipated across the Mid-
Mississippi Valley within the lee of a surface low analyzed near
the KS/MO border. This will create a general zone of recovery over
western TN into far southwest KY down through the Lower
Mississippi Valley with MLCAPE indices likely between 2500-4000
J/kg across the aforementioned area, and 2000-3000 J/kg for points
further east across TN and northern MS/AL. The western locations
will see an increase in convective coverage with locally heavy
rainfall initiating between 18-21z allowing for a modest flash
flood threat based on discrete convection alone. As we move into
the early evening hours, general convective pattern will shift from
discrete convection to more multi-cell and organized cell
structures with aid from the primary mesocyclone diving southeast
out of MO along the theta-E gradient. That`s when the convective
mode will favor upscale growth and a significant swath of very
heavy rainfall is anticipated through the path of the expected MCS.

Probability fields reflect the anticipated impacts this evening
within the bounds of the latest HREF EAS signals for at least 3" of
total precip the next 24 hrs, as well as a significant output from
the 10 and 100-year ARI exceedance probabilities. The former is now
running between 60-80% across the border of KY/TN to the northeast
of I-65 with a large swath of at least 40% probability expanding
west-northwest to east-southeast between Paducah to about 75 miles
east of Nashville. The 10-year ARI exceedance probabilities are
fairly similar to the 3" EAS markers with 100-year ARI settling
between 20-35% in that zone to the northeast of I-65. This
correlates to the area that will see dual impact from the morning
wave of convection and the expected MCS overnight leading to HREF
blended mean totals approaching 6-8" locally. An expanse of 3-4.5"
mean totals encompass the entire area from southwest KY through
northern and central TN leading to a robust areal coverage of heavy
rainfall and the necessity to expand the MDT risk a bit further
south to reflect the trends. The southern periphery of the SLGT was
also expanded to account for recent trends in guidance bringing
the southern and southwest extent of the primary MCS into northern
MS/AL, as well as northern GA as the bowing segment is forecast to
propagate to the southeast within that sharp theta-E gradient,
tapping into the favorable environment on the southern extent of
the complex. A secondary MCS is possible given the latest HRRR as a
shortwave ejects out of TX behind the primary shortwave
responsible for the setup. This will be a little less severe and
doesn`t favor the prolific rainfall totals from it`s predecessor,
however additional rainfall over areas that have been impacted
would add, "Insult to injury" and could exacerbate the flood threat
that is forecast to occur across TN.

The previous MDT was maintained with minor expansion on the western
and southern edges of the risk area. There is a non-zero chance for
a targeted High Risk somewhere within the confines of TN, but this
will be more of a nowcast as we monitor the evolution of the
prospectus MCS and the QPF footprint left in wake of the
disturbance. Regardless, a significant flash flood threat is likely
within the bounds of the MDT risk and could be locally significant
even within areas bounded by the SLGT risk. Elsewhere, the threat
across the Midwest did not feature any significant chances that
would allow from much deviation from the previous forecast.
Continuity was maintained for the locally heavy rainfall threat
under the guide of the inverted trough and upper forcing situated
across IA/MN/SD.

Kleebauer

...Previous Discussion...

A Moderate Risk has been introduced with this update across
portions of the Ohio and Tennessee Valleys for the potential of
multiple rounds of thunderstorms through early Thursday morning
that may lead to several/numerous instances of flash flooding,
some of which could be significant across southern KY and middle
TN.

Current radar imagery shows a few lines of thunderstorms moving
south of the Ohio River into KY, generally along/ahead of a cold
front that is draped across portions of Indiana southwestward
through southern Illinois. Aloft, the deep closed upper low remains
centered over the Northern High Plains with a broad area of
southwesterly flow stretching from the Southern Plains to the lower
Great Lakes. Embedded in that flow is a weak shortwave analyzed
over portions of AR/MO, moving northeast.

The expectation is that the current convection will continue to sag
south/southeast through KY, settling across southern KY near the TN
border by the start of the period before stalling out with the
loss the best nocturnal LLJ. This is expected to lay out a quasi-
zonal boundary that will be the focus for additional thunderstorm
development through the day as the airmass reloads with the
deepening low pressure over the Mid-MS Valley.

The environment today will be characterized by high PWs (1.5"+)
and a zone of sufficient instability (MUCAPE > 2000 J/kg). Most
importantly will be the increasingly favorable setup for training
convection especially by later this evening/tonight across middle
TN and southern KY with the deep layer mean flow becoming oriented
parallel to the storm motions, helping to foster backbuilding and
training convection.

The probabilistic data continues to support the higher end rainfall
and rain rate potential with the 00Z HREF probabilities for 5"
increasing to near 50% straddling along the KY/TN border. This is
supported by moderate values of 2" hourly rainfall total
probabilities later today/this evening. Finally, the HREF EAS
probabilities for 3" have increased from the previous forecast to
near 40 percent, again mainly along the KY/TN border region. All
this together supports the potential for multi-inch totals through
the period and potential for more significant instances of flash
flooding.

Elsewhere, the Slight Risk encompasses areas further north into IN
and OH where some hi-res guidance suggests convection may develop
that far north and be robust enough to support isolated but intense
rain rates, and some of these areas are a bit more saturated due to
recent heavier rainfall. Further southwest along the frontal
boundary, deeper and more intense convection is likely to develop
along the frontal boundary into AR but the more isolated nature and
higher FFGs may limit scope of any flooding there.


...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,
with PWs 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 and the latest HREF
probabilities support isolated/scattered instances of hourly totals
in excess of 1" and 24-hr totals up to 2" in places. Given this,
there is at least some flash flood risk so the Marginal Risk for
the area continues to look good.


Taylor


Day 2
Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS
TO THE LOWER MISSISSIPPI VALLEY...

...Eastern Texas to Lower Mid-MS Valley...
The strong spring system over the Central U.S. will begin to
elongate/stretch in a lengthy, positively 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 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 initially over eastern TX before slipping
south/southeast toward the Gulf Coast across portions of LA/MS/AL.
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 the latest
probabilistic data shows potential for isolated 2-4" totals in a
relatively short period of time. While there remains some
uncertainty in the placement of the heaviest, no significant
changes were made to the Slight Risk area and the message for
scattered instances of flash flooding.

...Central Appalachians to the Mid-Atlantic...
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.

...Midwest to Western Great Lakes...
Deeper moisture pooled along an inverted trough / TROWAL slowly
moving through the region will be the primary focus for embedded
heavy rain cores that may lead to flash flooding. Tall/skinny CAPE
profiles, an axis of higher/deeper moisture, and warm cloud depths
will promote efficient rain producing showers and thunderstorms
that could total 1-2" in spots and a Marginal Risk was introduced
for portions of this region.


Day 3
Valid 12Z Fri May 10 2024 - 12Z Sat May 11 2024

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

The advancing frontal boundary across the Southeast U.S. will be
the primary focus for deep convection Friday into Friday afternoon
and will be capable of producing rain rates in excess of 1-1.5"/hr
at times. Showers and thunderstorms are expected to be ongoing at
the start of the period, lingering from the deep, organized
convective line from the previous day. This activity will be in the
presence of a sufficiently high moisture pool, characterized by
dewpoints well into the 60s and low 70s and a PW axis of 1.5-1.75".
A subtle shortwave trough noted in the flow will be the main large
scale forcing mechanism to drive the convection that looks to
track through southeast Alabama, southern Georgia, northern
Florida and then eventually off the Southeast coast. Some
localized heavier rain totals and rain rates will be possible that
could pose an isolated flash flood risk.

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