821
FOUS30 KWBC 080052
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue May 7 2024

Day 1
Valid 1Z Wed May 8 2024 - 12Z Wed May 8 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO VALLEY, GREAT LAKES, TENNESSEE VALLEY, AND NORTHERN
PLAINS/ROCKIES...

...Great Lakes to the Tennessee Valley...

The existing Marginal Risk area across much of Illinois and into
northern Indiana was removed with this update since the main line
of storms has moved into central Indiana and western Ohio. The
latest CAM guidance suite suggests little in the way of additional
convection behind this line of storms. The best signal for locally
enhanced rainfall through 12Z will be across portions of central
and western Tennessee as the low-mid level flow becomes parallel to
an expected stalled frontal boundary across Arkansas and into
southeast Missouri. Some guidance has indicated a threat of
training near and ahead of the boundary during a nocturnal
convective pattern. Another area of concern is the ongoing
convection approaching the Ohio River, with some instances of
short-term convective training with high sub-hourly rainfall rates
across portions of southern Indiana into central/southern Ohio
within an axis of higher instability (MPD #237 is valid here until
430Z). Scattered areas of moderate to locally heavy rain are
expected to continue across portions of Wisconsin into Michigan,
but the hourly rainfall rates here should be lower than those areas
across the Ohio Valley.


...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 nuisance level
flooding concerns. The existing Marginal Risk area across Montana
was trimmed back on the eastern side of the outlook area to account
for the latest radar and CAM guidance trends.

Hamrick



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...

...20Z Update...

The Slight Risk area was maintained across the Ohio and Tennessee
Valley`s with the core of the threat along the Mid-Mississippi out
towards the tri-state MO/IL/KY borders with a primary QPF max
focused within western KY and points east/southeast over the KY/TN
border. An expansion further to the west and southwest into AR and
southeastern MO was executed given the trends in recent guidance
for the initiation of convection within the confines of the
aforementioned areas. The SLGT was also expanded into eastern TN
and south into northern MS/AL. The setup remains the same based on
the evolution, but there was a growing consensus in a higher impact
being felt further south compared to previous forecast issuance
based on both probabilistic output, as well as the ensemble QPF
footprint for the period. A higher risk across portions of KY and
TN was entertained given the focused QPF max and higher HREF EAS
probabilities for precip exceeding 1" (70-90%), and 2" (35-60%),
with a lower end probability depiction for exceeding 3" (10-15%),
centered over western KY into south-central KY and northern TN. The
question becomes more of how the overnight convection this evening
evolves as the positive feedback mechanism will drive the location
of the stalled frontal boundary that will become a primary feature
for enhanced low-level convergence and training within the warm
sector. Some guidance has trended south with this feature which
would play a pivotal role in the location of the primary QPF max
for the D2 period. The 12z HREF mean was still positioned over that
western KY corridor with an expansion in the higher means further
east, so that remains the focus for this forecast. If consensus
grows for that general area, there is an opportunity we could see
an upgrade to a MDT risk, but we want to make sure there is
continuity run-to- run for the placement of the areal QPF maximum
for an upgrade since the risk also implies higher confidence.
Regardless, this is a, "higher-end SLGT" for a large portion of the
Ohio and Tennessee Valley`s with locally significant impacts
possible within the confines of the warm sector.

Elsewhere, limited changes to the MRGL risk across the Midwest
with the anticipated inverted trough under the main upper
circulation. Locally heavy rain with totals between 1-2" will be
common within the confines of the MRGL risk area with a large
bullseye of 50-70% EAS probability for at least 1" and a
precipitous drop off to 10-20% for upwards of 2" within the same
bounds. This caps the potential overall, especially considering
rates generally maxing out around 1-1.5" just based on neighborhood
probabilities across northern IA, southern MN, and far southeastern
SD. Instability is generally meager, as well as PWAT anomalies
being only slightly above normal for climo. Thus, the potential is
fairly capped with a MRGL risk more than sufficient for the setup
in question.

Kleebauer


...Previous Discussion...

...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 AND LOWER MISSISSIPPI VALLEY....

...20Z Update...

The main shift in the D3 was a general movement to the west for the
previous SLGT risk issuance based on the latest ensemble QPF
trends, as well as NBM probabilities for exceeding 2" within the
period. There`s still some discrepancy amongst global
deterministic in the positioning of the cold front related to the
setup across the Southeast US with the GFS further north in
placement of the front and ECMWF further south, closer to the Gulf
Coast. Latest ensemble bias corrected QPF is a touch further south
compared to previous runs, but still within the field of error at
this lead. QPF maximum has shifted a bit further west into LA/MS
with a bullseye of 2.5-3" at a 72 hr lead. The setup is fairly
primed for some localized impacts over areas that have been hit
with a lot of rain recently. The ejection of the shortwave out of
the sub-tropical jet is still pretty well forecast with cell
initiation out across east TX. The forward propagation of the
shortwave and the positioning of the boundary will be the primary
focal points for the setup, and so far with have growing consensus
on the speed of the shortwave and some split in frontal placement.
Regardless, the setup will be conducive for locally heavy rainfall
with QPF max of up to 5" possible within a zone of training
potential along and just south of the cold front on Thursday
evening. A SLGT risk was maintained for continuity in the potential
and expanded a bit on the northern and southern edges to account
for the uncertainty in the evolution of the boundary.

There are no changes to the northern MRGL across the Mid-Atlantic
and Northeast as the thermodynamic environment remains favorable
for scattered convection capable of heavy rainfall within the
bounds of elevated PWATs focused over the area. A split in ML
guidance and primary deterministic add some uncertainty in the risk
for placement of the heaviest precip potential, so maintained
continuity given the likelihood of low-end MRGL somewhere between
the Mason Dixon and Upstate NY later Thursday.

Kleebauer


...Previous Discussion...

...Southeast...
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 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