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FOUS30 KWBC 061641 AAA
QPFERD

Excessive Rainfall Discussion...UPDATED
NWS Weather Prediction Center College Park MD
1241 PM EDT Mon May 6 2024

Day 1
Valid 16Z Mon May 06 2024 - 12Z Tue May 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN TO CENTRAL PLAINS AS WELL AS PORTIONS OF THE NORTHERN
HIGH PLAINS...

...16Z Update...
The forecast remains on track for the rest of today and tonight. A
strong low will slowly move north across the northern Plains, with
a trailing cold front and a dry line that will produce severe
weather in the Plains this afternoon into tonight and a nearly
stationary frontal boundary that will extend east across the
Ohio/Tennessee Valleys and Mid-Atlantic.

The slow nature of the low in the northern Plains will allow
anomalous moisture to continuously wrap around into central and
eastern Montana and support a prolonged period of moderate to
locally heavy rainfall. The existing Slight Risk area for this
activity was extended to the west to include more of Central
Montana where guidance suggests widespread rainfall totals of 1-3
inches will be possible through 12Z Tuesday.

In the Plains, conditions will be favorable for severe
thunderstorms with intense rainfall rates that could exceed 2-3
inches per hour at times. The existing Slight Risk covering
portions of the central and southern Plains was extended south into
portions of central/eastern Oklahoma and north to include
northwestern Iowa to capture the full extent of the convective line
where heavy rainfall will be most likely. These storms will be
moving quickly, which will help to reduce impacts from heavy
rainfall. However, 1-hour FFGs in this region are in the range of
1-2 inches, and 1-hour rainfall rates in stronger storms will
likely exceed this.

Elsewhere, only minor adjustments to the existing Marginal Risk
areas were needed to account for trends in model guidance. The
Marginal Risk area in the southern Plains was extended south into
northeastern Texas where showers and storms will develop in the
inflow region this afternoon. Storm motion will be northwards, and
there is some potential for training, which may result in instances
of flash flooding given the increased sensitivity of the region
after heavy rainfall last week.

Dolan


...Southern/Central Plains...
A strong shortwave trough, currently moving through the Four
Corners region, will eject out into the Plains later today/this
evening. This feature will open up as it does so, but take on a
negative tilt quickly and in response, there will be increasing
southerly flow across the Plains. In the low levels, moisture
advection will quickly bring 60F+ dewpoints northward and PWs above
1.25" while aloft, the increasing height falls will lead to
steepening lapse rates. With building instability and convective
temperatures reached by mid/late afternoon, explosive convection
is expected to start over portions of KS/OK in mainly discrete
storm modes before evolving into a large QLCS that quickly moves
eastward across portions of OK, KS, MO, IA, and AR. While storm
motions should be rather quick as the line matures, the strong
environment should support intense rain rates that exceed 2-3"/hr
at times and if there happens to be some localized cell
mergers/colliding clusters, some greater rainfall totals may lead
to instances of flash flooding. The latest guidance points toward
southeast KS, northeast OK and perhaps into southwest MO where the
potential for higher end rain totals exists. This lines up well
with the latest HREF neighborhood probabilities for 1-2" in a hour.
The Slight Risk remains largely unchanged from the previous
forecast with just some nudges on the southern end based on the
latest model trends.

...Northern High Plains...
As the aforementioned shortwave energy takes on the negative tilt,
a deepening surface low is expected to form over the Northern High
Plains. Deep moisture lifting northward will wrap around the system
and combined with the more than sufficient forcing will bring a
long duration period of moderate to locally heavy precipitation,
particularly this afternoon. There is strong model support for
widespread 1-2" totals through 12Z Tuesday and this rainfall
falling over areas with reduced FFG and lingering snowpack, some
instances of flooding will be possible.

...Ohio Valley to Mid-Atlantic...
Shortwave energy lifting through the lower Ohio Valley early this
morning will slowly slide eastward toward the Mid-Atlantic by later
this evening/tonight providing a modest amount of forcing/lift as
it does so. Ongoing showers and thunderstorms this morning across
portions of the Mid-Mississippi Valley and lower Ohio Valley are
riding along an existing stationary boundary and should slowly move
east/northeast through this morning. With daytime heating
(tempered by the extensive cloud cover), marginal values of
instability are expected to develop (upwards of 500-1000 J/kg
MUCAPE) and should promote additional development of showers and
thunderstorms across the region. Meanwhile, this feature will help
pull northward anomalous moisture into the region with the latest
guidance showing PWs exceeding 1.5-1.75" across the Ohio Valley as
well as from the Carolinas toward the coastal Mid Atlantic. These
values are between 2-3 std above the climatological normal for this
time of the year. Forecast soundings show fairly tall/skinny CAPE
profiles with deep layer near saturation. This should lead to
loosely organized and chaotic shower/thunderstorm clusters that
will have fairly slow storm motions. Given the PW environment and
storm speeds, these efficient rain producing showers and
thunderstorms could lead to instances of flash flooding,
particularly from the Ohio Valley through Central Appalachians and
Mid-Atlantic region. The 00Z HREF neighborhood probabilities for
1" hourly totals shows scattered values above 50 percent with a
slight signal (10-20 percent) for some storms capable of producing
2" in a hour. As a result, a Marginal Risk was introduced from
portions of the Ohio Valley eastward toward the Carolinas/Mid
Atlantic.

Taylor


Day 2
Valid 12Z Tue May 07 2024 - 12Z Wed May 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE GREAT LAKES TO OHIO VALLEY AS WELL AS NORTHERN HIGH
PLAINS/ROCKIES...


...Great Lakes to Ohio Valley...
Aloft, the strong storm system from Day 1 will evolve into a closed
mid/upper level low over the Northern High Plains with the leading
warm air advection/isentropic lift precipitation advancing through
the Upper Midwest and Great Lakes during the peak of daytime
heating. Locally heavy rainfall will be possible with marginal
values of instability expected and an increasingly moist
environment aided by strong southerly low level flow. Storm
motions should be rather quick though, at least from the Upper
Midwest to Great Lakes, however further south across the Ohio
Valley, the flow becomes more parallel to the storm motions and may
lead to a greater threat of training/repeating rounds (as well as
deeper instability to support more intense rain rates). Some
guidance points to a greater rainfall threat over the Ohio Valley
that could support the introduction of a Slight Risk in future
updates but uncertainty with how morning convection may influence
outflow boundaries and later thunderstorm development in the
afternoon kept confidence for an upgrade low at this point.

...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 3
Valid 12Z Wed May 08 2024 - 12Z Thu May 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY TO MID MISSISSIPPI VALLEY...

The deep/closed upper low over the High Plains begins to open up
and shift eastward through the Midwest/Great Lakes during the
period, with a strong embedded vort max expected to pass through
the Mid-MS Valley during the afternoon/evening hours Wednesday. At
the surface, this will lead to a deepening low pressure over the
lower Ohio Valley that then moves into the eastern Great Lakes.
South of this low, the air mass is expected to be warm and moist,
characterized by 60F+ dewpoints and PWs above 1.5". Deep convection
is likely to ignite over the lower Ohio Valley and Mid MS Valley
before moving east/southeast and with the flow nearly parallel to
the expected storm motions, some repeating rounds and training will
be possible, particularly across portions of southern Indiana,
Kentucky, Tennessee and Ohio where the flash flood risk lies on
the higher end of the Slight Risk probability range. Further to the
southwest along the advancing cold front across the Mid-MS Valley,
convection should be more isolated in nature, but could pose
intense rain rates and a risk for flash flooding.


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