205
FOUS30 KWBC 050036
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
836 PM EDT Tue Jun 4 2024

Day 1
Valid 01Z Wed Jun 05 2024 - 12Z Wed Jun 05 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF OKLAHOMA, NORTHEAST TEXAS, SOUTHWEST ARKANSAS AND NORTHWEST
LOUISIANA...

...01Z Update...

Other than trimming the Slight into western Iowa and Minnesota, and
adding a Marginal into southern West Virginia per the storms moving
through that region, no significant changes were made. The Moderate
remains contingent on a new MCS developing on the northwest side of
the Moderate in central Oklahoma and tracking southeast through the
night. Convective initiation has been slow to occur.

Wegman

...16Z Update...

The general premise from the previous forecast remains the same
with the focal point aligned over the southern plains into the
ArklaTex and Lower Mississippi Valley. Secondary maxima will occur
across the Midwest due to a culmination of low pressure propagation
into the central Midwest over IA/IL/WI, as well as large scale
ascent focused in-of the northern Midwest from an approaching
shortwave trough analyzed over the northern plains into
Saskatchewan. The inherited MDT risk was expanded a bit on the
northern and eastern periphery to account for trends in guidance
depicting a QPF maximum positioned on the northern edge of a tight
theta-E gradient bisecting central and northern OK down through
eastern OK into AR. 12z HREF neighborhood probabilities for at
least 3" was between 40-70% across much of the MDT risk area with
the maximum situated very close to the previously impacted area
from this mornings MCS leading to above average potential for flash
flood concerns within the area given the negligible recovery time
frame. Almost all of the area in the MDT is facing well above
average soil moisture (90-99th percentile), and is currently
facing regionally high stream flows as indicated via the latest
river forecasts and observations from the various River Forecast
Centers across the outlined area. There is still some uncertainty
on more specific details on the anticipated evolution and track of
the forecast MCS, but the bounds of the MDT risk encompass the
higher probability for impact over the course of the evening.

Further north, a cluster of thunderstorms will impact multiple
areas of the northern and central Midwest with a broad area of
probabilities for >2" pushing 25-50% within spots in IA/IL/WI/MN.
Convection across MN will be more linear in nature as a strong
shortwave pivots over the area with an ascent pattern favoring a
long line of thunderstorms capable of heavy rain that would induce
local flood concerns. FFG`s are lowest in south-central MN thanks
to previous rainfall, so the best chance for flooding will likely
lie along and north of US212 with the Minneapolis-St Paul metro up
into the north-central part of the state as the area of interest.
Further south, convective pattern will remain more scattered in
nature, but favorable surface based forcing will aid in locally
heavy rains extending from eastern IA through western and northern
IL. A few storms may produce rainfall exceeding 3" within a short
time frame and some smaller organized thunderstorm clusters could
enhance rainfall further, mainly within the confines of the
propagating low pressure motioning northeast through the period.

Kleebauer


...Previous Discussion...

We upgraded to a Moderate risk across portions of southeast OK,
northeast TX, southwest AR and northwest LA with this update. This
is with the expectation that these areas will most likely see two
organized convective complexes today...one this morning and another
tonight. Expect an MCS with some brief training will be ongoing at
12z across portions of southeast OK and northeast TX. Given the
favorable downstream environment, this convective complex is
likely to persist as it dives southeast across portions of eastern
TX, LA and southwest AR. The complex should become more progressive
with time, however short duration heavy rates are still likely to
drive some flash flood threat.

By later this afternoon into this evening additional forcing
moving into The Plains is likely to trigger convection over
portions of KS and OK. Given the degree of large scale forcing, the
extreme instability forecast, and the strong low level moisture
transport...upscale convective growth appears likely. This complex
should track over similar areas of OK, TX, AR and LA tonight. Even
without these two complexes much of this region has been quite wet
of late...with elevated streamflows and well above average soil
saturation. Combine those antecedent conditions with the potential
for two more rounds of organized convection today...and scattered
to numerous instances of flash flooding appear likely. This
warrants the upgrade to a MDT risk, with some significant flash
flooding possible.

This MDT risk is surrounded by a large Slight risk that stretches
from KS/MO all the way to MN and WI. Pretty strong large scale
forcing expected from IA into MN with a sharp mid level trough and
plenty of shortwave and upper jet energy to work with. Most
indications are that the front and convection will be progressive
in nature, which should end up limiting the upper bound potential
of rainfall totals. However also tracking a well defined shortwave
that is near the KS/MO border as of 08z. This feature will track
northeast today, and may act as a convective focus over portions of
IA, IL and WI by later today into tonight. Thus some of these
areas could see two rounds of convection today/tonight, locally
increasing the flash flood threat. Still generally not the
strongest HREF signal...strictly looking at HREF QPF probabilities
would suggest this is a borderline Marginal to Slight risk.
However will also note that the IA to MN and WI portion of this
risk does already have above average streamflows and soil moisture
from past rainfall. Thus flash flood susceptibility will likely
also be higher than normal...which combined with PWs around the
climatological 90th percentile...suggests some flash flood risk
exists and the Slight risk remains warranted.

A Marginal risk remains in place over much of the Southeast into
the TN valley and southern Mid-Atlantic. One MCV over northern MS
as of 08z, will track eastward and likely help trigger/organize
some convection over portions of TN, AL and GA today. Another
shortwave/MCV moving in from the west may help focus convection
over portions of the Southeast and/or TN valley tonight. Otherwise
plentiful instability and increasing moisture will support
isolated to scattered convection over a large geographic extent.
Still not seeing enough agreement on where any more organized
flood threat could evolve to upgrade any of this area to a Slight
risk. At this time think the broad Marginal risk is most
appropriate, with an isolated flash flood threat.

Chenard


Day 2
Valid 12Z Wed Jun 05 2024 - 12Z Thu Jun 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL APPALACHIANS AND MID ATLANTIC....

...Mid Atlantic...

Several shortwaves progressing through the mid-level flow east of
the Mississippi will traverse over the central Appalachians and Mid
Atlantic during peak diurnal instability allowing for numerous
showers and storms to develop and produce locally heavy rainfall.
Typically the anticipated pattern wouldn`t be prime for heavy rain
potential, but the key ingredient of anomalous deep layer moisture
will promote the risk within an area defined by lower FFG`s and
historical premise for flash flooding. Guidance has ramped up its
signal in the last series of runs with the latest hi-res
deterministic and associated HREF depicting some fairly elevated
totals during the time frame of impact. Mean QPF footprint from the
latest HREF shows a broad area of 1" or greater within the confines
of WV up through the southern Laurel Highlands, eventually spanning
eastward into the I-95 corridor with the primary urban zones
encompassing Baltimore/DC becoming the focal point. Instability
field isn`t overly impressive with the latest deterministic pinning
MLCAPE values between 500-1250 J/kg, a far cry from a prolific
thunderstorm setup normally. However, what makes up for the modest
instability is the available moisture ahead of the mean trough
pivoting eastward out through the Ohio Valley. Area PWATs are
forecasted to run between 1.25-1.5" out in the high country of WV
up through western PA with values closer to 1.75-2" further east,
mainly to the east of the Blue Ridge where surface flow will remain
south to southeasterly on the eastern flank of a surface ridge out
in the western Atlantic. The combination of marginal instability
and transient upper forcing will act within the +2-3 PWAT anomaly
present across the areas in question to create periods of heavy
rainfall from any convection that develops.

Neighborhood probability fields are fairly prolific for signals of
at least 2" within the area encompassing northern WV and points
east with a general 50-60% probability outlining the aforementioned
area, along with maximum probability between 70-90% focused over
the southern Laurels down through Cumberland and adjacent Allegheny
Front. These areas are higher risk historically for flash flood
potential due to terrain and run-off capabilities, along with some
embedded urbanization factors. Further to the east, the higher
probabilities for 2" are fairly similar with a max of 70% located
along some areas in central MD where urban sprawl presents a higher
risk for flash flood concerns just due to the expanse of
impervious surfaces. Lower end probabilities within 20-35% for at
least 3" exist along the Allegheny front down through the Potomac,
including the DC metro which is plenty to warrant an upgrade
considering CAMs being generous with the thunderstorm threat. A
SLGT risk was added to much of WV, southwest PA, central MD, and
the DC metro area including northern VA.

...Southeast TX into the Lower Mississippi Valley...

Remnants from a progressive MCS/MCV from the overnight D1 time
frame will advance southeast towards the Gulf coast with locally
heavy rainfall possible in the initial 6 hr window of the forecast
period across parts of east TX into LA. The remnant energy from the
complex will be monitored closely as current CAMs indicate a
redevelopment of convection later in the afternoon under the
focused circulation left over from the complex in question.
Sufficient low-level buoyancy within the convergence pattern near
the MCS/MCV will likely form a period of thunderstorms capable of
locally rainfall across southeast TX into west-central LA, the same
areas that have had issues with flooding over the past month.
Soils do look to have recovered closer to climatological norms,
but the deeper soils are still fairly saturated with NASA SPoRT
percentiles generally hovering between 80-90% over the area of
expected impact. FFG`s are generally higher, so unless we have some
focus of training, or if today`s slow progressing complex adds
more rainfall than previously anticipated, the threat is right on
the edge of a necessary upgrade. QPF mean is within 1-1.5" with
some local deterministic maxes around 3-4" where guidance
indicates a more viable impact from the presence of the expected
MCV. If this trend continues to be more aggressive with areal QPF
and potential, an upgrade will likely be warranted across the Piney
Woods area into west-central LA.

Kleebauer


Day 3
Valid 12Z Thu Jun 06 2024 - 12Z Fri Jun 07 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHEAST....

...20Z Update...

The previous MRGL risk was maintained with the current guidance
remaining steadfast on the proposed rainfall affecting portions of
NY state and New England. The biggest change was the extension of
the MRGL through portions of the Hudson Valley down through
Northeast PA, Northwest NJ, down into the Philadelphia metro and
surrounds. Cold front approach from the west will induce a period
of convection within a modestly unstable environment leftover from
the previous period. The front will clear much of the instability
out of the area in its wake, but local QPF maxima approaching
1-1.5" are possible within the corridor above. The time frame of
interest will lie between 12-00z with the front likely cleared out
of the region for the second half of the forecast. Recent ML does
have the expansion of the MRGL further south, so the correlation
factor does have merit for the extension. We will be able to fine
tune over the next several forecast on the expansion and
orientation of the risk area further south. In coordination with
the local WFO`s affected, the risk area was agreed with a low-end
threat lingering prior to the frontal clearance in the afternoon
hours.

Kleebauer


...Previous Discussion...

...Northeast...
An interesting setup for heavy rainfall over the Northeast on
Thursday. Stronger mid/upper level forcing moves in from the west,
and at the surface we`ll likely have prolonged convergence
along/near a north south oriented occluded front over NY/VT, and a
west east stationary front over southern New England. The
persistence of these features suggest some duration to the rainfall
is possible with this event. It seems like the main uncertainty
comes down to the degree of instability collocated with this better
convergence/forcing. The CSU ML tool does suggest Slight risk
probabilities over portions of the Northeast with this event. This
does not seem too unreasonable given the forcing and PWs in place.
However given the uncertainty on instability and the fact that
these areas have generally been dry of late with below average
streamflows and soil saturation...think the best course of action
is to stick with a Marginal risk for now.

Chenard


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