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FOUS30 KWBC 030837
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
437 AM EDT Mon Jun 3 2024

Day 1
Valid 12Z Mon Jun 03 2024 - 12Z Tue Jun 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL/SOUTHERN PLAINS, LOWER AND MIDDLE MISSISSIPPI VALLEY,
THE NORTHERN ROCKIES, AND WISCONSIN AND ILLINOIS...

...Plains to MS Valley...
High res guidance indicates a MCS should be ongoing at 12z this
morning across portions of KS and OK...and we are likely already
seeing the start of this complex over western KS as of 08z. Plenty
of downstream CAPE and moisture transport to support this complex
continuing into the morning hours, and there is pretty good model
agreement that this will happen. It should generally be progressive
as it moves off to the southeast across OK, southern MO, AR and
northeast TX. However some downstream convective development may
lead to cell mergers as this complex moves southeast through the
day. The downstream environment appears favorable for this
convection to make it as far south and east as northern LA and
central MS.

Even a progressive convective complex will have heavy rainfall
rates and 1-2"/hr rainfall...and where cell mergers occur we could
see 2-4" in an hour. Soil and streamflow conditions are still dry
over OK, but the rest of the area expected to be impacted has
elevated streamflows and more saturated soil conditions. Thus
isolated to scattered flash flooding will be a possibility with
this activity today.

Another low amplitude shortwave is likely to eject into the Plains
later today into tonight, which should result in additional
convective development over the TX Panhandle into OK. Given the
forcing, instability and low level moisture transport in place, it
seems probable that this activity grows upscale into another MCS
tonight. Of course lots of uncertainty on the details by this time,
however some of the same portions of OK/TX/AR that see convection
today could get this additional round tonight.

All in all a broad Slight risk seems warranted from portions of
OK/TX into AR, southern MO, northern LA and central MS. Plentiful
CAPE, moisture and forcing support one or more convective complexes
through the period. This combined with increasingly saturated
conditions over much of the area supports a broad region where
isolated to scattered flash flooding is likely today. While some
areas of higher end flooding is possible, a lack of confidence in
the exact region of greatest flash flood coverage precludes
anything higher than a Slight risk at this time.

...WI/IL...
A small Slight risk was added across portions of southern WI and
northern IL with this update. There is an MCV evident on
radar/satellite over northeast NE as of 07z this morning. This
feature is expected to track northeastward into southern WI by this
afternoon and should help focus convective development over
central/southern WI as instability increases. Cells will probably
be relatively quick moving, but still capable of heavy sub hourly
rainfall...and some chance we see a bit of backbuilding/training on
the south/southwest flank of convection. On the fence between a
Marginal or Slight risk, however with antecedent conditions quite
wet resulting in elevated streamflows and well above average soil
saturation...flash flood susceptibility should be higher than
normal and thus opted to go ahead with the Slight risk upgrade.

...Central/Southeast Virginia...
Will maintain a small Marginal risk across portions of southeast
VA into northeast NC. Not a unanimous signal in the 00z HREF
members, but there is still some support for locally heavy rainfall
near a weak boundary this afternoon. This is more of a conditional
threat, but given HREF 3"+ neighborhood probabilities are sill
around 15-30%, did not want to remove the recently introduced
Marginal risk.

...Pacific Northwest and Rockies...

No changes made to the Marginal and Slight risk areas as things
look to still be in good shape. The widespread moderate to heavy
rainfall will have pushed east of the western WA, however showery
conditions will linger as the trough axis pushes across. Briefly
heavy rain in these showers could result in some flooding concerns
given the amounts of rain that has fallen over the past 24 hours.

Heavier areal averaged rainfall is likely over the slight risk in
ID today...where 1-2" of rain is expected. Some embedded convective
elements on the backside of the rain shield could push hourly
rainfall up towards 0.5". This rainfall combined with snow melt may
result in some flooding concerns today.

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 PLAINS AND MISSISSIPPI VALLEY...

Another system moving into the Plains will bring a heavy rainfall
risk Tuesday into Tuesday night. 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. Just looking at this quick motion and model QPF fields
would probably suggest this is more of a Marginal risk type of
threat. However antecedent conditions will be quite wet over a lot
of this corridor...with streamflows and soil saturation levels
running much above average for the time of year. Thus flash flood
susceptibility will likely also be higher than normal...which
combined with higher PWs than this past event...suggests some flash
flood risk exists and the Slight risk remains warranted.

Large scale forcing drops off some as you go south into
KS/MO/OK/AR...however stronger instability and low level moisture
convergence expected here as the front aligns itself a bit more
west to east. These factors support some potential
training/backbuilding of convection for a period of time...although
even here the overall pattern is progressive. Tend to think the
higher QPF magnitudes will end up here, which combined with areas
of wet antecedent conditions, warrants a Slight risk.

The biggest change with this issuance was a large expansion of the
Marginal risk into more of the OH/TN valley into the Southeast and
southern Mid-Atlantic. The convective forecast is of low
predictability over these areas, but note some rather high QPF
output in several models, including the 00z GFS, GEM reg and UKMET.
These higher QPF values appear to be in part related to a
convectively enhanced shortwave that is expected to eject out of
the Plains Monday night and into this region Tuesday. This feature
should help organize a convective threat over this region...and the
setup does have some potential for a more organized flash flood
risk and potential category upgrade in the ERO. However this will
at least partially depend on how Plains convection Monday night
evolves and impacts the subsequent convectively enhanced shortwave
that is expected to move into the TN/OH Valley Tuesday.

The southern extent of this Marginal risk closer to the Gulf Coast
is more of a conditional risk. Some of the HREF members, including
the 06z HRRR, suggest an organized convective complex may move
into this area Tuesday morning.

Chenard


Day 3
Valid 12Z Wed Jun 05 2024 - 12Z Thu Jun 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND EASTERN U.S....

The frontal system moves east on Wednesday bringing a convective
threat to a large portion of the southern and eastern U.S.. A broad
Marginal risk stretches from TX across the TN/OH valley and into
portions of the Great lakes and Mid-Atlantic...pretty much
everywhere along and just ahead of this front. Plenty of moisture
and instability is expected with this system, which should drive a
heavy rainfall threat. The strongest synoptic forcing should be
over the Great lakes...however we are likely to see multiple low
amplitude shortwaves moving across the OH/TN valley and the Mid-
Atlantic as well. These shortwaves will at least partially be
convectively enhanced shortwaves driven by Plains and MS Valley MCS
development over the next couple days. Thus the exact location and
magnitude of these waves will be hard to predict with much lead
time.

These lower predictability features combined with the overall
progressiveness of the system, suggest a Marginal risk should
suffice for now. Do expect isolated flash flooding will be a
possibility anywhere within this large risk area...but too early to
pin down where a more concentrated risk may develop. Anticipate we
will eventually have one or more Slight risk areas valid for this
day given the favorable ingredients in play...but confidence in
getting the location right is not there at this point.

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