043
FOUS30 KWBC 040847
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
447 AM EDT Tue Jun 4 2024

Day 1
Valid 12Z Tue Jun 04 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...

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 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 Northeast...pretty much everywhere
along and just ahead of this front. Moisture is plentiful with this
system, with PWs over the climatological 90th percentile
(especially over the Mid-Atlantic). Instability is not extreme,
but should be sufficient for deep convective development. The main
limiting factor should be the progressive nature of the system
limiting the duration of heaviest rainfall. However we are likely
to see multiple low amplitude shortwaves moving east ahead of the
cold front, which should also act as a focus for convective
development. This suggests that some areas could see multiple
rounds of convection. Of course this also means a more messy
convective evolution, which may end up impacting destabilization.
All to say that while an isolated flash flood risk exists over a
large geographic extent, confidence on pinning down an area where
the flash flood risk is more organized remains low. So while an
embedded Slight risk may be needed somewhere from the TN Valley
into the Mid-Atlantic, still think the broad Marginal will suffice
for now.

Will need to keep an eye on southeast TX into the lower MS Valley.
It seems probable that an MCS will be moving across this area
Wednesday morning, and several of the higher res models suggest
this complex leaves behind a well defined MCV. This MCV could
become the focus for additional organized convective development
later Wednesday, which would result in a more focused flash flood
risk. While we may eventually need a Slight risk upgrade to
account for this threat, think it`s a bit too early for that at
this point...as this is conditional on how convection evolves
Tuesday and Tuesday night. So will keep things at a Marginal and
continue to monitor.

Also extended the Marginal risk into portions of the Northeast
with this update. While the stronger forcing holds off until
Thursday, it looks like there`s some weak shortwave energy over
NH/ME and about 1000-2000 j/kg of CAPE helping trigger isolated to
scattered convection Wednesday. Several HREF members have some slow
moving convection developing, with 2" neighborhood probabilities
over 40%. This setup may lead to a very localized flash flood
threat.


Chenard


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....

...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