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FOUS30 KWBC 051544
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1144 AM EDT Wed Jun 5 2024

Day 1
Valid 16Z 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....

...16Z Update...

Previous SLGT risk across TX was removed due to the progression of
this mornings MCS out of the area. Complex is currently in a slight
weakening/steady state as it moves towards the Gulf coast. The
general area of impact over southeast TX will experience rainfall
rates generally 1-2"/hr at peak intensity which will limit flash
flood concerns to isolated levels, mainly within urbanized areas
(Houston metro). Otherwise, the outflow from the disturbance will
exit off the coast with mainly scattered convection expected later
this afternoon with convective modes generally of the pulse
variety over southeast TX into southwest LA.

Across the Mid Atlantic, a well-defined mid-level perturbation is
currently moving over WV with scattered showers already developing
over the high terrain north of I-64. Increasing low-level
instability from diurnal destabilization along with the localized
ascent from the shortwave in question will lead to increased
convective coverage and magnitude with thunderstorm development
likely sometime after 17/18z. Any convection could cause issues
given the elevated PWATs over the entire Mid Atlantic as depicted
by a 1.86" reading on this mornings sounding out of KIAD. HREF
blended mean QPF is still running between 1-2" for an areal average
with some pockets of higher QPF between 2-4" depicted within the
individual CAMs members. The prospects for flooding are highest
within the terrain west of the Blue Ridge, as well as the urban
corridor(s) over Northern VA up through the DC/Baltimore metro,
extending into Southeast PA up to about I-78. Probabilities for
heavy rain prospects have increased over Southeast PA, likely due
to guidance indicating more organized thunderstorm potential moving
northeast out of the Mason Dixon. Forward propagation of the mid-
level disturbance is generally in line with the east-northeast
movement into the area, so synoptically this makes sense. The
previous SLGT was maintained with that expansion to the north on
the northeastern periphery of the risk area.

Mid-level shortwave over the ArklaTex will continue to press
eastward with an expected initiation of thunderstorms across
northeast LA into western MS by later this afternoon. Enough low-
level convergence from the eastern flank of the remnant MCS along
with ascent provided by the progressing shortwave should enhance a
small area of convection that could produce locally heavy rainfall
across the aforementioned area. The area of focus is relatively
small, but does have some urban centers (Jackson, MS) that are
within the zone of potential. Rainfall rates are generally expected
to be between 1-2"/hr at max intensity with probability fields
peaking at 25-30% over isolated areas in MS for >2"/hr. The threat
is on the "higher-end" of MRGL with the Jackson area the most
susceptible for impacts during activity. Area soils are moist from
the past several weeks of rainfall, so wanted to highlight this
area as one of the best spots outside the higher end risk areas.

The MRGL risk across the Great Lakes, Ohio Valley, and New England
were all maintained due to basic continuity for the convective
expectations this afternoon and early evening. Threat over
respective areas will be isolated in nature with much of the threat
confined to urban corridors over the Lakes/Ohio Valley and complex
terrain in northern New England.

Kleebauer


...Previous Discussion...

A Slight risk was maintained across portions of WV into southern
PA, northern VA and MD. Still looks like multiple rounds of
convection will impact this region today into tonight, resulting in
an isolated to scattered flash flood risk. One MCV currently over
eastern KY (as of 08z) will move northeast and help focus a
convective risk this afternoon across portions of this region.
Then by later this afternoon and evening another subtle shortwave
feature (currently over TN/MS/AL as of 08z) should help
trigger/organize additional convection over similar areas. Both
rounds should be fairly progressive in nature...however rainfall
efficiency should be high in this setup. Noting a deeply saturated
sounding with PWs over 1.75" and near 2" in spots, thus convection
should be capable of heavy hourly rainfall rates. The degree of
destabilization is probably the biggest question mark and main way
this event may under perform. However enough ingredients are in
place to warrant a continuation of the Slight risk.

The inherited Slight risk area aligns well with 00z HREF
probabilities, so only modest adjustments were needed. This
corridor has the highest 1" EAS probabilities (25-50%) indicative
of better convective coverage compared to the rest of the broad
Marginal risk area. 2" neighborhood probabilities are 40-70% and
3" probabilities as high as 30%. Once you get east of WV antecedent
conditions are quite dry resulting in high FFG. This should limit
the areal extent of flooding...however the urban corridor of
northern VA into MD will still be susceptible to flash flooding.

Elsewhere there is a large Marginal risk across portions of the
Southeast, TN/OH Valley and Great Lakes. A localized flash flood
threat exists over this broad geographic extent...however not
seeing enough of an organized more concentrated threat to warrant
any Slight risk upgrades at this time.

Will also carry a Marginal risk across portions of NH and ME for
isolated to scattered slow moving convection this afternoon. Some
weak vort energy east of the shortwave ridging should help trigger
convection with upwards of 1500 j/kg of CAPE anticipated. Cells
that form could be slow moving with some merging along outflows
possible. This may drive a localized flash flood risk.

Chenard


Day 2
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...
Will continue to carry a Marginal risk across portions of the
Northeast. From a heavy rainfall perspective this event does have
several things going for it. Plenty of large scale forcing and PWs
approaching the 95th percentile for early June, so rainfall
efficiency should be high. Looks like we should see one round of
rain early in the day...probably mostly stratiform but maybe some
embedded heavier cores. But more worried about the afternoon hours
when we should see another round of potentially more robust
convection. There will be a warm front in the area, so the concern
is we could see some training near that front. HREF probabilities
are pretty solid, with 2" neighborhood probs over 60% and 3" probs 20-30%
(and that`s only through 00z). The CSU ML tool also shows Slight
risk probs over the area. The dry antecedent conditions and high
FFG give some pause for any upgrade, and may keep this more in the
Marginal risk range.

At this point it`s probably more of a conditional threat depending
on a few factors. Can the first round of rainfall lower FFG at all
or will it be too transient in nature. Then do we see enough
destabilization to get intense afternoon convection, and does this
activity train near the warm front or do cell motions stay fast off
to the east. Given these questions, and after coordinating with the
impacted WFOs, will stick with a Marginal risk for now...but will
continue to monitor trends.

The Marginal risk extends south across NYC and towards
Philadelphia. It looks the we should see robust convection along
the cold front Thursday afternoon over these areas. This activity
will be capable of heavy rates, but should stay relatively quick
moving limiting the extent of the threat.


Chenard


Day 3
Valid 12Z Fri Jun 07 2024 - 12Z Sat Jun 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
KANSAS, NORTHEAST OKLAHOMA, AND SOUTHWEST MISSOURI....

A Slight risk was introduced with this update across portions of
KS, northeast OK and southwest MO. Looks like a good setup for
organized upscale convective growth Friday night. Plenty of
forcing, with mid level shortwave energy and right entrance upper
jet dynamics. In the lower levels we should have a warm front
draped across the area, and increasing 850mb transport within the
low level jet. The evolution of the 850mb moisture transport axis
does seem to support some northwest to southeast oriented training
of convection. The convective complex will probably move off to the
southeast as a whole at a decent clip...but it should be a broad
enough complex to support some prolonged heavy rainfall. PWs
increase above the 90th percentile for early June as well...so most
ingredients appear to be in place for a scattered flash flood
threat.

There is some latitudinal spread with exactly where the axis of
heaviest rain will fall...however we are beginning to see some
model convergence and a bit better run to run consistency. At the
moment most of the global and AI models point towards portions of
central to eastern KS, northeast OK and western MO being most at
risk. Thus this is where we will start with the Slight risk, and
can always adjust as the event nears.

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