FOUS30 KWBC 050819
QPFERD

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

Day 1
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....

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.

An MCS will be pushing south across east TX at 12z this morning and
we will carry a small Slight risk to account for this activity. 
While the MCS is diving southeast, there is some backbuilding and 
training occurring on the western flank of the activity. Recent 
HRRR runs suggest that some training will remain possible on the 
western flank of the MCS into the morning hours, which may drive 
an isolated to scattered flash flood risk.

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
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