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FOUS30 KWBC 132000
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Thu Jun 13 2024

Day 1
Valid 16Z Thu Jun 13 2024 - 12Z Fri Jun 14 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN
FLORIDA...

...16Z Update...

A SLGT risk was issued across portions of the central Midwest
through the Mid-Mississippi Valley for later this evening.
Advancing cold front will move south-southeast out of the northern
Midwest later this afternoon with an increasing convergence pattern
along and ahead of the front. Ahead of the boundary, some
significant increase in areal theta-E`s will prompt an improving
environment for convection and heavy rain potential. Soundings out
of KILX this morning were approaching the 90th percentile for PWATs
with a value of 1.37", and only expected to increase prior to the
incoming convective event. Latest 12z HREF neighborhood
probabilities have increased from prior runs for the upper
thresholds of >3" and >5" with the former running between 25-40% in
a broad area from the IA/MO border across into northern IN. 5"
probabilities are running between 10-15% across central IL which is
the consensus for best area to see the max potential rainfall.
Totals of 2-3" with upwards of 6" are going to be littered across
the outlined area within the SLGT as discrete supercell development
ahead of the front potentially garnering some scattered heavy rain
chances even prior to the upscale growth pattern along the boundary
as it slowly moves southward. In coordination with the central
Midwest WFO`s involved, the SLGT risk was agreed upon given the
favorable environment and forecasted probabilities.

The High Risk across FL was maintained from the previous special
update as the area from Fort Meyers/Naples over to the other coast
near Miami/Fort Lauderdale will see another round of significant
rainfall to exacerbate the remnant flooding in place from the past
few days. The highest threat aligns with the I-75 corridor across
into the urban areas extending from just north of FLL down into
Miami proper. Latest 12z HREF probabilities correlate well with the
expectation for at least 2-3" as the fields are both respectively
above 90% for the neighborhood probability, coast-to-coast along
the southern peninsula. Even more impressive is a swath of 20-35%
EAS probabilities for at least 3" running along the aforementioned
area which given the conservative nature of the EAS field is
statistically significant. The High Risk aligns very well with the
EAS field in question adding confidence to the issuance of the
risk. The rest of central and southern FL will see scattered
instances of significant rainfall with totals between 2-4" with
locally as high as 8" within the SLGT/MDT risks surrounding the
High risk area in place. Heaviest rainfall will occur this
afternoon through the evening before some marginal clearing of
precip overnight.

...1425Z Update...

After collaboration with the local Miami WFO, have opted to upgrade
a portion of the southern FL Peninsula to a High Risk to account
for significant impacts expected with the next round of heavy
rainfall over extremely sensitive areas that include the urban
corridor in Southeast FL through the I-75 corridor over Alligator
Alley. Additional totals of 2-5" are likely with locally as high as
10+" in isolated locations along the southwest coastal areas
extending through the interior of the Peninsula. Considering the
ongoing flooding with many reports of standing high water and the
historically low FFG indices for all 1/3/6/12 hour intervals, the
prospects for significant impacts will continue through the
afternoon and evening. Thus, the High Risk was issued to account
for the general expectation.

Kleebauer


...Previous Discussion...

...Florida...

Several inches of rain focused over South Florida yesterday which
has left soils water logged and have lowered the 1/3/6 hr FFG to as
low as 0.25 inch. The latest hi-res guidance show another very
heavy band developing potentially over the same areas by late
morning and into the afternoon hours. It will take very little
additional rain to aggravate ongoing flooding in rural and urban
locations. A Moderate was raised for a majority of the southern
counties which includes the Miami metro where areal averages with
be around 3 inches with isolated maximums up to 8 inches possible
per hi-res guidance.

Totals in the periods prior will be running between 3-12" for an
areal average across the above corridor with some maxes approaching
15" in the areas that have been hit the hardest over successive
days. Considering the repeated nature of the heavy precip,
expectation is for more flash flooding to arise late Thursday
morning through the afternoon as diurnal destabilization within the
convergent axis provides another round of significant rainfall for
those that have seen a prolific amount already.

...Midwest...

Shortwave over the Northern Plains will pivot around the northern
periphery of the mid-level ridge located across the Southwest U.S
with sights on the central Midwest by Thursday afternoon. At the
surface, a cold front will be pressing south-southeast over
portions of the Central Plains through the Mid-Mississippi Valley.
The coupling of the two distinct features in the midst of a
prolific thermodynamic regime located over the above area (MLCAPE
exceeding 4000 J/kg) will help initiate a robust line of convection
with cores capable of locally enhanced rainfall as they move
southeast with the forecast steering flow. The Marginal Risk was
maintained to allow for more consensus with guidance as they get
inside 24 hrs of the event.

While there continues to be some variance in where the convection
will be, consensus favors a line from the borders of
Iowa/Missouri/Illinois to east/northeast of Chicago where areal
averages of 1 to 3 inches are forecast. The environment may be
conducive for locally significant rainfall in the strongest
convective cores, particularly if backbuilding cells become
anchored to the front.

Campbell/Kleebauer


Day 2
Valid 12Z Fri Jun 14 2024 - 12Z Sat Jun 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN
FLORIDA...

...20Z Update...

The update for the SLGT risk across South Florida can be found in
the sub-heading below....

Elsewhere, other areas of note include the Central High Plains and
the Northeastern U.S with locally heavy rainfall forecast in each
area during the mid to late-afternoon hours heading into the
evening. Across the Northeast, a cold front will traverse the
region by midday with an increasingly favorable environment for
deep, moist convection ahead of the front. Instability is modest
overall, but the ascent pattern along and ahead of the cold front
will be sufficient for an organized line of thunderstorms to
initiate from New England to the Mid Atlantic, moving eastward
through the second half of the forecast period. Latest HREF mean
QPF is quite a bit more robust than the NBM, although the latter
has started to come around from its usual low bias at range.
Considering PWATs within the 1-2 deviations above normal range, the
chances for more efficient rainfall during the event are
considerably higher compared to other events in the past. A general
1-2" range is forecast from near PHI up through NYC and portions
of Southern New England with the heaviest precip focused around the
Lower Hudson into the NYC suburbs. A lot of this is due to the
combination of forecasted surface frontogenesis being maximized
over the Northern Mid Atlantic along with the favorable upper level
ascent within the RER of a jet streak moving over New England.
Given the latest forecasted precip and the dynamic factors becoming
more favorable, have expanded the MRGL risk further south and east
to include the Philadelphia and NYC metro areas along with the
surrounding suburbs.

There was only some minor adjustments necessary to the MRGL risk
across the High Plains as the upper level evolution remained
steadfast in its run-to-run continuity. Heaviest rain will fall
across the Front Range northeast of Denver/Boulder into Southwest
NE and Northwest KS before ejecting off into the northern Midwest
during the front half of the D3 period. Scattered signals for
totals exceeding 2" have been indicated within the latest
probability fields with the highest neighborhood probs of 60-70%
located within the Sand Hills of NE. This will be important as that
area has a much higher FFG interval compared to the surrounding
locales. This creates a better threat further south where the setup
is both favorable with those lower FFG`s comparatively. The threat
for significant flooding is low and with the coverage of heavier
rainfall likely more sporadic, a MRGL risk was sufficient to cover
the threat.

...South Florida...

No significant changes to the previous SLGT risk across South FL.
A lot on the potential is contingent on what transpires the prior
period in regards to antecedent conditions. The area will be wet,
but the prospects only increase if there is more significant
flooding with standing water present in any areas south of the Fort
Meyers to West Palm Beach line. The worst will be over compared to
previous periods as the front slowly sags south and the higher
PWATs begin degrading, albeit still pretty moist, but the ascent
pattern becomes less prominent leading to less organized waves of
convection. Recent trends with the ensemble QPF signal much less in
terms of the upper threshold of potential with the 90th percentile
QPF more in line with 3-5" compared to the 10+" we`ve had in
successive days. This will curb the higher end impacts to a degree,
but the receding waters will take some time before we are in the
clear. The the areas south of the line mentioned above will be
closer to the higher end of the SLGT risk with a chance for an
upgrade pending hi-res forecasts for thunderstorms during the
period. For now, will maintain wording of potentially significant
flooding, but will also mention the threat becoming more localized
as the ascent pattern loses some favorability compared to recent
periods.

Kleebauer


...Previous Discussion...

...Rockies and Plains...

A strong upper level disturbance exits northeast out of the Four
Corners with a strengthening low in the lee of the Rockies.
Scattered heavy rain signatures with some isolated totals exceeding
2" are forecast. Local enhancement of QPF is expected within the
higher elevations of northeast New Mexico otherwise the bulk of the
high QPF is expected across the Central Plains, roughly from
Boulder to Sioux Falls and could easily pick up 1 to 3+ inches of
rainfall. A vast majority of this region has 3-hr FFG near 1.5/2
inches however there are very isolated locations as low as 0.25
inch. A Marginal Risk is in effect from northeast New Mexico to
Iowa/Minnesota/South Dakota. There may be a need for a small Slight
Risk area with future updates if QPF trends continue to increase.

...New England...

A cold front will advance through Ontario/Quebec and reach the
Northeastern U.S during peak diurnal destabilization. Convection is
expected to develop along and ahead of the front and will be
capable of isolated heavy rain instances from Pennsylvania to New
England and into the Canadian maritimes. The highest QPF is
focusing across southeast New England where guidance suggests areal
average of 1 to 2 inches. The progressive nature of the convection
will limit the threat to marginal territories, although the
urbanization factors in-of the Lower Hudson up through southern
New England could produce some locally enhanced risks if a storm
overperforms.

Campbell/Kleebauer


Day 3
Valid 12Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
OF IOWA, MINNESOTA AND WISCONSIN...

..20Z Update...

The upper pattern across the Plains into the Northern Midwest is
still sufficient for a period of enhanced rainfall on Saturday
afternoon and evening. Latest ensemble bias correction and NBM both
agree on a core of 1-2" amounts with the upper quartile of NBM
output generally placing parts of southern MN and northern IA
closer to 3" which is pretty significant considering the setup a
full 48 hrs out. Recent ML output is a bit further west with the
core of the heaviest QPF, so it will be interesting to monitor if
the SLGT risk in place will get positioned a bit further west into
parts of the northern plains. Until there is some significant
deviation from the mean, have maintained continuity from previous
forecast.

Over FL, the threat for flash flooding will be waning as the
pattern responsible for the successive days of heavy precip will
finally be broken down with the deeper moisture field shifting back
west over the Gulf. Isolated to scattered thunderstorms are still
forecast over parts of South FL during Saturday afternoon which
could trigger some localized flood concerns thanks to the very wet
antecedent conditions. The threat is currently on the lower end of
the MRGL threshold, but at leads wanted to remain consistent and
will adjust if necessary as we monitor the pattern evolution.

Kleebauer


...Previous Discussion...

...Upper Mississippi Valley to Southern Plains...

During this period a cold front will be pushing through the
Northern Plains while a warm front lifts from the Central Plains
into the Upper Mississippi Valley. PW values of 1 to 1.5 inches
will be drawn into this system, reaching a peak of +2 standard
deviations above average over Iowa and Minnesota. There will be
strong mid-level ascent embedded with this pooled moisture which
will help enhance rainfall amounts and coverage from the Plains to
the Upper Mississippi Valley. Guidance is showing 1 to 3 inches
will be common although local maximums may reach 5 inches. A Slight
Risk is in effect from northwest Iowa to west-central Wisconsin. A
Marginal Risk area spans from North Dakota eastward to Lake
Michigan and southwest to Oklahoma.

...South Florida...

Convection is expected to linger nearing the decaying front
boundary and may produce an additional 1 inch or so. The soils will
be over-saturated and sensitive to any additional rainfall. A
Marginal Risk is in effect given the ongoing flooding concerns.

Campbell


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