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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0058Z Jun 13, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Wed Jun 12 2024

Day 1
Valid 01Z Thu Jun 13 2024 - 12Z Thu Jun 13 2024

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

The axis of heavy rain over southern FL has shifted south of the
hardest hit areas, and thus the Moderate risk was removed with this
update. However, areas of heavy rain are still occurring over
portions of the FL Keys into far south FL, where additional
rainfall of 2-5" is expected. This will continue to drive some
flood risk warranting a Slight risk. Another round of heavy
rainfall is expected over south FL on Thursday, and while most of
this should be after 12z, there are some indications that this
could begin to organize by 12z...thus will maintain the Slight risk
to account for this potential.

A Marginal risk was maintained over portions of the Upper Midwest
with convection moving across MN this evening. Locally heavy
rainfall from this activity will be possible as the storms move
into WI as well...although cell motions should be quick enough to
keep any flood risk localized in nature.


Day 2
Valid 12Z Thu Jun 13 2024 - 12Z Fri Jun 14 2024

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

...20Z Update...

Updates can be found in the sub-sections below....

...Florida...

A persistent convergence zone will bisect the southern Florida
Peninsula with locally heavy rainfall continuing over much of the
area south of the line of Tampa over to Port St Lucie and points
south. 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. Typically this area has a climatologically high
FFG for the 1/3/6 hr windows, but because of the significant
rainfall the past 48 hrs, antecedent conditions are more favorable
for flood concerns, especially the southeast and southwest urban
corridors of the state. Latest probability fields continue to yield
some impressive percentiles for totals exceeding 5" with a span of
40-60% extending from Sarasota down through Naples on the southwest
coast, and 30-45% located along the southeastern corner of the
state from West Palm down to Miami proper. 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. There is some uncertainty on specifics on where the
heaviest cores will occur and this setup is trending very delicate
given the unfavorable antecedent conditions. As a result, have
maintained a SLGT risk with wording for a higher-end SLGT and the
potential for targeted upgrades pending the radar evolution.

...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. Models are hinting at
the corridor from Chicago to points southwest being in the
cross-hairs of the heaviest precip threat with the HREF
probability fields outlining the areas with respectable
probabilities for exceeding 1" (70-90%) and 2" (45-60%) over the
course of the period. There is a significant drop off once you move
past 3" probabilities which is typically favored for the higher
risk forecasts, however the ambient environment is likely conducive
for locally significant rainfall in the strongest convective
cores. There is a prospect for backbuilding if cells become
anchored to the front, and this is documented within multiple CAMs
at the end of their respective runs. Issue is, the location of
where these occur are different with some favoring Chicago down
into the IL/IN border while others are much further west into
northeast MO to west-central IL. The MRGL risk was maintained to
allow for more consensus with guidance as they get inside 24 hrs of
the event. Expect a targeted SLGT risk somewhere in the middle of
the MRGL risk currently with the best chances lying along the
expected frontal placement by tomorrow evening.

Kleebauer


Day 3
Valid 12Z Fri Jun 14 2024 - 12Z Sat Jun 15 2024

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

...20Z Update...

There were only small adjustments based on the latest ensemble
forecast QPF across the respective risk areas. The SLGT risk across
FL is the headline currently with the ongoing heavy rain threat
continuing through the end of the week into the early part of the
weekend. Sheared out convergence zone across South Florida will
plague the region for at least one more period with locally heavy
rainfall anticipated south of the Sarasota to West Palm Beach line.
PWAT indices are expected to slowly degrade as the pattern
initiates the expected shift away from what we've seen over the
course of the week. Still, with the prior rainfall footprint
leading to much lower FFG's across the region, a risk of flash
flooding will be heightened despite some improvement in the
environment. Totals of 2-4" with locally higher are forecast across
parts of South FL leading to the SLGT risk continuation.

The Northeast U.S will see the approach of a cold front moving
through Ontario/Quebec, reaching the Northeastern U.S during peak
diurnal destabilization. Scattered showers and storms capable of
isolated heavy rain instances will occur from PA up through New
England and points east, especially into Southeast New England
where guidance maintains the highest precip forecast (1-2")
aligning well with the recent ML output from both the Graphcast and
AIFS from the ECMWF. 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.
The MRGL risk remains, but was extended south into northeastern PA
to account for the latest trends in guidance.

There are little changes with regards to the heavy rain potential
in the plains as 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 currently, but would not be surprised if
a small SLGT risk ended up being introduced in future runs as we
introduce CAMs output and associated HREF probability fields. The
MRGL will suffice for the time-being.

Kleebauer


...Previous Discussion...

...South Florida...

The frontal boundary will begin to sink south as a cold front
through central Florida during this period. Meanwhile tropical
moisture will persist across the state while now moving up the
eastern Seaboard. Guidance suggests this period to be less wet than
the last few days however the areal averages for southern and
central Florida are less than 2 inches. Given the multi-day heavy
rain event the threat for excessive rainfall and flooding concerns
will remain elevated for this period, therefore a Slight Risk
continues for South Florida and a Marginal Risk for central
Florida.

...Rockies and Plains...

Anomalous moisture over much of the Rockies and adjacent Plains
will migrate eastward with time as a low pressure system exits
into the High Plains. Some higher QPF is expected near the higher
terrain of northeast New Mexico and eastern Colorado. Much Nebraska
and the surrounding area will have be near or north of the warm
front 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.

...New England...

The shortwave and associated cold front will sweep through the
Great Lakes region and lift through the Northeast during this
period. Showers and thunderstorms are expected to spread much of
the Northeast however focusing the higher QPF values across
southern New England where tropical moisture will be advecting
northward into the region. Guidance does show variance on the
location and the max value but consensus seems to favor southern
New Hampshire/Maine where FFG is as low as 1.5/2 inches at this
time. Isolated instances of flooding may arise. A Marginal Risk
spans from southeast New York to Downeast Maine.


Campbell


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt