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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Tue Apr 11 2023

Day 1
Valid 12Z Tue Apr 11 2023 - 12Z Wed Apr 12 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE URBANIZED
AREAS OF SOUTHEAST FLORIDA FROM FORT LAUDERDALE SOUTH...

...16Z Update...

...South Florida...

In coordination with the MFL/Miami, FL forecast office, a Slight
Risk was introduced for much of the urbanized area of southeast
Florida from Ft. Lauderdale south with this update. 24 hour
rainfall totals in this area have exceeded 5 inches in some
localized areas, with more widespread rainfall totals over 4
inches between Ft. Lauderdale and Miami. CAMs guidance this
morning shows some potential for additional rainfall up to 4
inches in many of the same areas through tonight, though with
greater likelihood from Miami south through Homestead. In this
area, 24 hour totals up through the time of this writing were
generally in the 2 to 4 inch range. The MFL forecast office has
reported widely scattered flooding ongoing across this area due to
the repeated rounds of heavy rain.

A stalled out front draped south of the Keys is the primary
forcing for the heavy rain in this area. Light east to
northeasterly flow is resulting in training convection moving in
off the Atlantic. Recent convective flareups have produced
rainfall rates to 1.5 inches/hour, but those have been short-lived
and very localized. Thus, the primary flooding threat is
longer-term, as repeated rounds of convection with rainfall to
this intensity has been causing flooding in Miami and the
surrounding area. Since the front isn't moving, these same areas
are unlikely to get much break in the rainfall through tonight.
Instability will be relatively constant through tonight, and in
fact HREF probabilities suggest the greatest likelihood for heavy
rain in this area will be overnight. Thus, see no reason to think
the current radar presentation of widespread light to moderate
rain with embedded heavier elements is likely to change much
through tonight.

...Southern Louisiana...

No changes were made to the Marginal Risk area in southern
Louisiana. CAMs guidance remains in reasonable agreement that most
of the heavier rain that has the potential to cause localized
flooding will occur overnight tonight. Isolated very light rain is
occurring over portions of the Marginal Risk area, so unlike in
Florida, recent rainfall in this area is not a significant
contributing factor to the flooding potential. As an upper level
low over the area cuts off from the flow and intensifies tonight,
it will draw more Gulf moisture and heavier rain northward from
the Gulf into southern Louisiana tonight, which will cause the
locally heavy rain. Of note, quite a few river gauges upstream in
northern LA/MS/AR are in minor flood stage, and as such, the main
stem rivers are swollen and are unlikely to be much help draining
any localized flooding from the heavy rain. Despite reasonable
CAMs agreement, it's worth noting that there remains quite a bit
of variability model-to-model as to how far north the heavy
rainfall threat gets, with any southward adjustments diminishing
the potential flash flooding threat. 

Wegman

...Previous Discussion...

...Southern Louisiana...
Overall model agreement is pretty good with the evolution of the
closed 850-250mb closed low near the central Gulf Coast today into
tonight. However still seeing some spread with the low level
mesoscale features, which will likely be the main driver of how
much heavy rain makes it onshore during the day 1 period. The bulk
of the most intense convection will stay over the open waters,
however it does appear likely that at least some heavier
convective activity will impact portions of southern LA today into
tonight. 00z HREF probabilities of exceeding 3" get as high as
40-70% along coastal areas, and this seems to align fairly well
with the global model consensus as well...and fits with HREF
instability forecasts. This area is hard to flood, and this
rainfall may come periodically though the period, so even with the
potential for 3"+ amounts, think a Marginal risk will suffice.
Further north towards New Orleans, probabilities of exceeding 3"
are closer to 5-10%, which also seems reasonable. Will keep the
Marginal risk this far north to account for this low threat of
heavy rainfall, as amounts of this magnitude would likely pose a
localized urban flood risk if it materializes. 

...Southeast Florida...
Areas of heavy rainfall are again likely today across portions of
southeast Fl into the FL Keys. The better corridor of instability
shifts a bit south compared to Monday, so also expecting the
favored corridor of heavy rainfall to shift southward. The
environmental ingredients for heavy rainfall look pretty
good...with low level easterly convergent flow, instability of
~1000 J/KG, PWs getting towards the 75th-90th percentile, and low
level easterly winds with mid/upper level westerly winds favoring
slow cell motions. HREF neighborhood probabilities of exceeding 5"
are as high as 60-90% over the Keys, with 8"+ probabilities over
20%. Given the ingredients and decent HREF performance Monday over
southeast FL, hard to argue much with this output. Now whether
these more extreme rainfall amounts end up over a Key or over
water remains unclear, but pretty confident in areal averaged
rainfall of 2-3" over the Keys, and there will likely be pockets
of 5"+ amounts in the vicinity. Localized flash flooding is
probable with this activity today into tonight. The Marginal does
extend north towards Miami as well...as even though probabilities
of heavy amounts are lower here, this urban corridor is more
susceptible to flooding. So this extension accounts for this
susceptibility and at least some lower probabilities of 3-5"
rainfall amounts today.

...Northern ID into western MT...
Pockets of moderate rainfall will be ongoing at 12z this morning.
However snow levels will start dropping shortly after 12z, and so
no longer seeing enough rainfall potential to justify a Marginal
risk, as a large percentage of the QPF today/tonight will be snow
or a rain/snow mix.

Chenard


Day 2
Valid 12Z Wed Apr 12 2023 - 12Z Thu Apr 13 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG AND NEAR
THE CENTRAL GULF COAST REGION AND OVER SOUTHEAST FLORIDA AND
NORTHERN KEYS...

...Central Gulf Coast...
At least some heavy rainfall will move onshore by day 2 (Wed into
Wed night) across portions of the central Gulf Coast. By this time
the closed deep layer low looks to intensify, with a stronger
surface low reflection also expected. We are still seeing some
model spread with regards to how far inland and exactly where the
heaviest rainfall ends up, but overall guidance is beginning to
cluster better. The 00z EC/GFS and 00z GEFS mean and 12z EC mean
show pretty good overlap with their QPF forecasts and the Marginal
risk outline generally follows the depiction of these solutions.
Rainfall of 1-3" seems probable within at least some of the
Marginal risk, with some potential for locally higher amounts.

Initially most of the better instability will be offshore over the
Gulf of Mexico, but it should be gradually pulled northward
towards the coast into Wednesday night. However, exactly to what
extent and how far north will have an impact on rainfall rates and
thus also the flash flood potential. Whether these higher rates
end up confined offshore to just along the immediate coast, or if
they make it inland will be partially dependent on this northward
transport of instability. PWs with this system are not really that
impressive as the developing cutoff low will not have a great
tropical moisture connection. So while PWs will be high enough for
heavy rain, at the moment it does not look like we are looking at
very anomalous values (generally below 1.5"). This part of the
country is generally hard to flash flood, with high FFG. Combine
this with some uncertainty on the extent of onshore instability,
and thus coverage of higher rainfall rates, and still think the
flash flood risk is mainly localized in nature. Thus for now we
will stick with the Marginal risk over this region, although we
will continue to monitor trends and still a chance a more focused
Slight risk upgrade may eventually be needed.

...Southeast Florida...
Another day of heavy rainfall potential over southeast FL as a
warm front lifts northward across the area. Given the lifting warm
front, both higher instability and PWs will lift northward into
more of southeast FL. Thus should be a pretty good combo of
moisture and instability, along with slow cell motions given
easterly low level flow and westerly mid/upper level flow.
Generally it is a slightly modified version of the environment
that is over the FL Keys on Tuesday, and this setup supports a
localized flash flood risk along the southeast FL urban corridor.
Given the ingredients, there is some chance we will eventually
need a Slight risk upgrade, however there is enough uncertainty on
convective details at the moment to keep it a Marginal. We'll have
more high res guidance available to evaluate later today and
tonight which may help clarify some of the details.

Chenard

Day 3
Valid 12Z Thu Apr 13 2023 - 12Z Fri Apr 14 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST...

The closed deep layered low near the central Gulf Coast will lift
northeastward into the Southeast Thursday into Thursday night. As
it does it will be going through a weakening phase, gradually
opening up into a trough. Thus while there will be an increase in
ascent over the region, the magnitude of this deep layer ascent
should be on a gradual lowering trend through the period. PWs will
be above average, but without a robust subtropical connection
anomalies are not expected to be above the 90th percentile. Thus
while increasing convective coverage is likely, there remains some
question regarding the organization of storms and rainfall rate
potential with decreasing instability as you get away from the
coast. There is enough moisture/instability/forcing present to
indicate a scattered convective threat over a broad spatial extent
and model QPFs generally support pockets of 1-3" of rainfall. At
the moment this setup screams Marginal risk, with enough going for
it to support a localized flash flood risk, but not enough
environmental ingredients or QPF support for a more
organized/widespread flash flood threat. Of course we'll continue
to monitor trends and adjust the risk as necessary.

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


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