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FOUS30 KWBC 112020
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Tue Apr 11 2023

Day 1
Valid 16Z 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR ATLANTIC
COASTAL SOUTHEAST FLORIDA...

...2030Z Update...

...Southeast Florida...

The Marginal Risk area for the urban corridor of Southeast Florida
has been upgraded to a Slight Risk with this afternoon's update.
The meteorological setup continuing into Wednesday is not
substantially different from Tuesday. A slow-moving front will
continue to produce convection offshore, which will be slowly
advected into the coast. Plentiful instability and moisture will
persist, so there remains no big reason why anything will change.
The good news is that the upper level low over the Gulf will make
some progress towards beginning the front's northward movement up
the Florida Panhandle on Wednesday by supporting weak surface low
development over the northern Gulf near the coast of Louisiana.
The induced southerly flow east of the low center will allow the
front to finally begin moving. That said, it will take most of the
day before significant northward movement will occur. Thus, expect
many of the same areas getting drenched today to continue seeing
rain on Wednesday. One other small but notable change is that the
nearly stationary front stuck for at least the third consecutive
day means some of the forcing associated with the front is weaker
by Wednesday. Thus, the total amount of rain expected in the
Miami/Ft. Lauderdale area on Wednesday should be a little bit
lower than yesterday's or today's daily rainfall totals. The
trade-off of course is the area is already experiencing flash
flooding and 2-4 inches of rain are still expected on top of what
fell both yesterday and what falls today, so FFGs will be very low
by the time we get to Wednesday. Thus, the risk assessment for
Wednesday is for all intents and purposes the same as today, and
therefore required the upgrade.

By Wednesday night, the front will finally be moving north, so the
rain will slowly end from south to north.

...Central Gulf Coast...

Areas of heavy rainfall are likely to be ongoing across the
central Gulf Coast, particularly in southeast Louisiana at the
start of the day 2 period Wednesday morning. CAMs guidance is
still in fair disagreement on how far north and where the heaviest
rain will be at that point, and especially as the day goes on. For
sure the greatest threat for heavy rain remains in southeastern
Louisiana, but there is a fairly substantial gradient in that
heavy rain by the time New Orleans gets in on the rainfall. The
bayous of central Gulf coast and LA have very high FFGs, and in
coordination with the LIX/New Orleans, LA forecast office, opted
against any Slight Risk upgrades for this update. That said, the
daily totals of over 5 inches along the Mississippi River Delta
are concerning, particularly for the potential that FFGs may be
exceeded. However, since it will take about that much rain to
reach FFGs in that area, the Marginal Risk remains in place as
that should only result in isolated flash flooding. Any northward
shifts in the guidance would likely require an upgrade, as that
would result in more rainfall into the urban areas of New Orleans
and possibly Baton Rouge, Mobile, and Pensacola where FFGs are
locally lower, while any southward shifts could result in a
downgrade as the heaviest rain remains offshore. Certainly this
area will continue to be monitored.

Wegman

...Previous Discussion...

...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

The Day 3 outlook will be updated by 2030Z.


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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