National Weather Service Raw Text Product

926 
FOUS30 KWBC 120013
QPFERD

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

Day 1
Valid 01Z Wed Apr 12 2023 - 12Z Wed Apr 12 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH FLORIDA...

...Southern Louisiana...
The bulk of the most intense convection will stay over the open
waters as temperatures aloft cool while a closed low moves into
the far northwest Gulf of Mexico, however it does appear likely
that at least some heavier convective activity will impact
portions of southern LA later tonight as 500-1000 J/kg of MU CAPE
remains offshore/upstream and low-level inflow into the region
increases around a baroclinically-initiated convective low in the
central Gulf of Mexico/over the Loop Current which forms between
Tuesday evening and Wednesday morning.  The 18z HREF probabilities
of exceeding 3" in 12 hours get above 40% along coastal areas of
southeast LA with a non-zero risk of 5".  These sort of formations
are tricky, with model guidance sometimes shifting east by 100+
miles once a convective low forms in the Gulf of Mexico, so
despite model convergence on the expected scenario, model biases
with such systems suggest that uncertainty is higher than usual in
the scenario the guidance currently agrees upon.  Southern LA is
marshy and not easy to flood, and this rainfall is expected to
maximize in the early morning hours of Wednesday, should it occur,
so a Marginal risk will continue to suffice.  The Marginal risk
around New Orleans, despite the lowered chance of 3"+, was left in
place to account for any possible localized urban flood risk.

...Southeast Florida...
Areas of heavy rainfall are expected to renew in the morning hours
on Wednesday across portions of southeast Fl into the FL Keys as a
mesoscale low moves through the FL Straits around the southeast
periphery of a convective low forming across the central Gulf of
Mexico.  The environmental ingredients for heavy rainfall
exist...with low level east to east-southeast convergent flow of
~25 kts, instability of 1000+ J/KG, PWs of 1.75"+, and mid/upper
level west to southwest winds favoring slow cell motions and
providing enough effective bulk shear for organized convective
bands and the possibility of mesocyclone formation. HREF
neighborhood probabilities of exceeding 5" in 12 hours are high
close to Key West, with the probability of 3"+ highest over the
entirety of the Keys.  Given the ingredients available over
southern FL, it's hard to rule them out from additional heavy
rains during the early morning hours so the Slight Risk remains in
place, though it has been shifted somewhat southwest.  Over the
Keys, it's always a question of whether the high amounts occur
over the water or land, but isolated 5"+ amounts are quite
possible ashore.  Localized flash flooding is probable during the
early morning hours Wednesday.  The Marginal extends north towards
West Palm Beach as well...as even though probabilities of heavy
amounts are much lower there, the ingredients appear supportive,
and the South Florida urban corridor is more susceptible to
flooding.  Flash flood guidance values are modest over the urban
corridor due to the local 3-5" amounts which have already occurred.

Roth


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

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

...2030Z Update...

Mostly cosmetic changes were made to the Marginal Risk area
covering portions of the Southeast on Thursday. The upper level
low that starts the day over Louisiana will begin to open up into
a negatively tilted trough as it moves northeastward across the
Southeast. As the trough lifts northeastward, the associated rain
shield will move along with it. In addition to slowly diminishing
maximum rainfall totals, the area of real-estate expected to get
significant rainfall will increase due to the moving upper level
pattern. The very high FFGs from north Florida through Alabama and
Georgia will substantially limit the overall flash flooding
threat, so the Marginal risk will remain in place largely for
urbanized and other flooding-sensitive areas seeing isolated
flooding with any training cells. While the forecast QPF has
increased from the overnight issuance, the maximum potential
remains well below FFGs for almost all of the Marginal Risk area.
It remains to be seen how the forecast rainfall amounts will
change once CAMs get to resolve this event, but it appears
unlikely at this point that any upgrades to a Slight risk will be
needed.

Wegman

...Previous Discussion...

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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