FOUS30 KWBC 151319
QPFERD

Excessive Rainfall Discussion...Correction
NWS Weather Prediction Center College Park MD
818 AM EST Wed Nov 15 2023

Day 1
Valid 12Z Wed Nov 15 2023 - 12Z Thu Nov 16 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHEAST FLORIDA...

Latest satellite and radar composite shows a surface low south of
the central Gulf coast with a multi-wave pattern of weak
convection making their way onshore across South FL. The
convective pattern moving into the peninsula is in part to a
prevailing easterly boundary to mid-level wind field caused by the
southern periphery of a broad surface ridge over the east coast,
and a stationary boundary bisecting the southern part of FL. This
will be key in the convergence pattern that will develop later
this morning, carrying through the rest of the afternoon and
evening allowing for persistent heavy rainfall clusters to settle
over the Keys and the urbanized corridor on the southern tip of
the mainland. Model consensus has grown for a significant rainfall
event to unfold within the confines of the Keys later this
morning, extending north in the afternoon as a weak area of low
pressure develops in/near the FL Straits and moves northeast in
time. Deterministic QPF across much of the 00z suite indicates a
widespread 3-6" of rain within the aforementioned areas, but
maximum potential up to 10" is within the realm of possibility.
Forecast areal average soundings across South FL from much of the
CAMs indicate a general 500-1000 J/kg of skinny MLCAPE and PWAT's
between 2-2.2" by later this afternoon, a good 2.5-3 standard
deviations above normal. This is a textbook environment for upper
quartile outcomes as the PWAT field will fall within the 90-95th
percentile for climo, and the primary forcing will be present in
vicinity of the targeted area.

The probability fields are also very robust given the overall
consensus. 00z HREF neighborhood probability for 5" is a solid 90%
or greater across the Upper Keys into a zone stretching from
Homestead up to Miami. The 8" neighborhood probability is as high
as 70% within the corridor from Homestead through Key Largo,
matching the NBM 99th percentile QPF forecast within that zone.
There's still some deviations in where the heaviest rainfall will
occur as much of the setup will come down to low propagation after
genesis, as well as the speed of the surface reflection as it
makes the turn up the eastern FL coast and how the instability
fields will evolve in time. At this juncture, there is plentiful
support in guidance for a heavy rainfall axis to initiate within
the confines of the FL Keys, pivoting northward into the southern
FL Peninsula. General rainfall rates will settle between 1-2"/hr
given the very high probabilistic forecast from the latest hi-res
ensemble, but a 15-25% risk of up to 3"/hr is on the table within
the confines of Miami down into the Upper Keys. This will promote
a higher probability of flash flooding concerns within the urban
corridor down into the FL Keys north of Marathon. A Moderate Risk
was maintained from previous forecast issuance with a slight
extension south to include all of Key Largo based on the recent
guidance trends.

Kleebauer


Day 2
Valid 12Z Thu Nov 16 2023 - 12Z Fri Nov 17 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE EASTERN FLORIDA COAST...

The current MRGL was expanded to the south to include the Miami to
Fort Launderdale corridor given recent trends in guidance to
maintain locally heavy rainfall on Thursday morning, a compounding
effect on the expected D1 precipitation. This would exacerbate
flooding concerns through at least 18z before threat wanes.
Elsewhere to the north, a bit more consensus on the prospects of
locally heavy rainfall within the confines of the Melbourne-Orlando-Jacksonville triangle in due part to the proxy
of the surface low off the east coast of FL and the energy
transfer from the shortwave trough migrating eastward from the
Gulf. Latest blended guidance is bullish with totals of 2-4"
aligning portions of the eastern FL coast with some hint of
further impacts inland into central FL. A lot comes down to the
transfer of energy at 5H and whether there is a smooth transition
from the initial SLP in the Gulf to the low off the FL coast.
Where that handoff occurs would be important in deciphering local
precipitation maxima as the trailing energy over the coast would
allow for localized forcing to induce an area of heavier rain that
would likely train over the same area(s) until it vacates the east
coast. 6-hour 5 year ARI exceedence probabilities have increased
significantly between both the 12-18z and 18-00z time stamps on
Thursday with upwards of 25-30% now within an area encompassing
Melbourne to Orlando, as well as fixed further south along the
urban corridor from Port St. Lucie down into the Miami Metro. This
was one of the reasons for the general MRGL expansion, as well as
a nudge further west on the edge of the risk area. There's still a
bit too much variance for where exactly the heaviest rain will
occur, but the consensus is growing for somewhere near the Space
Coast down to Miami and out into central FL for there to be some
localized flooding concerns, especially given the PWAT anomalies
of 1-2 deviations above normal lingering for much of the state.

Kleebauer

Day 3
Valid 12Z Fri Nov 17 2023 - 12Z Sat Nov 18 2023

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4wCkZp686f9aPmRL2uSMesoHKZmz-HeKFX5Dvk09TGPi= Zjzw9OGJ-LQF4uXEkMPZMZh42mMKAgFlVqfqHPtRR-FqZWM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4wCkZp686f9aPmRL2uSMesoHKZmz-HeKFX5Dvk09TGPi= Zjzw9OGJ-LQF4uXEkMPZMZh42mMKAgFlVqfqHPtRYbxNqWo$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4wCkZp686f9aPmRL2uSMesoHKZmz-HeKFX5Dvk09TGPi= Zjzw9OGJ-LQF4uXEkMPZMZh42mMKAgFlVqfqHPtR1SaoIqM$=20


$$



=3D =3D =3D
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