FOUS30 KWBC 180012
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
712 PM EST Tue Dec 17 2024

Day 1
Valid 01Z Wed Dec 18 2024 - 12Z Wed Dec 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE EAST CENTRAL TO SOUTHEAST COAST OF FLORIDA...

..01Z Update..

The current synoptic pattern will yield similar convective output
across the eastern FL Peninsula with low-level convergence against
the coast due to prevailing easterlies within a corridor of above
normal PWATs and modest low instability present. Radar and
satellite composite indicate a focal point of frictional
convergence along the coast near Vero Beach with a cell cluster
anchored right against the coastline. Recent observations signal a
local max of just over 2" within the last few hours with moderate
to bordering heavy rain likely to continue until the cell finally
dissipates. Recent trends within hi-res deterministic indicate
isolated pockets of these types of events occurring from Coco Beach down
through Fort Lauderdale into the early morning hours thanks to the
unwavering easterly wind field helping to enact primed coastal=20
convergence when boundaries advecting off the nearby waters are=20
present.=20

Latest HREF neighborhood probability fields signal modest
30-50% chance of >5" across portions of FL with the max probs=20
located between Port St. Lucie down to Fort Lauderdale. Even with=20
that alignment, it's very plausible that just a slight north/south=20
displacement of the highest probs would indicate locally=20
appreciable rainfall worthy of flooding in any of the coastal urban
areas of eastern FL. Considering the current radar depiction and=20
collaboration with the local offices, decided to maintain general
continuity with a small extension northward given the radar trends
near Viera/Coco Beach.=20

Kleebauer


Day 2
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF
THE MID-SOUTH AND LOWER OHIO VALLEY ALONG WITH THE SOUTHEAST
FLORIDA COAST...

...TN into southern KY...
Have included a targeted Marginal Risk in the updated Day 2 ERO
across parts of western-central TN (especially northern portions)
into southern KY, which would include the Nashville metro area.
Amplifying upper trough to the west will allow for a more curved
upper level jet streak on the lee side of the trough -- which will
enhance the upper divergence and low-level moisture transport/flux
into the outlook area. The latest (12Z) HREF show more elevated
probabilities of 1"/hr rainfall rates between 12-18Z. In addition,
especially when considering recent heavy rainfall over the past
24+ hours (1-2 inches over parts of nw TN and sw KY), the latest 1
and 3 FFG values have come down while 0-40km soil moisture
percentiles per NASA SPoRT have climbed to 60-80% for most areas.
As such, the HREF neighborhood probabilities of 3 hourly QPF
exceeding 3 hour FFG peaks between 25-40% between 12-15Z Wed, when
in that short 3-hour window most areas should see additional
rainfall of 1-1.5" while some (isolated) locations receive closer
to 2+ inches.


...Southeast FL...
Basically carrying over the Marginal Risk area from Day 1 into the
Day 2 period, as much of the 12Z high-res CAMs show lingering
convective clusters along the se FL coast (mainly south of Vero
Beach). Continued favorable (onshore) low-level moisture
flux/convergence and sufficient deep-layer instability on Wed will
maintain elevated short-term rainfall rates, likely 1-2+ in/hr at
times during the day 2 period per the HREF probabilities. The 12Z
HREF meanwhile also indicates 50-70% 40km neighborhood
probabilities of at least 5 inches of additional rain during Day 2
(12Z Wed-12Z Thu), with 30-50% probs of 8+ inches. The NAM-CONUS
Nest, ARW1, and 12Z HRRR in particular are leading to these high
HREF probabilities.

Hurley


Day 3
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

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

Hurley


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!_NgqMpb88PXqjkJx9n9dNWlUDnQVs0ew7DBQVtyMs5iF=
1ZE6xYclFrbYeLc06se9k3VEm8cdOCpAW7vZ5xsnoF18qkQ$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!_NgqMpb88PXqjkJx9n9dNWlUDnQVs0ew7DBQVtyMs5iF=
1ZE6xYclFrbYeLc06se9k3VEm8cdOCpAW7vZ5xsnXKMlbx8$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!_NgqMpb88PXqjkJx9n9dNWlUDnQVs0ew7DBQVtyMs5iF=
1ZE6xYclFrbYeLc06se9k3VEm8cdOCpAW7vZ5xsn5tU4hBI$=20

=3D =3D =3D
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