FOUS30 KWBC 290802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Sun Dec 29 2024

Day 1
Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF SOUTHWEST OREGON AND NORTHWEST CALIFORNIA ALONG WITH THE
FOOTHILLS OF THE NORTHERN SIERRA NEVADA...

...Western Oregon and Northern California...

The tail end of a fairly strong atmospheric river will be impacting
especially southwest OR and northern CA this morning as a shortwave
trough and associated wave of low pressure advances into the
Pacific Northwest with a trailing cold front extending well to the
south and gradually moving inland across northern CA. IVT
magnitudes along and just ahead of the cold front will be on the 
order of 750 to 1000 kg/m/s across the northwest CA coastal ranges 
including Del Norte and Humboldt Counties. Lesser magnitudes will 
be advancing inland with the front into the Shasta/Siskiyou ranges 
and the northern Sierra Nevada. The IVT values should come down by 
18Z, but sufficient levels of onshore flow, convergence along the 
trailing front, and orographic ascent over the terrain should favor
areas of heavy rain lingering well into the 18Z to 00Z time frame 
across the foothills of the northern Sierra Nevada. The 00Z HREF 
guidance favors 0.50"+/hour rainfall rates at least occasionally 
impacting these areas, and the threat of heavy rain will also 
envelope the Bay Area where the coastal ranges here may also see 
some occasionally enhanced rainfall at least briefly with rates 
exceeding a 0.50"/hour before the front clears the area. For the
period, some additional 2 to 3 inch rainfall amounts will be
possible locally across the coastal ranges of southwest OR and
northwest CA, and also separately in the foothills of the northern
Sierra Nevada by tonight. A Slight Risk of excessive rainfall has 
depicted across these areas, and is an introduction in the case of
the northern Sierra Nevada foothills. Wet antecedent conditions
along with high streamflow will encourage there being at least a
small stream and urban flooding threat, and there may be a low-end
flash flood threat involving the more sensitive burn scar locations
where these heavier rains persist. Please consult MPD #1204 for
more details concerning the near-term heavy rainfall/flooding
concerns across the region.

...Central/Southern Appalachians into the Southeast...

An early-morning QLCS will be crossing through the eastern Gulf
Coast region and the broader interior of the Southeast as a strong
southerly low-level jet of 40 to 60 kts drives enhanced moisture
transport along with a nose of favorable instability just ahead of
an advancing cold front. This will be occurring as a surface low
and associated mid to upper-level trough rides northeastward up
into the Great Lakes region. The QLCS activity should at least clip
portions of the southern Appalachians and then advance across the 
Piedmont of the Southeast and eventually the coastal plain. Some 
of the more urbanized locations that see this QLCS passage may see 
a quick 1 to 1.5 inches of rain in less than an hour, and this may 
result in some urban runoff problems with a highly isolated threat
of flash flooding. In general, some of the moist flow into the central
and southern Appalachians this morning should favor at least some 
locally heavy rainfall totals that may reach upwards of a couple 
inches before the cold front clears the region. A Marginal Risk of 
excessive rainfall has been depicted across all of these areas.

...South Florida and the Keys...

The latest GOES-E IR satellite imagery shows a cluster of very
heavy showers and thunderstorms over the southeast Gulf of Mexico
in association with a low-level warm front that is attempting to 
advance northeastward toward the southern FL Peninsula. The 
convection has been focusing in close proximity to a well-defined 
instability gradient and there has been some well-organized 
convective cells with some mesocyclone characteristics evolving 
northwest of the lower FL Keys over the last few hours as seen in
the Key West radar. Deep moisture and elevated instability has 
already been favoring very high rainfall rates, but the challenge 
for today will be whether or not any of this organized convection 
can advance bodily inland across South FL. The 00Z HREF model 
consensus, 00Z NSSL MPAS guidance, and 06Z HRRR suggests Collier 
and Monroe Counties will see heavy rainfall this morning as this 
warm front pushes eastward at least toward the coast while also 
lifting up through the FL Keys. It is possible that Miami-Dade 
County could get into at least some peripheral bands of showers and
thunderstorms as well, some of which will be capable of producing 
heavy rainfall rates of 1 to 2 inches/hour. This would certainly 
introduce concerns for some urban flash flooding. Given the 
mesoscale nature of this setup, there is a concern for locally very
heavy rainfall totals (possibly 5+ inches over parts of Collier 
and Monroe Counties and portions of the Keys). Given the level of 
uncertainty with the coverage and the amounts, a Marginal Risk will
be depicted for the time being, but it would not be a stretch that
a targeted upgrade to a Slight Risk will be needed this morning to
address the threat for at least some areal flooding and 
potentially urban flash flooding.

Orrison


Day 2
Valid 12Z Mon Dec 30 2024 - 12Z Tue Dec 31 2024

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

Wegman


Day 3
Valid 12Z Tue Dec 31 2024 - 12Z Wed Jan 01 2025

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

Wegman
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