AWUS01 KWNH 132001
FFGMPD
CAZ000-ORZ000-140600-

Mesoscale Precipitation Discussion 1174
NWS Weather Prediction Center College Park MD
300 PM EST Fri Dec 13 2024

Areas affected...Northern California...Southwest Oregon

Concerning...Heavy rainfall

Valid 132000Z - 140600Z

SUMMARY...An approaching storm system accompanying a strengthening
atmospheric river will result in an areal increase of excessive
rainfall rates this afternoon and into tonight. Areas of flash
flooding and landslides are possible.

DISCUSSION...GOES-West satellite imagery shows a steady fire-hose
of Pacific moisture being directed at the U.S. West Coast today
with periods of rain well underway across northern CA and southern
OR. As the afternoon unfolds, the approaching warm front and storm
system will force southerly 925-850mb winds to accelerate into the
far northern CA coast. The warm front's approach will also force
freezing levels to rise as high as 5,000ft in some cases. This
provides both a deeper warm cloud layer and would allow for higher
elevations (up to around 5,000ft) to be at-risk for excessive
rainfall rates. By 00Z, the triple-point of the frontal system
will be tracking through the CA/OR border and the warm front will
reside along the CA Coastal Range. A surge in 850mb moisture flux
along the northern CA coast will accompany an IVT >750 kg/m/s that
surpasses the 99th climatological percentile according to NAEFS.
This robust IVT is the catalyst for a >12hr period for excessive
rainfall from as far north as southwest OR to as far south as some
of the northern Bay Area suburbs overnight.

12Z HREF probabilistic guidance shows high chance probabilities
(>70%) for >0.5"/hr rainfall rates around the Eureka area as early
as 21Z. These probabilities then spread as far north as the OR/CA
coastal border between 00-03Z. By 03Z, the cold front will be on
approach and the northern CA coast will become placed within the
storm's warm sector. Low-end MUCAPE values (generally <200 J/kg)
could be available for potential convective enhancement in
northern CA tonight, while the strong SW flow aloft supports
strong upslope enhancement into the Trinity/Shasta Mountains
between 03-06Z tonight. These two areas could see rainfall rates
approach 1"/hr in some cases.

Overall, through ~06Z Saturday, additional rainfall totals of 2-4"
are expected within most of the highlighted region with localized
totals surpassing 5" possible. The 12Z HREF depicts
low-to-moderate chance probabilities (30-50%) for 12-hr QPF >5"
along the coast near and south of Eureka, as well as in the
southwestern facing slopes of the Trinity/Shasta Mountains.
Previously saturated soils have recovered to some extent given the
drier than normal stretch of weather over the past couple weeks,
which should help limit the areal extent for potential flash
flooding. That said, the atmospheric parameters mentioned above
are more than enough to support the potential flash flooding and
landslides in parts of northern CA this afternoon and into
tonight.

Mullinax

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!4qLvtWeaBcIOYHZkoGSpo2fqD1WRjZoZ7yQJcmzH6-Uedhy7O7s0XLSR8fnL2zcKeXvT=
NoWtsLIC1SZ_CJNrFGFsldM$  for graphic product...

ATTN...WFO...EKA...MFR...STO...

ATTN...RFC...CNRFC...NWRFC...NWC...

LAT...LON   42442432 42422394 42172361 41792363 41342343=20
            40972324 40832293 41082230 40862213 40252285=20
            39652302 39112306 38872348 39072392 39622409=20
            40142453 40582460 41482437 42082453=20

=3D =3D =3D
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