AWUS01 KWNH 201309
FFGMPD
CAZ000-ORZ000-202100-

Mesoscale Precipitation Discussion 1160
NWS Weather Prediction Center College Park MD
806 AM EST Wed Nov 20 2024

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

Concerning...Heavy rainfall

Valid 201300Z - 202100Z

SUMMARY...Solid, persistent Atmospheric River to continue
throughout the morning into afternoon. Typical .25-.33"/hr rates
may occasionally tick up to .5" for an hour or so occasionally
with localized embedded pulses across Southwest facing terrain.
Solid 2-4" for terrain with 1-2" for lower slopes/valleys by 21z.

DISCUSSION...GOES-W SWIR and regional RADAR mosaic, shows the back
edge of the atmospheric river nearing the OR/CA boarder as the low
to mid-level drying under the core of the upper-level jet
continues to press the occluded front across W WA through to the
SW OR coast. The triple point appears in a traditional location
just north of Cape Mendocino before it jumps south as the front is
expected to reach the CAPE over the next few hours. While the
near record deep cyclone continues to slowly wobble north, there
remains still a bit of height-falls to allow for the front to
continue a slow sag southward. Well upstream, a subtle shortwave
near 35N and 140W is starting to amplify with downstream
baroclinic leaf forming between 140 and 130W; but that is well
away off, but will likely start to have some influence with
localized shortwave ridging and finally stall the southward
progression of the front, likely between 21-00z.

Southward drift relative to the coast remains a few miles an hour,
the core of 50-65kts of slightly cyclonic 850mb flow into
southwest facing orography will shift from S Humbolt county
currently, through Mendocino, likely reaching Sonoma and far
northern Marin county by 18-21z. With exiting of stronger
height-falls, winds will tapper but only slightly and remain at
50kts in the 850mb layer. CIRA LPW and RAP analysis show core of
highest sfc-700mb is now nosing into the coast and rain-rates are
likely to increase from the .25-.33" to .5"/hr at that nose for a
few hours before slipping south (with lingering .25-.5"). HREF
.5"/hr probability remains well above 75%, with a secondary maxima
along the lower slopes of the northern Sierra Nevada Range. The
strength of the WAA will likely keep freezing levels lifting up
from 2000 to 5000 ft throughout the day. Allowing the lower
slopes to see similar rainfall rates/impacts nearing 2-4" by 21z.
Valleys will obviously see much less, but still beneficial 1-2"
are probable (with exception of directly in rain-shadow of the SW
facing peaks).

As the AR plume, reaches its southward extent in the 18-00z time
frame, winds will still further slack, but PW will be increasing
as deep layer flow becomes unidirectional and each layer
contributes to near 1.5" (still well offshore); basically, as the
strength of flow decreases it will be bolstered by increased total
moisture and so rates are likely to remain fairly steady
throughout with only those localized embedded pulses reaching
above .5"/hr. While the soils continue to further saturate, with
increasing run-off as it does so; overall rates still do not
likely reach flash flooding concerns quite yet, but localized
slower rise flooding and nuisance/urban flooding will remain probable.

Gallina

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

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

LAT...LON 43012444 42922411 42582386 42142365 41592353
41102336 41032265 40992258 40992209 40622192
40152181 39602136 39232098 38702111 38332155
37832268 38392350 39202397 39972440 40582462
41222436 41822441 42262459 42652467 42972457


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