AWUS01 KWNH 020701
FFGMPD
NVZ000-CAZ000-021900-

Mesoscale Precipitation Discussion 0012
NWS Weather Prediction Center College Park MD
200 AM EST Sun Feb 02 2025

Areas affected...Northern California

Concerning...Heavy rainfall

Valid 020700Z - 021900Z

SUMMARY...An atmospheric river aimed at northern California will
continue to produce heavy rain with rates between 1/4 and 3/4
inches per hour possible.

DISCUSSION...An atmospheric river aimed at northern California
will continue to advect tropical moisture from near Hawaii
northeastward into the state today. The moisture has a tropical
connection due to an expansive area of high pressure located off
southern California joining forces with a cold core low off
Vancouver Island to squeeze the moisture into a relatively narrow
corridor (atmospheric river). PWATs between 1 and 1.25 inches are
associated with the moisture plume. This is 4 sigma above the
climatological normal as well as within the 90th to 95th
percentile compared to a 30-year average of PWATs for the area.
IVT values from CW3E GFS and EC guidance are around 800 kg/ms.
Over the past 24 hours, Anywhere from 1 to 4 inches of rain have
fallen across northern California, with the highest values in the
Sierras around and west of the Lake Tahoe region. NASA Sport soil
moisture imagery shows much of the surface soil layer has
saturated significantly compared to 24 hours ago, so most of the
rain that falls from here should convert to runoff.

Radar imagery shows a plume of rainfall occurring over much of far
northern California with rates generally between 1/10 and 1/4 inch
per hour ongoing. These rates are confirmed by the numerous
weather stations set up across northern California. A stationary
front draped over northern California (not shown) delineates the
remarkable surface convergence occurring within this atmospheric
river, with winds off the coast of Crescent City 20 kts out of the
north, and winds both off the coast of Mendocino as well as up the
northern Sacramento Valley are 20 kts out of the south. This
remarkable convergence is adding to the overall lift within the
atmosphere along the front, with almost all of the rain occurring
north of the front.

As a potent upper level shortwave approaches the coast over the
next 12 hours, HiRes models are in good agreement as to a slight
increase in rainfall rates in far northern California over the
next 4 hours or so, followed by a gradual southward drift of the
rainfall plume towards Point Arena and the northern Sacramento
Valley. Rainfall rates may locally increase to as high as 3/4
inches per hour. Along the coast, rates should diminish through
the predawn hours, with the rainfall tapering from north to south
through the morning. Expect 3-6 inches of rain for much of the
northern Sierras through 19Z, 2-4 inches of rain for the northern
Coastal Ranges, and lesser amounts within the Sacramento Valley
along the I-5 corridor and to the immediate lee (east) of the
coastal ranges.

Mudslides and rock slides are possible in the foothills and
mountains below 6,500 feet where the precipitation remains all
rain. Localized flooding is possible in the areas where the
heaviest rain occurs over flood-sensitive and low-lying areas.

Wegman

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

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

LAT...LON 41902181 41862023 41252007 39932011 39391980
38501957 38312007 38352115 39352158 40282212
40012270 39472270 38732243 38352235 38622251
38122328 38932398 39432394 40012417 40472452
40982421 41562316
$$
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