Mesoscale Precipitation Discussion 0016
NWS Weather Prediction Center College Park MD
202 AM EST Mon Feb 03 2025

Areas affected...Northern California

Concerning...Heavy rainfall...Flash flooding possible

Valid 030702Z - 031900Z

SUMMARY...Steady rain will continue into northern California today
with localized convergence bands of heavy rain embedded within the
broader atmospheric river. Localized rates over 1 inch per hour
and flash flooding possible.


DISCUSSION...Heavy rain associated with the ongoing atmospheric
river will continue through the day today. Just in the past 24
hours, rainfall amounts locally exceeding 6 inches have fallen
around and just south of the Redding area. This caused flooding in
several areas as noted by the local storm reports. For now the
band that caused that flooding around Redding has diminished in
intensity. However, many of the ingredients that led to the band's
formation remain in place. A stationary front remains in the area
acting as a convergence boundary between the colder air to its
north and the warmer air and southerly flow to its south. A
southerly low level jet from 850 to the surface remains strong
moving up the Sacramento Valley, with winds to 40 kts at 850 and
20 kts as noted by several stations in the Sacramento Valley south
of the area of rain. This low level jet and the orientation of the
terrain are locally enhancing convergence at the northern end of
the Sacramento Valley around Redding, as noted by localized
increased in the radar reflectivities around that area. Meanwhile,
plumes of moisture continue to move into the northern California
coast, where their initial uplift by the northern coastal ranges
are also responsible for heavier rains.

The jet stream remains largely zonal extending hundreds of miles
west out into the Pacific. The upper low to the north and the high
to the south continue to put the squeeze on the moisture plume,
increasing upper level winds and narrowing the moisture into a
100-200 mile wide corridor that is the atmospheric river. During
the day today, the upper low helping direct the jet stream will
retrograde west. This will begin to reorient the atmospheric river
from its current westerly flow to a southwesterly flow. This will
allow the atmospheric river to begin to drift southward down the
coast, while remaining in place over much of interior northern
California. This reorientation may also work to support the low
level jet, as the wind flow becomes more parallel with the
mountains.

HiRes guidance is in agreement that rainfall rates should remain
largely steady through the early morning, though again embedded
heavier bands remain very possible, especially at the northern end
of the Sacramento Valley. Where any bands form, localized flash
flooding in addition to the ongoing areal flooding cannot be ruled
out. A subtle surface low will approach the coast around 18Z,
likely enhancing the rainfall rates area-wide from west to east
and could cause additional flooding into the afternoon. Expect
another 2-4 inches of rain into the northern Sacramento Valley
through 19Z with 1-3 inches of rain expected elsewhere in the
highlighted area.

Wegman

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

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

LAT...LON   41912071 41622024 41282032 40912043 40342055
            39872043 39302035 39182094 39862145 40172184
            40302233 39802263 39442263 39162262 38912281
            38832334 38902386 39432391 39712388 40082424
            40392454 40732441 41102419 41622357 41882317
            41852176