AWUS01 KWNH 290426
FFGMPD
CAZ000-ORZ000-291625-

Mesoscale Precipitation Discussion 1204
NWS Weather Prediction Center College Park MD
1125 PM EST Sat Dec 28 2024

Areas affected...Southwest OR...Northern CA

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

Valid 290425Z - 291625Z

SUMMARY...Increasing rainfall rates associated with strengthening
atmospheric river activity will be impacting southwest OR and
northern CA overnight and through early Sunday morning. The wet
antecedent conditions and additional rains will promote concerns
for urban and small stream flooding, including a low-end flash
flood threat for any sensitive burn scar locations.

DISCUSSION...The latest surface analysis shows a warm front across
northern CA and with an offshore extension of this to a developing
surface low near 43N 130W. A cold front then extends well
southwest away from the low center. The low center will be
deepening overnight as it lifts northeastward toward the Pacific
Northwest overnight in conjunction with an amplifying shortwave
trough. The attendant warm front will slowly lift north with time,
but this coupled with deep layer southwest flow should still help
to focus a well-defined atmospheric river into the coastal ranges
of southwest OR and northwest CA.

GOES-W GeoColor satellite imagery shows the southwest extension of
moisture offshore of the West Coast, and the latest CMORPH2 and
NESDIS Blended Rain Rate data shows a pool of heavier rainfall
rates kust offshore in association with the advance of stronger
low to mid-level IVT magnitudes. The IVT values should increase to
750 to 1000 kg/ms/s in the 06Z to 12Z time frame across portions
of Coos, Curry, Del Norte and Humboldt Counties going from north
to south across southwest OR and northwest CA as the core of the
strong low to mid-level flow arrives ahead of the offshore cold
front and the south side of the aforementioned low center.

Enhanced warm air advection, moisture transport and orographic
ascent should favor rainfall rates increase to as much as 0.5" to
1.0"/hour across these coastal ranges. Rainfall rates may also
approach and briefly exceed a 0.50"/hour for some of the adjacent
terrain and counties extending inland into southern parts of the
OR Cascades and also down into the Shasta/Sisikiyou ranges of
northern CA including also eventually parts of the northern Sierra
Nevada early Sunday morning. Some of the heaviest overall rates
should tend to be associated with the final arrival and passage of
a cold front, and there may be some convective elements associated
with the front that will enhance the rainfall rates.

After the cold front passes through, the rates will then quickly
slacken, but additional rainfall totals over the next 6 to 12
hours are forecast to reach as high as 3 to 6+ inches for the
favored coastal ranges, and with as much as 1 to 3 inches
elsewhere. Some of these heavier rains may also reach down the
coast into the Bay Area Sunday morning.

Given the wet antecedent conditions and high streamflows, these
additional rains are likely to increase the concern for areas of
urban and small stream flooding, and there may be at least a
localized flash flood threat should some of these heavier rainfall
rates overlap any of the more sensitive burn scar locations.

Orrison

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!8dOWZUGZOi6H9JYNkcqb2e7NJkXCb7ced17aC56rmajOFfoUdP0hiVS36up0JBynBsbt=
KlEz6OyACTuHTemwR1GXNhc$  for graphic product...

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

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

LAT...LON   44712205 44262132 43742131 43252177 42792210=20
            42532272 42132301 41792271 41522238 41502185=20
            41262169 41012151 40382131 39562038 39022023=20
            38822057 39092111 39682161 40372216 40452237=20
            40322278 39782256 39342255 38702206 38012172=20
            37282169 36972215 37352271 38142330 39372423=20
            40592459 41602465 42482464 43212459 43962399=20
=20=20=20=20=20=20=20=20=20=20=20=20

=3D =3D =3D
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