AWUS01 KWNH 260301
FFGMPD
CAZ000-ORZ000-WAZ000-261500-

Mesoscale Precipitation Discussion 1190
NWS Weather Prediction Center College Park MD
1000 PM EST Wed Dec 25 2024

Areas affected...Western WA...Western OR...Far Northwest CA

Concerning...Heavy rainfall

Valid 260300Z - 261500Z

SUMMARY...Atmospheric river activity continues across the Pacific
Northwest with areas of heavy rain expected to persist into early
Thursday morning before weakening.

DISCUSSION...GOES-W Airmass RGB satellite imagery shows a strong
shortwave trough amplifying offshore of the Pacific Northwest, and
this is driving a rapidly deepening area of low pressure near 45N
131W. This approaching low center along with a strong upper-level
jet on the order of 130 to 150+ kts at 250 mb will continue to
drive a well-defined atmospheric river into the Pacific Northwest
overnight with a gradual southward shift of the higher PW axis/IVT
core by early Thursday morning as a cold front gradually arrives
and begins crossing the region.

IVT magnitudes are expected to peak over the next 3 to 6 hours
across the coastal ranges of far southwest WA, western OR and far
northwest CA with values impressively reaching as high as 800 to
1200 kg/m/s. However, the offshore energy and attendant cold front
is quite progressive and thus these enhanced IVT parameters are
not expected to persist for too long. The IVT values are forecast
by the 18Z GFS and 12Z ECMWF to steadily weaken by 12Z as the cold
front advances inland along with the corridor of stronger low to
mid-level flow.

Rainfall rates overnight are forecast to generally be on the order
of 0.25" to 0.50"/hour with occasional 0.50"+ rates expected for
especially the southwest facing slopes of the coastal terrain. The
most recent HREF guidance suggests portions of southwest OR and
far northwest CA will generally have the highest probabilities of
seeing these rates, with peak rates potentially here reaching as
high as 0.75"/hour as the cold front arrives later in the night.

By early Thursday morning, the rates are forecast to be weakening
overall, but there will be some post-frontal instability and
persistence of onshore flow to keep shower activity going that may
foster some additional brief heavy rainfall rates. A consensus of
the latest HREF guidance supports additional rainfall totals on
the order of 2 to 4 inches going through early Thursday morning
(15Z/7AM PST) across the orographically favored higher terrain,
and especially the coastal ranges.

Orrison

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!8M49YNk1dZh0T4I6mLeXxA08cPYpvQHh3Td323rWZpzqOpGY-HOXXE4-RDpq1HIAuSTx=
yjBfKQ0ENUv6OEEPMdRPsBE$  for graphic product...

ATTN...WFO...EKA...MFR...PDT...PQR...SEW...

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

LAT...LON   47872325 47542211 46972182 46442176 45442171=20
            44512185 43832202 43142230 42212222 41632232=20
            41052297 40082311 39822346 40102428 41142453=20
            42522470 44752434 46162436 47322425=20

=3D =3D =3D
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