AWUS01 KWNH 280307
FFGMPD
CAZ000-ORZ000-281300-

Mesoscale Precipitation Discussion 1198
NWS Weather Prediction Center College Park MD
1007 PM EST Fri Dec 27 2024

Areas affected...Southwest OR...Northwest CA

Concerning...Heavy rainfall

Valid 280305Z - 281300Z

SUMMARY...The latest in a series of atmospheric river surges will
be arriving overnight across southwest OR and northwest CA with a
new round of focused heavy rain over the coastal ranges.

DISCUSSION...The GOES-W Airmass RGB satellite imagery shows
another strong shortwave trough gradually approaching the Pacific
Northwest which will be favoring the arrival of a new area of
deepening surface low pressure near Vancouver Island by early
Saturday morning. To the south of this low center evolution, a
renewed deep layer surge of Pacific moisture and associated
atmospheric river activity will be overrunning the coastal ranges
of southwest OR and northwest CA.

Strong low to mid-level westerly Pacific flow overnight will allow
a warm front to approach the region and advance quickly inland
across northern CA, with a trailing cold front that will then
gradually begin to cross the coastal ranges by early Saturday
morning. IVT magnitudes will rise rather sharply over the next
several hours across especially southwest OR and far northwest CA,
including Coos, Curry and Del Norte Counties, with values
increasing to over 750 kg/m/s by 06Z.

Enhanced warm air advection and moisture transport into the
orographically favored terrain will likely result in rainfall
rates approaching and locally exceeding a 0.50"/hour. The 18Z HREF
guidance shows elevated probabilities (50 to 70 percent) of this
occurring with potentially some brief 0.75"/hour rates occurring
as the IVT values peak overnight just ahead of the approaching
offshore cold front.

The IVT magnitudes will then be dropping off in the 09Z to 12Z
time frame which will allow for the rainfall rates and shower
activity to at least diminish early Saturday morning, but the
aforementioned cold front that arrives will also be tending to
slow down which will keep the front in close proximity to the
coastal ranges of northwest CA through Saturday morning.

Additional rainfall amounts going through 12Z of 2 to 4 inches are
likely for especially the coastal ranges of southwest OR and
northwest CA, with isolated heavier amounts. Given the already wet
antecedent conditions and high streamflows, these rains are
expected to foster at least some localized concerns for runoff
problems and flooding.

Orrison

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!5S2qpY0yCgrlh_Xu6QoJd_NKFF2_brL_2Dv6RidVoWup7OYz2RXzF2_zPVJ7mN5N1bJ2=
MNpWRLOtTJjCB_69u5oY1Dw$  for graphic product...

ATTN...WFO...EKA...MFR...PQR...STO...

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

LAT...LON   43822300 43242210 42362196 41802203 41332170=20
            40812165 40012113 39812140 40222251 39862285=20
            39442332 39442397 40012446 40372455 41322425=20
            42042466 42782479 43542448=20

=3D =3D =3D
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