AWUS01 KWNH 251703
FFGMPD
ORZ000-WAZ000-260300-

Mesoscale Precipitation Discussion 1189
NWS Weather Prediction Center College Park MD
1202 PM EST Wed Dec 25 2024

Areas affected...Western WA...Western OR...

Concerning...Heavy rainfall

Valid 251700Z - 260300Z

SUMMARY...Strong deepening dynamics will back winds toward more
favorable orographic ascent by 19-20z and increase rainfall
efficiency to near .5"/hr in favored spots by 00-03z.  Runoff will
likely increase stream flows, but flooding is not expected quite
yet except for the most vulnerable/traditional locations.

DISCUSSION...GOES-W WV suite shows a compact shortwave crossing
the Southeast Gulf of AK at the far southeast edge of the global
scale trof that has been dominating the Bering Sea.  The compact
wave as a solid negative tilt to it associated with a broad strong
polar Pacific Jet with a 150-170kt 250mb streak supporting solid
low level strengthening wind profile with a strong warm front
starting to press eastward with 700-500mb moisture flux & WAA
resulting in lighter showers across W WA and NW OR at this time.=20
Low level winds continue to strengthen with 40-50kts at 850-700mb
per LGX VWP but remaining mainly parallel to the coast, but
increasing reflectivity and polar microwave passes suggest warm
front is approaching quickly and will veer the wind profile toward
21z across W WA and W OR. This will be accompanied by the core of
the sub-tropical moisture stream; with leading 850-700mb moisture
advected on 60-70kts of flow with IVT values increasing from 400
kg/m/s toward 600 kg/m/s.  Upslope component on the SW facing
Olympics and Willapa Hills will see first brunt with .33"
increasing to near, occasionally reaching .5"/hr rates by 00z=20
when winds start to peak near 80kts from the SW and IVT peaks near
850-900 kg/m/s.  Rainfall totals of 1.5-2.5" by 03z, are likely to
fully saturate the remaining capacity of the upper soils.=20
Currently, much of the area is about 70-80% capacity through 40cm
and while a lot, this is above average in the 70th percentile near
the coast but increasing toward 75-80th percentile in the higher
elevations and further inland.  As a result, increased run-off
will be channeled to the streams; and while this is a strong AR,
the duration is not expected to be long to result in any sizable
flooding but increase stream flows in preparation for subsequent
AR surges over the following days.

As the front passes winds will slacken and veer to more due west
and much of NW WA will be out of the core of the AR moisture plume
with .5-.75" PWATs...while shifting into W OR with 1 to nearing
1.25" total PWats.  This will maintain stronger rates in the
coastal range of W OR for a few more hours before the next
wave/lifts northward later on Thursday morning.  Still by 03z,
west facing orography in NW OR will experience 1-2.5" of rain even
to the tops of the peaks, while .5-1" totals are expected in the
lower valleys (less than .25" in traditional shadowed locations.=20
Similarly, a good soaking and setting the stage for a likely
active pattern of AR pulses though the weekend; however flooding
is not likely to be an issue with exception of the most
susceptible/traditionally flooded areas.=20=20=20

Gallina

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!63vZsNSo0SPkto2ZrbQyGX2WJrtCI8bhV8c3R_XIBrhaaKP1qCVuF8-99Dy7psvvqQ-i=
zaXcC8QSeEM7PLM8UuUHVrA$  for graphic product...

ATTN...WFO...MFR...PQR...SEW...

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

LAT...LON   48462444 47932366 47282319 46322307 44672328=20
            43912349 43612382 43492435 43922463 44862442=20
            45692426 46592439 47372467 47862493 48182502=20
            48312497=20

=3D =3D =3D
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