730
AWUS01 KWNH 191938
FFGMPD
WAZ000-200400-

Mesoscale Precipitation Discussion 1105
NWS Weather Prediction Center College Park MD
338 PM EDT Sat Oct 19 2024

Areas affected...Olympic Peninsula into the northern Cascades

Concerning...Heavy rainfall

Valid 191936Z - 200400Z

Summary...An ongoing atmospheric river is expected to produce an
additional 2-4 inches over the upslope regions of the Olympic
Peninsula and northern Cascades through 04Z. Hourly rainfall rates
in excess of 0.5 inches will be possible for several hours (mainly
the Olympics) with 36 hour rainfall totals of 4-8 inches.

Discussion...GOES West water vapor imagery showed an elongated
fetch of mid-upper level moisture streaming northeastward, ahead
of a shortwave trough, located near 44N 150W, into the West Coast
from southern Vancouver Island into northwestern WA. Layered PW
imagery showed this atmospheric river contained high moisture
content throughout the vertical column and Blended TPW data
indicated the moisture values within the plume were 150 to 200+
percent of normal. Moisture flux anomalies have weakened since 00Z
when the moisture axis was directed into southern Vancouver
Island, which has received over 10 inches of rain (preliminarily)
according to information from the NWS Weather and Hazards Data
Viewer, but high rainfall rates remained within the axis of
strongest moisture transport where peak precipitable water values
were near 1.5 inches along the coast. Recent observations across
the western Olympic Peninsula showed hourly rainfall between 0.3
and 0.5 inches, but with localized hourly totals in excess of 0.5
inches across western Clallam and Jefferson counties.

19Z visible satellite imagery and surface observations depicted a
cold front extending southwestward from north-central Vancouver
Island into the eastern Pacific. The cold front is forecast to
continue slowly moving toward the south and east over the next 6-9
hours, with continued hourly rainfall over 0.5 inches across the
western Olympic Peninsula through at least 00Z.

2 to 4+ inches of rain has been reported across the upslope
regions of the Olympics and northern Cascades over the past 24
hours. Additional rainfall totals of 2 to 4 inches are expected
though about 04Z, with rainfall intensity lowering between 00-03Z
as the moisture axis sinks south with the cold front and weakens.
This weakening will be in advance of the upstream shortwave near
150W, which is expected to re-align the offshore moisture axis in
a more south to north orientation, ahead of a forecast surface low
which is expected to approach northwestern WA after 12Z Sunday.

Otto

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...OTX...SEW...

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

LAT...LON   49002159 48952134 48882120 48782109 48642104
            48452102 48252108 48062115 47922120 47782128
            47632142 47542158 47572178 47732192 47962197
            48122204 48212222 48272240 48322257 48312279
            48302297 48172322 47942328 47772331 47522329
            47442332 47272340 47142381 47142429 47852471
            48312470 48432438 48622323 48992208