354
AWUS01 KWNH 122128
FFGMPD
ORZ000-WAZ000-130900-

Mesoscale Precipitation Discussion 1151
NWS Weather Prediction Center College Park MD
427 PM EST Tue Nov 12 2024

Areas affected...western WA

Concerning...Heavy rainfall

Valid 122123Z - 130900Z

Summary...Areas of heavy rain will move into western WA this
evening and overnight with occasional rainfall rates in excess of
0.5 in/hr, especially across the Olympics. Peak 12 hour rainfall
totals of 2 to 4 inches are expected across the favored upslope
regions of the Olympics and Willapa Hills.

Discussion...GOES West water vapor imagery at 21Z showed an upper
level trough centered about 900 miles west of the Pacific
Northwest coast, containing embedded shortwaves. One of the more
notable shortwaves/vorticity maxima was reflected in the visible
imagery with a well-defined swirl near 45N 143W. Surface and
visible satellite data identified an occluded cyclone extending
outward from a surface low near 50N 140W and widespread cloud
cover extending downstream from the associated IVT plume ahead of
the cold front.

As the upper trough and embedded vorticity maxima continue to
advance eastward through early tonight, a triple point low is
forecast to develop by a consensus of the latest numerical
guidance by 00Z just southwest of 50N 130W with northward movement
overnight. The attendant occluded/cold front will likely remain
progressive toward the south and east, reaching the Olympic
Peninsula in the 06-09Z time frame along with the maximum IVT,
forecast by the 12Z model consensus to near 800 kg/m/s along the
coast. 850 mb winds are expected to peak into the 70-80 kt range
from the SSW within precipitable water values of 1.0 to 1.1 along
the WA coast.

Rainfall intensity will gradually increase in the 00-06Z window
with peak rainfall rates possibly exceeding 0.5 in/hr in the
Olympics prior to 06Z (40-50 percent via 12Z HREF, but adjusted
for the 18Z HRRR and 18Z NAM_nest). Those probabilities of 0.5+
in/hr rates increase to just over 80 percent by 09Z for the
Olympics. The low level moisture transport will favor the highest
rates over SSW facing terrain, including the Willipa Hills,
although the probabilities for 0.5 in/hr in southwestern WA are
generally less than 10 percent for the overnight period via HREF
output. While rainfall intensity will be increasing overnight, the
greatest coverage of higher intensity rainfall will likely come
with the arrival of the frontal boundary though limited
instability may temper maximum rates a bit. Nonetheless, strong
forcing for ascent ahead of the approaching upper trough axis,
including increasingly diffluent flow aloft, should allow for
localized 12 hour rainfall maxima of 2-4 inches across the favored
terrain of western WA through 09Z. These rains are expected to
increase the potential for minor flooding across the region, but
mainly focused into the Olympics.

Otto

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

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

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

LAT...LON   48132389 48022339 47862310 47662299 46932314
            46282296 46232337 46262403 46472435 46802458
            47242477 47682501 48112467