045 
AWUS01 KWNH 030953
FFGMPD
CAZ000-ORZ000-032100-

Mesoscale Precipitation Discussion 0011
NWS Weather Prediction Center College Park MD
452 AM EST Mon Jan 03 2022

Areas affected...western/southwestern OR into northwestern CA

Concerning...Heavy rainfall

Valid 030951Z - 032100Z

Summary...Heavy rain will impact the Coastal Ranges of western and
southwestern OR into northwestern CA through 21Z. Localized 12
hour totals of 3 to just over 4 inches can be expected, with the
probability of higher rates lowering as a cold front reaches CA
later today.

Discussion...GOES West water vapor imagery through 09Z showed a
large closed low in the Gulf of Alaska slowly working south. At
the surface, a well defined cold front was analyzed just past
Astoria, OR, draping southwestward into the eastern Pacific.
Strong low level flow was estimated just ahead of the cold front
with RAP analysis data showing south-southwesterly winds at 850 mb
between 80-85 kt along the central to southwestern OR coast, just
ahead of the cold front. While precipitable water values were a
modest 0.6 to 0.8 inches within the moisture plume that was in
place ahead of the cold front, strong low level flow was allowing
for a small but potent plume of IVT near 700 kg/m/s. The locally
increased IVT values within the prefrontal moisture plume were
combined with weak instability (up to 250 J/kg), which has helped
to support observed rainfall rates between 0.7 and 0.9 in/hr in
northwestern OR earlier in the night.

Short term forecasts from the RAP and GFS take the cold front
steadily southward along the West Coast through 21Z. However, the
magnitude of IVT is likely near peak at the current time (09-10Z)
with forecasts showing weakening of the 850 mb flow through 12Z
along the southwestern OR coast. As the southeastern fringes of
the closed low edge closer to the coast, cooling in the 900-600 mb
layer will support shallow CAPE profiles up to 250 J/kg to as far
south as northern Curry County through 18Z. While low level winds
will weaken from their present values, 850 mb winds are still
forecast to be in the 40-50 kt range along the northwestern CA
coast along with some veering toward the WSW by 21Z.

The takeaway message will be the continued potential for 0.5 to
1.0 in/hr rainfall rates translating southward along the OR coast
early this morning, with the potential for 0.5+ in/hr rates
continuing into northwestern CA by early afternoon. Lower snow
levels inland will keep the threat for heavy rain focused to the
Coastal Ranges where 12 hour rainfall potential of 3 to 4 inches
will exist, although a localized 5 inch total cannot be completely
ruled out through 21Z. Localized flooding may result from the
intense rainfall.

Otto

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

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

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

LAT...LON   44712396 44642355 44342350 43702373 43152370 
            42732372 42272356 41702341 41102353 40632351 
            40072401 39902436 40422466 41262456 42872488 
            43822458 44592437 
