244 
FXUS66 KPQR 312221 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
221 PM PST Tue Dec 31 2019

SYNOPSIS...Rain will spread across the area this evening and
overnight. As a cold front drops southward later tonight, rain
transitions to showers by Wednesday morning. A much cooler airmass
will move over the area on New Year's Day, with showers in the lower 
elevations and snow in the Cascades. Showers decrease on Thursday 
with perhaps a mainly dry period for Thursday night into Friday as a 
warm front lifts north of the region. But, rain and cooler 
temperatures return Friday night and persist through the weekend. 

&&

.SHORT TERM...Tonight through Friday...Radar imagery shows
precipitation shield is slowly shifting southward, fueled by IVT in
excess of 600 kg/ms currently impinging on the north OR/south WA
coast. Models depict this IVT/AR to gradually weaken after 00Z, with
precipitation advancing southward through entire CWA. 

As upper level shortwave moves through the Gulf of Alaska this 
evening and tonight, cold front over the PacNW is pushed southward 
restoring a New Year's Day airmass far more typical of early January.
Snow levels begin a steady fall overnight, and drop to about 3500 
feet by this time tomorrow. With moist westerly flow, sustained 
orographic precipitation appears favorable for the Coast Range and 
Cascades. Given the steep low-level lapse rates and about 100 J/kg 
CAPE, expect to achieve snow advisory amounts between 6-10" at the 
Cascade passes. Will even add a slight chance of thunderstorms to 
the northern grids per SREF guidance; although, confidence is low 
since the convective cloud tops of most member profiles are not 
quite cold enough for electrification. 

Aforementioned cold front lifts back northward as a warm front
Thursday morning, and depending on the timing, the potential for a 
partly-to-mostly dry and mild break from Thursday afternoon into 
Friday appears likely. This break will be short-lived though, as 
another trough restores a cool and showery airmass for the weekend.
DRB

.LONG TERM...Friday night through Tuesday...Model/ensemble consensus
shows a moist, progressive westerly flow during the extended period.
The first shortwave arrives Friday night, followed every 36 to 48
hours thereafter. Freezing levels and precipitation amounts appear
seasonal during this time. Although heavy precipitation is not
expected, snow levels this weekend between 2500-3000 feet should be 
favorable for accumulation above 3000 feet and building the Cascade 
snowpack. This cool and occasionally wet pattern looks to continue 
through at least Tuesday. DRB

&&

.AVIATION...Generally VFR conditions across the area with mixture
of MVFR and IFR conditions along the south Washington and north 
Oregon coast. A warm front continues to slided towards the south
through the evening. Within the postfrontal air mass expect MVFR
conditions to develop inland, and IFR conditions along the 
coast. By 00Z Wednesday much of the area should be MVFR or lower.

Gusty south winds along the coast through the evening, with 
gusts 35-40 kt, with the possibility for isolated gusts up to 45
kt. Inland areas likely to experience 25-30 kt gusts through the
evening. There is a slight chance of thunder starting around 09Z
Wednesday for areas north of KUAO, this slight chance also 
includes the coast north ok KONP.

KPDX AND APPROACHES...Expect VFR to hold through the late
afternoon. Conditions likely to fall into MVFR around 00Z 
Wednesday, with the possibility of IFR after 03Z Wednesday. 
Slight chance of thunder starting around 14Z Wednesday. /42

&&

.MARINE...A warm front will continue slowly moving south through
the evening. South to Southwest wind gusts to 25 to 30 kt through
late this evening, with the potential for some isolated gusts up
to 35 kt. After looking at the most updated guidance as well as
an afternoon scatterometer pass over our waters showing that
winds are slightly weaker than expected, have adjusted the Gale 
Warning currently issued to match better with updated 
information.

The wind backs to west and subside below 35 kt tonight. However,
solid small craft advisory wind speeds will continue through
Wednesday night. Another warm front approaches the waters
Thursday. Operational models are showing some variance into the
weekend. It should be noted that Gale force winds will be 
possible starting Thursday afternoon through at least Friday 
evening, but timing and duration are questionable. 

Seas currently around 11 to 13 ft, but they will build to 14 to 
16 feet this evening and 18 to 20 ft by late Tuesday night.

Latest model guidance continues to show an abnormally long- 
period swell set within our waters. This secondary swell is 
forecasted to be around 4 ft at 25 seconds, but reach 10 ft at 
20 seconds by early Wed morning. The combination of this longer 
period swell and the pre-existing 14-15 second swell will result 
in wave heights around 20 ft or higher. Models show the two 
swell sets eventually becoming a dominant 18-20 second swell by 
late Wed morning. There will be a high likelihood for sneaker 
waves along the coast through at least Wed night. 

There is the potential for another round of 20-ft seas late in 
the week and into the weekend, but there is much lower 
confidence due to the large model discrepancies. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 10 PM this evening to 10 PM PST 
     Wednesday for Central Oregon Coast-North Oregon Coast.

     Winter Weather Advisory from 6 AM Wednesday to midnight PST 
     Wednesday night for Cascades in Lane County-Northern 
     Oregon Cascades.

WA...High Surf Advisory from 10 PM this evening to 10 PM PST 
     Wednesday for South Washington Coast.

     Winter Weather Advisory from 6 AM Wednesday to midnight PST 
     Wednesday night for South Washington Cascades.

PZ...Gale Warning until 7 PM PST this evening for coastal waters 
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Small Craft Advisory until 10 PM PST Wednesday for Coastal 
     waters from Cascade Head to Florence OR out 10 NM-Columbia 
     River Bar-Waters from Cascade Head to Florence OR from 10 
     to 60 NM.

&&

$$ 

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This discussion is for Northwest oregon and Southwest Washington 
from the Cascade crest to 60 nm offshore. This area is commonly 
referred to as the forecast area.