Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 102300
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
259 PM PST Sat Dec 10 2016

.SYNOPSIS...A series of low pressure systems will maintain showery
weather into next week...with temperatures remaining near or below
normal. Snow levels will dip down to the Coast Range and Cascade
Foothills at times. Showers will decrease Monday night. Tuesday and
Wednesday will be dry but very cold as east winds return to the area.
Precipitation including low elevation snow is possible Thursday or
Friday.

&&

.SHORT TERM...Tonight through Tuesday...A series of upper level
shortwave troughs embedded within a broad upper trough will support
showers across the Pacific Northwest through Monday. A couple of
surface lows approaching the Pacific NW will bring fast moving fronts
through the area with brief periods of more organized precipitation.

East winds continue through the Columbia River Gorge late this
afternoon, but winds west of the Columbia River Gorge entrance have
shifted to south or southwest...and temperatures in the Portland
metro area have warmed above freezing. Areas in the Columbia River
Gorge west of Bonneville Dam are beginning to warm above freezing,
but areas east of Bonneville are still below freezing. The first
somewhat organized band of precipitation is currently (2 PM) over the
Willamette Valley, and about to approach the Cascade foothills and
Columbia River Gorge.

Temperatures in the Central Gorge and Hood River Valley may rise
above freezing before this precipitation arrives, but may stay near
freezing with the initial onset, and have a brief period of freezing
rain before. Once the Troutdale to Dalles gradient turns west,
temperatures will rise through the gorge and limit the threat for low
elevation freezing precipitation. Otherwise, snow levels will be be
2500 to 3500 feet.

Isolated thunderstorms are embedded within the offshore showers, and
coastal areas, mainly north of Lincoln City may occasionally hear
thunder or see lightning tonight through Sunday morning. Do not think
any thunderstorms will move inland, but some of the inland showers
could contain small hail.

It is difficult to time the next batches of organized precipitation
(embedded within unstable showers) but estimate the next one will
move across the area late Sunday morning through Sunday afternoon,
and maybe another one Sunday night into Monday morning. All in all,
the interior lower elevations can expect around 0.75 inch of rain
from this evening through Monday afternoon, with around 1.25 inch of
rain for the coast, coast range, and Cascade foothills. The Cascades
could see another 10 to 20 inches of snow during this period. The
heaviest snow is expected late tonight through Sunday morning and is
captured by the current winter weather advisory.

Showers will decrease Monday night as high pressure tries to build
aloft. An upper low dropping south from offshore of Washington will
slow the upper ridge from amplifying over the Pacific NW. Daytime
temperatures for the interior valleys and the coast will peak in the
low to mid 40s Sunday and Monday with the lows in the upper 30s to
low 40s. As the low moves south along the coast Monday night and
Tuesday, cold high pressure moves south into eastern Washington from
Canada and east winds return to the area. The Troutdale to Dalles
east surface pressure gradient will strengthen to around 7 mb by
Tuesday night. TJ

.LONG TERM...Tuesday night through Friday. East winds and clearing
skies Tuesday night will result in very cold conditions Wednesday
morning with widespread temperatures below freezing and perhaps
widespread frost. East winds weaken a bit on Wednesday, but not
enough to warm temperatures. High temperatures on Wednesday will
remain in the 30s with another cold night expected Wednesday night.
The mid-range models diverge on the timing and position of a low
moving north from California, but the GFS forecasts that some
precipitation from this low will move over NW Oregon Thursday
afternoon. The ECMWF suggests that most of the moisture will stay
east of the Cascades except for maybe the Lane County Cascades.  THe
ECMWF suggests an offshore low will bring showers to the area
Thursday night, but the GFS keeps these showers offshore.
Temperatures will be very cold Thursday and Friday and any
precipitation would most likely fall as snow. So yes, there is the
potential for another low elevation snow event late next week, but
the forecast confidence is very low at this point. TJ

&&

.AVIATION...Frontal band extending from near a KTDO to KONP line
at 21Z. Generally VFR along the coast and interior lowlands, with
areas of MVFR and localized IFR. Expect a mix of VFR and MVFR this
through about 12Z Sun, then more MVFR and areas of IFR Sun morning
with a reinforcing shot of steadier precipitation. Passage of the
frontal band will result in a switch to SW wind direction in the
Columbia Gorge this evening. Mountains frequently obscured through
12Z Sun. As cigs lower late tonight and Sun morning higher
terrain, including the Oregon Coast Range and SW Washington
Willapa Hills will become obscured as well.

KPDX AND APPROACHES...VFR at the terminal as of 21Z, but incoming
frontal band will move across the terminal within the next couple
of hours, lowering conditions to MVFR. A mix of MVFR and low-end
VFR 00Z Sun through 12Z. Nearly all available guidance points to
MVFR or worse conditions after 12Z Sun and have trended in that
direction. E-SE wind at the terminal and eastern vicinity to
become S-SW early by early evening. Weishaar.

&&

.MARINE...A frontal boundary has moved through the waters and
indicated by W wind at buoy 029 and other offshore buoys. Some
convective-type elements behind the front as evidenced by brief 35
kt gusts at buoy 029. Small craft advisory wind speeds to persist
through 06Z Mon, with peak gusts around 30 kt. However, may see
some isolated 30-35 kt gusts over the north waters through this
evening.

12Z models show boundary layer wind speeds easing to 15 kt or
less between 06Z and 12Z Mon. Offshore pattern develops late Mon
and is expected to hold through mid to late next week. May
approach small craft advisory speeds Tue-Thu, especially over the
inner waters near coastal gaps.

Seas running 10 to 12 ft and are expected to hold in the 10 to 13
ft range through Sunday evening. A small spike to around 15 ft for
the north waters late tonight. Guidance suggests seas ease under
10 ft Mon morning. Offshore pattern will lower seas to 5 ft or
less late Tue through much of next week. Weishaar.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 2 PM PST Monday for Cascades in
     Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 2 PM PST Monday for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 10 PM PST Sunday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 4 AM PST Monday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 10 AM
     PST Sunday.
&&
$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.



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