Winter Storm and High Wind in Washington, December 19 to 20, 2016

Area Forecast Discussion...UPDATED
National Weather Service Portland Oregon
453 AM PST Mon Dec 19 2016

.SYNOPSIS...There will be a shift to a warmer and wetter weather
pattern this week as a series of fronts moves through the
region. However, cold air in and near the Columbia River Gorge will
be slow to scour out first thing Monday morning, will see mix of snow
and sleet or freezing rain across parts of the interior lowlands,
especially in the Gorge. With snow levels rising on Monday, all areas
turn to rain, aside from highest peaks in the Cascades. Wednesday
should be a dry day but another front will bring another round of
rain and mountain snow later in the week.
&&

.UPDATE...Temperatures along the lower Columbia River and the
Willamette Valley early this morning are still near or below
freezing as the precipitation is arriving. Light snow accumulations
are being reported in some areas whereas in other areas the snow was
very brief before changing to sleet or freezing rain. Freezing
precipitation in these areas are only expected to last a few hours
before changing to rain. However, with the timing of this wintery
precipitation during the morning commute have decided to expand the
winter weather advisory to include the lower Columbia River, the
Portland and Vancouver metro areas, and the central Willamette
Valley.

Previous SHORT TERM...Today through Wednesday...South winds warmed
temperatures along the coast above freezing right before rain ahead
of a cold front arrived. Some areas along the I-5 corridor along the
Columbia River have warmed just above freezing like near Kelso, but
other areas closer to the Willamette Valley like Scapoose have not.
The Willamette Valley is still entrenched with below freezing
temperatures.

Temperatures should warm within the next couple of hours along the
Columbia River and the Willamette Valley, either right before or
right after the moisture arrives. Therefore...there is still the
possibility that low elevations will see a burst of snow quickly
turning to sleet or freezing rain before all rain. This applies to
the Columbia River Gorge and Hood River Valley too, but these areas
will likely have a longer period (~ 6 hours) transitioning to more of
a liquid form.

Precipitation should reach the Willamette Valley around 4 am and
switch to all rain by 7 am, reach the Columbia River gorge around 6
am switching to rain around 10 am, and the precipitation will reach
the upper Hood River Valley around 8 am and switch to rain around 1
pm. Timing precipitation type change in these type of atmospheric
conditions is very challenging so these times are only a best
estimate. The areas with the highest uncertainty is the gorge east
of Cascade Locks and the Hood River Valley, as these areas tend to
hold on the cold air longer then we expect.

Fortunately, the timing of the heavier precipitation should arrive
just after the low elevation temperatures warm above freezing and
expect only light snow and ice accumulations for the Willamette
Valley.  THe Columbia Gorge and Hood River Valley may have higher
amounts due to the temperatures remaining below freezing longer
there. These areas are well covered by the winter weather products.
Unfortunately, the timing of questionable precipitation type for the
lower elevations falls within the morning commute window, and even
though accumulations are expected to light there is a threat for
challenging travelling conditions during a time of heavier traffic
which could be problematic.

So far the observed QPF is initializing well with the forecast, and
am fairly confident in the forecast rainfall amounts today through
this evening with the coast and coast range seeing 1 to 2 inches of
rain, the north Willamette Valley around 0.80 inch of rain and the
South Willamette Valley around 0.40 inch. Snow levels will rise to
near or above the Cascade passes (~5000 feet) late this afternoon or
early this evening. The Cascades and parts of the Cascade foothills
will see a decent dumping of snow this morning and early this
afternoon before the warmer air arrives and feel these areas are also
well covered in the winter weather product.

Satellite imagery shows the next low and front developing right
behind the current one. The low with this second front will bring a
chance for strong southwest winds to the coast this evening and
tonight and have issued a high wind watch for the entire coast.

Snow levels will remain high as this next front moves across the area
tonight into Tuesday afternoon ending the threat for additional snow,
but increasing the threat for flooding. This front will produce an
additional 1 to 2 inches of rain for the coast, coast range cascades
and Cascade foothills, and 0.5 to 0.75 inch for the interior low
lands from late tonight through Tuesday afternoon. Area rivers will
rise with this additional rain combined with contribution of snow
melt or increased run off, and some rivers may rise above flood
stage. Also can not rule out the possibility for urban and small
stream flooding. TJ

The rest of the short term discussion is from the previous issuance
(936 pm)...Significant snow accumulations will be limited to the
higher peaks of the Cascades during this time. But once the cold
front swings across the region Tue afternoon, snow levels will again
drop to around 3000 to 3500 feet ft. With cool onshore westerly flow,
showers will be dumping another 4 to 8 inches of snow over the
Cascades at that time.

Decreasing showers Tue night, with gradual clearing. High pres will
bring sunshine across region on Wed, though may have morning fog and
low clouds to deal with across the interior lowlands.	 Rockey.

.LONG TERM...No Changes. Previous discussion follows...Wednesday
night through Sunday...The active weather pattern continues through
the long term period. Models continue to show a frontal system moving
across the Pac NW on Thursday which will bring another round of
valley rain and mountain snow, with snow levels lowering to around
2000 ft by Friday morning. Model agreement with the timing of this
front remains consistent, but they continue to vary with its
strength. Moist onshore flow will keep showers over the area on
Friday with snow levels possibly lowering to around 1500 ft. Models
are starting to show a transient upper level ridge moving over the
forecast area on Saturday which could bring a brief period of drier
weather Saturday afternoon/evening. Another low pres system looks to
impact the region on Sunday, but there remain significant differences
with its evolution and subsequent impacts. As such, anyone with
travel plans over the Christmas weekend should monitor
the forecast closely in the coming days as things are likely to
change as models come into better agreement. /64

&&

.AVIATION...Gradually improving conditions in the interior valleys
early this morning, but still widespread IFR cigs. However,
visibility has improved from VLIFR and LIFR to a mix of MVFR and
VFR. Increasing cloud cover and lowering mid-level ceilings in
conjunction with a 5-7 kt SE to S wind has resulted in improving
vis and slow lifting of the stratus base. Precipitation reached
KAST just before 09Z, lowering conditions to MVFR. Precipitation
will spread south and east over the next few hours, initially falling
as snow, sleet or freezing rain in the interior valleys through
18Z. Valley -FZRA to persist longest from KPDX to KKLS and
through the afternoon in the Central Columbia River Gorge.

Overall flight conditions will be a mix of MVFR and IFR after
18Z. Model guidance shows SW wind 35-45 kt around FL020 by 15Z and
continuing through at least 12Z Tue. There is also potential for
LLWS at KPDX and near the Columbia Gorge through this afternoon. In
addition, wind gusts 50-60 kt likely to occur along the coast 04Z-
12Z Tue.

KPDX AND APPROACHES...VFR at 10Z with cigs around FL100. Cigs will
continue to lower over the next few hours, with precipitation
developing around 12Z. Initial precip type likely snow or sleet,
but will shift to a 1-2 hr period of -FZRA and finally rain by 17Z
or so. Areas east of the terminal, in the KTTD vicinity, likely
to see -FZRA through the morning. Cigs will likely lower to 035
around 13Z and then MVFR between 17z and 20z. E-SE surface wind
5-15 kt to continue through the morning. Combine this with 35-45
kt SW wind at FL020 and there is LLWS potential through the
afternoon. Weishaar

&&

.MARINE...Wind speeds over the waters increasing early this
morning as a front drops southeastward across the area. Current
Gale Warnings look good. However, trimmed the duration of gale
gusts as latest models, especially the NAM, RAP, and HI-RESWARW,
all show speeds diminishing after 18Z. Tightest gradient holds
across the the Central waters this afternoon, with gusts generally
25-30 kt. There may be a short period of sub-gale gusts over the
Central Waters this afternoon through early evening, but speeds
ramp up again later in the evening. Will leave the Gale Warning
for the central waters as is, despite a few hours of sub-gale
gusts.

Models continue to differ on the surface low development late
Mon night and early Tue morning. NAM continues to be the weakest,
while the ECMWF is the deepest, but more north. Generally went
with a compromise, with gusts to 45 kt over all waters 06Z-12Z
Tue. Models a little quicker weakening the gradient and lowered
wind speeds for Tue afternoon. High pressure settles over the
waters Tue afternoon and then shifts inland Tue night. Models in
somewhat agreement showing a weak front moving through the waters
Wed night for the potential for small craft advisory wind speeds.
Beyond Thu, models show more divergence. Per OPC went with the
ECMWF Fri-Sun.

Seas will quickly build to around 15 ft across the far north to 11
ft in the far south by 18Z, then remain near 15 ft over all waters
this afternoon and evening. Latest ENP run not quite as bullish
with seas Tue morning, showing 20-22 ft across the S Washington
and N Oregon waters. Seas gradually ease to around 15 ft by Wed
morning and stay there through at least Fri. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 1 PM PST this afternoon for Central
     Columbia River Gorge-Upper Hood River Valley.

     Winter Weather Advisory until 1 PM PST this afternoon for
     Western Columbia River Gorge.

     High Wind Watch from this evening through late tonight for
     Central Oregon Coast-North Oregon Coast.

WA...Winter Storm Warning until 1 PM PST this afternoon for Central
     Columbia River Gorge.

     Winter Weather Advisory until 1 PM PST this afternoon for South
     Washington Cascade Foothills-Western Columbia River Gorge.

     Winter Weather Advisory until 7 PM PST this evening for South
     Washington Cascades.

     High Wind Watch from this evening through late tonight for
     South Washington Coast.

PZ...Gale Watch from this evening through Tuesday morning for
     Coastal Waters from Cape Shoalwater WA to Cascade Head OR
     out 60 nm.

     Gale Warning until 10 AM PST this morning for Coastal Waters
     from Cape Shoalwater WA to Cascade Head OR out 60 nm.

     Small Craft Advisory for winds until 4 AM PST early this
     morning for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

     Gale Warning until 7 AM PST Tuesday for Coastal Waters from
     Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 4 PM
     PST Tuesday.
&&

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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.