Heavy Snow and High Wind in Oregon, December 19 to 20, 2017

Area Forecast Discussion
National Weather Service Portland OR
322 AM PST Tue Dec 19 2017

.SYNOPSIS...A strong cold front was moving inside 130W early this
morning and will move across southwest Washington and northwest
Oregon today for periods of heavy rain across the forecast area, and
quite windy conditions along the coast with windy conditions inland
as well. Snow levels will lower with the front this afternoon and
tonight with heavy snow in the mountains and also the foothills.
Showers will spread in behind the front tonight with thunder along
the coast and snow continuing in the higher terrain. Showers will
decrease Wednesday but continue into the afternoon. Expect dry
weather Wednesday night and Thursday that may continue into Friday,
though there is a small chance of rain and snow showers from a weak
disturbance Friday. Expect more dry weather with another round of
east winds and colder temperatures this weekend.
&&

.SHORT TERM...Today through Thursday...A strong cold front was out
near 130W early this morning and is forecast to move through
southwest Washington and northwest Oregon today for increasing rain,
heavy at times, with snow levels lowering dramatically this afternoon
and evening in the Cascades, the foothills, and the coast range.

This front will also produce rather strong winds along the coast and
windy conditions inland as well. Gusts at the coast, especially near
the headlands and beaches could get close to high wind criteria, with
a bit less in the coastal communities. The risk of high wind is
marginal and will stay just below high wind criteria in the forecast,
but will monitor closely. Winds in the valleys could reach 35 mph or
so.

The wind shift line and associated frontal trough is near the leading
edge of the precipitation band, with the main precipitation sloping
back into the advancing colder air, which is what we call an
anafront. This will allow some of the heavier precipitation in the
mountains to fall as snow before the frontal band moves through.
Behind the front colder air and showers in onshore flow will spread
in tonight and continue the heavy snow in the higher terrain, with
snow levels lowering to around 1500 feet.

The heavy precipitation with this front is due to the strong
dynamics/thermodynamics associated with this system, as well as the
tropical tap of moisture out near Hawaii. Precipitable water values
are in the 1-1.5 inch range, and are being transported by 850 mb
winds in the 50-60 kt range. The rather quick movement of the front
through the forecast area will limit any risk of flooding.
Precipitation values today and tonight at the coast and in the
mountains could reach 1.5-3 inches, in the valleys should be around
an inch or so.

After a few residual showers Wednesday evening near the higher
terrain, things clear out and will be dry Wednesday night through
Thursday. There could be areas of lower elevations fog or freezing
fog Wednesday night and Thursday as the colder post frontal air mass
promotes temperatures falling into the 20s Wednesday night under good
radiational conditions with a damp ground.

Surface pressure gradients are rather light Thursday so any fog and
freezing fog will be slow to lift and clear especially inland. This
could keep afternoon highs Thursday from reaching 40 degrees,
especially in the inland valleys. Tolleson

.LONG TERM...No Changes. Previous discussion follows...
Thursday night through Monday...Models are in general agreement that
longwave ridging will return to the west coast of North America
during the extended forecast period. This will produce cool upper
level northerly flow and mainly dry weather across the Pacific
Northwest. Easterly winds will also strengthen into at least
obnoxious levels across the western Columbia River Gorge and east PDX
metro during this time.

The main forecast challenge remains assessing how much cold air will
make it into the Pacific Northwest over the weekend. The EC remains
on the cold side while the GFS and many of its ensembles members
suggest a milder scenario similar to last week. Given the
considerable uncertainty, opted to trend the forecast towards a blend
of models with the disclaimer the forecast could easily end up 5-7F
warmer or colder than the current forecast suggests over the weekend.
Anecdotally, it seems models tend to be too aggressive bringing the
cold air into the region, both in terms of timing and intensity (i.e.
coldness). As a result, tried to also trend the forecast in a
direction that took this at least perceived bias into account.

It should be noted that the colder model solutions such as the EC and
GEM bring the upper level trough further westward on Friday. This
would support a stronger low level front dropping southward out of
British Columbia that could wring out a few showers west of the
Cascades before the colder and drier air moves into the region. Snow
levels will be low enough that just about anyone could see a light
rain/snow shower, but any precipitation it appears would be quite
light, so few impacts are anticipated at this time. /Neuman
&&

.AVIATION...Cold front fast approaching the coast early this
morning. Expect FROPA with a sharp windshift at the coast
between 16Z and 19Z and at inland TAF sites a few hours later as
the front moves south and east. The coast will remain a mix of
MVFR/IFR with MVFR conditions for the interior, but local IFR is
possible too. The front will move into the Cascades early
afternoon. There will be showers lingering behind the front
through the evening, but expect conditions will gradually improve
to predominantly VFR Tue evening in the more unstable post-
frontal airmass. There is a chance of thunderstorms along the
coast this afternoon and evening.

The cold front will bring some gusty southerly winds to the
region Tue morning. From 12-18z, gusts 35-45 kt possible at the
coast with inland gusts 25 to 35 kt.

PDX AND APPROACHES...Occasional MVFR and rain with possible IFR
conditions as the front approaches. FROPA is expected around 18Z
with showers and generally improving conditions with VFR
prevailing by 00Z if not sooner. Frontal passage will bring a
quick wind speed and direction change.
&&

.MARINE...A cold front is approaching the coast and is expected
to move ashore 7 and 10 AM this morning. Solid gales are will
occur ahead of the front. Frontal passage will be marked be sharp
windshift to the west or northwest. The parent low that is
associated with the front is 1004 MB as of 2 AM and located of
the central WA coast. The low is modeled to strengthen as it
comes onshore, making landfall around 995-997 mb. Feel good about
retaining the higher end gales and the associated warning with
winds peaking 40-45 kt and strongest across the central Oregon
coastal waters. The front will progress southward and past
Florence out of our waters by early afternoon. Seas appear to top
out at around 15 ft this morning with this system.

A secondary low is modeled to swing down tonight and Wednesday,
bringing NW post frontal winds on the gusty side and likely
warranting a trailing SCA for winds and seas. Winds decrease
later Wednesday and begin turning offshore through Friday as
strong high pressure builds inland. Will likely need additional
advisories for those easterly winds. Seas subside below 10 ft for
Thursday and beyond. /mh
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 4 PM this afternoon to 7 AM PST
     Wednesday for Cascade Foothills in Lane County-Northern
     Oregon Cascade Foothills.

     Winter Weather Advisory from 4 PM this afternoon to 7 AM PST
     Wednesday for Central Coast Range of Western Oregon-Coast
     Range of Northwest Oregon.

     Winter Storm Warning from 6 AM this morning to 6 AM PST
     Wednesday for Northern Oregon Cascades.

     Winter Storm Warning from noon today to 6 AM PST Wednesday for
     Cascades in Lane County.

WA...Winter Storm Warning from 4 PM this afternoon to 7 AM PST
     Wednesday for South Washington Cascade Foothills.

     Winter Storm Warning until midnight PST tonight for South
     Washington Cascades.

PZ...Gale Warning until 11 AM PST this morning for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 NM.

     Gale Warning until 1 PM PST this afternoon for Coastal Waters
     from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory for Rough Columbia River Bar until 7 AM
     PST this morning.

     Small Craft Advisory for Rough Columbia River Bar from 11 AM
     this morning to 9 PM PST this evening.

&&

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