Winter Storm and Heavy Snow in Washington, December 24 to 25, 2017

Area Forecast Discussion
National Weather Service Portland OR
930 AM PST Sun Dec 24 2017

.SYNOPSIS...Moisture streaming off the Pacific will overrun the cold
air currently in place to bring a mix of snow, sleet, freezing rain,
and just a plain old cold rain to the area today and tonight.

&&

.SHORT TERM...Tonight through Wednesday...Forecast this morning
continues to track as expected, although details in the precipitation
type and its evolution remains uncertain. Have received several
reports of sleet in the PDX metro, associated with the warm
front/isentropic lift moving northward. Although precipitation will
need to overcome drier air at low levels moving westward through
Columbia River Gorge, models indicate ascent/upward motion becomes
slightly more vigorous over northwest Oregon. Expect this band to
continue to produce mostly ice pellets and light freezing rain as it
moves northward, mixing with snow above 1000 feet, closer to the
Gorge, and under heavier bands. Surface low and associated cold
front moves onshore and into the Valleys between 00-03Z this
evening. Some models such as the HRRR are still developing secondary
low on the front farther south close to Newport, but consensus is
for the main low to arrive on shore between AST and HQM.The farther
north track is more conducive to a period of freezing rain in the
metro, probably snow east metro eastward, and transitioning to all
rain between Wilsonville and Salem southward. A more southern track
would push the freezing rain/rain transition farther south, and
perhaps change to more snow throughout the metro. For now though,
hedging toward model consensus, mixed precipitation types, and the
more northern track as depicted in current forecast. The remainder
of the discussion is unchanged. Bright

The KTTD-KDLS pressure gradient is quickly approaching 10mb with wind
gusts of 30 to 40 mph occurring across the Portland metro early this
morning. Mostly clear skies east of the Cascades overnight have
allowed temperatures to plummet into the teens and single digits
across the Columbia Basin. While this cold and dry air is being
modified as it moves westward through the Columbia River Gorge,
temperatures have  dropped into the low to mid 30s for most
Willamette Valley  locations. Just as importantly, dewpoints have
dropped into the low  to mid 20s, particularly across the Portland
metro area. As the  lower atmosphere moistens today, this should
result in the  temperatures wet bulbing into the 30-32F range even
though they will  likely hover in the 33-37F range beforehand, and
set the stage for a  wintry mix, particularly freezing rain.

Infrared satellite imagery shows plenty of cooling cloud tops
spreading across northwest Oregon and southwest Washington early this
morning. Doppler radar mosaic is beginning to pick up on light
returns as well, but so far, the bulk of this precipitation has been
in the form of virga. This precipitation is in response to an
elevated warm front around 750-650mb that will move northeastward
across the area today. Global models have been fairly adamant that
this warm front will intensify and spread 0.05-0.10" of
precipitation northeastward across the area today. However, some of
the higher resolution models suggest less QPF, and in fact suggest
most of the CWA could be dry, particularly south of the Columbia
River this afternoon. Given there is a fair amount of dry air to
overcome and ceilings are expected to remain above 6-8kft with this
first round of precipitation, suspect there is some legitimacy to
this idea.  Model soundings suggest any precipitation that falls
this morning could be snow and sleet for areas closer to the Gorge
and freezing rain for areas in the Coast Range and southern half of
the Willamette Valley. Temperatures in the southern half of the
Willamette Valley should moderate enough today that the freezing
rain threat appears short lived.

Deeper moisture associated with the front currently offshore should
have an easier time allowing precipitation to spread into the area
this evening. By this time, most locations should be above freezing
except across the Columbia River Gorge, portions of the Portland
metro and possibly portions of the lower Columbia River Valley. While
temperatures aloft will support snow in the Columbia River Gorge
today and tonight, areas farther west towards the Portland metro
will likely experience a period of freezing rain. Confidence is
highest that below freezing temperatures and rain will coincide
across the east PDX metro and West Hills, but certainly other
locations in the metro and lower Columbia River valley could see
freezing rain linger well into the evening hours. Given temperatures
will be close to freezing, not sure there will be much ice
accumulation on pavement away from the far east metro, but exposed
objects like elevated walkways, trees, etc. should accumulate at
least a tenth or two of ice. As low pressure shifts inland
overnight, east winds should weaken in the Columbia River Gorge,
which should allow temperatures to slowly moderate from west to
east. At this time, it looks like areas near Troutdale may not warm
above freezing until late Sunday night and likely much later on
Monday for areas closer to Hood River.

It should be noted there is one kink in the forecast and that still
relates to the track of the low pressure currently found offshore.
Models generally agree it will come ashore near the mouth of the
Columbia River late this evening. This will help warm air aloft to
spread into the region and create enough of a south-north pressure
gradient to allow weak southerly winds to warm up most of the
Willamette Valley. However, a model or two, and it seems to vary
from one model run to the next, suggest the dominant low pressure
could move towards Newport. This would help maintain colder air
aloft over the northern Willamette and lower Columbia River valleys
and keep precipitation more in the form of snow and/or sleet vs
freezing rain this afternoon and evening. It would also likely slow
the warm up expected to occur tonight across northwest Oregon and
southwest Washington. Again, most model guidance does not suggest
this scenario, but it does appear to be a small possibility.
We don't have a lot of observations to track the validity of this
possible solution, but pressure falls continue to be greatest along
the coast from KAST to KUIL so suspect the more northward low
pressure track suggested by most models will end up being reality.

Otherwise, precipitation should wind down across the area late
tonight and Monday before conditions dry out across the entire CWA
Monday night. Temperatures look to cool below freezing for many
locations Monday night. With models suggesting the next warm front
spreading precipitation into the area Tuesday, snow levels will
likely start off at least modestly low, particularly near the Gorge
so will need to monitor that situation closely once we get past the
next 24 hours. /Neuman

.LONG TERM...Tuesday Night through Saturday...A Pacific front brings
a valley rain and mountain snow on Wed. The ECMWF would suggest the
possibility of another lowland snow/freezing precip event. Models
continue to exhibit differences beyond Wed with GFS being more
amplified, taking frontal systems to the north, while EC has a more
zonal, progressive pattern into Fri, however both models show some
precipitation being brought into the area. Which should bring some
rain into lower elevations, and snow into the Cascades. Sat the
models start drying out the area. GFS is a little faster than the EC,
in this respect. Bishop

&&

.AVIATION...Weak frontal band near a KONP to KSLE line at 17Z
moving northeast. Flight conditions primarily VFR, but still some
areas of MVFR cigs between KEUG and KRBG. Have had reports of
-FZRA in the central and south interior valleys. Expect areas of MVFR
as this band moves northeast. Areas near the Columbia Gorge to remain
VFR through about 00Z Mon with east wind maintaining drier lower and
mid-levels. Primary precip associated with the incoming cold front
expected to reach the coast around 21Z and to the interior valleys by
00Z Mon. This will bring increasing MVFR to the area and a mix of
precip types. Precip south of KSLE likely to be mainly rain. Precip
north of KSLE to see a mix of -FZRA, sleet or even snow initially,
but gradually shift to rain. Areas near the Columbia Gorge will
remain snow or freezing through 12Z Mon. LLWS will also be a
potential for KPDX and near the Gorge after 00Z with increasing S-SW
wind just above the E-SE surface wind.

PDX AND APPROACHES...VFR through 00Z Mon. There may be some -SN
through 20Z, which may briefly lower vis to 5SM. Precip with the
cold front expected to reach the terminal around 00Z Mon. High
potential for -FZRA, sleet or snow 00Z through 03Z or so, then a
transition to -RA. Increasing chance of MVFR conditions after 00Z
Mon. East wind 15-25 kt with gusts up to 30 kt at the terminal
through the afternoon, but stronger to the east near the west end
of the Gorge. High potential for LLWS at the terminal late this
afternoon with increasing S-SW wind above the cold air layer as
the front moves through the area. Weishaar.

&&

.MARINE...Minimal change to current forecast. Gusty offshore
wind will maintain 20 to 25 kt gusts over the northern waters
through the morning. A cold front will move across the coastal
waters this afternoon and inland towards evening. This will
result in a shift to south to southwest wind, followed by another
shift to NW this evening. Current small craft advisory for wind
looks good.

Seas 4 to 6 ft this morning, but will build as a fetch of
longer-period seas reach the waters. Seas will push just above 10
ft by late this afternoon. Seas hold at 10 to 12 ft tonight into
early Mon, then slowly subside Mon afternoon and Tue night.
Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 10 AM PST Monday for Central
     Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

     Winter Weather Advisory until 4 PM PST this afternoon for
     Central Willamette Valley-Coast Range of Northwest Oregon-
     Lower Columbia-Northern Oregon Cascade Foothills-South
     Willamette Valley.

     Winter Weather Advisory until 10 PM PST this evening for
     Greater Portland Metro Area.

WA...Winter Storm Warning until 10 AM PST Monday for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Winter Weather Advisory until 4 PM PST this afternoon for I-5
     Corridor in Cowlitz County.

     Winter Weather Advisory until 10 PM PST this evening for
     Greater Vancouver Area-Willapa Hills.

     Winter Weather Advisory until 4 AM PST Monday for South
     Washington Cascades.

     Winter Storm Warning until 4 AM PST Monday for South Washington
     Cascade Foothills.

PZ...Small Craft Advisory for winds until 4 AM PST Monday for
     Coastal Waters from Cape Shoalwater WA to Cascade Head OR
     out 60 NM.

     Small Craft Advisory for hazardous seas from noon today to 6 PM
     PST Monday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 NM.

     Small Craft Advisory for winds until 4 AM PST Monday for
     Coastal Waters from Cascade Head OR to Florence OR out 60
     NM.

     Small Craft Advisory for Rough Columbia River Bar from 6 PM to
     10 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.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE PORTLAND OR
841 AM PST SUN DEC 24 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0840 AM     ICE STORM        1 SW LEWISBURG          44.62N 123.25W
12/24/2017                   BENTON             OR   TRAINED SPOTTER

            0.03" OF ICE ACCUMULATION ON NON-ROAD
            SURFACES.

&&

JP