Winter Storm in Washington, December 27 to 28, 2017

Area Forecast Discussion
National Weather Service Seattle WA
945 AM PST Wed Dec 27 2017

.SYNOPSIS...A weak system will move through Western Washington
today and tonight with mostly lowland rain or a mix. In Western
Whatcom county, colder northeast flow will result in snow and
freezing rain through Thursday morning. Snow and ice will accumulate
before changing to rain Thursday afternoon. Two fronts will move
through the area in quick succession Thursday afternoon and Friday,
for rainy weather with the snow level rising to around 5000 feet. An
upper ridge will bring drier weather this weekend, which could
continue into early next week.

&&

.SHORT TERM...Changes are being made to the near term forecast,
mainly for Whatcom county. Temperatures there are still below
freezing and dew points are in the 20s with light northeast flow.
Models show a weak warm frontal system moving through the area today
and tonight. Little in the way of rain or mixed precipitation is
expected today. However, warm advection rain should increase tonight
into Thursday ahead of the next system. Northeasterly gradient from
BLI-YWL increases and peaks near -13 by 12z Thursday morning. This
should keep the freezing temperatures in place at the surface over
Whatcom county through Thursday morning, with rising snow levels
elsewhere. Models vary on perception amounts tonight but 1 to 3
inches are possible looking over the average of all the models. A
transition to freezing rain is likely in Wrn Whatcom Thursday
morning. The 4km WRF is showing snow accumulation but other models
show warming just above the surface with westerly flow at 925/850MB
this should cause a switch over to freezing rain with ice
accumulation of roughly a quarter inch by noon. Ice amounts will be
lighter near the coastline and possibly little higher further
inland toward Lynden and Sumas. Confidence is fairly high that a
period of accumulating snow and freezing will occur, but timing of
this transition is somewhat uncertain. In addition, freezing rain
could last longer into the afternoon, especially close to the
Canadian border. For most areas in Wrn Whatcom, temperatures should
rise above freezing during the afternoon as a weakening surface low
arrives from the west.

Some light snow will fall over the Cascades through tonight but
accumulations look to be less than a few inches. More significant
snow is expected Thursday into Friday as a couple systems move
through. Snow levels fall mostly as mostly snow in the passes due to
the increasing easterly gradient. Will need to watch for possible
freezing rain in some of the lower passes or entrances. At this
point, advisory level snow amounts look most likely Thursday into
Thursday night.

Another strong low approaches Friday with rain increasing once
again. Snow may be heavy in the mountains. A watch may be required
for this system. The lowlands should be fairly well mixed out, so
lowland snow does not appear to be a threat at this time. Mercer

.LONG TERM...The post-frontal trough over Western Washington in
the wake of the Friday system will exit quickly to the east, and
an offshore upper ridge will build and progress across the area
Sunday and Sunday night. Aside from some light rain at the coast,
the ridge should bring dry weather with partly sunny skies on
Sunday. There is more uncertainty in the forecast for Monday and
Tuesday; the GFS shows a couple more days of dry weather, but the
most recent available run of the ECMWF shows a weak system moving
through the ridge. For now we will keep a chance of light
precipitation in the forecast. McDonnal

&&

.HYDROLOGY...Periods of moderate to heavy rain are likely Thursday
through Friday night, but snow levels will climb only to around
3500 to 5000 feet. This isn't a typical flood-producing pattern,
and it should at most present a threat only for the exceptionally
flood-prone Skokomish River.

&&

.AVIATION...A weak ridge of high pressure aloft will give WNW flow
through Thursday. Low level flow will remain light northeasterly
to easterly today. A weak warm front will lift northward tonight
turning the low level flow more southeasterly or southerly early
Thursday morning from about KSEA southward. The air mass is stable
and will have increasing moisture through Thursday morning.

An area of low level moisture in the interior from KPWT to KSEA
then into the southwest interior is giving MVFR to IFR conditions.
Other areas are seeing mainly VFR conditions with ceilings
050-060. The trend will be for moisture aloft to lower with time
and for low level moisture to gradually expand this afternoon
through Thu morning. Rather widespread IFR conditions can be
expected later tonight through Thursday with the weakening front
over the area and rather light pressure gradients.

The main problem area will be around KBLI where light
precipitation will begin as snow. There is the potential for 1-2
inches of snow at the terminal before precipitation there turns to
freezing rain or rain 06-09Z tonight. North of KBLI to the
Canadian border, there is the potential for significant ice
accumulation later tonight through Thursday night as shallow cold
air gets overrun by warmer air aloft. Albrecht

KSEA...MVFR conditions with the 020 stratus deck that is over the
terminal this morning may improve at times to SCT025 OVC050 20Z-
24Z today. Then expect CIGS to drop to OVC020 by 02Z and fall to
008-010 08Z-10Z tonight into Thursday as the warm front stalls
near the terminal. Expect -RA to be light with only a couple
hundredths of an inch of rainfall at most. Winds light
southeasterly through Thursday. Albrecht

&&

.MARINE...Lower pressure offshore with higher pressure inland will
keep the flow easterly or offshore today. A weak warm front will
lift northward into the waters tonight and stall through Thursday.
Small craft advisory east winds are likely to develop near the
west entrance to the strait on Thursday, and small craft advisory
southerly winds are likely to develop south of the warm front over
the coastal waters Thursday afternoon. There is a small potential
that gale force winds in portions of the coastal waters Thursday
afternoon, but the uncertainty is high enough that a gale watch
does not appear to be appropriate at this time.

Expect a cold front to move across the area Thursday night for
onshore flow.

A 1005 mb surface low will move ENE through southern portions of
the coastal waters and into the Puget Sound area on Friday. There
continues to be some inconsistency on how the models are handling
this feature. Strongest winds are likely to occur to the south of
the low track and after the low passes and there is a potential
for gale force westerlies over portions of the Strait of Juan de
Fuca later Friday.

A ridge of high pressure will move through the waters on Saturday
and east of the waters on Sunday giving light to light offshore
flow. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...Winter Storm Warning for Western Whatcom county from 6 PM this
     evening through Noon Thursday.

PZ...None.
&&

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