Heavy Snow in Washington, December 24 to 25, 2017

Area Forecast Discussion
National Weather Service Seattle WA
403 AM PST Sun Dec 24 2017

.SYNOPSIS...A weak low pressure system will pass across Western
Washington tonight and early Christmas morning bringing rain and
snow to the area. Lingering showers behind the front will keep the
chance of rain and snow going through the remainder of Christmas
morning. A upper level ridge will build off the Coast on Tuesday
for a brief dry spell. Another weak system will move through
Tuesday night into Wednesday followed by a warmer and wetter
system later in the week.

&&

.SHORT TERM...Satellite imagery shows approaching front out
near 132w at 11z/3am this morning. High clouds out ahead of the
front spread over the area overnight. With the clear skies early
on in the evening temperatures dropped below freezing and have
remained below freezing, in the 20s and lower 30s, at 3 am.

Quite the weather scenario developing today through Christmas
morning with the multiple solutions in the models adding to the
usual borderline snow scenario for western Washington.

Front offshore will become more north south oriented today which
will slow down it's eastward progress a little. Even with the
slower arrival cloud cover out ahead of the front will thicken
during the day. The low level is offshore but with the increasing
cloud cover it is going to be hard to get much warming. Have
gone with the colder model guidance which keeps highs in the mid
and upper 30s. Precipitation out ahead of the front beginning
along the Coast and over the Southwest Interior later this
morning. The air mass could still be cool enough over the
Southwest Interior during the onset of precipitation for the
precipitation type to be snow but if this is the case the
precipitation will quickly turn to a cold rain. Precipitation out
ahead of the front reaching the central Puget Sound by 00z/4 pm.
The air mass will be borderline cold enough for snow with the
1000-850 mb 1300 meter thickness line right over the Seattle
metro area. With surface temperatures in the mid 30s expect a mix
of rain and snow as the precipitation moves into the metro area
with the snow level around 500 feet. Further north around the
Everett area the precipitation will start out mainly as snow but
the precipitation will be light.

The front will move through western Washington tonight with the
associated surface low with the front moving inland near the mouth
of the Columbia. This will keep the winds northerly over area
which will help keep some cooler air in place. On the flip side
there is a slightly warmer air mass post frontal. The forecast
question for this evening into Christmas morning is how far north
will the warmer air get. 00z models runs were a little different
with this variable with the ECMWF keeping the Puget Sound
area in the cool enough for snow air mass while the GFS and other
models push the warmer air a little further north into the Puget
Sound area. With the surface low moving inland to the south have
trended the forecast more in the way of the ECMWF keeping the air
mass from the Puget Sound area northward cold enough for snow.
Even with the cooler air mass the other variable in play, how much
precipitation will we get from the frontal system, is not very
favorable for much snow with only up to a quarter inch or so of
water with the system with locations from about Everett northward
only getting a tenth or less. Behind the front some weak
convergence over the central Puget Sound extending back toward
Mount Rainier will keep some light precipitation going overnight.
By this time the air mass should be cold enough for snow in this
area with the 1000-850 mb 1300 meter thickness value as far south
as Chehalis even on the warmer models. Precipitation will be light
with the convergence, only up to a tenth of an inch of water.

End result of all this will issue a winter weather advisory for
locations between Everett and Tacoma including the Hood Canal and
the Cascade foothills for snow. With the easterly gradients
trapping some cold air over the Hood Canal and Bremerton areas
the advisory will start there first in the middle of the
afternoon. For the remainder of the areas where the advisory will
be issued the advisory will start in the early evening.
Precipitation for the most part will come to an end in the early
morning hours Christmas day behind the front and the advisory
will end for some locations. Will keep the advisory going for the
Central Sound to the Cascade foothills into the morning hours on
Christmas with another inch of snow possible with the weak convergence.

Total predicted snow amounts are...north of Everett an inch or
less...Central Puget Sound and Cascade foothills areas up to 3
inches with the highest amounts above about 500 feet and little if
any snow near the water...Hood Canal area with the earlier start
up to 4 inches possible. For the Southwest Interior and the Coast the
air mass looks to be too warm for much in the way of significant
accumulations.

Outside of the weak convergence Christmas morning showers will be
isolated and dry up in the afternoon hours.

Weak upper level ridge building offshore Christmas night for a
brief dry spell over the area on Tuesday. Temperatures will
continue to be below normal with highs in the mid 30s to lower 40s
and lows in the upper 20s to mid 30s.

.LONG TERM...Model solutions are coming together with a weak
negatively tilted front moving inland Tuesday night into Wednesday
morning. This could be another borderline snow scenario but the
precipitation amounts with the system look light at this time. A
warmer and wetter system will arrive later in the week quickly
removing the cooler air mass from the area. Felton

&&

.AVIATION...Northwest flow aloft over Western Washington early
this morning will back to westerly this afternoon as a Pacific
frontal system reaches the offshore waters, then continue tonight
as the system reaches the coast. Northerly offshore flow at low
levels will weaken. The air mass is dry, but it will become moist
today and remain moist tonight.

Skies were clear across Western Washington at 2 am. High clouds
will move over the region in the next few hours; ceilings will
gradually lower today, with widespread MVFR/local IFR conditions
likely this evening due to reduced visibility, low ceilings, and
rain/snow. Those conditions will continue tonight.

KSEA...Northeast wind 6-12 kt. Cloud cover will gradually lower
Sunday, with conditions probably deteriorating to MVFR late
today/early this evening. Light precipitation will probably begin
late afternoon, with up to 2 inches of snow possible at KSEA by
early Monday morning. McDonnal

&&

.MARINE...Moderately strong offshore flow will continue today as
a Pacific frontal system moves into the offshore waters. The
system will move inland tonight, followed briefly by onshore flow
on Monday. Small craft advisory winds will continue into tonight
for the central and west Strait of Juan de Fuca and into Monday
for the coastal waters. Elsewhere winds should generally be 20 kt
or less by noon today.

Weak offshore flow will prevail Monday night and Tuesday. A weak
system will move through the area Tuesday night and Wednesday.
Another -- somewhat stronger -- frontal system will affect the
area Wednesday night and Thursday, probably bringing small craft
advisory southerly winds to at least the coastal waters. McDonnal

&&

.HYDROLOGY...River flooding is unlikely for the next 7 days. A
period of heavier rain is possible around next Thursday night or
Friday. This will bear watching for the flood-prone Skokomish
River. Other rivers may experience rises but are not expected to
flood.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 7 PM this evening to 6 AM PST
     Monday for Everett and Vicinity-Tacoma Area.

     Winter Weather Advisory from 7 PM this evening to noon PST
     Monday for Bellevue and Vicinity-East Puget Sound Lowlands-
     Seattle and Vicinity.

     Winter Weather Advisory from 4 PM this afternoon to 6 AM PST
     Monday for Bremerton and Vicinity-Hood Canal Area.

PZ...Small Craft Advisory until 3 PM PST Monday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater 10
     To 60 Nm.

     Small Craft Advisory until noon PST Monday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 10 AM PST this morning for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until midnight PST tonight for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

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