Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231206

Area Forecast Discussion
National Weather Service Seattle WA
406 AM PST Fri Feb 23 2018

.SYNOPSIS...Cold and sometimes unsettled northwest flow aloft will
continue through next week. Another system will move southeast
across Western Washington later today and tonight with lowland
snow transitioning to rain. Moderate mountain snow is expected
into Saturday morning. Another cold wet system will dive south
into the area late Saturday night and Sunday, with another round
of moderate mountain snow, along with some localized areas of
lowland snow. A bit of a break in the action possible on Monday
before more systems moving down from the northwest arrive in the
middle of the next week.


.SHORT TERM...Some ideas this morning, really messy forecast for
the last week of February and snow fans in the metro area are
likely to be disappointed.

Satellite imagery shows high clouds out ahead of the next system
moving down the British Columbia coast just reaching the northwest
tip of the area at 11z/3 am. With the clear skies overnight
temperatures are chilly this morning with teens in many locations
with the rest of the area mostly in the 20s.

Next system arriving tonight with the precipitation out ahead of
the system spreading over Western Washington late this morning
into the afternoon hours. The air mass will be very dry ahead of
the precipitation with dew points in the teens and lower 20s.
1000-850 mb thickness values are below 1300 meters everywhere
except right along the Coast. On the flip side the southerly
surface gradients will be increasing and the KOLM-KBLI gradient
will be near plus 3 mb this afternoon. With the cold dry air mass
in place ahead of the system even with the increasing southerly
gradients precipitation should start as snow in most lowland areas
outside of the Coast this afternoon. With the flow aloft
northwesterly the Central Puget Sound will get rain or in this
case snow shadowed so expect the lightest precipitation amounts
around the Seattle metro area, only a few hundredths. More
precipitation for the remainder of the area especially in the
northern Puget Sound. Will issue a winter weather advisory for
Western Whatcom and Skagit counties as well as the San Juans for
this afternoon into this evening for up to 5 inches of snow with
the highest amounts expected inland in Whatcom county near the
Canadian border. Will also issue a winter weather advisory for the
Cascade Foothills and the Cascades beginning this afternoon. The
foothills will be the last location the where the low level
onshore flow will push the snow level off the surface. For the
remainder of the area snow amounts will be less than an inch. Highs
today will only be in the 30s.

System moving through the area this evening. Increasing onshore
flow from about Skagit county southward will lift the snow level
off the surface to around 500 feet give or take a couple hundreths
feet changing the snow to rain early in the evening for most
locations. The exception to this will be in the Cascade foothills
where the onshore gradients will not be strong enough to lift the
snow level off the surface. Precipitation amounts drying up in the
evening as the system moves through the area. Secondary shortwave
trailing the front moving into the area overnight. The air mass
will have moderated enough for the precipitation type with this
feature to be mostly rain in the lowlands. The foothills will
remain cold enough for the precipitation to remain in the form of
snow with up to 3 inches of snow on top of the 1 to 3 inches from
the initial system. The winter weather advisory will remain in
effect overnight for the Cascade Foothill zone. Snow will also be
piling up in the Cascades with the winter weather advisory there
also continuing throughout the night. Lows will be in the 30s.

Cooler air moving in behind the shortwave on Saturday with snow
levels hanging near 500 feet or so. The 00z model run was much
warmer with the lower levels with MOS high temperatures a few
degrees warmer than previous runs. One possible trouble maker for
snow is the possibility of a quick convergence zone over Snohomish
or King county behind the shortwave Saturday morning. This
feature could drive the snow level back down to near the surface
for a brief period of time before dissipating by the afternoon
hours. Snow will continue in the Cascades in the morning with the
snow rates decreasing significantly in the afternoon hours. Highs
on Saturday in the upper 30s and lower 40s.

Another cool upper level trough dropping out of the northwest
flow aloft Saturday night into Sunday morning. Decent onshore flow
ahead of the shortwave will moderate the low layers of the air
mass enough to push the snow level up near 1000 feet.

Air mass drying and cooling off again Sunday night. Another
convergence zone could produce some low snow levels Sunday evening
in Snohomish or King county.

.LONG TERM...Little change in the pattern through next week with
much cooler than normal temperatures and above normal
precipitation continuing. Next system embedded in the
northwesterly flow aloft arriving on Tuesday with another one
possible on Wednesday. Upper level trough digging toward the area
the latter half of the week will bring the low snow level scenario
back into the forecast after Wednesday. Felton


.AVIATION...Northwesterly flow aloft over Western Washington today
and into Saturday. The air mass will see moisture increasing today
as the next frontal system moves into the region. At the surface
southerly flow is expected.

VFR conditions continue to dominate the skies over W WA this early
morning with most locations reporting clear skies. Clouds will
increase throughout the morning with OVC conditions by late morning.
Once cigs start to fall...can expect to see low end VFR to MVFR
conditions by early afternoon and becoming more solidly MVFR and
locally dropping down into IFR as the form of snow and
as rain...becomes more widespread.

KSEA...Southerly wind 6-12 kt this morning before rising to 10-16 kt
in the afternoon. Clouds will increase throughout the morning with
cigs gradually lowering...eventually becoming low-end MVFR or
possibly IFR at times. KSEA is likely to receive about 1 inch of
snow, probably between 2 pm and 8 pm this afternoon and evening. SMR


.MARINE...Southerly flow will increase today as a frontal system
approaches then moves inland - Small Craft Advisories are in effect
for all waters. Strong onshore flow will follow tonight into
Saturday. Meanwhile, seas will build to 10 to 13 feet. Another
strong system may bring gales on Sunday. A weaker front will arrive
on Tuesday. 33


.HYDROLOGY...River flooding is not expected the next 7 days.


.CLIMATE...Thursday was the 5th day in a row with a high less than
40 degrees in Seattle. The last time Seattle had a streak of 5
days or more in a row with highs below 40 degrees in February was
February 12-18, 1990. Felton


WA...Winter Weather Advisory from 4 PM this afternoon to noon PST
     Saturday for Cascades of Pierce and Lewis Counties.

     Winter Weather Advisory from noon today to noon PST Saturday for
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties.

     Winter Weather Advisory from noon today to 8 PM PST this evening
     for San Juan County-Western Skagit County-Western Whatcom

     Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Saturday for East Puget Sound Lowlands.

PZ...Small Craft Advisory until 4 AM PST Sunday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory until 6 PM PST Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory from 1 PM this afternoon to 10 PM PST this
     evening for Admiralty Inlet.

     Small Craft Advisory from 1 PM this afternoon to 4 AM PST
     Saturday for Puget Sound and Hood Canal.


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