Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
FXUS66 KSEW 221743

Area Forecast Discussion
National Weather Service Seattle WA
943 AM PST Thu Feb 22 2018

.SYNOPSIS...A break between systems is occurring today, though
cold and dry air continues to filter out of western Canada and
into Western Washington. The next system will dive in from the
northwest on Friday, with precipitation moving spreading in from
the northwest from late Friday morning through Friday afternoon.
Low snow levels will continue as this system moves through the
area. Another system will follow quickly early Saturday. Cool
northwest flow aloft through the middle of next week, with cool
wet systems around Sunday and then next Tuesday or Wednesday.


.SHORT TERM...Northwest flow aloft will continue through early
next week. For today, a shortwave ridge is crossing the area, and
cold dry continental air continues to pour out of the Fraser
Valley past Bellingham, reinforcing the cold and dry air mass in

The next shortwave will dive down the B.C. coast on Friday, with
a secondary wave on its heels crossing the area early Saturday
morning. Westerly low-level flow in advance of Friday`s shortwave
will advect warmer maritime air onshore. The warm advection lift
will bring about precip. Precip will spread southeast down the
B.C. coast on Friday morning, reaching the northwest Olympic
Peninsula by late morning, then spreading across the rest of
Western Washington on Friday afternoon. Cold air mass will cause
precip to start as snow. Along the coast, precip will quickly
change to rain just an hour or two after beginning. Along the I-5
corridor from the Everett area on south, precip will change to
rain early Friday evening, after an inch or so of snow has
occurred. With onshore flow, orographic lift will enhance precip
over the mountains. In the Cascades, this could be an advisory-
level snow event.

By the time the secondary trailing shortwave arrives early
Saturday morning, most of the lowlands will be warm enough for
merely cold rain, though snow could linger in the eastern
lowlands, i.e. the Cascade foothill communities. Once the
shortwave passes, the air mass will cool down some again, with
snow level down to 300-500 feet by mid-day Saturday. Post-frontal
showers are possible, especially in a Puget Sound Convergence Zone
and the mountains. Haner

.LONG TERM...From Previous Discussion: The general trend of well
below normal temperatures with precipitation will continue through
the extended period. GFS and ECMWF in good agreement with another
low snow level, 1000 feet or less, system arriving on Sunday with
an upper level trough over the area on Monday. Model solutions
diverge after Monday with the ECMWF drying things out Tuesday and
Wednesday while the GFS brings yet another system down the British
Columbia coast Tuesday into early Wednesday with somewhat low
snow levels. Current forecast is more in line with the GFS
solution and with the inconsistencies in the models will stay with
the current forecast for the extended period. Felton


.AVIATION...Northerly flow aloft over Western Washington this
morning will continue today, then increase tonight and back to
northwest late tonight. The air mass will be generally dry, with
moisture increasing late tonight as the next weather system
approaches the region. At the surface moderate northerly flow
will continue today, ease this evening, and turn southerly late

There is still some lingering low clouds, generally around 1500
ft, especially in the south part of the forecast area; those
clouds should dissipate during the next couple hours, with only
high clouds above 12,000 ft over the area through tonight. The
exception to this is along the north side of the Olympic
Peninsula, where northerly low level flow is producing a local
deck of stratocumulus clouds around 4000-5000 ft.

KSEA...North wind 8-14 kt easing to 4-8 kt this evening and
switching to southerly around midnight. The low clouds around
1400 ft will continue to decrease this morning; otherwise there
should be no significant cloud cover below 12,000 ft through
tonight. The next chance for accumulation snow at KSEA appears to
be Friday afternoon. McDonnal


.MARINE...Moderate north to northeast pressure gradients will
continue today as a weak low over the far southern Washington
coastal waters drops south through Oregon. Small craft advisory
winds are expected through this evening over the coastal waters
and for today over much of the inland waters. Winds will ease
tonight as a weak ridge moves southeast across the area.

A strong surface trough will drop southeast into the waters later
Friday into Friday night, bringing small craft advisory south to
southeast winds to all of the waters. Winds will likely increase
further Saturday into Sunday as a couple of strong frontal
systems drop southeast through the waters. Small craft advisory
conditions are likely over all of the waters, and gales are
possible. McDonnal


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


PZ...Small Craft Advisory until 11 PM PST this evening 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.

     Small Craft Advisory until 3 PM PST this afternoon 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 until 1 PM PST this afternoon for Puget
     Sound and Hood Canal.


$$ is the U.S. government's official web portal to all federal, state and local government web resources and services.