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 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KSEW 022355
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Fri Dec 2 2016

.SYNOPSIS...Rain and mountain snow will taper to showers tonight. A
vigorous front arrives Saturday night, followed by a cold upper
trough Sunday through Tuesday. Snow levels will fall to around 500
feet with snow possible down to sea-level at times Monday and Monday
night. Cold and mainly dry weather develops Tuesday afternoon. A
system arriving Wednesday night and Thursday could bring some low
elevation snow before warm air brings rising snow levels.

&&

.SHORT TERM...Radar shows a well defined band of rain approaching the
inland waters, including Puget Sound. This appears to be the main
front as obs near the coast/Strait have switched around to the west.
Rain should switch to showers tonight with a possible Puget Sound
convergence zone bringing localized moderate rainfall to Western
Snohomish county. Snow levels jumped up today to around 5000 feet
south and 4000 feet north but should fall behind the front tonight
to around 3500 feet. A few inches of snow could fall in the higher
passes, and especially at along higher west facing slopes.

Showers linger through Saturday morning, the rain increases during
the afternoon ahead of the next stronger front. Snow levels will
fluctuate with this next system from 3500 to 4000 feet falling to
2500 feet late Saturday night and Sunday. Models continue to show a
wave forming along the confluent westerly flow aimed at the area
which enhances precipitation amounts along west facing terrain. A
surface low feature tracking south into Oregon may also lift
moisture over the area and keep WA on the cold north side of the
low/front. The WRFgfs and QPF on other models suggest 1 to 2 feet of
snow a good possibility in the mountains so a winter storm watch was
hoisted for the late Saturday afternoon through Sunday afternoon
period. It will rain in the lowlands but colder air aloft will start
to filter in Sunday night behind the front with lowering snow
levels.

Most models are in pretty good agreement on a lull in showers Sunday
night as a closed upper low drops south from Alaska toward the
Pacific Northwest. A weak 1015 mb surface low forms off the south
B.C. coast and tracks southeast across the area Monday. 500 mb
temperatures are very cold with this low, around -38C with unstable
conditions developing Monday as 1000-850mb thickness values fall to
1285-1290m. This would normally be cold enough in light or offshore
flow to bring snow levels down to sea-level. The 1000-500mb
thickness values are near 5200m, a bit higher than would normally be
associated with snow down to sea-level with any southerly flow. With
a surface low near Vancouver Island and moving over the area Monday
afternoon, it is difficult to tell if accumulating snow will make it
all the way down to sea-level with some southerly gradient. The best
chance for some accumulation will be with the convergence zone as
usual from far north king county to Snohomish county and locations
with a few hundred feet of elevation. The WRF4km is suggesting 1 or
2 inches could be possible, and locally more with the convergence
zone. For the greater Seattle Metro area southward, it would seem
Monday night would be the best shot for snow accumulation as colder
fraser outflow develops and squeezes out any residual low level
moisture. It is still too early to say how much and which areas will
get snow accumulations. Will refresh the special weather statement
as we are well outside any watch/advisory time-frame. Forecasts
should be monitored closely.

.LONG TERM...The coldest temperatures of the year are likely by
Tuesday, between systems. With some clearing and very cold air mass
aloft, lows could dip into the 20s for most areas. Highs may struggle
to reach 40 Tuesday.

The next system will approach on Wednesday. Models differ
significantly on how much cold air will be in place and the timing
of incoming precipitation. It is possible there could be several
hours of overrunning snowfall Wednesday night and Thursday. Most
likely any snow or frozen precipitation would switch over to rain
pretty quickly but some lowland accumulation is not out of the
question. By Thursday, the air mass will moderate and precipitation
will be all rain in the lowlands. Rain tapers to showers toward the
weekend. Mercer

&&

.AVIATION...Westerly flow aloft. A cold front moving across the
Olympic Peninsula at 23Z will push well east of the Cascades by
early this evening. The air mass will become unstable and somewhat
moist behind the front. Precipitation will rapidly transition to
showers with the passage of the front, then showers will become tied
to the terrain, the coastal zones, and a Central Puget Sound
Convergence Zone this evening through Friday.

Conditions will be mainly MVFR across the area through much of
tonight with some improvement at times at the lee of the Olympics
and mountains of Vancouver Island. VFR conditions can be expected
most areas on Friday. Albrecht

KSEA...South winds 11 to 14 knots. Gusts to 20 knots can be expected
this evening. Expect mainly VFR conditions at the terminal as
westerly flow aloft interacts with the Olympics to the west. Albrecht

&&

.MARINE...A cold front is moving through the inland waters this
afternoon. Onshore flow that is following this front will continue
through tonight. Small craft advisory southerly winds in the inland
waters will rapidly come to an end as the front passes by early this
evening. A period of small craft advisory westerly winds are
expected this evening over the coastal waters and through most of
tonight in the central and eastern portions of the Strait of Juan de
Fuca.

a 25 foot westerly swell is being reported by buoy 46036 this
afternoon. This swell will decay to about 21 ft in the outer coastal
waters and 17-19 ft over the inner coastal waters this evening.
Small craft advisories for hazardous seas will continue through
Saturday over the coastal waters and at the west entrance to the
Strait of Juan de Fuca.

Another frontal system will sweep east across the waters late
Saturday night and Sunday morning. This front is expected to bring
small craft advisory conditions to most of the waters.

Timing of the possible development of Fraser outflow over the
northern portions of the inland waters is not consistent with some
models showing moderate to strong outflow starting Monday evening
and others delaying the onset into Tuesday morning. At this time it
appears that north to northeast winds of 20 to 30 kt can be expected
during the period of outflow, though there is about a 30 percent
chance that gales may develop. Albrecht

&&

.HYDROLOGY...No river flooding is expected over the next week.
Mercer

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 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 for hazardous seas from 10 PM this evening
     to 6 PM PST Saturday 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 for rough bar until 6 PM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 3 AM PST Saturday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST this evening for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands-
     Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
     Of Juan De Fuca.

     Small Craft Advisory for hazardous seas from 6 PM this evening
     to 6 PM PST Saturday for West Entrance U.S. Waters Strait
     Of Juan De Fuca.

     Small Craft Advisory until 3 AM PST Saturday for Central U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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