Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 152212
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
312 PM PDT Sun Apr 15 2018

.SYNOPSIS...A low pressure system over Western Oregon will lift
north over Western Washington tonight through Monday night. Areas
of light rain or showers will increase over the area this evening
and persist into Monday as an upper low moves over the area. Snow
levels will start off above pass level this evening but lower to
the higher passes like Stevens and White pass on Monday. Weak high
pressure shifts across the area Tuesday with decreasing showers
through Wednesday. Drier and milder weather is still expected
Thursday and Friday. A front may bring unsettled weather to the
area by Saturday.

&&

.SHORT TERM...A closed upper low and -30C cold pool aloft is
shifting southeast nearing 130 w off the Oregon coast. An
associated short wave with 1006 mb surface low near the Oregon
coast is lifting northward this afternoon. Satellite imagery shows
bright cloud tops spreading into Western Washington but most of
this is mid and high cloud. Deeper moisture is not far to our
south and areas of light rain or showers should start increasing
over the area late this afternoon and evening. The flow aloft is
not very strong and precipitation will probably start off on the
light side. Also, mountain temps are running even milder than
yesterday with mid to upper 30s even at higher spots like Paradise
and Mount Baker. Snow levels should start to drop later tonight
but accumulations will probably remain above the passes and not
quite reach advisory criteria.

Colder air aloft moves over the region on Monday as the upper low
shifts across. Snow levels will plummet, falling to 3000 feet or
lower. This should bring some snow accumulation to the Cascades
including the passes. However, the flow direction is not initially
favorable for heavy snowfall but this changes by the afternoon as
onshore flow increases. Advisory amounts of snow (6+ inches in 12
hours) could fall Monday afternoon and evening. Given that snow
levels are currently running well above what models show, will let
the next shift evaluate the need for an advisory Monday/Monday
evening.

A weak ridge shifts east across the area Tuesday, followed by
split flow on Wednesday. Shallow instability may still produce a
few showers, especially coast and mountains. Any shower activity
should only produce light measurable precipitation.

.LONG TERM...The ECMWF still shows a decent break between system
Thursday and Friday. Dry and milder conditions are expected with
mostly clear skies. Another front arrives late Friday night and
Saturday with more rain and mountain snow. This system will be
progressive and not likely to produce significant precipitation.
Sunday could end up dry if the system moves east as quickly as
some models show. Highs may reach the low 60s around warmer parts
of the interior away from the water late in the week.

Mercer

&&

.AVIATION...Southerly flow aloft over Western Washington this
evening, with an upper level low offshore, will become light and
variable later tonight and Monday as the low moves over the area.
The air mass is moist and stable but will become slightly unstable
tonight. At the surface light northerly flow will become moderately
onshore tonight, then increase Monday.

Most ceilings this afternoon are VFR, with a few just below 3k ft.
Rain will spread from the south this evening. Ceilings will fall to
MVFR 2-3k ft 00Z-03Z and remain there tonight and much of Monday.

KSEA...VFR ceilings will give way to MVFR conditions as rain arrives
from the south around 00Z. Northerly gradients will switch to
southerly 03Z-06Z. Northerly winds 10-15 kt will become southerly
after 06Z tonight 6-12 kt. Southwest wind 10-20 kt is possible
Monday afternoon. CHB

&&

.MARINE...Onshore flow will resume tonight and increase Monday. A
strong onshore push Monday night will likely bring gale force
westerly winds to the coast, strait, and northern inland waters,
with small craft advisory strength winds elsewhere. A gale warning
is in effect for the coast beginning over the outer waters Monday
and spreading to the inner coast Monday night. A gale watch is in
effect for the strait and northern inland waters.

Winds will ease Tuesday. High pressure will develop over the area
Wednesday, giving light northerly winds. Onshore flow will resume
Thursday afternoon. CHB

&&

.HYDROLOGY...Most rivers are receding. The Skokomish River is the
only river that flooded, and it will fall below flood stage this
evening. The Skookumchuck River near Bucoda will crest just below
flood stage this afternoon. Additional flooding is not expected for
the next 7 days. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Refer to the latest flood bulletin.

PZ...Small Craft Advisory until 5 AM PDT 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.

     Gale Warning from 5 AM Monday to 5 AM PDT Tuesday 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 for rough bar until 8 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory until 5 PM PDT 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.

     Gale Warning from 5 PM Monday to 5 AM PDT Tuesday 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.

     Gale Watch from Monday evening through Tuesday 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.


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Refer to the latest flood statement bulletin.

PZ...Small Craft Advisory until 5 AM PDT 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.

     Gale Warning from 5 AM Monday to 5 AM PDT Tuesday 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 for rough bar until 8 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory until 5 PM PDT 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.

     Gale Warning from 5 PM Monday to 5 AM PDT Tuesday 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.

     Gale Watch from Monday evening through Tuesday 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.

&&

$$

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