Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 280429
Area Forecast Discussion
National Weather Service Seattle WA
929 PM PDT Mon Mar 27 2017
.SYNOPSIS...Rain will spread inland tonight as a warm front over
the offshore waters moves northeastward. The heaviest
precipitation with this front on Tuesday will be over the Olympic
Peninsula. A low pressure system will move into northern
Vancouver Island Wednesday morning with the associated cold front
pushing across Western Washington. Western Washington will see the
heaviest rainfall ahead of this southeastward pushing cold front.
Another trough will give showers to the area on Thursday. Dry
weather is expected on Friday and possibly for parts of the south
interior on Saturday. A weak front will bring a chance of rain
Sunday, with decreasing showers early next week.
.SHORT TERM...Showers have moved into the Cascades this evening,
but Pacific Northwest radars show warm advection rain spreading
onto the Olympic Peninsula and into the northern interior. This
rain shield is ahead of a warm front that extends from a 1004 mb
low southwest of Haida Gwaii to the south Oregon coastal waters.
The rain will push northeast into the area tonight through Tuesday
as the front pushes slowly northeastward.
A weak wave appears to be forming near 38N 148W this evening.
Short term models take this developing feature east-northeast as a
1006 mb low on the central or northern coast of Vancouver Island
Wednesday morning. The general model consensus is to have the
heaviest precipitation over the area from Tuesday evening into
midday Wednesday. Most of the models take the main low far enough
to the northwest of the area, limiting the strength of a
developing mesolow to the northeast of the Olympics. This results
in windy weather across the area, but not windy enough to require
the issuance of wind advisories or warnings. Another wave will
pass by to the south of the area Wednesday night. This wave may
slow the southeastward progress of the front that moves across the
area on Wednesday, but forecast models argue against this.
Snow levels in the mountains will rise from around 3000 feet
tonight and early Tuesday to near 6000 feet later Tuesday night
into Wednesday in the warm sector. Advisory snow amounts may occur
locally at Mt Baker, but the passes will change to rain before
significant snowfall accumulations occur and places like Paradise
Ranger Station in Mount Rainier National Park will see warmer
temperatures and lighter precipitation amounts. Will hold off on
issuing any winter weather advisories for the mountains at this
The landslide risk will increase over central and southern
portions of the forecast area starting later Tuesday and will
continue through late Wednesday or Thursday. A Special Weather
Statement for the elevated landslide risk was issued this
afternoon and will be maintained. Flooding with the incoming
rainfall is expected to be limited to the particularly flood-prone
Skokomish River in Mason County. A flood watch is in effect there.
An upper trough will bring showers to the area Thursday, but
precipitation coverage and intensity will generally be on the
decrease. A Puget Sound convergence zone may develop, bringing a
somewhat better chance of rain to Snohomish and King counties.
Highs will remain cool in the lower 50s.
The current forecast is in good shape; no updates are required.
.LONG TERM...From the previous long term discussion: Models are
in good agreement that Friday will be dry as an upper ridge builds
over the area. The flow will be light and temperatures should
warm up, possibly close to 60. Some rain may reach the coast and
north parts of the area Saturday but much of the area will be in
the warm sector of the next frontal system. Highs could be even
warmer, possibly into the low 60s around Puget Sound. If Sea-Tac
does reach 60, it will be the first time this year.
Light rain or showers will eventually reach much of Western
Washington by Saturday night and Sunday with somewhat cooler high
temperatures. Models have shifted around on the pattern early next
week. Climatology seems the best way to go and have maintained some
chance of showers and near average highs. Mercer
.AVIATION...Zonal or wly flow will prevail across the area overnight.
The low level flow will remain sly. Expect areas of MVFR CIGs and
VSBYs to become more wdsprd overnight.
KSEA...MVFR CIGs are expected overnight. There is a chance that CIGs
will lift into the VFR category during the day Tuesday. Winds will
remain sly 9-15 kt, possibly occasionally gusty during the day
.MARINE...A cold front well offshore will continue moving east
and will move across the Washington coastal waters late Tuesday.
Another frontal system will impact the area on Wednesday for the
possibility of gale force winds over the Strait of Juan de Fuca.
High pressure offshore with lower pressure inland will result in
onshore flow on Thursday.
.HYDROLOGY...A warm frontal system will bring 1 to 3 inches to
the Cascades and 2 to 4 inches to the Olympics Tuesday morning
through early Wednesday morning. While precipitation amounts will
only be moderate, currently saturated soils will cause increased
runoff. Most rivers will see rises, and a few could approach
bankfull on Wednesday.
Precipitation amounts looks sufficient enough to cause minor
flooding on the Skokomish river in Mason county. A flood watch was
issued for this threat from late Tuesday night through Thursday
morning. The most likely scenario is for minor flooding to develop
on the Skokomish river Wednesday morning and continue through
Wednesday night. See the latest flood watch for details.
The additional rainfall will also cause a further elevated risk of
landslides late Tuesday through Wednesday. Soils remain saturated
with the risk already elevated, but rainfall Tuesday into Wednesday
will increase the risk further. Up to 1 inch of rain could fall over
the interior lowlands from Tuesday morning through Wednesday
morning, with most areas somewhat less. Heaviest amounts over 1
inch will occur on the coast and over the mountains. Mercer
.CLIMATE...The rain today at Sea-Tac makes 45 days
with measurable rain since February 1st. Only two years in over 120
years of weather records in Seattle have had more rain days in
February and March, 1961 with 49 days and 2007 with 46 days.
Through 3 pm today the combined February and March precipitation
total for Seattle is 15.36 inches. This is the second highest total
on record. The record is 15.55 inches set in 2014.
There has been measurable precipitation on 23 out of the 27 days
at Sea-Tac this month. The record for the most days with
measurable precipitation in March in Seattle is 27 days in 1989.
There has been measurable rain every day so far this month at
Quillayute. The current rain day streak including this morning is
35 days in a row. The record is 47 days in 1990. Felton/Mercer
WA...Flood Watch from late Tuesday night through Thursday afternoon
for Mason County.
PZ...Small Craft Advisory until 6 PM PDT Tuesday 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-Puget Sound and Hood Canal-West
Entrance U.S. Waters Strait Of Juan De Fuca-Admiralty Inlet-
Northern Inland Waters Including The San Juan Islands.
Small Craft Advisory for rough bar until 9 PM PDT Tuesday for
Grays Harbor Bar.
You can see an illustrated version of this discussion at