Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131100

Area Forecast Discussion
National Weather Service Seattle WA
400 AM PDT Tue Mar 13 2018

.SYNOPSIS...A slow moving front will spread rain inland today
into the evening hours. A cool upper level low will remain off
the coast Wednesday then move south toward southern Oregon and
northern California later Thursday into Friday. Unsettled weather
will continue into the weekend.


.SHORT TERM...Satellite imagery shows negatively tilted front
offshore with increasing clouds over the area in the last hour or
two. Doppler radar indicating rain ahead of the front still
offshore. Wide variety of temperatures at 3 am/10z. Locations with
exposure to the east wind are still in the lower to mid 60s while
locations without the wind are as low to mid 40s.

Front offshore moving east a little slower this morning than
previously forecast. Most of the rain offshore on the Doppler is
remaining aloft. With the slower timing have cut back the pops
for the northern portion of the area this morning to the chance
category. Rain eventually spreading over the entire area this
afternoon but with the front weakening as it moves into the area
precipitation amounts will not be much, a quarter inch or less in
most places. Temperatures in locations with the east wind this
morning will have their highs this morning until the east wind
shuts off in the lower 60s. For the remainder of the area a much
cooler day today after the record breaking heat of yesterday with
lower to mid 50s common. Gusty easterly winds, 30 to 40 mph, will
continue in the foothills near gaps in the terrain with the
easterlies tracking down I-90 into the metro area.

Rain continuing this evening before the front falls apart over the
Cascades overnight. Most of the post frontal shower activity
getting wrapped around the upper level low offshore so will just
go chance pops for most of the lowlands. Lows will be in the upper
30s to mid 40s.

Model solutions were pretty consistent with the track of the
upper level low off the coast, leaving the low off the coast
Wednesday with the low moving southeast toward northern California
on Thursday. Models are much slower with the southeast movement
of the low later Wednesday into Thursday with the current
solutions leaving the upper level low off the Oregon coast on
Thursday. With the low closer to the area will have to leave at
least a chance of showers in the forecast for Wednesday through
Thursday. Highs both days will be in the upper 40s to mid 50s.

.LONG TERM...Inconsistency in the model solutions continuing into
the extended portion of the forecast on the 00z run. The ECMWF
has the upper level low stalling along the Oregon coast on Friday
with showers pinning around the low moving up into at least the
southern portion of the area. The GFS is staying with the idea
that the upper level low will move into northern California on
Friday. The GFS solution would result in a fairly dry day for
western Washington. Will keep a chance of showers in the forecast.
Confidence in the Saturday through Monday forecast low. Models
having a hard time getting a handle on where the next upper level
low goes off the coast and how long it takes for the low over
southern Oregon/northern california to dissipate. None of the
Saturday through Monday time frame is dry on both models so for
now the broadbrush chance of showers forecast will continue.
Highs will continue to be near normal, upper 40s to mid 50s.


.AVIATION...Moderate to strong southerly flow aloft over
Western Washington today as a frontal boundary over the central
Oregon coast this morning lifts northeastward across the area this
afternoon. Air mass will become increasingly moist this morning as
precipitation spreads across the region with areas of MVFR ceilings.
The air mass becomes somewhat unstable this afternoon and evening as
a broad upper level trough shifts onshore. At the surface, moderate
east winds early this morning will shift to southerly as onshore
flow develops in the wake of the front.

KSEA...VFR ceilings this morning with gusty east to southeast winds
10 to 20 knots through 15Z. Winds ease and shift to southerly later
this morning as front moves onshore. Precipitation is expected to
develop by around 18Z with periods of MVFR ceilings this afternoon
into this evening. 27


.MARINE...A low pressure system will move north well off the coast
today, then move back south Wednesday and Thursday. Earlier it
appeared gales might be associated with this low in the coastal
waters, but it seems the strongest winds will be well offshore. The
gale warnings have been dropped in favor of high end small craft
advisories. The west entrance and central strait will have small
craft advisory east winds today. There probably will be some 10 ft
swells reaching the coast later today and tonight.

Onshore flow will follow a front tonight and Wednesday. Offshore
flow is likely to develop Thursday as the low moves back south.
Winds will be fairly light Friday and Saturday as the low moves well
away from the region and gradients become light. CHB


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


.CLIMATE...The high on Monday at Sea-Tac was 73 degrees, tied for
the 4th warmest March day on record at the airport. The warmer
days 78 3/29/2004, 75 3/31/1987, 74 3/29/2004. There was also a
73 degree day on 3/28/2004. 73 degrees is the normal high in
Seattle between June 29th and July 2nd.

The warmest low temperatures ever recorded at Sea-Tac in March is
53 degrees on 3/1/1994. The daily low for today will occur this
evening so there will probably not be a all-time March warm low
temperature today but it does point out how rare it is for it to
be 63 degrees at 3 am in the morning in March in Seattle. Felton


PZ...Small Craft Advisory until 5 AM PDT Wednesday 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 for rough bar from 2 PM this afternoon to 5
     AM PDT Wednesday for Grays Harbor Bar.

     Small Craft Advisory until noon PDT today for Central U.S.
     Waters Strait Of Juan De Fuca.


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