Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191017

Area Forecast Discussion
National Weather Service Seattle WA
317 AM PDT Thu Apr 19 2018

.SYNOPSIS...An upper ridge over the area will bring dry weather
today into Friday morning. A frontal system will move quickly
across the area Friday night, followed by showers and sun breaks
on Saturday. A stronger upper ridge has the potential to bring the
longest stretch of dry weather this month beginning Sunday and
continuing into the middle of next week.


.SHORT TERM...Satellite imagery shows a few clouds lingering over
the mountains and the San Juans early this morning with mostly
clear skies over the remainder of the area. The clear skies
extended a couple hundred miles offshore. Temperatures at 3
am/10z were mostly in the lower to mid 40s with the cooler
locations in the mid to upper 30s.

Flat upper level ridge offshore moving over Western Washington
this afternoon. What little cloud cover that remains over the area
early this morning will dissipate by noon. Temperature dew point
spreads are small in many locations early this morning so expect
some fog to form before sunrise especially over the Southwest
Interior with the fog only lasting into mid morning. Today will
be the sunniest day of the month but the combination of
temperatures aloft not that warm, 850 mb temps in the plus 1 to 4C
range, and the low level flow remaining onshore will keep highs
just a couple of degrees above normal, in the mid 50s to mid 60s.

Next front to reach the area still well offshore with just
increasing high clouds overnight into Friday morning. Expect lows
in the mid 30s to mid 40s. If the high clouds are not too thick
could be a good morning for sunrise pictures.

Front approaching the area on Friday but with the front becoming
parallel to the flow aloft the eastward progress of the front will
slow down. Have cut back on the pops over the interior and
confined them to west of the Puget Sound Friday afternoon. Not
much sunshine with the increasing clouds ahead of the front.
Highs will be in the mid 50s to lower 60s.

Front quickly moving through the area Friday night with the front
into the Cascades 09z-12z Saturday morning. With the front moving
through quickly not going for much in the way of precipitation
with up to a half inch along the Coast and in the mountains with a
quarter inch or less in the interior. Snow levels will start out
above all the passes but lower down to around 4000 feet overnight.

Upper level trough moving through the area Saturday keeping a
chance of showers in the forecast. Convergence zone possible over
southern Snohomish and northern King county Saturday morning with
the low level convergence pattern remaining intact into the early
afternoon hours. High temperatures will cool back into the 50s.

.LONG TERM...Extended models in good agreement initially with a
dry trough moving through Western Washington on Sunday followed by
an upper level ridge on Monday. Low level flow goes
northerly/weakly offshore on Monday with the temperatures aloft
warming ( model 850 temps in the plus 6 to 8C range ). These two
factors will result in a sunny warm day across the area with highs
in the 60s. Some differences in the solutions start showing up on
Tuesday with the GFS bringing a splitting trough into the area
while the ECMWF keeps this feature further south and has warmer
temperatures aloft. On Wednesday the GFS brings a front through
Western Washington while the ECMWF has the front moving inland
north of Vancouver Island and the Canadian model has the front
even further north than the ECMWF. Current forecast is in line
with the drier and warmer ECMWF and Canadian solutions and with
the GFS being the outlier on this model run will stay with the dry
forecast through the entire extended period. Felton


.AVIATION...An upper level ridge will build into the Pacific
Northwest today with northwest flow aloft. The air mass is dry and
stable. Except for some areas of fog over mainly the southwest
interior this morning, expect a VFR day ahead with some high level
moisture arriving from the west late. At the surface, high pressure
will strengthen along the coast for an increase in onshore flow
this afternoon.

KSEA...VFR expected through tonight with an increase in high level
moisture toward evening. Surface wind NE 7 knots or less turning to
the NW this afternoon.  Wind backing to light S-SW after 06Z
tonight.  27


.MARINE...A ridge of high pressure will strengthen over the
offshore waters today then move slowly eastward into the interior of
Western Washington on Friday. Model solutions continue to show that
the combination of this strengthening ridge and differential heating
between the interior and coastal areas will result in the
development of Small Craft Advisory conditions over the eastern two
thirds of the Strait of Juan de Fuca this afternoon into tonight. As
such...inherited headlines look good with regards to timing and
coverage and will remain in place.

A moderate cold front will move through the waters Friday
evening. Small craft advisory south winds are expected to develop
over the coastal waters and at the entrances to the strait ahead of
the front. Will not do anything with this in the early morning
forecast package...but as models remain consistent...likely to see a
headline to cover this at some point within the next 24 hours.
Strong westerly flow will develop behind the front late Friday night
across portions of the Strait of Juan de Fuca. Earlier shift
suggested wind speeds might flirt with gale strength...but models
seem to be backing away from that solution.

Onshore flow will slowly weaken later Saturday through Monday.


.CLIMATE...Although not much rain is forecast for the Seattle
area Friday night into Saturday morning it should be enough to
push April 2018 into the 3rd wettest April on record. The current
April rainfall for Seattle is 5.34". The top 3 are 6.53" ( 1991 ),
5.89" ( 2013 ) and 5.37" ( 1996 ). The normal for the entire
month is 2.71" so there is a chance by Saturday Seattle will have
twice the normal April rain ( 5.42" ). A couple more numbers to
put things in perspective, normal precipitation in December for
Seattle is 5.35" and for January it is 5.57". Felton


PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Friday
     for East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from noon today to 2 AM PDT Friday for
     Central U.S. Waters Strait Of Juan De Fuca.


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