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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 181048

Area Forecast Discussion
National Weather Service Seattle WA
348 AM PDT Thu Apr 18 2019

.SYNOPSIS...A front will approach the area today and move through
Western Washington tonight into Friday morning. An upper level
ridge will build offshore giving the area dry weather over the
weekend. Frontal systems will brush by the area Monday and Tuesday
before a weakening front finally makes it into Western Washington
on Wednesday. 


.SHORT TERM /TODAY THROUGH SATURDAY/...Satellite imagery shows
cloudy skies over Western Washington early this morning. Doppler 
radar has light rain along the Coast and from about the 
King/Snohomish county line northward. With the cloud cover 
temperatures were in a narrow range, upper 40s to mid 50s. 

Frontal system offshore early this morning will have a hard time
moving inland today with the parent low associated with the front
moving northward into the Alaska Panhandle. This will lift the
front north as well. In addition, the west northwesterly flow 
aloft will result in a rain shadow for the Central Puget Sound. 
Will lower the pops from Seattle south today down to chance but 
keep likely pops for very light rain over the northern portion of 
the area. With the front lifting north Western Washington will be 
in the warm sector. Lots of cloud cover from Seattle north will 
prevent temperatures from warming up much this afternoon but south
of Seattle with some breaks in the cloud cover expect highs to 
warm into the mid 60s with highs near 70 possible in Lewis 
county. North of Seattle expect highs near 60. 

Front makes some eastward progress tonight with the front moving 
through Western Washington early Friday morning. Steady rain will
develop ahead of the front tonight and continue until frontal
passage. Lows tonight will be in the upper 40s to mid 50s.

Post frontal showers later Friday morning with showers continuing
into the afternoon hours as an upper level trough moves into the
area. Convergence zone developing over Snohomish county by
afternoon. Highs on Friday mostly in the 50s. 

Upper level trough dissipating over Western Washington quickly
Friday evening. Flow aloft becoming northwesterly which could push
the convergence zone down into the metro area before the zone
falls apart Friday evening. With some breaks in the cloud cover
overnight lows over the Southwest Interior will drop into the mid
and upper 30s. Lows in the 40s for most of the rest of the area.

Upper level ridge building offshore on Saturday with the surface 
gradients becoming northwesterly. This combination will dry out 
the lower levels of the air mass. Light flow early in the day will
keep some morning cloud cover around but by afternoon plenty of 
sunshine expected across Western Washington with highs in the 
upper 50s and 60s.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Extended models in good
agreement with a flat upper level ridge over the area on Easter
Sunday for another dry and sunny day. Highs will be mostly in the

Models also in good agreement in slowing down the system that was
timed into the area on Monday. Want to see a few more model runs 
before totally drying out the forecast for Monday. Will go drier 
Sunday night and lower the pops into the chance category on 
Monday. Highs will be in the upper 50s to mid 60s. 

Model solutions for Tuesday and Wednesday having some 
consistency issues with the strength and timing of the next front.
The GFS is stronger, wetter and faster than the ECMWf solution. 
Both models have the front moving over the top of an upper level 
ridge. With the model inconsistencies on timing and the ridge in 
the vicinity will just go with a broad brush chance pops forecast
for Tuesday and Wednesday. Highs mostly near 60. Felton 


.AVIATION...A frontal system will move through the area today and
tonight with west to southwest flow aloft. The air mass is moist
and stable.

Ceilings are mostly 4000-6000 feet early this morning. Ceilings
will gradually lower during the day today but much of the 
interior will likely stay VFR until late afternoon or evening. 
MVFR low clouds with pockets of IFR are expected tonight. 

KSEA...Ceilings 4000-6000 feet most of today with the terminal
somewhat rain-shadowed in westerly flow aloft. Ceilings should
lower tonight as the front pushes through. Southerly wind 5-10
knots. Schneider


.MARINE...A frontal system will move through the area today and
tonight with onshore flow developing behind the front on Friday.
Gale force west winds are looking less likely in the Strait 
Friday evening but there is still a small chance. Onshore flow 
will continue Saturday and Sunday. A second frontal system will 
reach the area on Monday. Schneider


.HYDROLOGY...Rivers coming out of the North and Central Cascades
will rise later today through Friday but remain well below flood
stage. River flooding not expected across the area for the next 7
days. Felton


WA...Small Craft Advisory from 6 AM this morning to 3 PM PDT this 
     afternoon for San Juan County.

PZ...Small Craft Advisory from 6 AM this morning to 3 PM PDT this 
     afternoon for Northern Inland Waters Including The San Juan 

     Small Craft Advisory until 8 PM PDT 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.

     Small Craft Advisory from noon today to 8 PM PDT this evening 
     for Puget Sound and Hood Canal-West Entrance U.S. Waters 
     Strait Of Juan De Fuca.