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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 151026

Area Forecast Discussion
National Weather Service Seattle WA
326 AM PDT Sun Sep 15 2019

.SYNOPSIS...A cold front will move through the area this morning.
A cool upper level trough will follow for tonight and Monday. 
Next system arriving on Tuesday with an upper level trough over 
Western Washington Wednesday. An upper level ridge will build 
offshore but not close enough to keep systems embedded in the 
northerly flow aloft from moving over area late in the week.


.SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows
cloudy skies over Western Washington at 3 am/10z. Hard to pick up
the front on the satellite imagery. Surface observations put it 
just east of Hoquiam to Port Angeles to near the San Juan Islands 
at 10z. Expect a brief round of heavy rain with the frontal 
passage. Port Angeles got 0.55 inches of rain last hour. 
Temperatures at 3 am were in the mid 50s to lower 60s. 

Front sagging southeast through the remainder of the area this 
morning with steady rain coming to an end. Satellite imagery has 
a brief break between the front and the post frontal shower 
activity, which will move over Western Washington this afternoon.
Possible convergence zone near the east entrance of the Strait of
Juan de Fuca by late afternoon. Highs will be well below normal, 
in the lower to mid 60s.

Cool upper level trough moving over Western Washington late
tonight into Monday. Jet stream is well south of the area with the
trough axis moving through the area late Monday afternoon.
Convergence zone dissipating early Monday morning. Enough 
instability with the trough axis to increase the shower coverage 
Monday afternoon. Just a chance of showers for most locations in 
the morning. Lows tonight in the upper 40s to mid 50s. Highs on 
Monday remaining cool, in the 60s.

Trough axis moving east of Western Washington Monday night but 
the break in the action will be short lived with the next system 
moving down out of the Gulf of Alaska east of 130W by 12z Tuesday.
Lows Monday night in the 50s.

Another cold front moving through Western Washington on Tuesday
making for another wet day across the region. Highs will only be
in the lower to mid 60s, Felton  

agreement Wednesday and Thursday with another cool upper level
trough moving over Western Washington on Wednesday with the 
trough kicking out to the east Wednesday night. Weak upper level 
ridge nosing into British Columbia on Thursday with northerly flow
aloft. Models in general agreement that the ridge axis is too far
to the west to prevent weak systems from sliding down the back of
the ridge into Western Washington late in the week. Model timing 
of these features are inconsistent leading to a broadbrush slight 
chance of showers forecast for Friday and Saturday. Highs will 
warm a little toward the end of the week but remain near to a 
little below normal, in the 60s and lower 70s. Felton


.AVIATION...A surface front will exit the area this morning. 
Southwesterly flow aloft with southerly flow at low levels. The air 
mass is moist and stable this morning becoming slightly unstable 
this afternoon. Ceilings have been MVFR to IFR with the front 
overnight and will likely linger through the morning then improve to 
around 2500 to 3500 feet by early afternoon. A weak Puget Sound 
Convergence Zone is possible during the afternoon hours, most likely 
remaining north over Snohomish County.  

KSEA...MFR conditions will linger through the morning, especially 
with steady rainfall. Ceilings will improve to VFR midday as rain 
transitions to showers. Southerly winds 6 to 8 knots this morning, 
becoming 8 to 10 knots with gusts to 15 knots by early afternoon. 


.MARINE...A front moving through the interior waters will keep 
small craft advisory southerlies winds to most of the inland waters 
through this afternoon.  Winds and seas will ease Sunday night into 
Monday. Another strong system will arrive around Tuesday, with 
possible gale force southerlies over the coastal and parts of the 
interior waters. 


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


PZ...Small Craft Advisory until 5 PM PDT this afternoon for Admiralty 
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-Puget 
     Sound and Hood Canal.