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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 192146
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
246 PM PDT Wed Sep 19 2018

.SYNOPSIS...A weak upper level trough will give a small chance of
showers tonight as it moves across the area. A warm front will
give some rain, mainly to the coast and northern interior later
Thursday into Thursday night. A cold front, associated with a 
stronger low pressure system moving into Vancouver Island, will 
give rain and breezy conditions to the area later Friday into 
Friday night. Showers will decrease Saturday into Sunday as high 
pressure builds in. Strong offshore flow will give dry conditions
to the area next week. 

&&

.SHORT TERM...Radars and satellite imagery show a thin line of
showers associated with a weakening trough over the western
Olympic Peninsula this afternoon. This band will move inland this
evening. Behind the trough, the air mass is slightly unstable and
will result in partly to mostly cloudy conditions across the area
with a continue small threat of showers. With the increase in
moisture and cloud cover, we expect temperatures tonight to be a
bit more mild that what we saw this morning.

The next upstream system, a weak 1012-1014 low moving eastward
north of a broad upper ridge centered at 135W, will stretch out
into a warm frontal feature offshore that will push onshore over 
the southern Washington coast Thursday afternoon and lift 
northward through the area Thursday night. Highest rain chances 
and rainfall amounts are expected to be over the Olympic Peninsula
and in the interior north of Everett to include the northern 
Cascades. Behind the warm front, expect to see partly to mostly 
cloudy skies in the interior on Friday, and it appears that it 
will warm up into the upper 60s and lower 70s Friday afternoon
from Seattle southward. 

12Z deterministic model solutions appear more consistent now in
taking a 985-990 mb low onto the NW tip of Vancouver Island late
Friday afternoon with its associated cold front sweeping eastward
through the area Friday evening. The low itself slowly weakens at
it sags southeastward down Vancouver Island, but it appears that
there will be sufficient pressure gradient behind the front and to
the south of the low to produce some breezy conditions across the
area Friday night and Saturday. Albrecht

.LONG TERM...Onshore flow with clouds and showers will gradually
decrease Saturday into Sunday. Showers will gradually become
confined to the mountains and to a possible Puget Sound
Convergence Zone, especially later Saturday into early Sunday.
Temperatures will generally range from the mid 40s to mid 50s at
night, then into the 60s during the day.

Models show very good consistency and continuity next week as a
strong upper level ridge builds offshore and low level high 
pressure builds over British Columbia. The developing pattern 
results in the development of moderate to strong low-level 
offshore flow beginning on Monday and continuing through the 
upcoming week. Expect mostly clear skies next week with 
temperatures ranging from the 40s to lower 50s at night to the 70s
during the day. A model blend that favors the warmer GFS solution
was used to construct the temperature forecast. Albrecht

&&

.AVIATION...A weak cold front will push inland tonight for
isolated light showers across Western Washington. The flow aloft
is northwesterly. Onshore flow will increase behind this front
with low clouds and MVFR ceilings likely overnight. A warm front
will spread rain to the coast Thursday afternoon. 33


KSEA...Increasing clouds tonight with MVFR cigs by 12z. S winds at
the surface to 10 kt. The air mass will remain moist and stable on
Thursday as a warm front clips the region. 33


&&

.MARINE...Onshore flow will increase tonight behind a cold front - Small
Craft Advisory winds are possible through the Strait of Juan de 
Fuca and Northern Inland Waters. A stronger Pacific frontal system
will arrive Thursday night and Friday with increasing southerly 
flow over the waters. The flow will turn onshore Friday night and 
Saturday as this system moves inland. Offshore flow will develop 
early next week as a thermal trough forms along the coast. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday 
     for Central U.S. Waters Strait Of Juan De Fuca-East 
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland 
     Waters Including The San Juan Islands.

&&

$$

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