Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 171634

Area Forecast Discussion
National Weather Service Seattle WA
934 AM PDT Sat Mar 17 2018

.SYNOPSIS...Upper level low pressure over the Pacific Northwest
will bring scattered light showers to Western Washington this
weekend and Monday, but for the most part the weather will be
dry. An upper ridge will move across the region Tuesday for dry
weather. A frontal system will bring a return to wetter weather
Wednesday through the end of the week.


.SHORT TERM...A 5390 meter upper low is over southeast Oregon this
morning, as it continues to move slowly east-northeast across the
region. Light northerly flow aloft prevails over Western
Washington, while the low level flow is beginning to turn weakly
onshore. There is more cloud cover over the area this morning,
especially from the Puget Sound region southward, and 9 am
temperatures ranged from the mid 30s to the mid 40s.

The upper low over southeast Oregon will weaken and move slowly
into central Idaho late today, becoming a non-player in our
weather by this evening. Another upper low will form over
Vancouver Island late this afternoon and drift slowly south-
southwest over our offshore waters through Sunday. This will cause
the light northerly flow aloft to become weak southerly. At the
same time low level onshore flow will increase, bringing more
moisture into the lower 5000 ft or so of the atmosphere.

The cool air aloft, low level moisture, and daytime heating will
lead to weak instability and scattered showers today and Sunday;
showers will be most numerous during the afternoon and evening
hours, and over the mountains. Much of Western Western will miss
out on the showers, and where precipitation does occur it will be
mostly light. There will be some sun breaks as well, and
afternoon temperatures will be near normal -- mainly in the lower
to mid 50s. Lows will be in the mid 30s to lower 40s.

Models are having some trouble pinning down the movement of the
upper low by Monday, which is pretty typical model behavior. It
looks increasingly like the upper low will continue southward
into the Oregon offshore waters on Monday. (Yesterday, models
were leaning toward the idea that the low would drift further
west, with a small upper ridge building northward over the Pacific
Northwest.) So it`s starting to look like Monday`s weather will
be pretty similar to what we see this weekend. McDonnal

.LONG TERM...Here is the long term section from the early morning
forecast discussion -- After another mostly dry day Tuesday, a
deep upper trough off the coast will turn the upper flow southerly
by Wednesday. Models bring moisture from the south on Wednesday
and especially Thursday. The upper low is forecast to move inland
on Friday. The pattern is cloudy and moist, and by Friday it will
be cool as well. The snow level will fall to 1000 feet or so
Thursday night and Friday. After highs in the 50s all week, on
Friday the high temperatures may not get above the 40s. Burke


.AVIATION...The region will remain under cyclonic flow aloft
today for light and variable flow aloft. The low level flow will
be light onshore. Areas of mainly IFR CIGs this morning will
dissipate or lift by early this afternoon. However, expect areas
of MVFR CIGs to persist over the coast. The air mass will become
weakly unstable this afternoon for a few showers.

KSEA...IFR CIGs (006-009) will lift into the MVFR category range
(010-015) later this morning or scatter out altogether. At any
rate, expect VFR conditions to return by 1900 UTC. MVFR CIGs may
return late tonight or early Sunday. Expect winds to increase from
the south-southwest near 10 knots later this morning. Light
southerly winds will prevail tonight through Sunday morning.


.MARINE...Higher pressure offshore with lower pressure inland
will result in onshore flow through this evening. The flow will
weaken overnight. Expect weak onshore or westerly flow to prevail
across the area Sunday and Monday.


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


PZ...Small Craft Advisory from 5 PM this afternoon to midnight PDT
     tonight for East Entrance U.S. Waters Strait Of Juan De

     Small Craft Advisory from 3 PM this afternoon to midnight PDT
     tonight for Central U.S. Waters Strait Of Juan De Fuca.


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