Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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360
FXUS66 KSEW 152217
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
320 PM PDT Tue Aug 15 2017

.SYNOPSIS...Higher pressure aloft building over the region
coupled with low level onshore flow will maintain seasonably mild
summer weather into Friday. A weak upper level weather system is
expected to bring some cooling and a small threat of showers Friday
night into Saturday before higher pressure rebuilds into the area
early next week with temperatures rebounding.

&&

.SHORT TERM...Generally westerly flow aloft across the Gulf of
Alaska over the top of a strong upper high near 40N/145W this
afternoon. A few weak shortwaves embedded in the flow. One brought
some high clouds to Western Washington today. The next near 135W
early this afternoon will bring some more higher clouds again
Wednesday. The following stronger shortwave crossing around 165W
this afternoon is expected to help raise heights aloft over the
region further Thursday for likely the warmest day this week.

At the surface, weak onshore flow should increase tonight and expect
some areas of morning clouds to develop Wednesday morning. The
onshore flow is a bit stronger Wednesday night for more Thursday
morning marine clouds. The sun should break out by afternoon with
seasonable or a bit warmer than average mid August temperatures.

By Friday, the progs are consistent with height falls aloft and a
transition to bringing what is left of the 165W shortwave onshore
Friday night into Saturday for an increase in cloud cover and a
small threat of showers mainly across the northern part of Western
Washington and the mountains in orographic flow. Buehner

.LONG TERM...The main energy of the incoming upper trough Saturday
will be in British Columbia with the southern tail sweeping across
Western Washington with a small threat of showers. This system will
bring some cooling with daytime temperatures dipping a bit below
average.

It appears former Typhoon Banyon located near 30N/163E early this
afternoon may play a key role in our forecast early next week. On
satellite imagery, this system looks quite weak. Mid-range progs
concur that as this system continues to become extra-tropical, it
will build a downstream upper ridge over the Pacific NW Monday for a
warming and clearing trend. The Seattle-North Bend OR surface
gradient turns offshore, meaning any marine cloudiness Monday
morning may be limited to the immediate coast leaving a nearly clear
sky for the eclipse. This trend can change and bears watching, but
that appears to be the case right now. Buehner

&&

.AVIATION...An upper level ridge will remain centered offshore
thru tonight. This will keep the flow aloft from the NW. Contd low
level onshore flow. Expect areas of LIFR CIGs/IFR vsbys to dvlp
after 0600 UTC, mainly over the coast. Overall, the stratus/fog
should be a little more widespread tomorrow morning compared to this
morning.

KSEA...VFR, but there will likely be some low stratus and fog in the
vicinity of the airport after 5 AM Wednesday (1200 UTC). Northerly
winds of 10 kt or less will become light and variable overnight.
Winds will generally be SW 5-10 kt during the day Wednesday.

&&

.MARINE...
Onshore or westerly flow of varying strength will continue through
this weekend due to persistent high pressure offshore with lower
pressure east of the Cascades. There is a possibility that wind
speeds could briefly reach gale force over the central Strait of
Juan de Fuca this evening and Wednesday evening. The probability of
gale force winds over the central and eastern Strait of Juan de Fuca
will be higher Friday evening.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle/

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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