Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 250333
Area Forecast Discussion
National Weather Service Seattle WA
833 PM PDT SAT SEP 24 2016
.SYNOPSIS...A weak front over British Columbia will keep high
clouds over Western Washington tonight with spotty light rain in
the north. A strong upper ridge will build Sunday and Monday for
sunny warmer weather. A weak front will move through Monday night,
but generally dry weather is expected Tuesday through Thursday. An
upper trough will bring a chance of showers Friday and Saturday.
.SHORT TERM...An upper level ridge is building over the region,
extending northeast from the 5960 meter upper high well west of
California. At the same time a weak warm front is moving over the
north side of the ridge into British Columbia, and this feature is
bringing mid and upper clouds to Western Washington with spotty
light rain in the north. At 8 pm temperatures were in the 50s to
The upper ridge will continue to build tonight and Sunday, and
models are still forecasting 500 mb heights over Western
Washington to climb to around 5900 meters late in the day. The
ridge will push the frontal band north of the forecast area, and
the air mass will dry as the low level flow turns light northerly
or even weakly offshore. The day will probably begin with some
cloud cover, but Sunday should become sunny with highs mainly in
the upper 60s and 70s and a few spots into the lower 80s.
The upper ridge will move east of the Cascades Sunday night and
into the northern Rocky Mountains on Monday. Its weak thermal
surface trough will move across Western Washington on Monday
though, and the lower atmosphere will warm several degrees. Monday
will be the warmest day in the stretch for the interior, with
much of the lowlands in the mid 70s to lower 80s. For Seattle --
SeaTac, officially -- we will be close to the record high of 78
for the day.
Onshore flow will increase quickly Monday evening, pushing the
marine layer inland Monday night. In addition, a weak front and
its upper trough will move in from the west later Monday night for
a chance of precipitation late Monday night and Tuesday morning.
.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- Models generally agree that a long wave
trough will carve out over the eastern Pacific Wednesday through
Thursday, then will swing eastward across the area Friday through
next weekend. The general trend of the models has been to sharpen
the trough offshore and slow its forward progress somewhat. Hence,
Wednesday and Thursday should be mainly dry with some fog in
favored valleys during the late night and morning hours. Rain
chances will increase Thursday night through the weekend with the
incoming upper trough, and temperatures will fall to below normal
values for late September and early October. Albrecht
.AVIATION...A mid level deck of clouds at 8 to 10kft this evening
will thin out and dissipate overnight as a strong upper ridge
continues to build into the region. There probably wont be enough
time with clear skies for much fog to form, but if it does it will
be over SW WA where skies will be clear for more of the night, so
Olympia and Hoquiam may see a few hours of fog at daybreak.
KSEA...This mid level deck of clouds should thin and clear out
overnight. Sunny with light and variable winds, and then a light
north breeze later in the day for Sunday.
.MARINE...A warm front is brushing the outer WA coastal waters
tonight. A surface high over western WA will keep the flow light
through Sunday morning. Light northerly flow will develop Sunday
afternoon before turning more onshore or westerly on Monday.
The onshore flow will strengthen Monday night in response to the
passage of a cold front. This will lead to the possibility of
gale force westerlies over parts of the Strait of Juan de Fuca.
The onshore gradient will weaken on Tuesday.
You can see an illustrated version of this discussion at