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fxus66 ksew 211528 

Area forecast discussion
National Weather Service Seattle Washington
828 am PDT Mon Aug 21 2017

Update...there are no significant updates to the forecast this
morning. Stratus and fog in the shallow, surface-based marine
layer is limited mainly to the coastal and inland waters and the
larger river valleys.

&& upper ridge will move across the Pacific northwest,
bringing mostly sunny and warmer weather to western Washington
today through Wednesday. A weak upper trough will move across the
area Wednesday night and Thursday for cooler weather and a chance
of showers, especially for the coast and mountains. Another ridge
will bring dry and warmer weather this weekend.


Short term...high clouds that passed through the area yesterday
have pretty much cleared out as high pressure aloft build over the
Pacific northwest. The ridge will maintain dry weather the next
couple days with light northerly flow. Clear skies this morning has
allowed for radiational cooling with areas of fog and low clouds
forming in place. Satellite fog product imagery shows this low level
moisture expanding rapidly along the coast and reaching down the
Strait and through the Chehalis gap into parts of the southwest
interior. While tops on these clouds are likely quite low, probably
less than 1-2k ft, it does appear that the low clouds could deepen
and penetrate further into greater Puget Sound than was expected.
Low clouds or fog could reach most of the interior waters and
surrounding coastlines except perhaps not reaching the north
interior and possibly the east lowlands and foothills. The good news
is that the shallow cloud tops should burn off rapidly, hopefully by
9 to 10 am just before the eclipse peaks. If clouds do hang in and
expand over the next few hours, the best chance of viewing the
eclipse will probably be east of Lake Washington and locations with
some elevation. Hopefully the shallow nature of the fog and stratus
will allow it to burn off quickly this morning. Once the shallow
moisture mixes, sunny skies will prevail the rest of the day. Highs
will warm into the low 80s today and again Tuesday with no rain

The pattern begins to change by Wednesday as an upper trough
approaches the region. Heights will fall and the flow will become
onshore with cooler high temperatures. Models seem to hold back on
the deeper moisture until the main trough axis shifts through early
Thursday. This will be the highest chance for measurable rain
around western Washington, although models agree that amounts will
be unimpressive. Orographic enhancement should squeeze out a bit
more rainfall along west facing slopes, perhaps a tenth of an inch
in spots.

Long term...high temperatures will struggle to reach above 70 on
Thursday with the chance of showers ending by evening. Long range
models still agree on building another ridge on Friday through the
weekend. The latest GFS is not as amplified with the ridge compared
to past runs. However, the European model (ecmwf) still builds 500 mb heights above
5800m by Saturday and Sunday. Highs could reach the low to mid 80s
if this pans out. Mercer


Aviation...light northwest flow aloft continues over western
Washington this morning ahead of an upper ridge just off the
Pacific northwest coast. The flow will switch to west-southwest
midday today as the ridge moves inland this afternoon and tonight.
At the surface, high pressure remains offshore with lower
pressure east of the Cascades, for weak low level onshore flow.

The air mass is dry except for a shallow surface-based marine
layer mainly over the coastal and inland waters and in the larger
river valleys. The inland stratus should generally clear by 17z,
with stratus on the coast clearing by 19z. Stratus/fog should be
even more limited late tonight and Tuesday morning.

Ksea...north to northwest wind 6-12 kt. It looks like stratus/fog
this morning should remain over the sound. Mcdonnal


Marine...high pressure off the coast with lower pressure east of
the Cascades will maintain varying degrees of low level onshore
flow into western Washington through late this week.

Small Craft Advisory west winds are likely in the central and
eastern Strait of Juan de Fuca each evening through early morning,
although gale westerlies are possible Tuesday and Wednesday nights
as a weak upper level trough approaches British Columbia and
tracks inland Thursday. Mcdonnal


Sew watches/warnings/advisories...
Pz...Small Craft Advisory until 5 am PDT Tuesday for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.



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