Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 291548
Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Thu Sep 29 2016
.SYNOPSIS...Dry southwest flow aloft will bring mostly sunny weather
through Friday, except for patchy morning low clouds and fog. An
upper level trough will bring showery and cooler weather Friday
night through the weekend. Unsettled weather will likely continue
early next week in westerly flow aloft.
.SHORT TERM...There is a bit more fog this morning than yesterday.
There is also a patch of stratus on the west side of the north
Sound. Otherwise skies are mostly clear except for a band of cirrus
moving across the area. By afternoon most areas should be sunny.
Highs today will be in the 60s, just touching 70 in the south.
An upper level low moving south off the coast will turn upper flow
more southerly tonight and Friday. Previous and current model runs
generally hold off any rain until Friday night, and even then limit
it to the coast and mountains. Friday therefore is likely to be dry
with increasing high and mid level clouds. Highs will be in the 60s.
The most noticeable change Friday may be the lowering snow level,
which by afternoon will be 6000-7000 feet, down from its current
level of 10000 feet or so.
Saturday looks like the wetter day this weekend. Models agree that
rain from an impulse spinning north around the upper low will spread
over Western Washington on Saturday. The amounts appear light and
the more mesoscale models do not have rain over every part of the
interior lowlands. However the trend is clear and will probably
increase pops for Saturday and Saturday night, especially over the
mountains and coast. Highs Saturday will be in the lower to mid 60s.
.LONG TERM...Previous discussion...The secondary low center mentioned
above will drop into the base of the large trough Saturday night,
and should be W of the southern Oregon coast (near 43N/128W) early
Sunday morning. Unlike the previous two shortwave troughs, the GFS
and ECMWF take this feature to the SE heading toward N CA Sunday
afternoon. Moist S flow aloft will still be present over W WA Sunday
morning for a chance of showers, but it now looks like the shower
activity could be decreasing Sunday afternoon as the secondary low
moves farther away. POPs are still broadbrushed in the chance range
for Sunday, but could be refined in later forecasts if the models
stick with this solution. The air mass will remain cool on Sunday
with highs in the upper 50s to lower 60s and mountain snow levels
around 5000 feet.
The models are in broad agreement that the flow across the PacNW
will be generally westerly Monday through Wednesday. The GFS ECMWF
and Canadian models still show weak systems reaching the area but at
different times. With no consensus in timing of these features, the
current broadbrush chance of showers remains the best forecast. Kam
.AVIATION...An upper level low will spin offshore today with
southwesterly flow aloft. The air mass over Western Washington is
dry. There is patchy fog on the coast and in the interior with
mainly cirrus clouds aloft otherwise. The patchy fog will burn
off late this morning with VFR conditions expected. Onshore flow
will increase overnight with stratus clouds pushing into the
central sound. 33
KSEA...Mainly high clouds today for VFR conditions. North winds at
the surface to 10 kt. 33
.MARINE...Onshore flow will prevail across Western Washington
today with high pressure offshore and lower pressure inland.
Westerly winds will increase through the Strait of Juan de Fuca
this evening - a Small Craft Advisory is in effect. A weak frontal
system will spiral offshore on Friday then diminish as it moves
inland Friday night. Another weak surface low will track down
130W on Saturday then shift into southern Oregon on Sunday. A
stronger Pacific frontal system will reach the area on Monday. 33
PZ...Small Craft Advisory from 6 PM this evening to midnight PDT
tonight for Central U.S. Waters Strait Of Juan De Fuca-East
Entrance U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at