Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 210350
Area Forecast Discussion
National Weather Service Seattle WA
850 PM PDT TUE SEP 20 2016
.SYNOPSIS...An upper trough will gradually shift south over the
region the next couple days. The Western Washington lowlands will
have mostly sunny weather Wednesday and Thursday, with scattered
afternoon and evening showers over the mountains. A weak front
could bring some rain Friday, then a strong ridge will bring dry
and warmer weather Saturday through Monday.
.SHORT TERM...An upper trough/upper low over the Pacific
Northwest will gradually shift south and east over the next
couple days. Showers and isolated thunderstorms were mostly
confined to the Cascades and Southwest Interior earlier today but
these are currently fading with the loss of daytime heating.
There should be another round of convection Wednesday afternoon in
roughly the same locations as today but the air looks a bit more
stable with the upper low sagging south. This should preclude any
thunder and showers should be more isolated. Elsewhere will be
mostly sunny with highs close to normal.
The upper low will shift further south and east away from Western
Washington on Thursday with an upper ridge building just
offshore. The air will become more stable and it should be a dry
and mostly sunny day.
Models continue to show a weakening front reaching the area
Friday. A little rain is likely near the coast but further inland
will probably not get much as the front falls apart. Schneider
.LONG TERM...Previous discussion...The models have also come into
better agreement that the next weak front will probably fizzle
offshore or be shunted northwards, as a large upper ridge begins
to build over the region. We have removed the mention of
precipitation from the forecast for Saturday and Saturday night.
The ridge will build strongly Sunday with 500 mb heights rising to
around 5900 meters. Then it will shift eastward Monday. Both days
should be generally sunny, and we have raised high temperatures
considerably. Much of the area should have highs in the mid 70s to
lower 80s both days. A weak front will probably move into the
region Tuesday. McDonnal
.AVIATION...A week upper level trough over Western Washington will
slowly dig southwest and offshore through Wednesday morning. Flow
aloft will remain light and northeasterly, with northerly low level
flow through Wednesday. Lingering showers over the Cascades will
diminish this evening as the air mass trends drier and more stable.
Overall VFR conditions will prevail tonight, however radiational
cooling and light winds will help patchy IFR fog form in sheltered
areas, especially over the the South Puget Sound and Southwest
Interior. Conditions will improve all areas to VFR levels by
KSEA...VFR conditions this evening. Patchy fog remains possible late
tonight or early Wednesday morning. The likely scenario will be fog
forming near the terminal in the low lying and sheltered valleys to
the east. Northerly winds 6-12 KT.
.MARINE...Northerly winds 10-15 knots over the interior waters will
ease overnight. Winds will strengthen Wednesday afternoon and
evening over the coastal waters, with small craft advisory criteria
winds possible, especially over the southern zones. Moderate onshore
flow will develop Thursday as higher pressure settles offshore.
Models now show a front reaching the coast Friday with more
widespread small craft advisory southerly winds. Winds will be
lighter over the weekend as high pressure develops aloft and at
You can see an illustrated version of this discussion at