Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PDT Wednesday may 27 2015
Synopsis...relatively weak low level onshore flow will prevail for
the next several days...with above normal temperatures and mostly
sunny days. An upper low will bring showers and cooler weather
the first part of next week.
Short term...the air mass remains unstable over the Cascades with
the visible satellite imagery loop showing late day cumulus
development...mainly near the Cascade crest. A batch of
thunderstorms did manage to drift southward out of b.C. Into Whatcom
County over the past couple hours. This activity should
diminish shortly with loss of daytime heating. Models keep negative
lifted indice's over the upper west slopes of the Cascades to the crest the next
couple afternoon and evenings. Showers are in the forecast but
isolated thunderstorms might need to be added near the crest and far
north. It will remain stable and dry elsewhere.
A weak upper level low will migrate slowly southward along the b.C. Coast
into the weekend. The short term affect will be to maintain weak
onshore through Saturday bringing marine stratus along coast into
the Strait and through the Chehalis gap during the late night and
morning hours. Gradients are marginal for bringing stratus beyond
the far S sound into the Tacoma/Seattle/Bremerton areas. Assuming
even a partial shallow stratus push...models still show 850mb temperatures
warming from about +2 to 3c Thursday and Friday with some mixing.
The trend should be toward slightly warmer temperatures
inland...possibly reaching near 80 around greater Puget Sound sound by
Friday. Will need to watch the strength of onshore flow as a heavier
marine influence could hinder high temperatures. A more pronounced
marine push is likely sometime around Sunday with moderating
temperatures and more clouds.
Long term...Sunday will be another mostly sunny after some morning
low clouds. Highs might even warm a little as an upper low
approaches from the southwest and low level onshore flow eases.
The aforementioned upper low will move over Oregon on Monday.
Moisture coming up from the south with this low will bring an
increasing chance of showers to the area. Highs will cool into the
60s. The upper low will move inland to the south...close enough to
keep showers and clouds over the area through Tuesday with highs
remaining in the 60s.
Weak high pressure aloft will build over the region on Wednesday.
This should allow things to dry out and there will probably be some
sunshine by afternoon. Schneider
Aviation...an upper ridge over the offshore waters combined with an
upper trough over Idaho will give light northerly flow aloft. The
air mass is dry and stable with a marine layer offshore that will
push into portions of the interior during the late night and morning
Stratus bases along the coast are 008 mean sea level with visibilities
generally 5sm or higher. Low level flow is light to moderate onshore
this evening. The 00z nam12 and the 02z hrrr both show the stratus
moving inland through the gaps in the terrain making it eastward to
Whidbey Island and Everett on the north and to around ksea on the
south. Expect ceilings 003 at ksea early Thursday morning and 1/2sm fog at
kpae. All stratus and fog will burn back to the coastal zones by
Ksea...mostly clear skies tonight. Stratus with bases 003 will touch
the terminal from about 13z to 18z. Light northwest winds early this
evening will turn southwesterly to 5 knots after 08z then will return to northwesterly
about 20z Thursday. Albrecht
Marine...the subtropical ridge along about 140w combined with lower
pressure east of the Cascades will give light onshore flow through Friday
or Saturday. The flow though the central and eastern Strait of Juan
Delaware fuca will likely increase to Small Craft Advisory criteria during
the late afternoon and evening hours each day. Onshore pressure
gradients will increase somewhat Friday evening as an upper level
disturbance drops into central British Columbia from the northwest. Albrecht
Pz...Small Craft Advisory central and east Strait of Juan Delaware fuca
until 1 am PDT Thursday.
You can see an illustrated version of this forecast discussion at