Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
855 PM PDT Sat Aug 30 2014
Synopsis...westerly flow aloft will maintain a unsettled and cooler
than normal weather pattern over western Washington through
Wednesday. Weak weather systems embedded in the flow aloft will
bring periods of light showers mainly Sunday and Tuesday into
Wednesday morning. An offshore upper level ridge will move over the
region after Wednesday for warmer and drier weather into next
Short term...weak showers drifting off of the Olympics this evening
have been forming a narrow band across the area from sea-tac to
South Seattle. These showers should diminish by around 06z/11 PM as
the air mass becomes more stable.
A stronger than normal 110kt westerly jetstream will be the primary
factor in the weather pattern through middle week. Small weak upper
level shortwave troughs embedded in the west-northwest flow aloft will move
periodically across West Washington. A weak shortwave trough near 135w at 02z/7
PM will move across the area Sunday morning. This wave is not very
strong to begin with and the dark slot behind the wave on water
vapor imagery has been filling in...indicating a weakening trend.
This wave should bring a minor increase in shower activity Sunday
morning...but not enough to go with likely probability of precipitation. Vorticity forecasts
suggest another weak wave trailing the first feature...which should
maintain scattered showers Sunday afternoon.
The models have been having an understandably difficult time in
resolving these small weak waves...and are still having a hard time
with the third wave Sunday evening. This feature is the stronger
looking one on water vapor imagery out near 155w at 02z. Previous
model runs brought the wave more directly across the area...but
latest models take it over S b... brushing the north part of West Washington
Sunday evening. Current probability of precipitation may be a little low for Sunday and may
be a little high Sunday evening.
The jetstream will shift north over S b.C. On Monday as a flat weak
shortwave ridge moves across the area. Monday should be generally
dry and slightly warmer with a bit more sunshine as well.
The models agree on bringing a stronger higher amplitude shortwave
trough across the area Tuesday afternoon through Tuesday night.
Models have been rather miserly with the probability of precipitation so it would not be
surprising to see future guidance hit it a little harder. At this
point the nam12 looks it has a reasonable solution.
With the active weather pattern early this week...maximum temperatures
will remain below normal in the 60s to lower 70s. Kam
Long term...previous discussion from the 333 PM afd...overall
confidence for the big picture in the extended period a bit higher
than overnight. Medium range models continue to show an upper level
trough moving into the region Tuesday afternoon and evening bringing
another round of showers to the region through early Wednesday
morning. Models all show the zonal flow over the northeast Pacific
breaking down at this point into a higher amplitude pattern as a
ridge builds into the Gulf of Alaska. Models now in better agreement
on the downstream development of dry split flow over Pacific
northwest Thursday followed by the large scale ridge shifting east
Friday through the weekend with continued dry and warmer weather.
Aviation...an upper level trough over the region will shift eastward
tonight with light northwest flow aloft. Low level onshore flow behind the
surface trough will dominate through Sunday. Moist and weakly
unstable atmosphere. Scattered showers.
A Puget Sound sound convergence zone will focus most shower activity
between Tacoma and Everett. The convergence zone should
dissipate later tonight. There could be periods of MVFR at the
kbfi/ksea terminals tonight...with MVFR becoming more widespread
late tonight and Sunday morning. Clouds should lift to VFR with
scattering by midday Sunday.
Ksea...a Puget Sound sound convergence zone will keep shower activity
going through much of tonight...then just a few showers through
Sunday. Ceilings mostly VFR this evening...with MVFR ceilings developing
later tonight and Sunday morning. Improving ceilings after 18z.
Marine...westerly flow behind a trough will maintain small craft
winds in the central and east Strait through most of tonight. The
flow will weaken on Sunday. A weak decaying surface trough will
increase onshore flow slightly on Sunday night. Another stronger
trough will bring increased onshore flow Tuesday and Wednesday.
Pz...Small Craft Advisory central and eastern Strait of Juan Delaware fuca
through 3 am PDT Sunday.
You can see an illustrated version of this forecast discussion at