Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT Thursday Jul 24 2014
Synopsis...precipitation over western Washington will decrease
today as the upper level low over southeast British Columbia moves
into Alberta. There may be a few sun breaks in the afternoon as the
air mass begins to dry. High pressure will start building over the
region Friday...bringing sunny warmer weather through the middle of
Short term...a 5590 meter upper low is over southeast British
Columbia this morning and moving slowly east-northeast. The air mass
over western Washington remains quite moist...with middle and high
level moisture wrapping around the north side of the low and moist
onshore flow at low levels. Precipitation has been steadily
decreasing this morning though...and at 8 am was limited mainly to
the northwest interior...Everett area...and the central and north
The upper low will move into southern Alberta late today...and the
wrap around moisture will move northeast with it. The air mass over
western Washington will remain generally moist below 6000 or 7000 feet
though as onshore flow continues. Some showers should continue over
the Cascades and there will probably be a weak Puget Sound sound
convergence zone north of Seattle...but outside of that showers
should be light and spotty at most. There will probably be some sun
breaks in the afternoon.
Westerly flow will prevail over western Washington tonight as the
low moves into Saskatchewan...and an upper ridge will just begin
building over the region on Friday. Low level onshore flow will veer
to northwest...and the air mass will gradually dry. Clouds over
western Washington tonight and Friday morning should give way to
partly to mostly sunny skies Friday afternoon. High temperatures
will be back into the 70s over most of the area.
The upper ridge will continue to build Friday night and Saturday...
with its axis along or just west of the Rocky Mountains. The 500 mb
heights over western Washington will climb to around 5820 meters...
and onshore flow will be weak. There will be some marine layer
clouds at night and Saturday morning...especially at the coast...but
the day will be sunny and warmer yet. Mcdonnal
Long term...here is the long term section from the early morning
forecast discussion -- long range models are fairly consistent in
strengthening a large high pressure ridge over the interior western U.S.
Sunday through the middle of next week. It appears the ridge axis
will extend through the northern rockies which should result in SW flow
aloft over Washington. This upper flow pattern favors light west-northwest surface
gradients which may bring some clouds to the coast but probably not
making much progress inland. GFS MOS gives low to middle 80s Sunday
through the middle of next week. The European model (ecmwf) is stronger and further
west with the ridge which could result in temperatures approaching
90 degrees. Either way it will be a big change in the pattern with
dry weather and above average temperatures. Mercer
Aviation...the upper level low now over southeast British Columbia
will continue to move east today. Moderate northwest flow aloft will
develop as an upper ridge builds over the area. Low level onshore
flow will strengthen today. Air mass is unstable and moist this
morning but will stabilize and dry this afternoon and evening.
Ceilings are generally MVFR 2-3k feet this morning except for some IFR
patches mainly in the north interior. There will be an overall
improving trend in the afternoon and evening...except mesoscale
models show the development of a Puget Sound sound convergence zone over
the central sound late in the afternoon and evening.
Ksea...MVFR conditions with showers will become more VFR after 21z.
There is some chance a pscz over the central sound will affect the
terminal. Southwest wind 8-12 knots. Chb
Marine...moderate onshore flow has already developed with Small
Craft Advisory strength winds already occurring in the Strait. Have
updated the grids and forecasts for 20-30 knots westerly winds in the
Strait beginning this morning. Moderate northwesterly winds over the
outer coastal waters will ease this afternoon.
A more typical onshore flow pattern will persist through the weekend
with Small Craft Advisory westerlies in the Strait possible at times
during the afternoon and evening hours. Chb
Pz...Small Craft Advisory outer coast...central and east Strait...
northern inland waters.
You can see an illustrated version of the forecast discussion at