Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PDT Monday may 25 2015
Synopsis...a surface ridge offshore will maintain moist low level
onshore flow across western Washington this week. Moderate onshore
flow will keep skies mostly cloudy through Tuesday...but weaker
onshore flow from middle week Onward along with higher pressure
building over the region will allow more afternoon sunshine and a
warming trend. The air mass over the Cascades will continue to
become slightly unstable late in the day for a chance of showers
near the crest.
Short term...the forecast was updated this evening to remove
thunderstorms over the Cascades of Whatcom and Skagit counties. The
upper level low is now spinning over eastern Washington with T-storms
remaining east of the crest tonight. The rest of the forecast is
The upper level low will spin over the Washington/Idaho border through Tuesday.
Showers will mainly affect eastern Washington with a few showers clipping
the Cascades. Otherwise...The Lowlands will be dry but mostly cloudy
as onshore flow persists.
The upper level low will weaken and exit east on Wednesday while
high pressure builds in from the west. We may see a break in the
clouds Wednesday afternoon for a few sunbreaks and warmer weather.
There is still enough moisture and instability for a chance of
showers in the Cascades Wednesday afternoon and evening...with
thunderstorms possible near the crest. 33
Long term...previous discussion...GFS/ECMWF/Canadian all bring
another upper level low down the b.C. Coast Wednesday through Friday
morning...when per the European model (ecmwf)/Canadian it will be centered over the
central b.C. Coast. The GFS does not bring the low as far S and is
quicker in sending it east over the weekend. None of the models show
any precipitation reaching West Washington...but it will probably prevent the upper
level ridge over the Pacific northwest from building any higher than 5760
meters. Onshore flow should remain fairly weak for partly to mostly
cloudy mornings and afternoon sunshine. Maximum temperatures are expected to
top out in the 70s each day this weekend.
All three models agree on progressing the ridge quickly east Sunday
night...allowing an upper level trough to approach the region from
the SW on Monday. Kam
Aviation...a weak closed upper low will move southeast through
eastern Washington tonight through Tuesday...reaching northeast
Oregon late Tuesday night. Light northwest flow aloft. Moderate
onshore flow at the surface will keep the air mass moist in the lower
levels and somewhat moist aloft. Air mass is stable except slightly
unstable over the Cascades.
Mostly high end MVFR ceilings this evening will lower to 1-2k feet
overnight. Ceilings will lift Tuesday afternoon to 3-5k feet. Patchy
morning drizzle is possible over The Lowlands and somewhat more
likely along the coast.
Ksea...the deep marine layer will remain intact for the next 30
hours along with broken-overcast cloud coverage. Ceilings will fall below
2k feet tonight and Tuesday morning...then lift near 4k feet in the
afternoon. Southerly wind 3-7 knots. Chb
Marine...high pressure will remain offshore with lower pressure inland all
week long. Small Craft Advisory strength winds of 15 to 25 knots will
occur this evening and most evenings this week. The lowest chance of
Small Craft Advisory strength westerlies will be on Thursday when
onshore flow should be weakest. Chb
Pz...Small Craft Advisory central and east Strait.
You can see an illustrated version of this forecast discussion at