Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
845 am PDT Sat may 30 2015
Synopsis...low clouds cover the Puget Sound sound area...the coast...and
the southwest interior. These clouds should retreat to the coast by
afternoon. High and middle level clouds will increase Sunday as an
upper trough moves toward California...with showers over the area
Monday through Tuesday as the trough lifts northeast. Another weak
trough will bring a chance of showers Wednesday and Thursday.
Short term...satellite imagery shows the result of a moderate
marine push last night...with stratus covering the coast...the
southwest interior...and most of the area around Puget Sound sound. The
maximum extent of clouds appears to have occurred at 15z and they
will evaporate back to the coast by afternoon. Skies will be mostly
sunny today with highs in the 70s.
Stratus will push inland again tonight and retreat again Sunday
afternoon. Meanwhile an upper trough will develop off northern
California and this will stream middle and high level clouds into
Washington from the south. Beginning Sunday evening there will be a
chance of thunderstorms in the Cascades with the shower chance
becoming more widespread on Monday. The pattern of divergence aloft
suggests thunderstorms are possible over The Lowlands on Monday but
negative lifted indices are confined to the Cascades. Highs on
Sunday will be similar to today but lower heights and lots of
moisture will keep temperatures in the 60s on Monday. Burke
Long term...previous discussion...showers are likely Monday night
and Tuesday as the upper trough lifts across the area. The models
are not in especially good agreement in the longer range...but it
looks like another weak trough will move into the Pacific northwest
Wednesday or Wednesday night...followed by some upper ridging on
Aviation...an upper level low over central British Columbia
combined with an upper level ridge over the SW U.S. Will give light
west-southwest flow aloft over West Washington through Sunday. The air mass aloft is dry
and stable and no convection is expected over the area through
A surface ridge offshore is maintaining weak onshore flow over West Washington
this morning. Cool moist marine moisture has spread inland from the
coast across much of the interior lowlands. The marine layer is
shallow with estimated cloud tops somewhere between 1100 and 1700
feet. At 15z the stratus was still pushing north toward kbvs but the
eastward spread looked like it was stalling near Lake Sammamish.
The stratus is expected to burn back to the Washington coast
18z-20z...but may not make it all the way back to the coastline.
Good VFR conditions should prevail this afternoon with only some
Onshore flow will increase again this evening which will produce
another surge of marine moisture inland Sunday morning.
Ksea...stratus covers ksea this morning with bases ovc009 and tops
reported around 1100 feet. Weak onshore flow will continue this
morning which should delay the burn-off until around 19z. Ceilings will
rise slightly through 19z. Good VFR conditions are expected after
19z with just a few high clouds over the area. Surface winds will
remain SW 4-8 knots through around 23z then shift to west 3-7 knots. Kam
Marine...a surface ridge well offshore combined with lower pressure east
of the Cascades will give light to moderate onshore flow through the
weekend. Onshore flow was weakening this morning with the uil-bli
pressure gradient down to 1.5 mb at 15z. Small Craft Advisory winds are still present
mainly in the central Strait but should diminish by 18z.
Onshore flow will increase again this evening but should be weaker
than last night...remaining sub-gale in the central and east Strait.
Winds over the remaining Washington waters are expected to remain 20
knots or less through Sunday. Onshore flow through the Strait is
expected to be light Sunday afternoon and Sunday night.
An upper level low moving in from the SW will shift the onshore
pressure gradients to a SW direction Monday morning. Southerly winds
over the Puget Sound sound and the northern inland waters may approach
Small Craft Advisory criteria on Monday...with westerly Small Craft Advisory winds in
the Strait Monday evening. Kam
Pz...Small Craft Advisory central and east Strait of Juan Delaware fuca
7 am PDT through 11 am PDT and
4 PM PDT this afternoon through 5 am PDT Sunday.
You can see an illustrated version of this forecast discussion at