Scientific Forecaster Discussion
fxus66 ksew 221559
Area forecast discussion
National Weather Service Seattle Washington
859 am PDT Thu Jun 22 2017
Synopsis...a building upper ridge will provide dry weather and a
warming trend through the weekend. The ridge will shift inland early
next week with onshore flow developing. Temperatures will start to
moderate Monday with a return to more seasonable temperatures
Tuesday and Wednesday. Dry weather will persist but night and
morning clouds will return next week.
Short term...other than a thin ribbon of low stratus along just
off the north coast of western Washington, skies are severe clear
across the region. Low level flow is becoming more northerly as a
ridge of high pressure extending from haida gwaii southward along
about 135w noses into south-central British Columbia. The high
building to the north will give northeasterly flow aloft and
northerly flow at the surface, the start of a good warming trend.
After a cool start this morning with temperatures in the 40s to
lower 50s, expect strong June sunshine to pop highs well into the
70s. The dry air mass in place and clear skies will allow
temperatures to fall back to 45 to 55 tonight. High temperatures
on Friday will be in the 70s, except lower to mid 80s several
miles east of Puget Sound and in the interior south of Seattle.
Warming really ramps up on Saturday as adiabatic compression does
its job at lower levels and temperatures aloft warm with the
incoming upper ridge. Offshore flow will be aided by the
development of a thermally induced trough now over northern
California into western Oregon and the southern portions of
western Washington. Temperatures will rise well into the 80s most
areas with some lower 90s seen inland from the beaches on the
north coast and a few miles east of Puget Sound, and in the
interior south of Seattle.
Current forecasts are in good shape this morning. No updates are
Long term...from the previous long term discussion: models agree
that the upper ridge axis will remain over western Washington
through Sunday, then begin to flatten and shift slowly inland on
Monday. Thermally induced surface low pressure along the coast
will shift inland over the interior lowlands Sunday afternoon.
Offshore flow will become light in the afternoon with a lack of
sea-breeze effects producing the hottest day of the year around
greater Puget Sound. GFS/European model (ecmwf) MOS give 95/92 for sea-tac, either
of which would break the record of 88 set in 2006. Given this is
day 4, decided to keep temperatures little changed which mirror
the more conservative European model (ecmwf) guidance.
A transition to onshore flow begins Sunday evening into Monday. The
switch looks to occur at the coast first Sunday night, then
gradually develop inland sometime on Monday. Both the GFS/European model (ecmwf)
appear to show onshore winds reaching Puget Sound in the afternoon
which would halt afternoon warming from reaching much above 80.
Stayed close to MOS which gives low 80s but did indicate some upper
80s east of Puget Sound near the foothills where onshore flow may
arrive a bit later.
Stronger onshore flow should flood marine air and stratus into
western Washington Monday night and Tuesday morning, repeating again
on Wednesday. There should be partial afternoon sunshine each day
but highs will fall back closer to average. The dry pattern appears
likely to continue but will need to watch a trough that will brush
the north part of the area Tuesday and Wednesday. Mercer
Aviation...an upper level ridge will continue to slowly intensify
today and into tomorrow with moderate northwest flow aloft today,
becoming lighter tonight and Friday. The air mass will be stable,
with dry conditions persisting. Clear skies are expected area-wide.
Ksea...clear skies for the next 30 hours with prevailing north
wind around 5-10 kts. Haner/smr
Marine...a northeast-southwest oriented surface ridge will remain
about 400 nautical miles northwest of Cape Flattery through
Saturday. Meanwhile, a strong heat low over California and
southern Oregon will set up northerly gradients over the waters.
Inherited headlines have been allowed to expire...but models
continue to suggest that winds over the coastal waters may creep
back into Small Craft Advisory criteria by this evening. Will evaluate with incoming
12z data before making decision for afternoon forecast package. On
Sunday gradients will weaken, then onshore flow will quickly
increase on Sunday night. Haner/smr
Pz...Small Craft Advisory until 9 am PDT this morning for coastal
waters from Cape Flattery to James Island 10 to 60 nm-
coastal waters from James Island to Point Grenville 10 to
60 nm-coastal waters from Point Grenville to Cape
Shoalwater 10 to 60 nm.