Scientific Forecaster Discussion
fxus66 ksew 231601
Area forecast discussion
National Weather Service Seattle Washington
901 am PDT Fri Jun 23 2017
Synopsis...an upper ridge will bring dry weather with a warming
trend through the weekend. High pressure aloft will shift inland
on Monday with onshore flow developing. Temperatures will moderate
closer to average with highs mainly in the upper 60s to to low 70s
during the early to middle part of next week. Dry weather should
continue next week but with areas of low clouds giving way to
afternoon sunshine over the interior.
Short term...skies are clear across the region this morning with
dry low level northerly flow as a ridge offshore builds and
progresses slowly eastward. 500 mb heights build close to 5900m
this weekend as the air mass warms; low level offshore flow and
compressional heating will add to the warmth of the boundary layer
this weekend. Afternoon high temperatures will be close to record
levels on Saturday and will likely break records on Sunday. Dry
weather will prevail through the weekend.
Pressure gradients this morning are northerly and cross Cascade
gradients are gradually shifting to offshore. The northerly flow
will hinder high temperatures slightly at locations near the
water, including Seattle, today. Areas inland from the Puget Sound
and in the southwest interior zones will see the biggest increases
in temperature today. Highs today will be in the 70s north, near
80 on the coast, and in the lower to mid 80s in the south
The hottest day at the coast is expected Saturday, including places
like Forks and Hoquiam as offshore winds increase. Areas around
the southwest interior and greater Puget Sound to the south of
Everett will also reach near 90. Forecasts generally follow a
mean of the raw 2 meter temperatures from the mesoscale models.
The latest 12z NAM model output statistics are 5 degrees hotter
for Saturday in the interior, and don't appear realistic.
Models continue to show the thermally induced low shifting inland
over Puget Sound on Sunday with easterly cross Cascade gradients
peaking during the morning to midday. This will bring the hottest
temperatures of the year to the I-5 Metro areas including Seattle
and Bellevue. 2 meter temperatures from the GFS and NAM show highs
in the lower to mid 90s from the Seattle Metro area southward in
the interior. MOS values from the 12z NAM appear a couple degrees
too warm and are being discounted. With a couple days expected to
reach near or above 90 in the Metro area fairly early in the
season, a heat advisory is in effect for the Seattle-Tacoma-
Bellevue Metro areas southward to the southwest interior and east
toward the Cascades for midday Saturday through Sunday evening.
Forecasts are in good shape. No updates are planned. Albrecht
Long term...from the previous long term discussion: a transition
to onshore flow still looks to occur on Monday. The latest 00z
model runs still show a weak push Sunday night which may bring
some low clouds to the coast partially into the lower Chehalis
gap. It looks like a more gradual push scenario with any low
clouds clearing quickly inland and still abundant afternoon
sunshine but with gradually increasing onshore flow. Upper heights
will also be falling. Foothills may hold out with one more hot
day in the mid to upper 80s but current timing of the push would
halt temperatures from warming much above 80 by early to mid
afternoon over most the area.
A weak trough brushes mainly to the north of the area Tuesday.
Models are mostly dry although a stray shower could occur over the
northern Cascades. The odds of anything measurable are very low and
will keep the forecast dry. Lower heights and stronger onshore flow
will bring much cooler temperatures. Highs will be more seasonable
in the upper 60s to low 70s through Thursday. Clouds will be
stubborn along the coast which will also intrude inland during the
night and morning hours. However, partly to mostly sunny skies are
expected inland during the afternoon and early evening hours.
Aviation...an upper level ridge will prevail over the region for
northwest flow aloft. Low level northerly flow will become offshore or
easterly tonight. Expect VFR conditions.
Ksea...VFR. Winds will be northerly 7-14 knots, strongest this
afternoon and evening.
high pressure over British Columbia with lower pressure over
Oregon will result in northerly flow today. The flow will become
offshore or easterly tonight due to a thermally induced trough of
low pressure setting up just off the coast. Offshore flow will
then persist for much of this weekend.
Onshore flow will return late Sunday afternoon and then strengthen
Monday and Tuesday for gale force westerlies across parts of the
Strait of Juan de Fuca.
Washington...heat advisory from noon Saturday to 9 PM PDT Sunday for Bellevue
and vicinity-Bremerton and vicinity-east Puget Sound
lowlands-Hood Canal area-Seattle and vicinity-southwest