Scientific Forecaster Discussion
fxus66 ksew 220423
Area forecast discussion
National Weather Service Seattle Washington
923 PM PDT Sat Apr 21 2018
Synopsis...an upper ridge will build offshore Sunday for a sunny
afternoon. The ridge will move inland Monday, bringing a stretch
of sunny warm days that should continue through Thursday. An upper
low moving into Oregon on Friday should result in cooler weather
and more clouds.
Short term...surface high pressure will nose into western
Washington from the Pacific through Sunday. Weakening onshore flow
and increasing northerly flow will lead to an overall clearing
trend over the next 30 hours, aided by large-scale subsidence.
Late Sunday, an upper ridge axis will move inside 130w, and a
thermal pressure trough will develop along the Oregon and
Washington coast on Sunday night. The upper ridge axis will pass
overhead on Monday and into far eastern Washington on Tuesday, but
the low-level offshore flow will continue all the way into
Wednesday. Several days of offshore flow will lead to a
substantial warming trend, especially with afternoon temperatures.
High temperatures are forecast to reach 70f on Monday over the
coastal side of the olympic peninsula. By Tuesday, 70+ degree
temperatures will expand to include the I-5 corridor from Everett
southward, with mid-upper 70s forecast over southwest Washington.
Otherwise, the offshore flow and upper level high pressure will
dry out the air mass, so few or no clouds are expected from
Sunday evening through at least Wednesday. Haner
Long term...from previous discussion: the upper ridge will
remain over the region Wednesday and Thursday, with light offshore
flow continuing, and the lower part of the atmosphere warming. On
Thursday, the 850 mb temperatures should be around +14c; that
should be the warmest day over the interior lowlands, with highs
ranging from the upper 60s in the north to the lower 80s in the
south. The pattern will probably break down Friday and Saturday,
as an upper low begins moving northeast from the California
offshore waters into Oregon. That should at least induce a switch
to onshore flow, resulting in cooler weather with highs in the
upper 50s and 60s. It could produce some showers as well. Mcdonnal
Aviation...an upper ridge axis will develop off the coast on
Sunday, and surface high pressure will nose into western
Washington on Sunday. Moderate northwest flow aloft tonight,
becoming northerly on Sunday. Further scattering of clouds is
expected through Sunday. The air mass will become increasingly dry
Ksea...northerly surface wind component for the next 30+ hours.
Clouds will scatter fairly quickly through 07z. Just enough low-
level moisture on Sunday morning that a few clouds in the
1,500-2,000 foot range cannot be ruled out in the first few hours
after sunrise on Sunday, but this should be short-lived. By late
Sunday morning, confidence is high that clear skies will be in
place and then persist for a few days. Haner
Marine...onshore flow will weaken tonight and early Sunday. Seas
around 10 feet over the coastal waters will slowly subside
A thermally induced trough of low pressure will set up
along the Oregon and Washington coasts from Sunday night through
Wednesday morning, leading to light northerly offshore flow. Haner
Pz...Small Craft Advisory for hazardous seas until 3 am PDT Sunday
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.
Small Craft Advisory for rough bar until 3 am PDT Sunday for
Grays Harbor bar.
Small Craft Advisory for hazardous seas until midnight PDT
tonight for coastal waters from Cape Flattery to James
Island out 10 nm-coastal waters from James Island to Point
Grenville out 10 nm-coastal waters from Point Grenville to
Cape Shoalwater out 10 nm-West Entrance U.S. Waters Strait
of Juan de Fuca.
Small Craft Advisory until 3 am PDT Sunday for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.