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Area forecast discussion
National Weather Service Seattle Washington
844 am PDT Tue Apr 24 2018

Synopsis...upper level ridging over Washington will keep
conditions dry through Thursday. Surface high pressure over
southeastern British Columbia will give low level offshore flow.
Warm conditions, with near-record high temperatures, are expected
today through Thursday. An upper level low will approach from the
southwest late in the week bringing a return to seasonably cool
temperatures and a chance of showers Friday and into next weekend.

&&

Short term...an upper level ridge extends from southeastern
British Columbia into central Oregon this morning. At the
surface, a 1038 mb high over southeastern British Columbia combined with a
thermal trough of low pressure that extends northward from the
central Oregon coastal waters is giving decent low level offshore
flow. Easterly cross-Cascade pressure gradients are over 6 mb and
will contribute to rapid warming through mid afternoon. The
warmest temperatures today will likely be seen along the west
slopes of the Cascades and over the inland portions of the coastal
strip just west of the Olympics due to adiabatic compression with
the offshore flow. The immediate Puget Sound area and locations
near the water in the northern interior of western Washington will
stay cooler with northerly return flow off the chilly water.
Temperatures are running over 10 degrees above where they were
yesterday at this time, so it should not be hard to hit mid 70s in
the Seattle area today. The forecast was updated to increase
maximum temperatures today by about 4 degrees above the previous
forecast.

A short wave trough will move by to the north of the area later
this afternoon through Wednesday morning. This trough will bring
an increase in cirrus across the region and will relax the
offshore flow somewhat as 500 mb heights fall a few decameters.
Building surface high pressure behind the trough to the north
will have the effect of increasing northerly flow somewhat on
Wednesday. Despite the backing in the low level flow, temperatures
on Wednesday afternoon will likely be near where they are today
due to the warmer morning start and still warm air mass.

Thursday will be the warmest day of the week, and the year so far,
in the interior as offshore flow strengthens once again in the
morning, then the coastal thermal trough shifts inland during the
afternoon. Temperatures along the coast will peak early in the
day, then will drop in the afternoon as onshore flow increases.
Temperatures on Thursday will be around 80 degrees across much of
the interior.

All good things come to an end. The thermal trough will continue
to shift eastward late Thursday afternoon and a strong marine push
can be expected to bring cooler marine air inland starting
Thursday evening. Albrecht

Long term...from the previous long term discussion: a change
over to onshore flow starting as early as Thursday evening will
start to signal a change in the pattern. An upper level low over
the Pacific making its way toward southern or/northern California will
push up showers from the southwest up into West Washington late Friday
morning/early Friday afternoon. While models do not particularly
agree on the details of this low...its movement/track or the
presence of any secondary systems that may either follow it
(ecmwf) or fold into it (gfs)...the broad strokes end up lining up
with showery weather expected for much of the upcoming weekend
and temperatures cooling to more seasonal values with afternoon
highs in lowland locations during this time frame only reaching
the mid 50s. Smr

&&

Aviation...the air will remain dry and stable. There will be some
cirrus through tonight. Low level offshore flow will persist and the
thermally induced low pressure trough is centered along the Oregon
coast.

Ksea...some cirrus with an afternoon and evening northerly breeze.

&&

Marine...the thermally induced low pressure trough is centered
this morning along the Oregon coast near Cape Blanco. Small Craft
Advisory strength easterlies are forecast this morning and again on
Wednesday morning in the vicinity of Cape Flattery. The thermally
induced low pressure will move inland on Thursday with onshore flow
developing Thursday night and Friday. Westerly flow through the
Strait should peak on Friday night.

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory until 1 PM PDT this afternoon for coastal
waters from Cape Flattery to James Island out 10 nm-West
Entrance U.S. Waters Strait of Juan de Fuca.

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