Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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622
FXUS66 KSEW 232130
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
230 PM PDT Mon Apr 23 2018

.SYNOPSIS...A ridge aloft will move into Eastern Washington
tonight and persist into Thursday. Surface high pressure over
Southeastern British Columbia combined with a developing thermal
trough of low pressure along the coast will give low-level
offshore flow. The ridge aloft and offshore flow at lower levels
will give highs into the 70s across much of the area Tuesday and
Wednesday, and highs will reach into the lower 80s in some
locations on 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 through next weekend.

&&

.SHORT TERM...Other than a few cirrus, conditions are clear across
the region and temperatures are warming rapidly as offshore flow
develops. To varying degrees, high pressure aloft and low level
offshore flow will give very nice weather to the area through
Thursday!

After highs near 70 today, we will see highs into the 70s on
Tuesday (about 5 degrees higher than today) with a possible lower
80 in the far south interior. Low temperatures tonight will fall
into the 30s in the typically coldest locations (like Arlington
and Olympia Airport), but will generally be about 5 degrees warmer
than this morning. Low temperatures will warm further on Tuesday
night as the air mass continues to warm.

The high temperature forecast for Wednesday is a bit trickier than
the one for Tuesday. A weak shortwave trough will be moving over
the top of the west coast ridge Tuesday night allowing heights to
fall a bit and offshore flow to relax somewhat. Low level
thickness forecasts from all of the models, however, show a
slight increase by Wednesday afternoon as ridging and offshore
flow strengthens. So, temperatures on Wednesday are likely to be
the same or a degree or two higher than on Tuesday. High
temperatures Tuesday and Wednesday will threaten the record daily
maxima at many locations, but will likely fall just short.

Thursday will be the warmest day of this upcoming warm spell and
will be the warmest of the year so far. Current thinking is for in
the mid 70s to lower 80s across the interior and potentially on
the coast as the upper level ridge rebounds over the area and
offshore flow peaks. Daily record highs are a bit warmer on
Thursday, so again records will be threatened - but temperatures
will likely fall just short.

Forecast temperatures are a blend of the previous forecast and a
mean of the various short-term models. Albrecht

.LONG TERM...There continues to be rather large differences
between the GFS solution and the ECMWF/Canadian solutions today.
All models agree that a marine push will begin Thursday evening,
but the ECMWF/Canadian solutions hold most of the precipitation
just to the south of the forecast area as a weaker shortwave
trough rotates around a low that will move inland over the
southern half of Oregon. The GFS is much more robust with the
strength of the shortwave trough and is farther north with its
track. The GFS develops a rather heavy rain band with a
deformation zone over Western Washington Friday through Saturday
night. The GFS total precipitation amounts are well over an inch
at SeaTac and some solutions are wetter still. The ECMWF is much,
much more tame. At this time, a model blend was used to populate
the forecasts and were weighted more heavily toward the ECMWF. The
models are all showing good continuity with their previous
solutions; maybe one of them will `give in` on the next couple of
forecast cycles.

One sure bet for the upcoming weekend will be much cooler temperatures.
Temperatures will fall significantly on Friday. A model blend
would suggest upper 50s for highs on Friday while the ECMWF would
suggest highs more into the mid 60s. A weighted blend toward the
ECMWF was used today. After Friday, temperatures fall to a few
degrees below for the weekend as deeper cloud cover and
precipitation chances increase and the air mass aloft continues to
cool. Albrecht

&&

.AVIATION...An upper level ridge is over the region with dry
stable air. At the surface there is high pressure centered over
southeast British Columbia and thermally induced low pressure
extending north along the Oregon coast.

KSEA...Clear skies and a northerly breeze.

&&

.MARINE...High pressure inland and thermally induced low pressure
along the Oregon and Washington coast will persist until about
Thursday morning. Northerly flow down Puget Sound should peak
around 20kt this afternoon and evening but overall the flow
should become become more easterly for some areas. Small craft
advisory strength easterlies are forecast at the West Entrance
of the Strait of Juan de Fuca around Cape Flattery. The thermally
induced low pressure will move inland on Thursday with
onshore flow developing Thursday night and Friday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from midnight tonight to noon PDT Tuesday
     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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