Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182229
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
329 PM PDT Sun Jun 18 2017

.SYNOPSIS...High pressure aloft will prevail for dry and warmer
weather on Monday. A weak cold front will bring a threat of rain
to the region Monday night and Tuesday. Expect onshore flow to
keep temperatures near normal Tuesday and Wednesday. Anticipate a
warming trend the latter part of this week.

&&

.SHORT TERM...
Clouds still blanketed much of the CWA this afternoon. There were
also lingering pockets of light rain or drizzle over mainly the
northern part of the forecast area. This activity was winding down
and should end by late this afternoon now that the warm front was
moving into Canada. Otherwise, expect some clearing over at least
the southeast part of the CWA this evening.

An upper level ridge of high pressure will prevail over the region
through Monday for drier and warmer conditions. Temps on Monday
should be about ten degrees warmer than today on average,
particularly across the interior.

Expect a weak cold front to approach the area from the west Monday
night. This front should move across the CWA Tuesday morning for a
threat of showers. The onshore surface pressure gradient will
tighten Tuesday afternoon and evening in the wake of the cold front.
Expect a Puget Sound Convergence Zone (PSCZ) to develop as
westerly flow strengthens across the Strait of Juan de Fuca. The
PSCZ is forecast to dissipate by late Tuesday evening. Needless to
say, temps will be cooler on Tuesday.

Look for morning stratus across the lowlands Wednesday morning to
give way to mostly sunny skies Monday afternoon. Low level onshore
flow will persist; thus, expect daytime temps to be near normal.

.LONG TERM...
The medium range solutions were still having trouble deciding
whether a trough of low pressure or a deep closed upper level low
will impact the Pacific Northwest at some point in time during the
extended period or, perhaps, not at all. For each cycle and for
each model, there is a different solution. Decided to stick with
the idea of a warm upper level ridge dominating the weather
pattern over the Pacific Northwest until the medium range
solutions come to an agreement and conclusively decide whether
there will be a low pressure system during this period or not.

Temps are forecast to climb well above normal this weekend, with
some lowland sites climbing into the 80s. If the axis of the
thermally induced trough of low pressure extends farther north
into the CWA and shifts off the coast, temps could end up warmer
than currently forecast.

&&

.AVIATION...An upper ridge over the area tonight will shift inland
on Monday with westerly flow aloft becoming southwest. At the
surface, weak high pressure will be over southern Vancouver Island
tonight for light northerly low level flow. Onshore flow will
develop on Monday as high pressure builds offshore and lower
pressure moves into Eastern Washington. The air mass is stable.
Moisture will be decreasing tonight and Monday morning.

Widespread low clouds are slowly lifting and breaking up this
afternoon. MVFR low clouds are expected to become VFR early this
evening and then scatter entirely later tonight.

KSEA...Low clouds should continue to lift and eventually scatter
this evening. Some mid level clouds will hang around until Monday
morning, then mostly clear. South wind 4-8 knots should switch to
north this evening. Schneider

&&

.MARINE...Weak high pressure will be over southern Vancouver
Island tonight for light northerly flow. Onshore flow will develop
on Monday. Small craft advisory west winds are expected in the
Central and Eastern Strait Monday evening.

A weak front will move through the waters on Tuesday. Stronger
onshore flow will develop behind the front later Tuesday and
Tuesday night. Gales are possible for the Central and Eastern
Strait Tuesday night. Small craft advisory westerly winds are
likely most other waters.

Moderate onshore flow will prevail on Wednesday. Onshore flow will
weaken on Thursday and then turn more northerly on Friday.
Schneider

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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