Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 271617
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PDT WED JUL 27 2016

.SYNOPSIS...Upper level ridging and weak low level onshore flow
will remain in place over Western Washington through Friday. This
pattern will produce sunny days with temperatures well above
normal. Temperatures will cool over the weekend as an upper level
trough moves into the area. The cooler weather will continue into
next week with another trough moving into the area.

&&

.SHORT TERM...Satellite imagery shows stratus along the coast, down
the strait and through the lower Chehalis valley. This stratus is
quite shallow and will evaporate back to the coast this morning.
Under sunny skies and with heights in the upper 580s, highs today
will be in the 80s inland with 70s on the coast.

Thursday will also have temperatures well above normal with a flat
upper level ridge over the area and light surface gradients. 500 mb
heights and 850 mb temperatures will peak on Thursday with 500 mb
heights approaching 590 dms and model 850 mb temperatures in the
plus 16-19c range. Thursday looks to be the warmest day in the short
term with highs in the interior in 80s to near 90 and highs on the
coast in the mid 70s.

Friday will be a transition day across the area with the low level
flow turning onshore during the day. By 00z Saturday models have the
KOTH-KSEA gradient as high as plus 5 mb and the KHQM-KSEA gradient
up over plus 3 mb. Temperatures aloft will cool just a touch on
Friday. The coast will see the greatest cool down Friday with the
increasing onshore flow resulting in 5 degrees or so of cooling. For
the interior model guidance indicating up to 5 degrees of cooling
but with the biggest increase in the onshore flow during the
afternoon persistence is a good forecast for the interior with
highs near Friday`s readings...in the 80 to near 90. Burke/Felton

.LONG TERM...Previous discussion...Extended models in good agreement
with a marine push Friday night and an upper level trough moving
over the area on Saturday for a cooler day with morning cloud cover
for the interior and just some afternoon clearing on the Coast. Model
solutions differ on Sunday with the ECMWF kicking the trough to the
east while the GFS keeps the trough over Western Washington. Both
the ECMWF and GFS ensemble solutions are similar to the operational
run. Will stay with the current forecast that leans toward the GFS
solution which calls for cooler tempers and a slight chance of
showers. Models are pretty consistent for Monday and MOnday night
with a weak upper level low reaching the area keeping the forecast
on the cool side for early August. The ECMWF once again is quicker
to kick this feature versus the GFS. For now will stay near
climatology for the highs on Tuesday. Felton

&&

.AVIATION...A flat ridge of high pressure will give light
westerly flow aloft. At the surface, high pressure over the
offshore waters and lower pressure east of the Cascades will give
northwesterly flow at lower levels. The air mass is dry and stable
with low level moisture mainly over the coastal waters and
zones.

Visible satellite imagery shows low stratus over the coastal
waters then inland through the Chehalis gap and into the south
interior to the southern portions of Puget Sound. This stratus is
shallow with bases at 200-400 feet and tops 1000-1500 feet.
A patch of shallow stratus and fog around Whidbey Island and
around KBVS this morning will dissipate by 17Z while the stratus
and fog over the south interior rapidly erodes back to the
coastline by 19Z. Good VFR conditions are expected this afternoon
and tonight, although very low stratus and some fog will reform
along the coast.

The air mass will dry somewhat with subsidence through Thursday
morning. However mesoscale models show the reformation of a thin
fog or stratus layer over portions of the inland waters early
Thursday morning that may result in conditions similar to this
morning from about KPAE northward and from KSHN westward through
the Chehalis gap. Albrecht

KSEA...Clear skies through tonight. Shallow stratus and fog that
reforms to the south of the terminal Thursday morning and another
batch around KPAE should not impact operations at the terminal.
Winds northwest 5-9 kt this afternoon shifting to 02008kt after
04Z then diminishing overnight. Albrecht


&&

.MARINE...A ridge of high pressure over the offshore waters will
give moderate to strong northwesterly flow from off the central
portion of Vancouver Island southeastward through the outer
portion of the Washington coastal waters for the next several
days. The long fetch of northwesterly winds will result in a steep
wind wave or fresh swell that will result in hazardous sea
conditions, especially later Friday through Saturday. A small
craft advisory is in effect for the outer portions of the coastal
waters through Thursday evening and will likely be extended into
Friday with the next forecast package.

Higher pressure offshore combined with lower pressure inland
is giving low level onshore flow to the inland waters. Expect
small craft advisory conditions late this afternoon and tonight
through the central and eastern portions of the Strait of Juan de
Fuca. Late Thursday afternoon through Thursday night should be a
repeat. Onshore flow will increase significantly late Friday and
will continue through Saturday as an upper level trough of low
pressure moves across the region -tightening surface pressure
gradients. Gale conditions are a good possibility for the central
and east portions of the strait starting Friday afternoon, and the
inland waters bordering those zones -such as the southern
portions of the Northern Inland Waters and Admiralty Inlet- may
see small craft advisory conditions. Albrecht

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
     Thursday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT Thursday 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.

&&

$$

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You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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