Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 281554
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PDT Wed Jun 28 2017

.SYNOPSIS...A surface ridge offshore will maintain onshore flow into
early next week. This will produce typical early summer weather with
mild temperatures, morning low clouds, and afternoon sunshine. An
upper level ridge will weaken the onshore flow and produce a minor
warming trend Thursday and Friday. Stronger onshore flow will bring
more cloud cover this weekend, and possibly light rain to the coast.

&&

.SHORT TERM...Satellite imagery shows stratus has filled in most of
the lowlands this morning. The mountains, especially the Olympics,
are mostly sunny. Today will be quite similar to Tuesday with low
level onshore flow continuing. Model cross sections show the marine
layer depth pretty similar to yesterday. End result is a persistence
forecast for today with the stratus burning back to the coast
midday. High will be similar to Tuesday`s readings with mid 60s
along the coast and mid 60s to mid 70s inland.

The upper level ridge offshore will move east later today and
tonight. 500 mb heights will rise over the area with the ridge axis
just offshore at 12z Thursday. Low level onshore flow will weaken,
resulting in less morning cloud cover for the interior Thursday
morning. With less cloud cover and the warming temperatures aloft,
highs on Thursday will be a few degrees warmer with mid and upper
60s on the coast and 70s common over the interior. Lows tonight
will mostly be in the 50s.

The upper level ridge will move through the area Thursday night into
Friday morning with 500 mb heights peaking in the lower to mid 580
dms. Onshore pressure gradients don`t begin to increase again until
Friday afternoon making for a mostly sunny day across most of the
area on Friday. Friday will be the warmest day in the forecast
period with some lower 80s from Seattle south. North of Seattle 70s
will be common with highs near 70 on the coast. Burke/Felton

.LONG TERM...Previous discussion...Extended models in good agreement
for the weekend with zonal flow aloft and low level onshore flow on
Saturday for morning clouds and some afternoon sunshine over the
interior. A weak trough approaches the Coast on Sunday for a slight
chance of showers in the morning. Low level onshore flow remaining
intact through Monday with a weak trough remaining over the area for
a continuation of the morning clouds with some afternoon sunshine
scenario for the interior. Some differences show up in the models
for the 4th with the GFS beginning to build a ridge while the
ECMWF keeps the weak trough in the area. Both solutions are dry.
Current forecast is closer to the less cloud cover/ a few degrees
warmer GFS solution. Will stay with that idea for the morning
forecast package. Felton

&&

.AVIATION...Upper level high pressure offshore will slowly make its
way eastward today with flow aloft mainly from the northwest. Low
level flow will remain onshore. Low level stratus are expected to
linger through the morning hours...resulting in cigs generally
remaining in the MVFR range in many locations...before starting to
break up between 18-21Z...with partly sunny skies expected afterward.

KSEA...Above discussion applies...although would narrow the time
frame for stratus burn-off slightly...probably happening between 18-
20Z. Winds southwesterly 6-10 kts turning northerly by this evening.
SMR

&&

.MARINE...Higher pres offshore with lower pres E of the Cascades
will maintain onshore flow of varying strength through the week.
Inherited forecast suggests the prospect for low end SCA winds for
the Strait this evening and tonight. Will await new model data and
make decision on whether or not a headline is needed for afternoon
forecast package. This onshore flow is expected to strengthen Friday
afternoon and evening for the possibility of gale force winds over
parts of the Strait of Juan de Fuca.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory for central and east strait late this
afternoon and tonight.

&&

$$

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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