Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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927
FXUS66 KSEW 281542
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
842 AM PDT Fri Apr 28 2017

.SYNOPSIS...A flat ridge of high pressure aloft will move into the
area today giving drier weather and temperatures close to normal
for late April. The drier weather will be interrupted by a fast
moving frontal system that will spread light rain across the area
Saturday afternoon and night followed by decreasing showers
Sunday. Higher pressure aloft is forecast to rebuild Monday into
the middle of next week. Passing nearby weather systems will
likely bring some clouds and a threat of light rain at times, but
temperatures should rise above early May averages.

&&

.SHORT TERM...A rather flat upper level ridge around 134W
longitude early this morning will shift eastward into the area
tonight then east of the Cascades on Saturday. The NWS doppler
radar at Camano Island has been reporting some light showers from
about Mount Vernon southward into King county early this morning
that are moving southward, but these showers will rapidly diminish
as the offshore ridge approaches and should be totally out of the
picture by about 11 AM this morning. Satellite imagery shows
clouds over the Cascades extending westward across the Central
Puget Sound area and from the coastal zones out into the coastal
waters. Expect the clouds over the lowlands to dissipate midday,
while they linger in the mountains as surface heating results in
cumulus development.

After a dry night tonight, conditions will gradually deteriorate
from the west during the day on Saturday as the frontal system mow
around 145W approaches the area. Models are consistent in bringing
rain onto the coast during and the western portions of the
northern interior during the day then into the interior late in
the afternoon or early in the evening. The front itself will move
across the area during the evening hours then push off to the
east. day and Saturday night. The big forecast question for
Saturday concerns high temperatures. The current forecast shows
precipitation holding off late enough in the day in the interior
for temperatures to rise into the upper 50s from about Seattle
southward - a reasonable looking forecast. But model guidance
shows thick enough clouds and enough of a rain chance by afternoon
to hold temperatures 3-5 degrees below the current forecast (also
reasonable).

After Saturday night`s frontal system and rain, showers will taper
off on Sunday as another ridge approaches the area from the west.
Showers in the morning will become confined to a convergence zone
and the mountains as the day wears on, and temperatures will rise
into the mid 50s to around 60 later in the afternoon as sunshine
becomes more abundant.

An update was made to increase showers in the interior this
morning and to show areas of morning clouds today. Other updates
are not required this morning. Albrecht

.LONG TERM...From the previous long term discussion: The big
question as we head into May is will the rebuilding upper ridge be
strong enough to deflect weak weather systems passing by on the
west side of the ridge Monday through Thursday. Once again,
mid-range guidance offer divergent solutions.

Collaborated with neighboring NWS Portland and the Pacific Storm
Prediction Centre in Vancouver on this issue overnight. The
consensus was the forecast will reflect some cloud cover and a
relatively small threat of light rain as passing systems ride up the
west side of the upper ridge in a warm advection type pattern. At
least high temperatures should rise to above early May readings,
particularly by Wednesday. Heights sag by Thursday and may suggest a
transition back to cooler somewhat wetter weather. Buehner

&&

.AVIATION...High pressure offshore today will move over Western
Washington tonight. Flow aloft will be moderate northerly. The
air mass is moist in the lower levels and generally dry aloft,
with drying at all levels this afternoon and tonight. There are
some 3k ft ceilings around Puget Sound this morning, but also
large clear areas. The afternoon should be VFR all areas. Isolated
showers this morning will end by noon.

KSEA...Southwest winds 5-10 kt will shift to northerly again late this
afternoon. Winds should switch back to southerly by late evening. CHB

.MARINE...Onshore flow will weaken today. A weak westerly push
will bring renewed small craft advisory strength winds to the
strait this evening, and have issued an advisory. Otherwise,
lighter winds are expected across the waters. Expect another
switch from southerly to northerly winds in Puget Sound this
afternoon and evening.

A front will bring at least small craft advisory strength southerly
winds to the coastal waters by Saturday afternoon. Winds will spread
to inland waters with some advisory level winds possible, mainly north
of Puget Sound. Strong onshore flow Sunday will weaken Monday and
Tuesday with lighter winds expected. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 11 PM PDT this
     evening for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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