Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211048
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
348 AM PDT Fri Apr 21 2017

.SYNOPSIS...An upper level ridge will move into the area today.
This combined with developing offshore flow in the lower levels
will result in sunny skies by afternoon and high temperatures a
few degrees above normal. The dry spell will be short lived. A
series of weather systems will take aim at Western Washington
beginning Saturday and continuing into the middle of next week.

&&

.SHORT TERM...Satellite imagery shows upper level ridge offshore
with the ridge axis just west of the coast early this morning.
Convergence zone showers have come to an end but cloudy skies
remain over Snohomish county back across the Sound into the
Quimper peninsula. Sc deck also hanging on along the coast. Where
skies have cleared, fog has formed with visibilities at Olympia
and Shelton below a mile. Wide variety of temperatures at 3
am/10z. Where skies have cleared over the Southwest Interior
temperatures have dropped into the 30s while with the cloud cover
hanging on temperatures in the metro area are in the upper 40s
and lower 50s.

Upper level ridge will continue to move east today with the ridge
axis over the Cascades at 00z Saturday. KOLM-KBLI gradient near 0
at 10z and will go negative this morning. Cross cascade gradient
around plus 4 mb at 10z but it will also go negative this
afternoon as surface high pressure builds east of the Cascades.
Model cross sections continue to show easterly winds up to 700 mb
this afternoon. High temperature guidance has a big spread today
with the GFS MOS keeping highs near normal while the NAM MOS is 5
degrees or more above normal. With the transition to offshore flow
have trended the forecast toward the warmer NAM guidance with
most places getting into the 60s today and warmer locations with
good exposure to the downslope winds off the mountains like the
Cascade foothills pushing 70.

Dry weather continuing overnight as the next in what will be a
series of systems to reach the area becomes negatively tilted and
stays offshore. Lows will mostly be in the 40s.

Front moving through Western Washington Saturday afternoon with
rain beginning along the Central coast mid morning spreading up
into the Bellingham area early in the afternoon. Highs will cool
back down to near or a couple of degrees below normal...in the 50s
and lower 60s.

Negatively tilted upper level trough following the front.
Current model runs have slowed down the eastward movement of this
feature which is to be expected with it becoming more negatively
tilted. Some post frontal showers lingering around Saturday
evening but for the most part Saturday night looks pretty dry.
Lows will be in the 40s.

Upper level trough moving into Western Washington on Sunday with
the trough axis right over the area at 00z Monday. Good low level
onshore flow will add plenty of low level moisture for the trough
to work with resulting in widespread shower activity Sunday
afternoon. Highs will only be in the lower to mid 50s.

.LONG TERM...Multiple runs in a row of the extended models have
shown the next weather system moving into Western Washington on
Monday. The jet is over Southern Oregon/Northern California so we
will not get a direct hit from this system but there is plenty of
moisture associated with it so even though we are getting the
northern portion will still up the pops to categorical for
Monday. A little wrinkle in the extended runs this morning with
the following system moving inland a little further south on
Tuesday. Not far enough south to take the chance of rain out of
the forecast but if the trends continue precipitation on Tuesday
could end up being fairly light. Model solutions diverge on
Wednesday with the GFS keeping a weak trough over the area while
the ECMWF has another organized front approaching the area by the
end of the day. The GFS is much slower with this feature with the
front not arriving until Thursday night while the ECMWF is 12 to
18 hours faster and much stronger with the surface low associated
with the front. For now will stay with chance pops on Thursday.
Felton

&&

.AVIATION...Northwesterly flow aloft with an upper ridge building
over the area, though just for one day. There are likely to be a
few areas of morning low clouds and patchy fog, but after those
dissipate it will be sunny. Surface high pressure over the area
will shift inland and light easterly low level flow will develop
through tonight. But high clouds will increase later today ahead
of another front that arrives Saturday.

KSEA...A patch of morning clouds will give way to a sunny day. Winds
will be light and variable this morning then a north breeze will
develop for the afternoon and evening.

&&

.MARINE...High pressure over the area will shift east and light
and variable winds will become northerly and then more easterly
tonight. A front will move across the area on Saturday with
southerly winds. The weather pattern will be active next week with
two or three significant weather systems likely to affect the
region.

&&

.CLIMATE...Seattle precipitation total since October 1st at 44.06
inches through the 20th. The record for October through April
precipitation of 44.52" was set in 2015-16. The record will mostly
likely be broken later Sunday or Monday. Last season there was
"only" 128 days with precipitation between October 1st and April
30th in Seattle. So far this season there has been 141 days with
more on the way before the end of the month. Last year the total
precipitation for April in Seattle was 1.19 inches. We will end up
with more than triple that number this year.

Temperature wise the normal high for Seattle goes up to 60 degrees
on Saturday. The normal high will not drop back down below 60
degrees until October 17th. The normal high peaks at 77 degrees
between July 19th and August 16th.

RIP Harry...Felton

&&

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

$$

www.weather.gov/seattle

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



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