Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182227
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
327 PM PDT WED MAY 18 2016

.SYNOPSIS...
An upper level low pressure system near the coast of southeastern
Alaska will drop southeastward into the area on Thursday, then
will drop into southern Oregon on Friday. Systems rotating around
the low will affect the region through the upcoming weekend.
Another upper low will drop southeast into the area by the middle
of next week.

&&

.SHORT TERM...
A sprawling vertically sitting just off the western coast of
southeastern Alaska is moving toward the area. Models all show the
upper low moving over western Washington on Thursday, then
dropping into southwestern Oregon on Friday. The main forecast
challenge in the short term is for rain chances and temperatures
on Saturday as models have difficulty placing the main center of
the upper low and a deformation zone rotating around it to the
north and northwest.

The front ahead of the incoming upper low sits from central
Vancouver Island to the Washington coastal waters this afternoon.
Onshore pressure gradients will increase late this afternoon and
evening as the front moves off to the southeast of the area. The
increased flow will produce a good increase in showers along the
west slopes of the mountains as moisture deepens, and will
produce a convergence zone initially around Skagit and north
Snohomish counties tonight that slides southward into southern
Snohomish and King counties later tonight into Thursday. Mountain
snow levels are expected to drop to about 4500 feet tonight and
4000 feet on Thursday as the cool upper low moves into the area.
Snow showers are likely on the higher Cascade passes and some
locations above 4000 feet by Thursday could see 2 to 4 inches.
While accumulations are not expected to be a problem on the
roadways, a Special Weather Statement was issued to alert people
traveling across the higher passes and into the backcountry of
hazards associated with the late season snow showers.

The 12z models all drop the center of the upper low into southern
Oregon and Northern California on Friday. With the low to the
south and the air mass somewhat unstable but with no focusing
mechanisms, Friday may not be too bad of a day in the lowlands
with only a slight chance of showers and partly sunny skies.
Convection will likely be confined to the higher terrain during
the afternoon and evening hours.

On Friday night and Saturday, models wrap a deformation zone
around the north and west sides of the upper low. The Canadian
GEM continues to be much more robust in bringing the deformation
zone westward into western Washington for a wetter and cool day
Saturday. The GFS is farthest east, taking the upper low more into
the Idaho Panhandle. The ECMWF is between the two extreme
solutions. Both the GFS and ECMWF result in warmer max
temperatures and lower pops across the area on Saturday. A model
blend was used as a first guess to the POP and temperature grids
while the ECMWF sky field was used as a first guess for cloud
cover. Confidence in the forecast for Saturday is low.
Albrecht

.LONG TERM...
A weak and dirty ridge with low level onshore flow moves across
the region later Sunday. The various model solutions suggest
rather deep marine moisture with dribs and drabs of precipitation
at times and temperatures a few degrees below Sunday and Monday.
Another upper low will likely drop into the region around the
beginning to middle of next week for a repeat of the weather late
this week. Forecasts generally follow a model consensus. Albrecht

&&

.AVIATION...
An upper level trof will settle over the region
overnight. Strong sw flow aloft will weaken and become variable
overnight. The low level flow will become strong onshore later
today and will persist overnight.

Expect areas of MVFR cigs to become more widespread overnight.
There will likely be areas of LIFR cigs/MVFR vsbys mainly within a
Puget Sound Convergence Zone (PSCZ) that will dvlp late today or
early this evening in its typical location (near the King/
Snohomish County line). The PSCZ may drift south into central
King County late tonight or early Thursday.

The mtns will be obscd at times.

KSEA...
Expect MVFR cigs to become prevalent over the terminal
about 0600 UTC. A PSCZ will dvlp late today or early this evening
near its typical location. There is a 50% chance that the PSCZ
will drift south over the terminal after 0900 UTC. If this occurs,
expect a wind shift from sw to nly. There is also a chance that
the winds over KBFI could become nly while remaining sw over the
terminal for a brief period late tonight.

&&

.MARINE...
Strong onshore flow in the wake of a cold front will gradually
weaken on Thursday due to lower pressure developing over the
coastal waters. The flow will strengthen on Friday in response to
higher pressure building over the coastal waters and falling
pressure east of the Cascades.

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 2 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.

     Small Craft Advisory from 6 PM this evening to 9 AM PDT Thursday
     for Coastal Waters From Cape Flattery To James Island Out
     10 Nm.

     Gale Warning until 6 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 3 AM PDT Thursday for Northern Inland
     Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PDT Thursday for Admiralty Inlet.

&&

$$

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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