Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201057
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
357 AM PDT Tue Jun 20 2017

.SYNOPSIS...A cold front will move through Western Washington this
morning. A Puget Sound Convergence Zone could keep some showers
going in the afternoon between Seattle and Everett. An upper ridge
will build offshore beginning tonight. The ridge will shift
closer to the area this weekend with the warmest days of the year
possible Saturday and Sunday.

&&

.SHORT TERM...Satellite imagery shows weak cold front offshore
near 128w at 10z/3 am. Mostly cloudy skies over most of Western
Washington the thickest cloud cover over the northern portion of
the area. Temperatures at 3 am were on the mild side, in the mid
50s to lower 60s.

Cold front moving through western Washington late this morning.
Doppler radar not picking up and precipitation west of Western
Washington at 10z. All of the models squeeze out a hundredth or
so in places as the front moves through so will stay with the low
chance pops this morning. Best chance for measurable rain will be
in the convergence zone behind the front. Gale force winds
expected in the Strait with lighter southerlies central and
southern Puget Sound. With the gales there is the potential for
the convergence zone to set up slightly south of its favored
position, over King county in the afternoon. Highs will be near
normal, in the 60s and lower 70s.

Convergence zone dissipating this evening. For the remainder of
the area just partly to mostly cloudy skies. Lows tonight will be
in the 40s to lower 50s.

Upper level ridge building offshore beginning late tonight and
continuing into Thursday. While 500 mb heights slowly rise over
the area the ridge axis remains well offshore. Surface flow
onshore northwesterly on Wednesday with little change in the
pattern on Thursday. Some lingering low level moisture Wednesday
morning for areas of morning clouds. The low clouds will burn off
by midday leaving mostly sunny skies for the entire area. With the
low level onshore flow High temperatures will remain near normal
in the 60s to mid 70s on Wednesday with just a couple of degrees
of warming on Thursday.

.LONG TERM...It`s a split decision on Friday in the models with
the NAM and ECMWF bringing a dry trough down out of the north
Thursday night/early Friday inducing stronger onshore flow. The
GFS and Canadian not only don`t have this feature they both build
the upper level ridge over the area. Large variability in the
temperature guidance with the cooler models 5 to 10 degrees less
than the warmer solutions. Current forecast leans toward the
cooler solutions, which still have highs will above normal...in
the 70s to mid 80s. With the lack of consistency in the models
will stay with the current forecast. The weekend looks very warm
with the potential for the warmest days of the year Saturday and
Sunday ( warmest day so far in Seattle 86 on May 28th ). Thermally
induced trough making its way up the coast later Saturday into
Sunday. Model 925 and 850 mb winds are light but both have an
easterly component. 500 mb heights rising into the mid 580 dms.
Have bumped up the temperatures a couple of degrees over the
weekend with upper 70s to lower 90s forecast for both days. Low
level flow turns onshore during the day on Monday with 10 degrees
or so of cooling for the interior and more along the Coast. Felton


&&

.AVIATION...A weakening cold front will approach the
region tonight from the offshore waters and move through Western
Washington Tuesday morning. The air mass will remain stable and dry
at low levels this evening with VFR conditions.  Low stratus will
move into the area late tonight into early Tuesday morning with
isolated showers. Expect IFR to low MVFR conditions to settle into
the coast by late evening, followed by the strait and interior by
around 11Z. IFR and MVFR conditions will improve to VFR levels
behind the front around midday Tuesday. The development of a Puget
Sound Convergence Zone behind the front may help lower conditions
persist in portions of the central Puget Sound area into early
Tuesday afternoon.

KSEA...VFR conditions this evening will gradually lower early
Tuesday morning as stratus at 800-1100 AGL develops over the
interior around 10/11Z until around 19Z Tue. There is some chance
that lower ceilings and showers will persist into the early
afternoon hours behind the front with the development of a
convergence zone. Northerly winds 4-7 knots will becoming variable
this evening, switch to south 7-10 knots after 06Z. Albrecht

&&

.MARINE...Offshore high pressure is driving onshore flow this
evening which will receive a boost with developing ridging over the
coastal waters ahead of an offshore cold front. A small craft
advisory remains in effect for the central and eastern portions of
the Strait of Juan de Fuca for tonight.

A weak front will move through the coastal waters Tuesday morning
with increasing onshore flow behind the front Tuesday afternoon and
evening. A gale warning is in effect for the eastern two thirds of
the Strait of Juan de Fuca for Tuesday afternoon and evening. Small
craft advisory westerly winds are expected over most of the
remainder of the waters late Tuesday into Tuesday night.

Moderate onshore flow will continue Wednesday, weakening on Thursday
then becoming northerly on Friday with high pressure over southern
British Columbia. Over the coastal and offshore waters, persistent
low level northwesterlies will result in the potential development
of a steep short period swell.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 5 AM PDT
     Wednesday for Admiralty Inlet-Coastal Waters From Cape
     Flattery To James Island 10 To 60 Nm-Coastal Waters From
     Cape Flattery To James Island Out 10 Nm-Coastal Waters From
     James Island To Point Grenville 10 To 60 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out
     10 Nm-Northern Inland Waters Including The San Juan Islands-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 PM PDT this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 2 PM this afternoon to 3 AM PDT Wednesday for
     Central U.S. Waters Strait Of Juan De Fuca-East 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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