Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 261605
Area Forecast Discussion
National Weather Service Seattle WA
905 AM PDT THU MAY 26 2016
.SYNOPSIS...An upper level trough will continue to dig further
into British Columbia today where it will remain anchored for much
of the weekend. This pattern will allow a series of embedded
shortwave troughs to affect the region through the weekend keeping
cool and showery weather in the forecast through Sunday. Conditions
will improve on Memorial day as an upper level ridge builds offshore
with sunnier and much warmer weather following into midweek as the
ridge moves inland.
.SHORT TERM...An upper level trough over Northwestern
British Columbia will continue to work south and east today before
effectively settling over central British Columbia for the weekend.
The anchored upper level trough will allow a series of embedded
shortwave troughs to rotate into Western Washington...boosting
onshore flow and showers. Temperatures will remain on the cool side
today with highs only in the upper 50s to lower 60s. The first of
these embedded shortwave troughs will rotate into Western Washington
late in the day today bringing breezy conditions to much of the
North Interior and San Juan Islands with windy conditions for
Admiralty Inlet and the East Strait tonight. Abundant low level
moisture and lift with upper level shortwave will be enough to
produce showers tonight...especially over the north. In addition to
the upper level forcing...the solid onshore flow will produce a Puget
Sound Convergence Zone this evening over Snohomish County. Expect
the convergence and heavier showers to shift southward into northern
King County as the onshore flow fully develops overnight. Lows
tonight will be in the upper 40s and lower 50s.
The proximity of the upper level trough over British Columbia
coupled with the moist onshore flow will keep showers in the
forecast Friday...especially over the Central Puget Sound with
lingering low level convergence. It will be another cool day with
solid cloud cover and highs around 60. Snow levels will lower to
near 4500 feet...affecting the higher passes of the Cascades.
The second embedded shortwave trough will rotate around the
anchoring upper level low and into Western Washington Saturday
morning. The primary impact with this feature will be an enhancement
to lingering showers through the day Saturday. Temperatures will
remain cool with highs over much of the interior reaching into the
low 60s and high 50s for the coast and strait.
.LONG TERM...from previous discussion...Extended models in good
agreement on Sunday with the trough remaining overhead with another
shortwave moving through the area. Some differences in the models on
The ecmwf is quicker to build the upper level ridge offshore and
dry out western Washington. The gfs leaves the upper trough in the
vicinity in the morning. Have gone ahead and changed the forecast
for Monday to dry the afternoon out. The models are back in
agreement on Tuesday with a rapid warm up as the upper level
ridge offshore moves over western Washington and a thermally
induced trough develops along the coast resulting in good offshore
flow for the area. Increased the high temperatures on Tuesday into
the 70s for most of the area. On Wednesday both models show the
upper level ridge shifting east with the ecmwf stronger with the
ridge. Low level flow goes weakly onshore during the day. For now
have only cooled down the temps along the coast significantly for
Wednesday with another warm day for the interior. Felton
.AVIATION...An upper level trough over British Columbia will move
southeast today and tonight. Westerly flow aloft today will become
northwesterly tonight. Low level onshore flow will continue. The air
mass is stable with moisture mostly in the lower levels, becoming
somewhat unstable tonight. A Puget Sound convergence zone is likely
by late afternoon or evening, possibly affecting the KPAE/KSEA/KBFI
terminals with shifting winds and showers.
Ceilings in marine stratus are mostly 2-4k feet this morning. They
will lift to 3-5k feet this afternoon and evening with some clearing
also likely. Marine stratus with MVFR ceilings will return overnight.
KSEA...Discussion above applies. Southerly wind 8-12 KT becoming
northwesterly after 00z. CHB
.MARINE...Onshore flow will strengthen again this afternoon and
evening. Westerly gales are likely to develop in the central and
east strait this afternoon and evening. Small craft advisory
strength winds are likely in the northern inland waters and in
Admiralty Inlet tonight.
The onshore pattern will repeat Friday into the weekend but will
gradually weaken. Northerly or offshore flow may develop early next
PZ...Gale warning central and east strait.
Small craft advisory coast, west entrance, northern inland
waters, Admiralty Inlet.
You can see an illustrated version of this discussion at