Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 290432
Area Forecast Discussion
National Weather Service Seattle WA
932 PM PDT SAT MAY 28 2016
.SYNOPSIS...Showers will continue tonight then gradually diminish
through Sunday as an upper level low over British Columbia moves
slowly east. A strong upper level ridge will build over the region
Monday, bringing mostly sunny and warmer weather for Memorial Day,
then continuing through the middle of next week.
.SHORT TERM...The large upper level low over B.C. will hang around
through Sunday, then begin drifting east Sunday night as an upper
level ridge builds just offshore. This will keep showers in the
forecast through Sunday evening before the low is far enough east
for showers to end.
For tonight, a weak warm front moved inland this afternoon and is in
the process of dissipating. A very weak cold front just west of the
north coast will follow tonight. The main impact with from both
features is to keep the lower air mass quite moist. Weak lift
provided by the cold front and an upper level shortwave trough will
keep scattered showers going tonight. Westerly onshore flow has
already re-developed in the Strait of Juan de Fuca so a convergence
zone may end up developing this evening, a little sooner than the
GFS and HRRR indicate.
Scattered showers will continue on Sunday with the low still in the
vicinity but should be diminishing for good Sunday evening. Models
indicate the main shower areas on Sunday will be over the Cascades
and Olympics, and over Puget Sound and the SW interior, particularly
from Seattle to Everett in the convergence zone. The convergence
zone should sag south over Seattle in the afternoon.
With the low heading east Sunday night, shower activity is expected
to end by midnight. Drier northerly flow aloft should allow clouds
to gradually diminish overnight. There is a good chance there will
be some areas of low clouds still hanging around the interior Monday
morning, but these should be gone by mid-day for a sunny afternoon.
Models remain in good agreement that the upper level ridge just
offshore will continue building on Monday then shift over the WA
coast on Tuesday. Northerly low level will prevail on Monday, but
will shift to easterly offshore flow Monday evening as the upper
level ridge axis moves over the coast. The easterly flow will pump
temperatures up and allow a thermally induced surface trough to form
along the coast. GFS/ECMWF MOS guidance for Sea-Tac shows highs in
the lower 70s Monday, warming to near 80 on Tuesday as the offshore
flow cranks up. Kam
.LONG TERM...previous discussion from the 3 PM AFD...a similar
pattern Wednesday, with both the upper ridge and the thermal trough
moving a bit east, should make that day the warmest day inland with
highs mainly in the 80s. On Thursday and Friday the surface flow
turns onshore, which implies at least some morning clouds. A recent
idea in the GFS is upper level moisture arriving from the south.
Have introduced a chance of thunderstorms in the Cascades Thursday
afternoon and evening.
The GFS brings a weak system onto the coast Friday so have put a
chance of showers there. Models become dry again on Saturday. High
temperatures will fall from around 80 on Thursday into the 70s on
Friday, then rebound somewhat on Saturday. Burke
An upper level trough over b.c. and Washington will shift eastward
Sunday. Light northwesterly flow aloft becoming northerly by Sunday
night. Generally stable and moist in the lower levels.
Cigs will vary across the area mostly ranging from MVFR to VFR.
Showers may bring cigs down to IFR at times briefly. Scattered
showers can be expected to continue through Sunday...especially over
higher terrain and in a possible central Puget Sound convergence
zone which may affect the KSEA/KBFI/KPAE terminals. Improvement to
VFR conditions is expected Sunday afternoon.
KSEA...South to Southwest wind 8-12 KT. Mainly MVFR cigs through
Sunday morning before improving to mostly VFR stratus.
However...periods of IFR during showers is likely tonight. The
convergence zone could approach the terminal Sunday morning with a
chance of a northerly wind shift less than 10 KT. DTM
Post-frontal onshore flow through Sunday. Small Craft Advisory westerly
winds through the Strait of Juan De Fuca is likely tonight and again
Sunday afternoon and Evening. Winds will generally stay below 20 KT all
other waters through Sunday afternoon.
High pressure will build offshore on Memorial Day. This will give the
potential for northwesterly winds up to 25 KT over the coastal zones
Monday afternoon and evening. Winds will become northerly inland and
dominate on Tuesday as a thermally induced trough of low pressure
nudges northward along the western Oregon and south Washington coasts.
The trough will push inland Wednesday with a marine push likely in
the evening. DTM
PZ...Small Craft Advisory Central and East Strait through late
You can see an illustrated version of this discussion at