Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 292222
Area Forecast Discussion
National Weather Service Seattle WA
330 PM PDT FRI JUL 29 2016
.SYNOPSIS...Low level onshore flow will become strong this evening.
Marine stratus will reach into the lowlands both Saturday and Sunday
mornings and both days will be dry but cooler. An upper level low
will move across western Washington on Monday and Monday night,
bringing scattered showers and lower temperatures. Sunny skies and
higher temperatures will return by the middle of next week.
.SHORT TERM...There is virtually no stratus left, even on the coast,
which makes a full-blown marine push with stratus to the Cascade
crest unlikely tonight. However with strong onshore gradients some
stratus is likely to form in situ tonight. Have put clouds over the
southwest interior to about SeaTac and also down the strait Saturday
morning. Clouds will evaporate somewhat during the day on Saturday,
but onshore gradients will remain strong Saturday and Saturday
night. This should be enough to complete the marine push job by
Sunday morning, when clouds will probably cover the lowlands. High
temperatures will fall into the 70s on Saturday and be a degree or
two lower on Sunday.
On Monday an upper level low will approach the coast and it will
move inland Monday night. Models are in broad agreement about this
feature though the 12Z runs have shifted it a bit north. This has
moved the area of greatest instability, around the region of
divergence aloft, into Canada. As a result the thunderstorm chance
seems to be confined to the north Cascades, though ordinary showers
are possible nearly everywhere. Burke
.LONG TERM...On Tuesday the upper low will move into eastern
Washington. The shower chance will diminish as it moves away, until
only the Cascades have a chance of showers Tuesday night. Tuesday is
likely to be the coolest day of the week, as heights in the low 570s
along with plenty of clouds and persistent surface onshore flow
conspire to keep temperatures in the 60s and low 70s. Heights
rebound later next week, with most days seeing 580dm. Temperatures
will be closer to normal. Burke
.AVIATION...Dry stable air over much of Western Washington...with
only a very thin line of clouds lingering along the coast.
Marine layer cloudiness is expected to redevelop along the coast
this evening and tonight...then push inland somewhat by daybreak
Saturday. Model guidance remains in disagreement on this but have
opted to stick with GFS solution that does bring some stratus as far
east as Sea-Tac...thus giving rise to possibility of MVFR cigs in
some locations during the early to mid morning hours tomorrow. SMR
KSEA...Clear skies with northerly winds 8-12 kts. Winds expected to
become more southerly overnight tonight and slow to 5-10 kts. Some
marine stratus possible by 12Z Saturday...dropping cigs down to
.MARINE...High pressure is offshore with lower pressure east of the
Cascades. Just this set up alone will make conditions prime for
small craft advisory winds over coastal waters today and into
Saturday afternoon. As onshore flow increases tonight with an upper
level trough moving into the area...expecting gales over the
northern and central outer coastal waters so gale warning for later
today will remain in place. Models continue to show gale speed winds
over portions of the outer coastal waters into Saturday. Will not
act on this for afternoon package but will pass on to next shift.
The gale warning for the Strait of Juan de Fuca for a strong
westerly push tonight continues to look good...if maybe a little
early in its start time. Models remain persistent that gales will
kick up as upper level low pushes inland north of the area...so
overall pattern and forecast looks good and opted not to tamper with
it. Current solutions continue to show that another round of gales
may be possible for Saturday night in the Strait as well. SMR
PZ...Small craft advisory in effect through late tonight for inner
coastal waters and southern outer coastal waters.
Small craft advisory in effect through this afternoon for
central and northern outer coastal waters.
Small craft advisory from 7 PM PDT this evening through Saturday
morning for Northern Inland Waters.
Gale warning from 5 PM PDT this afternoon through Saturday afternoon
for central and northern outer coastal waters.
Gale warning from 5 PM PDT this afternoon through late tonight for
central and eastern Strait of Juan de Fuca.
You can see an illustrated version of this discussion at