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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 122228

Area Forecast Discussion...RESENT
National Weather Service Seattle WA
328 PM PDT Sun Aug 12 2018

.SYNOPSIS...A strong warming trend is expected Monday and Tuesday
due to the cutoff of cooling onshore flow. Warm temperatures are 
expected all the way to the coast on Tuesday, with Southwest 
Washington nearing record highs. Onshore flow will return in time
Wednesday to cool the coast but will wait til Thursday to take 
the edge off heat over the interior. A re-strengthening of upper
level high pressure next weekend will start the next warming
trend. No precipitation for the week ahead.


.SHORT TERM...Upper heights are rising quickly this afternoon
following the departure of yesterday's upper low. Skies will
continue to clear this evening, but a thick layer of smoke aloft
will keep a hazy, milky looking sky in place through Monday
morning. Otherwise, just some short-lived stratus in the morning
along the coast and in the lowlands along the I-5 corridor, but 
skies will clear quickly by mid-day.

Through Tuesday, a strong upper ridge axis will nose from the 
Pacific into southwest B.C., keeping the weather dry over Western 
Washington. At the surface, forecast gradients suggest low-level 
flow will turn weakly offshore for the next two mornings, with 
only weak onshore flow for a few hours during the PM diurnal peak 
on Monday and Tuesday. With a lack of cooling onshore flow and 850
mb temps climbing to about 19C on Tuesday, temperatures will 
quickly climb over the next two afternoons. By Tuesday, looking 
for highs of 90+ from Seattle and Bremerton southward. The 
forecast high of 94F at Olympia on Tuesday is only one degree shy 
of the record high for the day. Even the coastal side of the 
Olympic Peninsula will climb well into the 80s on Tuesday.

An upper trough will develop off the Oregon coast on Tuesday night
and Wednesday, with a turn to southwest flow aloft over the
forecast area. The 12z and 18z NAM runs suggest a surge of marine
stratus will spread up the Oregon coast on Tuesday night, 
reaching Grays Harbor and the rest of the Washington coast on Wed 
morning. The arrival of marine air along the coast will cool 
afternoon temps back into the 60s there, but shallow marine air 
will arrive too late over the interior on Wed to provide anything 
more than just a few degrees of relief. Haner

.LONG TERM...Models have been admirably consistent through next
Saturday. There has been a good deal of run-to-run inconsistency 
next Sunday, or Day 7, but some consensus is developing today.
They show a weak upper trough moving across the Northwest U.S. on 
Thursday, with the best height fall/rise couplet and upper lift 
passing east across Oregon. Not expecting precip over Western 
Washington with this feature, but the initiation of moderate low- 
level onshore flow will bring temperatures on Thursday and Friday 
afternoons back down into the low 80s over the typically warm I-5 
corridor of Southwest Washington, with marine stratus nosing into 
the coastal gaps both mornings. 

Strong upper ridging will nose in from the Pacific on Saturday.
Ensemble means then show a strong low amplitude ridge axis off the
coast next Sunday, with a positive height anomaly bullseye off the
coast of Vancouver Island. This pattern would assuredly lead to
another warming trend next weekend. 

Bottom line for the week ahead: No rain and above normal
temperatures.  Haner


.AVIATION...An upper level trough extending from east-central
British Columbia to off the Oregon coast will slowly weaken
through Monday. Light NW flow aloft. The air mass is stable and
dry with patches of low level moisture expected during the 
morning on Monday in the interior and more widespread low level
moisture along the coast. Light pressure gradients will dominate 
at lower levels. 

Observations and satellite imagery show clouds decreasing 
rapidly from northeast to southwest this afternoon. A smoke layer 
originating from forest fires over British Columbia extends from 
about 7000 to 10000 ft MSL. Expect VFR conditions to dominate 
through tonight. Most forecast models show clear skies with LIFR 
CIGS limited to the coastal waters and immediate Pacific coastline
Monday morning. The NAM12 shows the development of a shallow 
stratus layer 500-1000 ft MSL near or north of KPAE that will 
quickly stream southward to include the KBFI and KSEA terminals 
14Z-18Z combined with north winds 8-10 kt. At this time will limit
the LIFR conditions to the coast and to KPAE and will just 
indicate a low scattered layer at KSEA as confidence on this 
development is low to moderate. Any shallow moisture will quickly 
burn off by 18Z leaving VFR conditions in the afternoon. Albrecht

KSEA...VFR conditions through tonight with an elevated smoke layer
070-100 MSL. NW wind 7-10 kt becoming N 7-9 kt after 03Z then NW
again after 19Z Mon. There are some signals that a LIFR ceiling 
with north winds 6-9 kt could occur 14Z-18Z Monday. However
confidence is not high. Hence will include a SCT003 layer in the
forecast during this period for the terminal. Albrecht


.MARINE...Light north to northwest flow is expected through 
Tuesday. Stronger northwesterly pressure gradients are expected to
develop Tuesday night or Wednesday over the outer coastal waters and
continue through the rest of the week over the rest of the coastal
waters as high pressure builds offshore. Albrecht