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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
860 
FXUS66 KSEW 121631
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
931 AM PDT Sun Aug 12 2018

.SYNOPSIS...Residual clouds and a few light showers this morning 
will give way to hazy afternoon sunshine. Higher pressure aloft 
will then bring dry weather and a warming trend through Tuesday, 
and possibly Wednesday. Increasing onshore flow will moderate 
temperatures a bit by Thursday as high pressure weakens over the 
region. Warmer temperatures are possible again next weekend.

&&

.SHORT TERM...Yesterday's upper low is already exiting across the
Canadian Rockies, with northwest flow aloft in its wake. A few
lingering sprinkles and light rain showers near Cascades and
foothills will diminish this morning and dissipate this afternoon.
Visible satellite imagery nicely shows a large elevated area of
smoke now spreading southeast across the region, likely sourced
out of B.C.. The Canadian smoke model shows concentrations within
the elevated smoke layers increasing through this afternoon, so
hazy skies will await those who are waiting for lower clouds to 
scatter out. 

A lingering upper trough will drape itself across Southwest
Washington through tonight with little or no sensible impact. The
tail end of the lingering trough will pinch off into a closed
upper low off the Oregon coast on Tuesday, bringing a return to
S-SW flow aloft over Western Washington on Tue night and Wed. 

Cooling low-level onshore flow will be quite weak through Tuesday,
and drying of the air mass will mean diminishing clouds and more
hazy sunshine. Therefore expect a substantial warming trend away
from the coast through Tuesday. Haner

.LONG TERM...From Previous Discussion: Models show a weak upper 
trough approaching the region on Wednesday. The ECMWF is a bit 
slower than the GFS, so the timing of onshore flow is a bit in 
question. MOS guidance shows some slight moderation in 
temperatures Wednesday, so will stick to the idea that highs will 
ease off by a few degrees. The trough will open up and cross the 
area Thursday, with the only real noticeable change being slightly
cooler highs in the upper 70s to low 80s, still above average. 

Westerly onshore flow picks up behind the dissipating trough on 
Friday which typically does not bring much low cloud inland but will 
keep temperatures in check. By the weekend, some models indicate the 
onshore flow weakening as higher pressure builds offshore. This will 
probably bring a return to much warmer temperatures and continued 
dry weather next weekend. Mercer

&&

.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 will stabilize
and dry this afternoon through tonight. Light pressure gradients 
will dominate at lower levels. 

Observations show a main ceiling at around 4500 ft this morning
with lower clouds at around 1500 ft seen with a left over
convergence zone in the central Puget Sound area and some ceilings
600-1500 ft along the coast. Expect the lower layer to dissipate
by about 18Z and the 4500 ft layer to scatter out by mid
afternoon. One rather reliable model shows a low stratus layer 
forming in the central Puget Sound area 12-17Z Monday for a 
potential for LIFR conditions affecting the KPAE, KPWT and KSEA 
terminals in northerly flow. Other models show a dry boundary 
layer on Monday. Will introduce a sct003-005 group into these 
terminals for Monday morning for now. Albrecht

KSEA...Ceilings near 4000 with a scattered layer at 1500 ft. Both
layers will scatter out as mentioned above with scattered clouds
at 5000 ft later this afternoon and clear skies tonight. Light SW
winds this morning becoming northerly 6-10 kt this afternoon and
evening. Heads up: there are some indications that a LIFR ceiling
with north winds 6-9 kt could occur 13Z-18Z Monday. Albrecht

&&

.MARINE...Light pressure gradients will give way to light
northerly to northwesterly flow this afternoon through Tuesday.
Stronger northwesterly pressure gradients are expected to develop
Tuesday night or Wednesday and continue through the rest of the
week as high pressure builds offshore. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

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