Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 230400
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PDT MON AUG 22 2016

.SYNOPSIS...
A weakening of low level onshore flow is expected as the upper
level low pressure exits to the east, while an upper level high
pressure begins to strengthen across Western Washington. The
result will be a strong warming trend across the area through late
this week. By Saturday there will be the return of cooler air and
even some possible showers by Sunday as an upper level low
pressure system brushes through the Pacific Northwest.

&&

.SHORT TERM...
Temperatures topped out this Monday afternoon generally from near
to slightly below normal. SeaTac saw a high of 72 degrees compared
to a normal of 76. Some cloud development was noted this afternoon
and evening but is steadily melting away following sunset. A quiet
night with temperatures dipping slightly below normal can be
expected. Readings in the lower to mid 50s in the urban areas and
mid to upper 40s in the outlying locales should be expected. Some
patchy fog should also be expected in favored spots, as Monday
afternoon dewpoints dropped into the mid to upper 40s which may be
close to the lows in the favored spots tonight.

For Tuesday expect the warming trend to begin as highs will crest
from near to slightly above normal and more 80s returning. 500 mb
heights will rise about 6 to 8 dm by 00z Wed and climb above
580dm. The 850 mb temps will climb as well from around +10 to +14C
by Tuesday evening. This will take place as the mid level
troughing slides east with mid level ridging building in from the
Pacific. Wednesday will feature even more warming with an offshore
flow developing and pushing 850 temps even higher. Highs will
settle into the middle and upper 80s with around 90 in the
southwest.

.LONG TERM...From previous discussion. Model guidance in recent days
has been struggling with the details of the weather for the
coming weekend. For now, they at least agree that an upper trough
will dive southeast through B.C., with a turn to cooling onshore
flow in advance of the trough axis. However, the GFS begins the
onshore flow and marine push on Friday, while the ECMWF holds off
until Saturday. So this means that the GFS shows cooling westerly
flow getting underway on Friday, while the Euro shows one more
warm or hot day Friday before cooling begins on Saturday.

Models today are trending weaker with the incoming trough for the
weekend. Though timing is about 18 hours different, neither the
GFS nor ECMWF bring 500 mb heights much below 570 decameters. As a
rule of thumb, this should keep shower activity light and mainly
confined to the northern half of the forecast area.

It appears that the upper trough will exit east across the Rockies
a week from today, with rising upper heights and the start of a
warming trend over western Washington. The 12z GFS seems to be an
outlier by dropping an upper low southward out near 135W next
Monday. The 18z GFS looks more like the 12z ECMWF in exiting the
upper trough on Monday and showing the rising upper heights.
Haner

&&

.AVIATION...An upper level ridge will build offshore tonight
through Tuesday. The flow aloft is northerly. The air mass over
Western Washington is dry and stable. Low level marine clouds will
mainly affect the coast overnight. Low clouds will burn off
Tuesday afternoon with VFR conditions expected. 33

KSEA...Light north winds and VFR conditions. 33

&&

.MARINE...Onshore flow will ease tonight. Winds will turn
northerly Tuesday through Thursday as a thermal trough forms along
the coast. Onshore flow will develop on Friday as low pressure
shifts inland. Onshore flow will continue through the weekend. 33

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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