Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 172232
Area Forecast Discussion
National Weather Service Seattle WA
332 PM PDT SUN JUL 17 2016
.SYNOPSIS...The Pacific Northwest will remain under low pressure
aloft through midweek for continued somewhat unsettled conditions.
Overall, the best chance of showers will be over the interior,
the Cascades in particular.
Scattered thunderstorms developed over the Cascades as expected.
The steering flow was from east over the north Cascades; thus, the
potential exists for a thunderstorm to drift across the northwest
interior lowlands (from northern Snohomish County northward) later
this afternoon and evening. The thunderstorms over the central
Cascades were moving slowly north, so the threat of thunderstorms
over the lowlands in southern Snohomish County southward was
starting to appear unlikely. The storm propagation vector was
slightly west, so cannot completely rule out an isolated
thunderstorm over the Cascade foothills this evening, especially
in Snohomish County. So far, there hasn`t been any lightning
strikes over the Olympic but there will probably be a lightning
strike or two by sunset.
Meanwhile, the Pacific Northwest will remain under cyclonic flow
aloft through Wednesday. This will keep the weather pattern over
the area somewhat unsettled. Overall, the best chance of showers
will be over the interior, the Cascades in particular. There will
also be a threat of thunderstorms over the interior (especially
the Cascades) again on Monday. The steering flow will develop a
westerly component Monday night and this should end of the risk
of thunderstorms over the CWA.
The GFS solution was faster in kicking the upper level trof
eastward on Thursday. Decided to go with the slower ECMWF solution
and kept a risk of showers in the forecast.
Beyond Wednesday, high pressure aloft will prevail over the
region for dry weather. The low level flow will remain onshore, so
expect late night and morning low clouds, especially over the
coast. Temperatures will be near normal for late July.
.AVIATION...A broad upper low over Western Washington will shift
west over the offshore waters tonight and Monday with weak
southerly flow aloft developing. At the surface, onshore flow
will prevail with high pressure offshore and lower pressure
There are currently some mid clouds across the area with mostly
clear areas as well. There are also some VFR low clouds -
ceilings around 4000 feet - over the South Sound, East Puget
Sound Lowlands, and towards the coast. These 3500-4500 foot
ceilings will likely linger into tonight. At the same time, MVFR
low clouds should increase along the coast and spread inland
There will be isolated to scattered thunderstorms over the
Cascades late this afternoon and evening. This activity should
die off after sunset with the loss of daytime heating. An
isolated thunderstorm might drift west into the lowlands east of
Puget Sound but the threat is pretty low as storms should be
weakening as they move into the marine layer.
KSEA...The terminal is right on the edge of an area of 3000-4000
foot ceilings this afternoon and these clouds will likely flirt
with the terminal for a few hours. Otherwise, there will be some
mid clouds at times through early Monday morning. MVFR low clouds
will likely reach the terminal Monday morning shortly after
sunrise. Thunderstorms are expected to remain east of the terminal
this evening. West to northwest wind 4-8 knots will ease and
become more southerly later tonight. Schneider
PZ...Small Craft Advisory in effect from 8 PM this evening til 5
AM Monday for the central and eastern Strait Of Juan De Fuca.
An illustrated version of this forecast discussion can be seen at