Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 290455
Area Forecast Discussion
National Weather Service Seattle WA
955 PM PDT SUN AUG 28 2016
.SYNOPSIS...The region will lie between a low pressure system
offshore and strong high pressure over the Rockies through
Tuesday. Expect the offshore low pressure system and associated
occluded front to approach the area Tuesday night. The front is
anticipated to move across the region on Wednesday.
There was some spotty light rain over the far northern part of the
CWA at this time; otherwise, there was a general clearing trend
taking place over the area.
Meanwhile, the upper level trough offshore will continue deepening
while the downstream ridge amplifies. This will cause the flow
aloft over the region to back to southwest. The occluded front
will become parallel with the front and will not make much headway
east. Therefore, anticipate increased sunshine and warmer
temperatures over much of the CWA on Monday.
The flow aloft will become more southerly on Tuesday in response
to further amplification of the synoptic scale features. There
should be more stratus present Tuesday morning than on Monday
morning due to increased onshore flow; thus, expect temperatures
to be slightly cooler on Tuesday.
Expect the upper level trough and associated occluded front to
approach the Pacific Northwest Tuesday night. The models have
slowed this system down from the previous runs, which was not
surprising. It now appears that much of the interior will remain
dry Tuesday night. The occluded front will move inland on
Wednesday, possibly dissipating as it moves east, while the upper
level trough remains offshore.
.LONG TERM from the previous discussion...
The large upper level low offshore will move slowly inland across
the region Thursday through Friday. It looks like about three
upper level shortwave troughs rotating around the main low will
move across W WA. This should be a mostly cloudy and showery
period with temperatures a little cooler than normal. The model
solutions have been inconsistent for this period so confidence is
only moderate. Confidence is lower for Saturday and Sunday. For
now I will maintain the broad brush chance of showers forecast.
.AVIATION...An upper level trough will dig off the Pacific
Northwest coast tonight and Monday. Light southwest flow aloft.
Gradual drying at levels tonight except residual low level moisture
at the coast and near the surface over the southwest interior.
Mid level cloud deck over the northern portion of the area will lift
north overnight into Monday morning. Clear skies to the south.
Patchy low clouds and fog could produce a few hours of IFR
conditions at KHQM and KOLM early Monday morning otherwise mostly
KSEA...Northwest wind 4-6 kt becoming light after midnight.
Just a few high clouds clouds through Monday. Felton
.MARINE...Light surface flow over the waters tonight. A weak front
will reach the coastal waters and stall Monday through Monday night.
Winds will remain just below small craft strength over the coastal
waters, then possible reach small craft strength Tuesday before the
front reaches the coast Tuesday evening. The front will weaken and
push through the inland waters Tuesday night with winds likely to
stay below 20 kt. Another front may reach the area late Thursday.
This system could be strong enough for another round of small craft
advisory winds over the coastal waters. Felton
An illustrated version of this forecast discussion can be seen at