Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 260246
Area Forecast Discussion
National Weather Service Seattle WA
746 PM PDT SUN SEP 25 2016
.SYNOPSIS...A strong upper ridge will move across the area Monday
for a warm and mainly sunny day. A weak cold front will bring a
chance of light precipitation Monday night and Tuesday morning.
Expect partly to mostly sunny days Tuesday afternoon through
Thursday with dry southwest flow aloft. Then an an upper trough
will bring cool showery weather Friday through next weekend.
.SHORT TERM...A strong upper ridge build over the region last
night and today, with 500 mb heights over Western Washington
rising to around 5900 meters. Highs today were mainly in the 70s
with sunny skies and offshore low level flow.
The upper ridge will move east of the Cascades tonight and into
the northern Rocky Mountains Monday. The air mass will warm
further tomorrow as the thermal trough moves across Western
Washington, and highs across the interior lowlands should be in
the mid 70s to lower 80s. If there`s a fly in the ointment it`s
that the latest model runs show the weak cold front upstream of
the upper ridge approaching more quickly than previous runs. There
is already some cirrus over the forecast area, and it could
increase Monday. It`s even possible that some light precipitation
will reach the northwest corner of the forecast area Monday
The cold front and its upper trough will move through Western
Washington Monday night and Tuesday morning. The main threat of
precipitation will be in the north Monday night, and then in a
weak Puget Sound convergence zone and along the west slopes of the
mountains Tuesday. Any precipitation amounts should be light.
Tuesday should become partly sunny during the day as the threat of
showers decreases. Highs will be back into the 60s to lower 70s.
Relatively dry southwest flow aloft will prevail Tuesday night
and Wednesday as an upper trough begins to amplify well offshore
near 137W. Wednesday should be mostly sunny with highs again in
the 60s to lower 70s. McDonnal
.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- Deep troughing around 135W late Wednesday will shift
slowly east through the period, finally moving across Western
Washington On Sunday. Long term models bring rain chances into the
area late Thursday night into Friday morning. Showers are likely
across much of the area next weekend. Snow levels will lower to
around 6000 feet on Friday and 5000 feet Saturday and Sunday.
Higher mountain passes like those on the North Cascades highway,
may see some snow next weekend. Albrecht
.AVIATION...An upper ridge and surface high pressure will shift
inland through Monday. Offshore flow will keep late night and
morning fog limited to the sheltered valleys around daybreak. A
cold front will reach the area Monday night.
KSEA...Some cirrus at times, a northerly breeze, patchy valley fog
may form around daybreak Monday bit that will not effect Sea Tac.
.MARINE...Northerly flow will turn westerly Monday afternoon and
strengthen Mon night as a cold front moves through Western WA. This
could result in gale force westerlies over the central and eastern
portion of the Strait of Juan de Fuca where a Gale Watch is in
effect. Onshore westerly flow will turn more northerly and decrease
PZ...Gale Watch from Monday evening through late Monday night for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at