Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 291006
Area Forecast Discussion
National Weather Service Seattle WA
305 AM PDT MON AUG 29 2016
.SYNOPSIS...Western Washington will remain between a low pressure
system offshore and higher pressure aloft over the Rockies through
Tuesday. A frontal system off the coast is expected to drift onshore
Tuesday night and Wednesday bringing a little rain. The offshore
lower pressure system aloft is then forecast to move onshore
Thursday and Friday producing some light showers and cooler than
average early September daytime temperatures.
The broad upper level trough was over the NE Pacific early this
morning with a series of embedded shortwaves in the flow aloft.
Upper level ridges were over the Rockies and along around 160W. The
shortwaves digging southward from the northern Gulf of Alaska have
been elongating the upper trough over time and that trend will
continue into Tuesday.
This pattern will result in the flow aloft over western Washington
backing from SW today to more southerly Tuesday. The baroclinic zone
extending SW from Vancouver Island should continue to pivot to a
more south to north orientation off the coast through Tuesday.
With light surface gradients under generally clear skies overnight,
areas of morning clouds and fog should give way to considerable
sunshine today. Today will likely be the warmest day of the week
with highs a bit warmer than on Sunday. As the baroclinic zone
creeps closer to the coast tonight and Tuesday, cloud cover should
increase with rain developing along the coast Tuesday evening and
working its way inland through Wednesday morning. With the
baroclinic zone stretching out and weakening, do not expect
significant rain amounts.
For those interested, today marks one year following the late summer
wind storm on August 29th. Look for a public info statement on
this topic by sunrise this morning. Buehner
The mid range guidance concurs on nudging the offshore upper trough
inland Thursday and Friday. This system will offer some showers
along with cooler than first of September high temperatures. The
trend is also for the current high amplitude flow pattern across the
north Pacific and North America to become more zonal.
As we head into the coming holiday weekend, the progs tend to
differ. The Canadian and ECMWF are in general agreement that the
Pacific NW will have slowly rising 500 millibar heights and a drying
trend. The GFS offers an incoming flat upper trough. The current
forecast maintaining a threat of showers into the weekend is okay
for now. Stay tuned as these weather feature details become more
.AVIATION...An upper level trough will dig off the Pacific
Northwest coast today and tonight. Light southwest flow aloft. Air
mass stable with areas of low level moisture this morning.
Light surface gradients this morning combined with some light
precipitation on Sunday has led to areas of low clouds from about
KPAE northward. Ceilings below 1000 feet will slowly lift to above
1000 feet later this morning. The stratus will burn off by midday.
For the Puget sound area southward temperature dew point spreads are
small this morning so patchy fog and low clouds will form around
sunrise. what does form will burn off by mid morning. Just a few
high clouds this afternoon and evening.
KSEA...Still a four degree spread (56/52) at 09z at the terminal
with light winds. Possible that some patchy fog forms around the
terminal but the thought at this point is that what does form will
be too shallow to get up to the airport. Light winds becoming
southerly 4 to 8 knots later this morning. Winds trying to come
around to northwesterly late this afternoon/early evening but
probably will end up around 240-260 degrees. Felton
.MARINE...A weak front will stall over the offshore waters
today and tonight. Winds will remain below small craft strength over
the coastal waters, then reach small craft strength Tuesday in at
least the outer coastal waters 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 will reach the area late Thursday or Friday. 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