Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PST Wednesday Nov 25 2015
Synopsis...a strong ridge of high pressure will build over the
Pacific northwest through at least Monday of next week. The weather
will be dry and chilly with most places falling below freezing the
next few mornings. Low pressure may bring light rain back into the
area by Tuesday or possibly Wednesday of next week.
Short term...a period of dry offshore flow and chilly temperatures
will dominate the weather over the next several days. High pressure
aloft will strengthen off the Pacific northwest coast through
Thursday...then move directly over the region Friday and Saturday.
Surface pressure gradients will continue to weaken tonight with
generally light wind by Thursday afternoon. This will lead to poor
ventilation by Thursday night and Friday as inversions strengthen
over western Washington. Given that models show this pattern to
persist for over 72 hours...it seems prudent to go ahead and issue
the air stagnation advisory which will go into effect Thursday
evening and run through at least noon Monday. High temperatures will
run in the 40s through the weekend with lows in the 20s to middle 30s.
Cold pocket locations like Olympia could reach the lower 20s the
next few morning. There could also be some patchy freezing fog but
the air mass is dry enough to inhibit anything widespread.
Long term...high pressure aloft remains in place through at least
Monday and possibly Tuesday. Models remain inconsistent today
regarding the timing and strength of a low pressure system that may
reach the area by middle next week. Some models keep the area dry even
into Wednesday. This would require an extension of the air
stagnation advisory but there is plenty of time for models to settle
on a solution. Any fronts that do manage to reach the area will
probably be weak with light precipitation amounts. Kept a chance of
rain in the forecast starting Monday night as some GFS runs have
shown some threat of rain that early. Mercer
Hydrology...flooding is not expected for the next seven days.
Aviation...a large strong upper level ridge centered offshore will
maintain cool dry NE flow aloft over West Washington through Friday...shifting
to dry east flow aloft this weekend. A surface ridge will remain over
southern b.C. Which will maintain dry north to NE low level flow.
The air mass will remain dry and stable through the weekend with
minimal fog or stratus in the morning. Morning air temperatures will
be near or below freezing so isolated freezing fog is possible. NE
low level flow over the north interior will produce gusty surface winds
at times through Friday.
Ksea...clear skies. North-NE surface wind 8 to 15 knots through 00z...then
easing to 5-12 knots after 00z. Kam
Marine...a strong surface ridge over southern b.C. Will maintain
light to moderate north-NE flow today. Pressure gradients will gradually
weaken through this afternoon so the small craft advisories
currently in effect should end this evening. The NE offshore flow
pattern is expected to persist through this weekend.
This period of King tides should produce the highest tides around West
Washington this year the next couple of days. Fortunately strong high
pressure over the area appears to be doing the job of keeping The
Tides below the highest astronomical tide level...which is the
nominal level when tidal overflow might begin. Winds are in the
process of easing which will allow wind waves to subside...and not
produce minor coastal flooding during the peak tides. Kam
Pz...Small Craft Advisory all inland waters and coastal zones
153...173 and 176.
You can see an illustrated version of this discussion at