Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Seattle Washington
900 am PST Wednesday Nov 25 2015

Synopsis...a strong upper ridge offshore will maintain dry
northeast flow aloft through Sunday...with cool sunny days and
cold nights for western Washington. Windy conditions over the
north interior...caused by cold air flowing out the mouth of the
Fraser River in southern British Columbia...will gradually ease
today. Wet weather will probably return on Tuesday.


Short term...Fraser outflow across the north interior is gradually
easing this morning. Winds are still gusting to around 35 miles per hour around
western Whatcom County but just breezy conditiions elsewhere. Forecast
pressure gradients drop off quickly this afternoon so expect much calmer
winds by tonight. With a strong upper level ridge offshore through
Thursday...dry weather and mostly clear skies will continue. Radiational
cooling overnight will allow for rather chilly morning lows in the middle
20s to around 30. Cold pocket locations like Olympia and Shelton could
dip into the low 20s Thursday and Friday mornings. Highs will mostly
be in the low to middle 40s through the end of the week.

Models show the upper level ridge to center directly over the Pacific
northwest by Friday with light flow. This will allow inversions to
build stronger with air stagnation likely a issue through the weekend.
With 72+ hours of stagnant conditions likely...will issue an advisory
with the afternoon forecast package covering the Thursday night through
Sunday time frame. The mountain zones above the inversion layer will
be excluded as will the coast where more mixing naturally occurs. Mercer

Long update from previous discussion...the upper high will
weaken somewhat Saturday and Sunday as it moves from British Columbia
into Alberta. Dry northeast flow aloft will continue over western
Washington. The offshore flow at the surface will weaken further...and
the air mass over the forecast area will gradually moderate. Light
southerly flow aloft will probably resume on Monday...but models
are delaying the arrival of the first weak frontal system...which
will probably move into western Washington on Tuesday. Mcdonnal


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. Kam


Sew watches/warnings/advisories...

Pz...Small Craft Advisory all inland water and coastal zones 153...173
and 176.




You can see an illustrated version of this discussion at

National Weather Service Glossary of Abbreviations