Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PST Monday Mar 2 2015
Synopsis...the weak weather system over Washington will move south
of the area this morning. Skies should be mostly sunny this
afternoon with a breezy north wind. Generally dry weather will
prevail the remainder of the week and into the weekend...under the
influence of an upper ridge and low level offshore flow.
Short term...a weak upper shortwave has been digging southeast
across Washington overnight. While much of the Puget Sound sound region
remained dry or nearly dry...this feature brought light
precipitation to the rest of western Washington. At 2 am skies were
still mainly cloudy...but precipitation was spotty and light. The
air mass is rather cool with temperatures in the middle 30s to middle 40s
and the snow level around 1500 feet.
Northerly flow aloft will continue today ahead of the upper ridge
offshore along 140w. The shortwave will move south of the forecast
area this morning...with the air mass drying from north to south.
Skies should be mostly sunny by late today. The northeast surface
pressure gradient has been increasing overnight...and this will
continue today as surface high pressure moves south in western
Canada. Much of the forecast area will have breezy north or
northeast winds...which will be strongest in the north interior of
western Washington near the mouth of the Fraser River.
Dry northerly flow aloft will continue tonight through Wednesday...
as the offshore upper ridge moves slowly toward the Pacific
northwest. Clear skies and easing northeast surface winds will mean
overnight lows in the upper 20s to 30s both tonight and Tuesday
night. There will be patchy fog or freezing fog during the early
morning hours in the south Puget Sound sound region and south interior.
Tuesday and Wednesday will be mostly sunny with highs mainly 45 to
Long term...the upper ridge will move inland Wednesday night and
Thursday...with an upper ridge building over the region Friday and
during the weekend. A weak system moving over the north side of the
ridge could brush western Washington on Thursday or Thursday night.
Some model runs keep the forecast area entirely dry...and if any
precipitation falls it will be light and most likely limited to the
north coast and north Cascades.
The models are in rather poor agreement for the entire period
Thursday through Sunday...and the run to run consistency of
individual models has not been very good either. For now suffice it
to say that an upper ridge will probably build over the region
Friday and during the weekend for more dry weather.
Hydrology...flooding is unlikely for the next 7 days.
Aviation...an upper ridge centered offshore will keep the flow
strong northerly through tonight. Moderately strong low level northerly flow will
become Ely tonight. Areas of ceilings in the 1-3k feet range over the
interior this morning will dissipate by 2000 UTC.
Ksea...VFR conditions are anticipated through this evening...although
cannot rule out occasional ceilings near 015 this morning. Winds will be northerly
at generally 10-15 knots...with occasional gusts near 25 knots possible
strong high pressure over British Columbia with lower pressure over
Oregon will result in moderately strong northerly flow today. A
1037 mb high developing over Montana with lower pressure over the
coastal waters will cause the flow to become easterly or offshore
So far...the northerly pressure gradient has been running weaker than
what the models predicted. For example...the GFS model predicted
that the Bellingham to Williams Lake /British Columbia/ pressure
gradient would be -8.4 mb at 4 am this morning. The observed pressure
gradient was -3.9 mb. Therefore...there is a chance that the
forecast speeds might be a little too high in some places
especially for tonight.
Pz...small craft advisories are in effect for all waters.
Gale watch in effect from 6 PM this evening through Tuesday
morning for the northern inland waters.
You can see an illustrated version of this forecast discussion at