Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
830 PM PST Wednesday Dec 24 2014
Synopsis...the weather over the next two days will be mostly dry
though skies will be mostly cloudy and there may be isolated showers
at times. Rain is more likely Friday night through Saturday night as
a system arrives from the north. Snow will fall in the mountains.
Drier and cooler weather will begin Sunday and last into next week.
Short term...radar shows showers in a broad convergence zone north
of Seattle and also a few showers on the central coast. Flow aloft
is northerly and at the surface is onshore. Models show the surface
flow becoming weakly offshore Thursday and Friday while northerly
flow aloft continues. The upshot should be mostly cloudy skies but
mainly dry conditions Thursday and Friday with highs in the 40s and
lows in the 30s. With nighttime temperatures that low any showers at
night could have snow...but after tonight probability of precipitation are generally low.
On Saturday a weather system moves into the area from the north.
Rain is likely over The Lowlands with this system and snow is a sure
bet in the mountains. A foot or more of snow is possible with the
snow level around 2000 feet. Precipitation will taper Sunday. Burke
Long term...previous discussion...high amplitude ridging offshore
is expected to shift eastward to around 130-135w as a strong high
pressure over southeast British Columbia drops down into eastern Washington. High heights and high
pressure will result in sunny days and clear nights. Expect daytime
temperatures in the middle 30s to lower 40s and lows mainly in the 20s
if the GFS solutions are correct while they will a bit colder if the
12z European model (ecmwf) verifies. Some moderate Fraser outflow is likely behind
the front sun and sun evening but will die off quickly as the ridge
shifts east of the area. Albrecht
Hydrology...flooding is not expected for the next 7 days.
Aviation...northerly flow aloft will prevail over western
Washington tonight and Thursday...as a high amplitude upper ridge
moves east over the northeast Pacific. Low level onshore flow will
gradually ease during the period. The air mass will be moist and
Scattered showers and low MVFR to IFR conditions continue this
evening mainly at the coast and in a weak Puget Sound sound convergence
zone. The convergence zone should weaken overnight and dissipate
early Thursday morning...remaining north of Seattle. Elsewhere in
the forecast area conditions are likely to remain low-end VFR to
high-end MVFR with scattered-bkn020 broken-ovc040 ovc070. There will be some
improvement Thursday afternoon.
Ksea...south to southeast wind 6 knots or less. The convergence zone
should remain north of both kbfi and ksea. Ceilings should remain in
the 2000-4000 feet range tonight and Thursday morning. Mcdonnal
Marine...onshore flow will ease tonight. Small Craft Advisory will
remain in effect for the coastal waters and West Entrance Strait due
to west swell 13-16 feet subsiding to 10-11 feet Thursday.
Generally light flow will prevail over the coastal and inland waters
of western Washington Thursday through Friday night with weak high
pressure over the region. A weak front will move through the area
from the northwest on Saturday. Northerly offshore flow will develop
Sunday and Monday as high pressure moves south through British
Some of the higher astronomical high tides of the year will occur
the next few days. With light wind and weak high pressure Thursday
and Friday...positive tidal anomalies and tidal overflow are
Pz...Small Craft Advisory coastal waters and West Entrance Strait of
Juan Delaware fuca.
Small Craft Advisory for rough bar Grays Harbor bar.
You can see an illustrated version of this forecast discussion at