Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
845 PM PST Sat Dec 27 2014
Synopsis...scattered showers will continue this evening as an upper
level trough crosses the area...with a chance of showers Sunday.
Another upper level trough will cross the area Sunday evening
bringing rain or snow showers. High pressure building over British
Columbia will push modified Arctic air south Sunday night...giving
mostly sunny and cold weather over the area through middle next week.
Short term...radar shows showers continuing over the coast and
mountains with other areas drier. There is a sort of convergence
zone in Snohomish County with most of the precipitation heading
toward the Cascades in response to outflow from the Strait. Lows
tonight will mostly be in the upper 30s under partly cloudy skies.
Sunday will be mostly cloudy with a chance of showers and highs in
the low 40s. The wind will switch to northerly during the day over
most of the area as Fraser outflow begins. This outflow will
strengthen Sunday night and Monday. The colder and drier air mass
will push any remaining moisture out of the area by Monday. During
this process late Sunday and Sunday night there could be snow
flurries. The current forecast has the snow level falling to 300
feet or so Sunday night...then even lower as the air mass dries. It
is Worth noting the uw WRF-GFS has no snow in The Lowlands though
there is some over the northern foothills of the Olympics where the
northerly surface flow will be upslope. In any case by sometime on
Monday probability of precipitation fall to zero as skies clear.
Monday night and Tuesday will have clear skies. Overnight lows will
fall into the 20s with upper teens not out of the question. Highs on
Monday will be in the middle and upper 30s...and a few degrees lower
than that on Tuesday. Burke
Long term...previous discussion...the models maintain a strong
upper level ridge offshore through midweek...keeping the weather
mostly sunny cold and dry through Thursday. The ridge will be
weakening over time.
The Thursday through Saturday period is still in question and I
expect the models will remain inconsistent into early next week. As
the upper level ridge weakens...warm southern branch flow around a
large low north of Hawaii flows north and merges with the cold northern
branch flow coming S across the Gulf of Alaska. The two branches merge
offshore with the resulting jet aimed roughly at the pacnw. This
will be an active weather period for West Washington but the models are going
to have a hard time settling on whether the flow over the area will
be colder northern branch or warmer moister southern branch. So far
the European model (ecmwf) has been a little more consistent in showing a shortwave
reaching the area Friday and Saturday. The GFS has been more
variable. A chance of rain has been kept for Friday. Saturday shows
a chance of rain...with rain or snow over the northern zones since
the latest guidance was a little colder.
Hydrology...flooding is not expected for the next 7 days.
Aviation...northwest flow aloft over the Pacific northwest will
continue tonight and Sunday. Low level onshore flow will gradually
ease. The air mass will remain moist and stable. The Puget Sound sound
convergence zone...which extended from southern Whidbey Island to
around Stevens Pass at 8 PM...will remain north of Seattle as it
weakens tonight and dissipates Sunday morning.
VFR to local MVFR conditions continue this evening with sky cover
roughly scattered-bkn025 broken-ovc040 bkn-ovc070. Showers have become
limited to mainly the coast and convergence zone. Conditions will
probably lower somewhat tonight...then improve slowly on Sunday.
Ksea...southwest wind 10-16 knots gusting 24 knots...easing to 6-12 knots
later tonight and Sunday. Mcdonnal
Marine...moderately strong onshore flow this evening will ease
Northerly offshore flow will develop Sunday...as surface high
pressure starts moving south through British Columbia and a weak
surface low develops along the Washington coast. The strength of the
northeast flow will probably reach its peak Monday or Monday night.
Small Craft Advisory northeast winds are likely for the coast...with
gales possible for the northern inland waters and the Strait
beginning later Sunday night and Monday. A gale watch is in effect.
Offshore flow will ease Tuesday and Wednesday as the high moves into
the northern Rocky Mountains. Mcdonnal
Washington...Winter Storm Warning through late tonight for the Cascades
from Snohomish County through Lewis County.
Winter Weather Advisory through late tonight for the
Pz...Gale Warning central Strait of Juan Delaware fuca and East
Small Craft Advisory all other marine zones.
Small Craft Advisory for rough bar Grays Harbor.
Gale watch Sunday night through Monday night for northern
inland waters and Strait of Juan Delaware fuca.
You can see an illustrated version of this forecast discussion at