Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT sun Mar 29 2015
Synopsis...a stalled frontal boundary will bring some light rain at
times to the coast and north interior through Monday. The front will
push inland Monday night bringing rain to the rest of the area. A
trough of low pressure will move into the region Tuesday through the
end of next week. The upper trough will give cool and showery
weather to the area and snow levels will fall to around 3000 feet in
the mountains Tuesday night or Wednesday.
Short term...radar shows warm advection rain extending from the North
Washington coast across the north interior of western Washington this morning. Everett
reported some off and on sprinkles earlier and it has stayed dry to
the S. The GFS/NAM-12 appear to be performing well and keep quantitative precipitation forecast
limited to the far northern part of western Washington through Monday. Elsewhere...it
will be partly to mostly cloudy with highs above average in the low
to middle 60s.
An upper trough will dig southward toward the Pacific northwest Monday afternoon as a
1000 mb surface low tracks into central British Columbia. The
associated cold front will bring rain to the coast by the
evening...then quickly spreading inland. Models depict a rather
vigorous system with much cooler air behind the front. Snow levels
ahead of the front will be around 6000 to 7000 feet...then falling
to near 3000 feet by later Monday night and Tuesday. Strong onshore
flow and an unstable air mass will keep showers and breezy
conditions going through Tuesday afternoon. There could even be some
isolated thunderstorms given the gfs40 forecast lifted indice's of -1 to -2c.
Will hold off mentioning thunder for now but will consider adding a
mention in the next forecast package. High temperatures will be
noticeably cooler...only in the middle 50s Tuesday. As for the
mountains...the western slopes of the Cascades should pick up several
inches of snow given some added orographic enhancement. Because of
the showery pattern it may be tough to get solid advisory level
amounts of snow. The exception might be the Cascades near Stevens
Pass where a convergence zone may form. Mesoscale models are showing a
heavier quantitative precipitation forecast bullseye around the S Snohomish/north King County Cascades
near Stevens Pass. Something to consider once we get closer to the
Long term...from previous discussion...an upper level trough will
give cool and showery weather to the area on Wednesday. Thursday may
see some decrease in shower activity and a little more sunshine as a
short wave ridge moves through the region.
Both the European model (ecmwf) and GFS models show cool and showery conditions
Friday through next weekend with snow levels 2500 to 3000 feet in
the mountains. Ensembles are rather consistent in showing the change
to the cooler weather pattern with an upper trough over western
Hydrology...flooding is unlikely over the next 7 days.
Aviation...a broad flat upper level ridge over the Pacific
northwest will maintain fairly moist westerly flow aloft over West Washington
through Monday morning. A nearly stationary front is draped across
southern b.C. And extends SW from Vancouver Island across the
offshore waters. The front will continue to bring a little light
rain at times across the far north part of West Washington...from kuil to kclm to
VFR conditions are expected to continue over the interior with ceilings
generally bkn050-060 with higher overcast layers. Ceilings will probably be a
little lower over the north interior. MVFR ceilings are expected on the
coast with possible IFR conditions over the north coast closer to the
A cold front will approach the area Monday...crossing the coast late
Monday afternoon and the interior Monday evening.
Ksea...VFR conditions are expected to continue through tonight with
cloud layers generally scattered-bkn050-060 bkn-ovc080-100. Surface winds
will remain southerly 5-10 knots. Kam
Marine...a small weak surface ridge will remain over western
Washington through Monday morning. Lower pressure will continue over
British Columbia and the waters offshore where a baroclinic zone
extends southwestward from Vancouver Island. This pattern will
result in moderate S flow across the coastal waters through tonight.
S flow over the inland waters is expected to weaken below advisory
criteria this afternoon.
An approaching cold front will increase winds over the coastal
waters on Monday...and back to advisory levels over the inland
waters. The cold front is expected to cross the coast late Monday
afternoon and the inland waters Monday evening. Moderate to strong
westerly onshore flow will develop over West Washington Monday night behind the
cold front. Gale force winds are likely in the central and east Strait
of Juan Delaware fuca Monday night and Tuesday...and Small Craft Advisory winds are
expected over the coastal waters and the waters adjacent to the
There will be a westerly swell of 10 to 12 feet over the coastal
waters through Tuesday...with rough bar conditions as well.
Onshore flow will weaken Tuesday night as a weak ridge tries to
build over the area. However the GFS now shows an upper level
shortwave trough crossing the region Wednesday. This feature may
cause another brief period of moderate onshore flow Wednesday night.
Pz...Small Craft Advisory all waters except the central Strait.
Small Craft Advisory Grays Harbor for rough bar.
Gale watch for Monday night and Tuesday central Strait and
East Entrance Strait.
You can see an illustrated version of this forecast discussion at