Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
309 PM PDT Friday Mar 14 2014
Synopsis...Post-frontal showers will continue across the area this
evening. A stalled system will spread more rain across the area
Saturday and Sunday. An upper level trough will push the front
southeast of the area Sunday night...allowing showery weather to
take over. An upper level ridge will bring a dry period Tuesday and
Wednesday. More rain will probably arrive Thursday.
Short term...Post-frontal onshore flow continues across western Washington
this afternoon...with an energetic Puget Sound sound convergence zone
over Southern Island and southern Snohomish County. The pscz is
producing brief heavy downpours...small hail and a few
cloud-to-ground lightning strikes. The uw WRF GFS and hrrr show the
pscz persisting into the evening hours along the Snohomish/King
County line. The convergence zone is also producing locally heavy
snow in the Cascades near Stevens Pass - a Winter Weather Advisory
remains in effect through early this evening. Elsewhere...we are see
just scattered showers. These showers will taper down after midnight
Expect more rain on Sat as a warm front clips western Washington. The
baroclinic band will stall over southern b.C. Saturday
afternoon...then slowly slide southeast into western Washington Sat night through
Sunday. Rain will be heavy at times...especially over the north
coast and in the mountains. Models are showing 2-5 inches of precipitation
in the Olympics and north Cascades with 1-2 inches in the central
Cascades. The snow level is around 5000 feet in the north Cascades
with Mount Baker right on the edge - heavy snow is possible. The
interior could see another inch of rain through this period...with
1-3 inches along the coast. Heavy precipitation will force sharp rises on
the rivers and minor flooding is possible. Will refresh the
hydrologic outlook. This system will exit late Sun night with heavy
Northwest flow will prevail on Monday as high pressure builds offshore.
Models show showers mainly in the interior and Cascades. 33
Long term...upper level ridging will bring dry weather to western
Washington through at least Tuesday. This ridge may linger into
Wednesday...although the GFS is now kicking it out Wednesday afternoon as our
next trough approaches. The Euro however maintains the ridge over
the Pacific northwest on Wednesday for dry weather. Confidence is low but will stick
with the slower Euro solution for now. Showery weather is forecast
through the end of the week as a series of upper level disturbances
roll through western Washington. Temperatures will remain close to normal. 33
Hydrology...a frontal system will bring fairly significant
precipitation to the Olympics and north Cascades Saturday night and
Sunday...with the snow level climbing to around 6500 feet. Models show
2-5 inches of precipitation through the period. If the model solutions are
close to what does happen...then flooding could occur on the
Skokomish River and maybe some of the most flood-prone rivers
flowing off the west slopes of the northern Cascades. A flood
potential outlook has been issued to discuss the possibility.
Aviation...northwesterly flow aloft tonight will become westerly
Saturday. A baroclinic zone will develop north of the area Saturday
then sag south Saturday night. At the surface onshore gradients will
ease tonight then become increasingly southeasterly on Saturday. The
air mass is moist and slightly unstable. It will stabilize and dry
somewhat tonight...then moisture will increase all levels Saturday
afternoon and evening.
A vigorous Puget Sound sound convergence zone lies over southern Snohomish
County with scattered showers elsewhere. In showers and the pscz
ceilings are MVFR. Elsewhere there are clear areas or VFR ceilings.
The trend will be toward more widespread VFR this evening...then
ceilings will fall on Saturday as clouds thicken and lower. By
Saturday afternoon and evening most ceilings will be MVFR.
Ksea...convergence zone does not seem destined to reach the terminal
tonight...though a stray shower cannot be ruled out. Generally VFR.
Southwesterly winds 14g22kt will ease this evening then become
southeasterly 4-8 knots after 06z. Chb
Marine...moderate onshore flow will ease tonight. Small craft
advisories have ended over all waters except the central and east
Strait and Puget Sound sound. Winds in those zones will ease this evening.
A frontal system will move through the area Saturday afternoon and
night. This system will bring another round of Small Craft Advisory
winds...with gales forecast for the northern and central coastal
zones and the West Entrance. High pressure will build offshore
Sunday night and Monday with lower pressure inland. Westerly gales
are possible at times in the Strait Sunday and Monday...then
gradients diminish on Tuesday. Heavy west swell in the 15-20 foot
range will reach the coast Sunday through Monday. Chb
Washington...Winter Weather Advisory north and central Cascades until 8 PM
Pz...Gale Warning northern and central coast zones and West Entrance.
Small Craft Advisory central and east Strait and Puget Sound sound.
.Small Craft Advisory for rough bar Grays Harbor bar.
You can see an illustrated version of the forecast discussion at