Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
301 PM PDT Monday Mar 30 2015
Synopsis...a cold front will spread rain across the area tonight
followed by an upper level trough Tuesday for showers and cooler
weather. Showers will diminish Thursday as weak ridging builds over
the area ahead of a frontal system that will affect the area on
Friday. An upper level low will maintain a chance of showers through
Short term...a warm front remains over southern b.C. This afternoon
with western Washington in the warm sector. As a result...temperatures are pretty
mild across the region with highs mainly in the 60s. South winds are
increasing as the cold front starts to slide S/southeast into western Washington.
This front will bring wetter weather to the region tonight as it
moves in. Rain will mainly affect the coast this evening...with rain
spreading into the interior near midnight tonight. There is plenty
of moisture with this front and we will see wetting rains across the
An upper level trough and onshore flow will maintain cool and
showery weather through Tuesday. Snow levels will drop to around
3000 feet with a few inches of snow in the mountains. A Puget Sound sound
convergence zone may enhance amounts in King County. Models also
show a negative lifted index and cannot rule out isolated
thunderstorms Tuesday afternoon. Highs on Tuesday will be in the low to middle
50s - a few degrees below normal.
The air mass will remain cool and unstable as we head into Wednesday
with a trough still overhead. Low level onshore flow will prevail
for scattered showers. The air mass will stabilize and dry out as we
move into Thursday. By then a weak upper level ridge will build over
the Pacific northwest. 33
Long term...the next Pacific frontal system will arrive on Friday
for another period of wet weather across western Washington. Probability of precipitation were
nudged upwards as both the GFS and European model (ecmwf) show signs of agreement.
The forecast through the weekend and into early next week remains
unsettled as an upper level low spins offshore. This system will
continue pumping moisture inland for a chance of showers each day.
The air mass will remain cool with temperatures slightly below normal. 33
Hydrology...flooding is unlikely over the next 7 days.
Aviation...a cold front along 130w at 21z will reach the Washington coast
around 05z then cross the interior around 09z. Moist westerly
onshore flow will develop behind the front. Flow aloft will be SW.
The air mass was fairly dry and stable at 21z with good VFR
conditions everywhere except for MVFR ceilings along the coast. Ceilings
throughout West Washington are expected to lower to MVFR tonight in rain with
and behind the cold front. Gusty S surface winds will develop
tonight ahead of the front and will be gusty in spots late tonight.
A broad upper level trough will move over West Washington by Tuesday afternoon
with the air mass becoming unstable...with showers and a chance of
Ksea...good VFR conditions are expected through 05z tonight with
ceilings lowering to bkn030-050 with rain as a cold front reaches the
area roughly 07z-09z. MVFR ceilings are expected late tonight. Surface
winds will remain S-SW 6-12 knots through 06z then rise to SW 15-25 knots
behind the front. Kam
Marine...a cold front along 130w at 21z will reach the coast late
this evening the move across the inland waters a little after
midnight. Strong onshore flow will develop through the Strait
immediately behind the front. Tonights forecast was based on the
morning hrrr model runs which is the only model that is now
consistently indicating gale force winds in the Strait. The 12z
Canadian lam also shows a couple of areas of gales. Small Craft Advisory S winds are
expected over the inland waters and over the coast tonight and
Weaker onshore flow will prevail Wednesday and Thursday as a ridge
builds offshore. There should be enough west pressure gradient through
the Strait for Small Craft Advisory winds in the evenings. Another cold front will
probably reach the area Friday. Kam
Pz...Small Craft Advisory coastal waters...entrances to the
Strait...Admiralty Inlet...northern inland waters...Puget Sound
sound and Hood Canal.
Gale Warning central Strait of Juan Delaware fuca and East Entrance.
Small Craft Advisory Grays Harbor for rough bar.
You can see an illustrated version of this forecast discussion at