Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
830 PM PDT Monday Mar 30 2015
Synopsis...a cold front will spread rain across the area tonight.
An upper level trough will follow 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...rain is just reaching the coast and it should spread
inland after midnight or so as a cold front moves across the area.
Showers will follow Tuesday along with what may be a persistent and
strong Puget Sound sound convergence zone centered in King County. The new
NAM run shows it lasting until about 12z Wednesday. Cold air will
arrive after the front and high temperatures on Tuesday will be in
the low to middle 50s...ten degrees lower than today.
The snow level will fall to 3000 feet or so and stay there through
the end of the week. This means there will be snow in the passes.
Depending on where the convergence zone sets up there could be
significant snow in the passes. The forecast currently has about an
inch of water in the quantitative precipitation forecast grids around the convergence zone through
12z Wednesday and the models are in good agreement. If all that
falls as snow then a Snow Advisory may be needed for at least the
higher passes including Stevens and white.
Another impulse arrives Wednesday afternoon and then things dry out
Thursday. Highs will be in the low 50s Wednesday then rise a bit on
Long term...previous discussion...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...southwest flow aloft will continue tonight and early
Tuesday as a cold front moves through the area. An upper trough will
follow the front Tuesday along with moist onshore flow. The air mass
will remain moist and become unstable Tuesday.
Widespread VFR conditions continue this evening with scattered-bkn070
bkn-ovc160...but should lower to MVFR and local IFR tonight -- first
along the coast and then inland -- as the cold front just offshore
moves into western Washington. Conditions will improve gradually
Tuesday as moist onshore flow continues...but there will be showers
and a chance of afternoon thunderstorms. There will also be winds
locally gusting to 25 knots with and behind the front.
Ksea...south to southwest wind 6-12 knots...rising to 12-18 knots gusting
25 knots beginning around 2 am. Rain should begin around 11 PM with
conditions deteriorating to MVFR overnight. Mcdonnal
Marine...a cold front just offshore will reach the coast late this
evening then move inland overnight. Strong onshore flow will follow
the front later tonight and Tuesday. Small Craft Advisory winds will
prevail for the forecast area through most of Tuesday...with
westerly gales expected in the central and eastern Strait of Juan Delaware
Onshore flow will weaken Wednesday and Thursday as high pressure
builds over the region. Another cold front will move through the
forecast area Friday.
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