Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
943 PM PST Sat Mar 8 2014
Synopsis...a cold front will move across western Washington
overnight. Once the front passes...rain will taper off to showers
and wind will diminish. An upper trough will move through on Monday
for cooler showery weather. A strong upper ridge will provide dry
and mild weather Tuesday and Wednesday.
Short term...a cold front is now making its way onto the coast. The
heaviest precipitation rates at a given location appear to be just in
advance of the front...fueled by SW 850 mb winds of 50-60 knots and
precipitable water values well in excess of 1 inch. (The 00z kuil
sounding showed a precipitable water of 1.08 inches...and a freezing level of 8500
feet.) With the front along the coast...the interior of SW Washington and
the south Puget Sound sound region are getting their most intense rainfall
now. Worrisome is the fact that observed rainfall has been a good
bit higher over the southern Olympics over the past 6 hours than
forecast. More on this in the Hydro section. But precipitation should taper
off quickly to mere showers over the next 1-3 hours as the front
moves east to the Cascades.
Behind the front...freezing levels will start a gradual drop through
Monday morning. By Sunday evening...the snow level should drop below
Stevens Pass...and then below Snoqualmie Pass on Monday morning. A
little shortwave ridging on Sunday afternoon will put a lid on shower
activity...making it a predominantly dry afternoon for most. But then an
upper trough axis will pass across western Washington on Monday morning...providing a
little shower enhancement at that time. Showers will be fairly
disorganized...as most of the energy and lift with the upper trough
axis will be well to the south over southern Oregon and northern cal.
Upper heights build quickly on Monday night...with a substantial ridge
over the Pacific northwest by Tuesday afternoon. 500 mb heights will exceed 5700 M...and
low-level offshore flow will develop as well. A sunny day with high
temperatures over the SW interior flirting with 60 degrees is my
expectation for Tuesday. Haner
Long term...from previous discussion...an upper level ridge over
the Pacific northwest will maintain dry and stable weather across western Washington
Wednesday and Thursday...possibly into part of Friday as well. We should see
some sun on these days with temperatures in the 50s to lower 60s. Models
then show a weak upper level disturbance rolling through southern
b.C. Friday afternoon or evening which may bring a few showers back to
the region. Both the GFS and Euro try to rebuild the ridge over next
weekend for another break in the weather. 33
Hydrology...rainfall has exceed the forecast this evening over the
southern Olympic Mountains not surprisingly...the skokomish is tracking
up faster than forecast as well. Now appears that the Skokomish
River will exceed the major flood stage of 17.5 feet.
Rfc guidance also takes above flood stage on Sunday the Puyallup at
Orting...the Cowlitz at Randle...and the Snoqualmie at Carnation.
The time window is soon approaching to issue warnings for these
three locations...especially if rainfall meets or exceeds
expectations on those basins' headwaters. Stay tuned.
Otherwise...rises are expected on all other area rivers.
Rainfal will taper off quickly in a few hours with no additional
hydrologically significant precipitation expected over the next few days.
Will surely be watching water makes its way down the relatively flat
and slow-to-drain Chehalis river long after the rain is done. Rfc
guidance currently keep all points along the Chehalis below flood
stage...but the margin of error will make this a situation to watch
as well. Haner
Aviation...strong SW flow aloft will continue tonight and Sunday. A
cold front has moved inland over West Washington as of 04z and will gradually
weaken overnight. Rain is still widespread and moderate in intensity
in spots...but with the back edge of the precipitation just reaching
the coast...rain will diminish by midnight. The air mass will remain
quite moist with widespread MVFR and local IFR conditions through
18z. VFR conditions are expected after 21z Sunday as the air mass
Ksea...continuing rain will keep the air mass moist for IFR ceilings and
MVFR/IFR visibility. MVFR ceilings below 2000 feet are expected 12z-18z.
Drying Sunday afternoon should raise ceilings to VFR. Surface winds will
remain S 7-10kt. Kam
Marine...wind shifts and rising pressures along the coast indicate
the cold front moved inland early this evening. S-southeast flow has
weakened below even Small Craft Advisory levels over the coastal and north inland
waters...so the gales have ended. The pressure gradient through
Puget Sound sound has increased a little so I will leave the Small Craft Advisory for Puget Sound
sound in place until midnight as planned. S winds have increased to
14g23kt at S sound land stations...kolm and ktiw...so there is still
a chance the Small Craft Advisory will work out.
Swell on the coast has risen above 10 feet so a Small Craft Advisory for hazardous seas
has replaced the gales. Small Craft Advisory for rough Grays Harbor bar conditions
will continue through Sunday.
Other minor problem is how strong the west onshore flow will be on
Sunday with the weak surface ridge building along the Washington coast.
Could see west 15-25 knots in the central Strait...but model winds have
been running sub-sca...while the pattern suggest Small Craft Advisory winds. I will
hold off on the advisory until the 00z WRF-GFS arrives.
A stronger surface ridge will approach the area from offshore on
Monday...so onshore flow will increase a little. The surface ridge
will move over the region on Tuesday then hang out through middle-week
for a period of weak winds. Kam
Washington...flood watches and/or warnings remain in effect for parts
of the County Warning Area.
Pz...Small Craft Advisory for Puget Sound sound and Hood Canal.
Small Craft Advisory coast for hazardous seas.
Small Craft Advisory for rough bar Grays Harbor bar.
For the graphical afd...visit