Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
920 am PST sun Nov 23 2014
Synopsis...a front was over southwest Washington this morning.
Heavy showers and thunderstorms were embedded in the front. Periods
of heavy snow will continue in the mountains today. After a break in
the weather later tonight and Monday morning...a warm front will
reach the area Monday afternoon. There will be wet weather Tuesday
and Wednesday...especially in the mountains.
Short term...a front gave some heavy rain to portions of western Washington
around daybreak. At 9am the front is over SW Washington with some heavy
showers and embedded thunderstorms with gusty winds. There is also a
band of heavy showers just north of the Snohomish County line at
9am. Snow will fall in the mountains. The Portland to Bellingham
gradient is +9mb for some blustery weather...the gradient will
weaken this afternoon and evening surface high pressure builds into
western Washington and Oregon. A dirty upper ridge and warm front moving over the
top of the ridge will poke into western Washington later Monday and
Tuesday. This will mean periods of rain with a higher snow levels in the
Long term...previous discussion...confidence in the long term
forecast for Wednesday through Saturday is not very high. The broad
upper ridge will probably move east of the area on
Wednesday...leaving western Washington near the confluence of a
moist southwest fetch and a cooler drier northerly flow over western
Canada. Earlier model runs generally favored the idea that the drier
northerly flow would prevail Thanksgiving day and Friday. More
recent runs...especially of the GFS...show the wetter pattern
developing. We increased probability of precipitation into the likely to categorical range
for Wednesday...but we have decided to stand pate on the forecast
for Thanksgiving day through Saturday. Mcdonnal
Hydrology...a sort of mental blend of the models solutions the past
couple days suggests a chance of river flooding for the rivers
coming out of the north Cascades by Wednesday. The GFS continues to
focus rather heavy rain in the north Cascades while the 12z NAM
surprisingly only snows a 48hr total of about three inches of rain
spread out along all of the Cascade Range. I have not seen the European model (ecmwf)
yet...but will take that into consideration for a Hydro outlook
Aviation...strong west flow aloft morning...becoming moderate northwest
flow this afternoon and tonight. Air mass moist and unstable...but the
moist layer will become more shallow tonight.
Active line of east-west oriented thunderstorms has been spreading
onto the coast south of Grays Harbor for the past 2 hours...with
some individual cells showing signs of rotation and significant
organization...ie these are strong storms. Main impact within our
forecast area will be in Lewis County for the rest of this morning.
Otherwise...a few band of showers follow the cold front from earlier
this morning. Trend will be toward decreasing shower coverage and
decreasing wind for the rest of today.
Ksea...a band of showers appears likely through 19z...with the
outside possibility of a single lightning strike. Then a decreasing
trend on shower chances and coverage this afternoon and evening. Winds at
the surface and aloft will also start decreasing this afternoon. May be
eyeballing a convergence zone north of the terminal this
afternoon...perhaps as far south as downtown or north Seattle from
Marine...cold front passed earlier this morning. Is now being
followed by SW flow and a westerly surge through the Strait. This
has prompted a Gale Warning for the central and eastern
Strait...with small craft winds elsewhere. Winds will quickly
subside this afternoon and evening. West swell along the coast will easily
continue above 10 feet for the next 24 hours.
Flow will shift to southeasterly and increase somewhat as a warm front
approaches the waters from the SW on Monday. Then the warm front is
expected to pull up stationary somewhere from between central
Vancouver Island and the Washington waters. Expect rather strong SW
flow on the order of 20 to 30 knots to the south of where this front
stalls Monday night through Wednesday. Haner/albrecht
Washington...Winter Storm Warning Cascades above 3000 feet.
Pz...Gale Warning central and eastern Strait of Juan Delaware fuca.
Small Craft Advisory remaining waters.
Small Craft Advisory for rough Grays Harbor bar conditions.
You can see an illustrated version of the forecast discussion at