Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Seattle Washington
947 am PDT Thursday Oct 30 2014

Synopsis...a wet Pacific storm will bring significant rainfall
to much of the area this afternoon through Friday morning. The
cold front will move across region late Friday morning...with the
steady rain tapering off to showers behind it. A weak low pressure
system aloft will slowly move across the area Friday night through
Saturday for somewhat unsettled conditions.

&&

Short term...
the cold front offshore has slowed down a bit due to waves
developing along the southern part of the frontal boundary.
Also...the front was oriented nearly parallel to the flow.

Rain rates are expected to pick up this afternoon in response to
strong upward forcing. The south facing slopes of the Olympics
will likely see rain rates of a 0.10 to 0.2 inch per hour this
afternoon through tonight. In fact...a couple of gages on the
olympic range recorded rain rates of near 0.2 per hour this
morning but this will be short lived. National Weather Service Doppler radars showed
a relative dry slot moving into southwest Washington at this time.

Expect locally heavy rain to develop on the south facing slopes of
the Cascades tonight...continuing into early Friday morning. The
steady precipitation will become showery Friday afternoon behind the cold
front. The upper trough will move over the region Friday night and is
expected to be east of the County Warning Area Saturday evening. Satellite imagery
indicated that the air mass behind the front was quite unstable.
Thus...starting to wonder whether the forecast for Friday night and
Saturday is too optimistic.

Long term from the previous discussion...
confidence not very high in the extended period early on with the
models losing their continuity both run to run and among
themselves on the 00z run. The European model (ecmwf) was slower in exiting the
trough on Saturday...which slowed down the approach of the next
system that was timed into the area on Sunday. Now it is Sunday
night into Monday. The GFS was a little faster with this system
but still slower than previous models. With the GFS solution
showing the most continuity at this point...will stay with the
likely probability of precipitation on Sunday. Models were in better agreement with a wet
day on Monday...with a system moving through the area Monday
night/Tuesday morning. Both the GFS and the European model (ecmwf) were showing an
atmospheric river scenario setting up for the middle of next
week...with the European model (ecmwf) being the stronger and wetter of the two
models. Kept rain in the forecast through at least the middle of
next week. Felton

&&

Hydrology...
a rather slow moving frontal system will bring steady precipitation
western Washington through Friday morning. The largest amounts
will fall over the olympic and north Cascades with up to 3 inches
of rain in the next 24 hours. The snow level will remain high 7000
to 8000 feet. This amount of rain could once again drive the
exceptionally flood-prone Skokomish River above flood stage. In
addition...the Nooksack river will probably rise significantly.
Flooding there is unlikely but not out of the question...and a
hydrologic outlook addresses that situation.

At this time flooding is not expected on the other rivers in the
forecast for the next 7 days. Felton

&&

Aviation...SW flow aloft. The air mass is stable and moist.

Satellite and radar imagery show a frontal system over the coastal
waters moving slowly east. Pressure gradients are easterly ahead of
the front today and that will keep ceilings 3000-5000 feet or higher
in the interior today. The only exception will be along the north
slopes of the Olympics where upslope stratus around kclm keeps
conditions there IFR. The front moves onshore later this afternoon
then stalls as it moves inland tonight. Expect ceilings and
visibilities to drop to marginal VFR tonight as rainfall increases
and pressure gradients collapse. Albrecht

Ksea...expect light rain to fall from a ceiling 5000 feet or higher
most of today with east to southeast winds 9-12 knots. Ceilings will
fall to 2000-3000 feet with -ra as the front moves into the interior
and stalls and pressure gradients become weak. Visibility with the front
00z-03z this evening may occasionally fall to 5sm -ra br. Albrecht

&&

Marine...satellite and radar show the incoming front over the
extreme outer coastal waters slowly moving east. The front will move
onto the coast during the late afternoon hours then will stall over
the inland waters late this afternoon and evening. Small Craft
Advisory southeast winds are occurring ahead of the front over all
waters except Puget Sound sound and Hood Canal and Admiralty Inlet.

Expect winds to diminish as the front moves inland and dissipates
tonight. The problem of the day concerns a wave that appears on
satellite over the central Oregon offshore waters. The 12z GFS
develops a wave that moves NE across the area early Friday morning
and is followed by strong southwesterly flow. The 12z NAM and 00z European model (ecmwf) do
not show this wave but take a weaker impulse inland well to the
south of the area. Will maintain the weak wind forecast for Friday
at this time.

Weak high pressure will build into the area later Friday into Sat. Then an
active period of weather is expected beginning sun and continuing
through next week as a series of fronts move across the waters.
Albrecht

&&

Sew watches/warnings/advisories...
Washington...Flood Watch in effect from 6 PM this evening through late
Friday night for Mason County.

Pz...small craft advisories are in effect for all waters except
the Admiralty Inlet and Puget Sound sound/Hood Canal.

&&

$$

Www.Weather.Gov/Seattle

You can see an illustrated version of this forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

National Weather Service Glossary of Abbreviations