Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
900 PM PDT Thursday Oct 30 2014
Corrected remarks about Skokomish River in hydrology section
Synopsis...a Pacific frontal system will bring rainfall to western
Washington tonight. The cold front will cross the Cascades early
Friday morning...and rain will turn to showers and decrease.
Saturday should be partly sunny with light spotty showers. A weak
front will probably bring some light rain on Sunday. Rainy and
locally windy weather is likely most of next week.
Short term...a Pacific frontal system is moving slowly through the
Pacific northwest this evening...with areas of rain continuing
across western Washington. Precipitation amounts for the storm so
far have been roughly 0.50 to 1.50 inches for the coast and
mountains...with locally almost 3 inches over the Olympics. Up to
0.75 inch has fallen in the interior lowlands. The snow level is
still around 7500 feet.
The system will progress slowly eastward with the cold front
crossing the Cascades late tonight or early Friday morning. Steady
rain will turn to showers...which will probably taper off quickly on
An upper trough will move to the coast late Friday night and inland
Saturday. However the portion of the trough that moves into western
Washington will be weak and disorganized...with the core of the
upper trough digging southeast across California. A forecast of
partly sunny skies and a chance of showers looks good. For the
showers that do occur...precipitation amounts will be light and
coverage might just be scattered. The snow level will be around 4500
to 5500 feet.
A small flat upper ridge will move into the Pacific northwest
Saturday night and Sunday...with a weak warm front following closely
behind the ridge axis. Models are having difficulty with the timing
of this system. It will bring some rain to western Washington on
Sunday...but some model runs show slower timing with the rain moving
in Sunday night. Mcdonnal
Long term...here is the long term section from the afternoon
forecast discussion -- confidence in this period was not high due to
continued disagreement and lack continuity in the medium range
solutions. It did appear that western Washington will probably receive
significant rainfall during this period. The medium range solutions
seemed to be converging on the idea of a significant rainfall
producer impacting the County Warning Area during the Monday/Tuesday time frame. Have kept
probability of precipitation above climatology during the extended period.
Hydrology...about 1 to 2.75 inches of rain has fallen over the
Olympic Mountains...with the heaviest amounts along the southeast
slopes. Radar and satellite imagery show the back edge of the
heaviest rainfall moving east of the Olympics though...and models
show only another 0.3 to 0.6 inch of rain in the skokomish basin
through Friday. The Skokomish River near Potlatch is rising...and
actually tracking slightly ahead of the forecast. After much
deliberation I will go ahead and issue a Flood Warning this
evening...but confidence is not especially high. It could fall just
short of flood stage.
The other river we have been concerned about is the Nooksack
river...because the other focus of heavy rainfall is around Mount
Baker. Flooding on the Nooksack looks unlikely though.
Flooding is not expected on other area rivers during the next 7
Aviation...SW flow aloft. The air mass is stable and moist...
becoming more unstable Friday as cooler air moves in aloft. The
heaviest rain with this frontal system at 8pm was mainly east of I-5
and moving into the Cascades. The radar shows precipitation should become
more showery tonight and Friday.
Ksea...a band of heavy rain moved through the metropolitan area this
evening and now the radar suggests the precipitation will be more showery.
The infrared satellite image still shows a frontal band draped over
the area...so showers could turn back to rain again but certainly on
Friday the cooler more unstable air will work in over the area for
just scattered showers.
Marine...looks like a front went through the area with westerlies
in the Strait this evening...Race Rocks has been up around 30kts for
a few hours...the buoys and Smith Island in the Strait have not
shown much of the westerly push...but with race blowing pretty
strong I have a Small Craft Advisory up for the whole Strait for
this evening. Another breezy spot at 8pm was the area north of
Bellingham...and Sandy Point had gusts to 28mph...but the winds over
the water and throughout the San Juans were not more than about
10kts so I don't have an Small Craft Advisory for the northern waters. The winds over the
coastal waters behind this front should turn to northwest 15kt
tonight...but then become more SW again Friday as a surface low
pressure trough moves through the coastal waters. Weak high pressure
will build across western Washington Saturday. More active weather
is on the way Sunday through early next week...with more frontal
systems expected to move across the waters.
Washington...Flood Watch in effect from 6 PM PDT this evening through
late Friday night for Mason County.
Pz...Small Craft Advisory Strait of Juan Delaware fuca until 11pm.
You can see an illustrated version of this forecast discussion at