Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
930 am PDT Friday Oct 24 2014
Synopsis...a deep low will reach the coast this afternoon and then
fill as it moves northeast. Rain and windy conditions will occur
this evening over western Washington. Sunday and Monday will be
showery. Another weather system will arrive Tuesday. Wet and breezy
weather is likely at times through the end of the week.
Short term...a deep low off the southern Oregon coast will move
northeast today...reaching the Washington coast this afternoon and
moving inland over western Washington tonight. Windy conditions will
develop over most of the area tonight...easing late. Showers will
follow Sunday and Monday with the next system arriving Monday night.
The depth and track of the incoming low pressure center vary
slightly in the latest models. Most agree on the basic idea that the
low will move onto the coast around kqhm 23z-01z...then fill and
move northeast until it crosses Admiralty Inlet around 03z. Windy
conditions will develop on the coast this afternoon and spread
inland this evening.
The strength of the wind over the Seattle areas will depend on the
north-south pressure gradient. The kpdx-kbli gradient varies from
about 10 to about 14 mb in the latest models. In general high wind
starts around 12 or 13 mb. For now have kept the idea of around a 12
mb gradient for a few hours this evening...implying high end
advisory winds or 20-35 miles per hour gusting to 50 miles per hour with perhaps a few
exposed areas going a little higher. A Wind Advisory covers most of
western Washington except for the far north.
As far as weather...a rain band is moving through the metropolitan area
this morning but radar indicates there could be a break of a few
hours. Rainfall amounts over the interior will be fairly light but
will be heavier on the coast. Rain will fall everywhere tonight
before turning more showery late. A decent convergence zone is
likely late tonight and Sunday morning. The pattern remains showery
the rest of Sunday...with some showers still around Monday. Burke
Long term...previous discussion...the next system will approach the
Pacific northwest on Tuesday with the advancing warm front bringing rain back
into western Washington. Probability of precipitation were boosted as models mostly agree on this and
rain shadowing will not be an issue. A 985-990 mb surface low will
start to fill as it tracks east-NE across northern Vancouver Island by
Tuesday night. This system will produce strong southeast pressure gradients
so Wind Advisory level winds are probable from around Whidbey Island
northward and along the coast. Gradients never turn southwesterly through Puget Sound
sound so much less wind expected here. This will be a wet system
with 24 hour rain amounts of 2-4 inches likely in the Olympics and
There will be a lull in showers between systems on Wednesday.
However...it will not be completely dry. The 00z GFS/European model (ecmwf) are in
better agreement on bringing another system into the area on
Thursday. But models are implying this system will split...so it may
not be well organized. There will be a chance of rain at times the
rest of the week.
Hydrology...the Skokomish River appears to be bottoming out near 15
feet this morning. It will rise again in response to heavy rain
today and tonight. It is possible the river will get close to flood
stage late today or tonight. For now the forecast takes the stage at
Potlatch to between 15.5 and 16 feet. Flood stage is 16.5 feet.
A break in the rain will allow the skokomish to recede river Monday.
Another front Monday night and Tuesday will bring 2-4 inches of rain
to the skokomish basin. Will need to keep a watch on this but the
current forecast keeps the river below flood stage.
Otherwise...flooding on other rivers is unlikely over the next 7
Aviation...southwest flow aloft will continue over the area
today...then veer to westerly tonight as a vigorous frontal system
and its upper trough moves through western Washington. The air mass
will be moist and somewhat unstable.
We have mainly VFR conditions across the forecast area this
morning...with locally lower conditions in rain and fog. Conditions
will deteriorate late this afternoon and evening as the approaching
frontal system reaches the area...and as moist onshore flow follows
the front. There could also be isolated thunderstorms along the
coast...but they are unlikely over the western Washington interior.
Winds will increase as the surface low associated with the frontal
system tracks northeast across western Washington this evening with
areas of sustained 20 to 30 knots winds and gusts to 45 miles per hour.
Ksea...winds will continue to be variable to 15 knots today...but they
should trend toward northeast during the day as a surface low
approaches western Washington from the southwest. Winds will
increase rapidly early this evening as the low tracks northeast
across the forecast area...becoming southwest 20-30 knots gusting 40
knots. Winds will probably peak around midnight. Mcdonnal
Marine...a 992 mb low will move NE across the coastal waters late
today...making landfall along the central Washington coast late this
afternoon. Gale warnings are in effect for the coastal waters for
this. The low will track northeast across western Washington this
evening...and southwest flow will increase rapidly in its wake. Gale
force west or southwest winds are likely for the coastal
waters...the entire Strait of Juan Delaware fuca...and Puget Sound sound and
Hood Canal. High-end Small Craft Advisory winds are a good bet for
Admiralty Inlet and the northern inland waters.
High pressure will move across the forecast area Sunday. A warm front
will reach the area late Monday...with a cold front moving through
from the west Monday night and Tuesday. This feature will likely
produce gales over the coastal waters and some inland waters.
Washington...Wind Advisory for the coast...southwest interior...east Puget Sound
sound lowlands...and Puget Sound sound/Hood Canal area.
Pz...Gale Warning coastal waters...Strait of Juan Delaware fuca...Puget Sound
sound and Hood Canal.
Small Craft Advisory Admiralty Inlet and northern inland
You can see an illustrated version of this forecast discussion at