Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
330 am PDT Friday Oct 24 2014
Synopsis...a warm front offshore will produce locally strong
east wind along the Cascade foothill early this morning. Another
stronger low pressure system will bring locally windy conditions to
portions of western Washington later today and tonight. It will also
be wet with locally heavy rain possible. Sunday and Monday will be
showery. More rain is expected Tuesday with another advancing
frontal system. Wet and breezy weather is likely at times through
the end of the week.
Short term...a warm front lifted northward overnight and radar shows most
the rainfall has shifted over extreme northern areas early this morning.
The cross Cascade gradient continues to climb with sea-eat near near
-7.5 mb at 3 am. The wrfgfs-4km shows gusty east winds along the
foothill continuing into the morning hours before easing. Mesoscale west
observation are showing gusts to near 35-40 miles per hour at sites around Enumclaw.
With the climbing gradient the wind gusts should reach 45 miles per hour at
times so the advisory remains in effect through 9 am PDT. The
Bellevue zone was removed as stronger winds show no signs of
extending that far.
The main feature of interest is the low pressure system spinning off
the northern California coast. Models appear to have a much better handle on the
complex structure of this low as it organizes and moves NE toward
our area. Its central pressure is already down to near 985 mb and a
bit more deepening is possible over the next few hours given the
numerous lightning strikes which often indicate a deepening system.
These strikes are also occurring to the north/NE of the low which should
generally tell the direction the low will track...in this case to
Latest 00z/06z models are now in much better agreement so there is
slightly more confidence that the low will consolidate rapidly today
and track inland near Hoquiam. Even the 00z nam12 which was still
bringing the low too far S for wind has shifted north with the low track
on the 06z run...bringing it NE through Puget Sound sound this evening.
Virtually all the other models show this consolidation and general
track with only slight variations. The European model (ecmwf) is the most alarming
with its more north-northeasterly track just west of Puget Sound sound resulting in lots
of southerly gradient...+14 mb pdx-bli...all through way up to the north
interior. This solution would probably give high end advisory for
much or all of the interior. Most other solutions show a much
shorter period of southwesterly winds with the initial sharp pressure rises
behind the low late this afternoon and evening. The nam12 and
wrfgfs-4km both indicate 3 hour pressure rises of +8 to +9 mb from
the coast to the SW interior/Puget Sound sound region behind the low.
Those models then surge the westerlies down the Strait and produce
strong winds into Whidbey Island and maybe The Headlands of the eastern
Strait. This scenario would cut off stronger winds to the S...so
Puget Sound sound southward would have winds easing by midnight. Not all models
agree on these details but this is the best guess scenario at this
time. Will issue a Wind Advisory for the coast...SW interior...Puget Sound
sound/Hood Canal...and the eastern Strait/Whidbey Island zones. Most
models cut off stronger winds before reaching Everett so that zone
will be left out of the advisory. Will also issue an Special Weather Statement to cover
the rest of the area indicating breezy to locally windy weather
possible...but sub advisory. At something will be in place in the
event the European model (ecmwf) is correct in producing strong southerly gradient all the
way up the interior.
Strong Post-low onshore flow will prevail on Sunday with rain
changing to showers. It will be noticeably cooler with highs only in
the middle 50s...though it will feel colder given the breeze.
Long term...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 will rise again in response to
heavy rain today and tonight. Since it is still running high from
the previous rainfall...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
A break in rain will help water recede on the Skokomish River
Monday. However...another front will bring up to 2-4 inches of rain
to the skokomish basin. Will need to keep a watch on this but
current projections keep the Skokomish River below flood stage.
Otherwise...flooding on other rivers is unlikely over the next 7
Aviation...a cold front just offshore will move inland this
morning...followed by an upper level and surface low this afternoon
and this evening. The air mass will moisten with VFR ceilings this
morning lowering to predominant MVFR ceilings with spotty IFR ceilings late
this afternoon and evening.
A surface low will move across Astoria around 21z and across West Washington
00z-06z. Strong west-SW flow behind the low will produce gusty surface
winds in spots...sw20-35g45kt. The air mass will become more
unstable as the upper level low approaches...with isolated
thunderstorms possible along the coast and over the SW part.
Ksea...the terminal is in a lull at 11z between a warm front and the
cold front just offshore. Rain will return 15z-19z as the cold front
crosses the area. Precipitation will continue after that as the upper level
and surface low follow. The air mass will become more unstable as
the upper level low nears the area with isolated thunderstorms
possible. Ceilings should gradually lower through bkn030-050 through
around 22z then reach the MVFR range after that. Winds aloft
fl050-fl100 will be S 30-55kt from 15z-00z. North surface winds will
shift to southerly 5-10kt after 15z as the cold front approaches.
Surface winds sw15-30g35kt will develop after 00z. Kam
Marine...a 995 mb surface low associated with an approaching cold
front will move NE across the coastal waters this morning. A 990 mb
low will move NE across Astoria this afternoon and over Puget Sound sound
late this afternoon. This is a bit of a complex pattern and models
are not quite in lock step with respect to timing...track...and wind
strength in various locations. Confidence is only moderate. The
first low should raise southerly gales along the coast with Small Craft Advisory
winds over parts of the inland waters. The second stronger low
should generate west-northwest gales over the coastal waters and should raise
full gales in the Strait this evening. Pressure rises behind the low
should raise winds in Puget Sound sound but gales are a little more
tenuous here. Models have the weakest Small Craft Advisory level winds over Admiralty
Inlet and the north inland waters...but gales are not out of the
After the second low exits to the NE tonight...a surface ridge will
gradually build inland over the region late tonight and Sunday.
Another vigorous frontal system will approach from the west on
Monday...then move inland Monday evening. Gales are possible for the
coast and north interior. Kam
Washington...Wind Advisory for the east Puget Sound sound lowlands through 9 am
PDT this morning.
Wind Advisory for the coast...southwest
interior...east Puget Sound sound lowlands...and Puget Sound sound/Hood
Canal from late this afternoon through tonight.
Pz...Gale Warning coast...Strait and Puget Sound sound.
Small Craft Advisory Admiralty Inlet and northern inland
You can see an illustrated version of this forecast discussion at