Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
352 PM PDT Wednesday Oct 22 2014
Corrected minor typo in the second paragraph of the marine segment.
Synopsis...disturbances moving around an upper trough offshore will
continue through Thursday. The air mass is unstable...so there will
be a chance of thunderstorms on the coast tonight and throughout the
area on Thursday. Shower activity will diminish Thursday night and
Friday morning. While it will remain rather cloudy Friday
afternoon...a brief and weak ridge of high pressure aloft combined
with low level offshore flow will give drier weather. A low pressure
system developing off the Oregon coast will move across the area
late Saturday or Saturday night. Active weather will continue into
Short term...satellite imagery and radars show the front that moved
across the area early this morning is now nearly stationary from the
Cascades into northwestern Oregon. A broad and rather deep upper
level trough is centered around 145w longitude. Upper level
disturbances are rotating around this trough and are riding
northeastward along the front across the Pacific northwest. The
first disturbance enhanced precipitation today as it moved northeast
across the area. The GFS shows another wave now over the Oregon
offshore waters shifting NE across the area tonight. The axis of the
heaviest precipitation amounts will likely be shifted about 50 to
100 miles farther to the south and east of where the rain was seen
today. Yet another disturbance...this one now seen as what appears
to be a developing instant occlusion north of the front near
135w...appears that it will move across the area on Thursday.
Thunderstorm activity that was seen earlier today over the coastal
waters has mainly moved into Vancouver Island...but some isolated
lightning strikes can still be seen over the offshore waters. Some
thunderstorm activity can be expected over the coastal waters
tonight and there is a chance that a few lightning strikes will be
seen along the coast and in the western Olympic Mountains. The air
mass is expected to be a bit more unstable over the interior
Thursday afternoon as the instant occlusion moves NE across the
area. Lifted index values fall to around -2.1 and convective available potential energy rise to
about 500 j/kg in the Seattle area 21z Thursday per the nam12 and the GFS
is slightly more unstable. Showers with a chance of thunderstorms
will be in the forecast for the entire area on Thursday.
Showers diminish Thursday night and come to an end Friday morning across
much of the area as a deep low develops off the northern California
and southern Oregon offshore waters. This low appears to be
developing in response to a system dropping southeast from the Bering Sea
as it phases somewhat with the remains of Ana northwest of Hawaii. The
latest nam12 and GFS solutions show the low reaching its maximum
depth at around 985 mb off the central Oregon coast while the
Canadian and European model (ecmwf) solutions are 5-10 mb deeper. Offshore flow and
short wave ridging aloft may bring a rather dry period...albeit
cloudy...to the area from late Friday morning into early Friday
The low quickly becomes vertically stacked in all of the models off
the Oregon coast...then it matures...moves northeast...and gradually
decays as it moves NE across our area Sat evening. Depending on the
depth and track of the low...it may get windy over portions of
western Washington later Saturday into Saturday night. But with the
low decaying and wind field expanding...expect the threat of high
wind in the forecast area to be below 25 percent. Some model
solutions...like the 12z Canadian solution...keep the track of the
low south of the forecast area. This is not unreasonable given the
general split flow pattern being seen over the western U.S. Of late.
Temperatures will be seasonable for this time of year and it will
remain rather cloudy as we go through the next several days. Albrecht
Long term...showers can be expected behind Saturday nights low
pressure system on Sunday. Beyond that time...models are having a
tough time with the details. The European model (ecmwf) has a storm track to the
north of the area with frontal passages here through the middle of
next week. The storm track on the GFS is just to the south of the
area...still a wet one for western Washington but less windy. The
Canadian solution is closer to the European model (ecmwf). In any case the pattern
will remain active. Albrecht
Hydrology...a Flood Warning remains in effect for the Skokomish
River in Mason County. The skokomish at Potlatch crested around 17.2
feet at noon today. It is expected to fall below flood stage tonight
and gradually recede Thursday through Friday.
Wet weather is expected to continue through the upcoming week. But
rainfall amounts are not expected to drive any rivers above flood
Aviation...a slow-moving cold front over the interior is expected to be
east of the Cascades by 0600 UTC Thursday. It now appears that the unstable
air mass will remain over the coastal waters through tonight.
Meanwhile...strong SW flow aloft will continue over the region through
tonight. The low level flow will remain southerly.
Expect areas of ceilings and visibilities in the 1-3k feet and 3-5sm range...
respectively...through tonight. However heavy precipitation will result in
occasional IFR cigs/vsbys.
Ksea...ceilings have lifted into the VFR category but occasionally ceilings in the
2-3k feet range can be expected through part of tonight. Ceilings near 2k feet
may become predominant late tonight or early Thursday.
a 1001 mb low developed over the Oregon coastal waters this
afternoon and this has slowed down the cold front over the Puget Sound
sound region. The cold front should be east of the Cascades by late
this evening. Will need to watch for the possibility of a burst of
southerly winds near 25 knots over the waters from Point Grenville
to Cape Shoalwater out to 60 nm late this afternoon through this
evening as the low moves off to the NE.
Beyond this afternoon...numerical weather models were not in good
agreement with the track...timing...and strength of the low that is
expected the end of this week. Depending on which model is
correct...this system could produce anywhere from high end gale or
low end storm force winds over parts of the area to inconsequential
wind speeds. The forecast reflects the consensus. Thus anticipate
gale force winds over much of the waters Saturday night.
Washington...Flood Warning in effect for the Skokomish River in Mason
Pz...Small Craft Advisory in effect for hazardous in effect for
the coastal waters and West Entrance to the Strait of Juan Delaware
Small Craft Advisory for rough bar in effect until 6 am
Thursday for the Grays Harbor bar.
You can see an illustrated version of this forecast discussion at