Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
158 PM PST Wednesday Nov 25 2015
Synopsis...a blocking pattern is developing over the Pacific
northwest with an upper high over southern b.C. And lows over the
Great Basin and well off the West Coast. This will provide dry but
rather cool weather over the area that will likely last through most
if not all of the Holiday period over southwest Washington and
northwest Oregon. We will also see brisk offshore flow developing
through tonight that will continue through much of the weekend. The
dry pattern may break down early next week but confidence in the
timing is highly uncertain at this point.
Short term...the upper low that brought colder and damp weather to
southwest Washington earlier this week was in the Great Basin this
afternoon...and the wraparound moisture that lingers over the central
Oregon Cascades area will decrease this evening. Northeast wind aloft
has been increasing over our forecast area...between the low in the
Great Basin and the upper high building along the b.C. And Washington
coasts. This building upper high will continue to drift onshore into
b.C. Through Thanksgiving day and Friday that will result in a
blocking pattern through most of the weekend. This pattern will
produce dry but cool weather over the area.
Initially...we will have decent northeast flow aloft across the area
through at least Friday and likely continuing Saturday. The models
show some breezes developing today under the northeast flow aloft...
but the stronger offshore gradients kick in tonight and continue into
Saturday. Thus the east winds through The Gorge will pick up tonight
and be about as strong as or perhaps a little stronger than that
which occurred this past weekend. Looking for gradients peaking in
the 8 mb range...perhaps 9... which would support gusts to around 60
miles per hour at Corbett and Rooster Rock...and at least 70 or 80 at Crown
There is a small chance of some patchy fog and freezing fog near
Eugene and north of Kelso late tonight and early Thursday...but the
chance is not high. Otherwise drier air working through the area will
result in a minimum of morning fog the next couple of days. As the
offshore flow increases...could see the clouds and fog increase at
the east end of The Gorge...possibly affecting Parkdale and Hood
River the next few days.
The main weather message for the next few days will dry weather...
the increasing east winds...low wind chills in The Gorge...and the
cool low level air mass that will allow areas that decouple from the
wind at night to fall into the lower 20s. Tolleson
Long term...Saturday night through Wednesday...blocking pattern
keeps high pressure over our area...meaning dry and cool conditions
through the weekend. GFS has trended back toward breaking down the
Rex block earlier and bringing rain back in Monday night in to
Tuesday. Am a bit skeptical because the European model (ecmwf) is holding on to a
slower solution and models tend to try to break these blocking
patterns down too early. If the GFS does pan out...we may return to
an active weather pattern with multiple chances of rain and snow
starting Monday night through the rest of the week. If the European model (ecmwf) is
correct...blocking pattern doesn't break down and bring precipitation back
into the area until later in the week. Hedging toward a blend of the
two...with precipitation returning later on Tuesday or Wednesday. -McCoy
Aviation...VFR conditions expected this evening and overnight.
Offshore gradient will strengthen through the Columbia River gorge
overnight resulting in gusts of 30-40 knots at the west end of the
Columbia Gorge later tonight into Thursday.
Kpdx and approaches...VFR conditions tonight and Thursday.
Marine...winds will become east-northeast over the coastal waters
tonight as thermal trough extends northward into SW Oregon coastal
area. Strongest winds will be in southern waters...10-60nm from
around Cape Foulweather south with gusts around 25 kts. Wind speeds
gradually diminish late Friday through the weekend. Next round of
stronger winds in the Tuesday/Wednesday time frame.
Seas around 8 to 9 feet tonight. Seas decrease to around 5 to 6 feet Thursday
and remain there through early sun. Seas gradually increase to around
10 feet Sun night. Tw
Pz...Small Craft Advisory for winds until 4 am PST Thursday for
waters from Cascade Head to Florence or from 10 to 60 nm.
Small Craft Advisory for rough Columbia River bar until 6 PM
PST this evening.
Small Craft Advisory for rough Columbia River bar Thursday
afternoon through Thursday evening.
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This discussion is for northwest Oregon and southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.