Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland Oregon
856 PM PDT Tue Oct 25 2016
Synopsis...a warm front will lift north into Lane County a little
before midnight then reach southwest Washington around sunrise. The
bulk of the precipitation looks to be confined to the coast...Oregon
Coast Range and the Willapa Hills of SW Washington. The cold frontal
boundary follows close behind the warm front. The cold front hugs the
coastal zones Wednesday then gradually pushes east late Wednesday
through Thursday. A complex upper trough offshore will continue to
produce unsettled weather into early next week.
Short term (tonight through friday)...a slight lull in the action
this evening...but things will change overnight. The big picture has
a weakening and departing upper low along the British Columbia coast.
However...a rapidly-developing low is taking shape near 40n 137w.
This feature has a warm front pushing into far SW Oregon as of 02z.
The developing surface low appears to be on the edge of the NAM model
domain...but various models deepen the low to around 992 mb by 12z,
by which time it is along 130w longitude off the central or south
Blended total precipitable water graphics valid 01z suggest 1.2 to
1.3 inches at the leading edge of the warm front and up to 1.7 inches
near the low center. The first few frames of the 00z NAM are in and
it has the leading edge of the warm front reaching Lane County near
06z and then the Columbia River around 12z Wed. NAM also keeps the
bulk of the precipitation along the coast and in the higher coastal
terrain. This differs from the 18z GFS, which had plenty of quantitative precipitation forecast
inland overnight and into Wed morning. The 850 mb wind flow remains
southeast to S overnight, but finally shifts to the SW Wed morning as the
majority of the warm-frontal precip moves north.
The offshore low-level gradient will strengthen overnight, which will
result in 35 to 45 mph gusts at the west end of the Columbia Gorge.
The 00z NAM has the surface low moving north along the 130w longitude
line Wed. This looks to be far enough away for any high wind threats
for the coast. In fact, at 21z the NAM shows 60 kt 850 wind barbs off
the north Oregon coast. This lasts for about 3 hours before the 850
mb to 975 mb wind speeds diminish. Not inclined to go any higher than
55 mph gusts for The Headlands Wed afternoon.
The warm front will move north of the Columbia River late Wed
morning, but a cold front follows close behind and will continue rain
over the area. Models want to hold the cold front close to the coast
and higher coastal terrain Wed. Interior Valley areas may not see
much rainfall Wed as models show re-development along the front over
SW Oregon. Finally, a kicker disturbance pushes the frontal boundary
inland Wed night, but it takes its time moving east. The cold front
will move southeast of the area by Thursday evening and showers will
decrease Thursday night with isolated showers lingering through
Friday morning. Another low and frontal system approaches the
area from the south on Friday. Snow levels will remain between 6500
and 7000 feet tonight through Friday. Weishaar
Long term...Friday night through Tuesday...wet pattern continues
through early next week. No particularly strong systems appear to be
on the horizon. Trough on Friday night into Saturday splits into two
circulations within the trough, the first of which brings a front
through our area from the southwest Friday night into Saturday. The
low pressure center associated with this front stays off the coast
and actually gets pulled back westward with the approach of the next
low pressure center to the south. There could be a drier period
between these two systems late Saturday or early Sunday, with some
uncertainty on the timing of this drier period. Then the front
associated with the next low moves up from the southwest bringing
rain back to the area during the day on Sunday. These two
circulations consolidate into one and weaken as they move onshore
late Sunday. Models diverge significantly on the next system, but
the general consensus is that we will continue to see systems every
24-36 hours through early next week. -McCoy
Aviation...predominately VFR Wednesday with some MVFR along the
coast as rain moves in after 06z. May also see some MVFR
conditionsinland with the rain but confidence is lower there.
Kpdx and approaches...mainly VFR conditions with some MVFR
possible as rain spreads up from the south late tonight. Bowen
Marine...another low, a little weaker than the previous system,
will move north tonight along 130w, off the Oregon coast late
tonight and Wed, to off the Washington coast Wed night. This
system will bring increasing winds to the southern waters late
tonight and over all the coastal waters Wed. At this time it looks
like gale force winds are likely to develop over the outer waters
starting Wed morning and spreading inland Wednesday afternoon.
Have upgraded gale watch to a warning and extended it across all
of the waters. Winds should diminish again late Wed night as the
low moves north.
Seas continue to decrease tonight with the departure of the first
low, but will begin to build again early Wed as the next system
moves up. Due to a weaker anticipated intensity of the low, seas
are likely to peak a little lower than the last system, peaking
late Wed afternoon and evening in the upper teens offshore and
lower teens nearshore.
Late in the week looks a little calmer on the coastal waters as a
significantly weaker system moves in Fri. Bowen
Pz...Gale Warning from 4 am to 9 PM PDT Wednesday for waters from
Cape Shoalwater Washington to Florence or from 10 to 60 nm.
Small Craft Advisory for hazardous seas until 1 am PDT
Wednesday for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Gale Warning from 11 am to 9 PM PDT Wednesday for coastal
waters from Cape Shoalwater Washington to Florence or out 10 nm.
Small Craft Advisory for rough Columbia River bar until 3 am
Small Craft Advisory for rough Columbia River bar from 1 PM to
5 PM PDT Wednesday.
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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.