Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
859 am PDT sun may 3 2015
Synopsis...a light west to northwest flow aloft will keep some low
clouds along the coast but mild and a bit above normal temperatures over the
inland areas today. The approach of a low pressure system later
tonight and Monday will turn the flow more southwesterly and deepen
the marine layer near the coast. The associate weak cold front will
approach the coast late Monday and move through southwest Washington
and northwest Oregon Monday night for a little light rain. The
associated upper low will spread increasing showers and possible
thunderstorms over the area Tuesday. The upper low will linger over
the inland portion of the western U.S. Wednesday and Thursday with
showers and possible thunderstorms becoming more diurnally driven...
and becoming more focused near the Cascades later in the week. Snow
levels will be lower to around pass elevations Tuesday. Another low
pressure system is possible early next week.
Short term...as expected...the marine inversion lowered this morning
as a weak cyclonic westerly to slightly northwesterly flow of air
aloft will continued. This maintained a shallowing marine layer near
the coast...but inland spread barely made it into the coastal gaps
and valleys. Look for plenty of sunshine today with temperatures pushing the
middle 70s inland. Look for continued breezy near the coast and some
decent breezes in the valley in the evening.
The next low pressure system continues to be forecast to approach the
Pacific northwest late tonight and Monday. This initially turns the
flow aloft more southwesterly later tonight and Monday... deepening
and solidifying the marine layer along the coast by Monday...and
probably spreading some more low clouds inland Monday morning both in
the north and through the southern coastal gaps. Recent model runs
are a bit faster with the approach of the low...and the initial cold
front may begin to reach the north coast with a little light rain
late Monday before moving onshore through the forecast area Monday
The main upper low center will be to our north...but enough energy
will swing around the southern periphery of the low to sag the cold
pool into our area by Tuesday. Look for increasing showers and
possibly some thunderstorms to spread into the coast late Monday
night and inland Tuesday. Small hail...gusty winds...and heavy
showers are possible Tuesday. Snow levels will lower to around pass
elevations on Tuesday...but amounts will tend to be on the lighter
Long term...no changes. Previous discussion follows...
Tuesday night through Saturday...the low center will be setting up
over central and eastern Washington Tuesday night and Wednesday...
but there is plenty of energy rotating around the back side of the
low to keep some showers and a few thunderstorms in the forecast. A
lot of energy will be digging into California and Nevada by
Thursday...but there is still a chance of showers rotating northward
back into our area. Look for the showers to become more
diurnal...driven by daytime heating...as we move through the extended
period. Some afternoon and evening instability may linger near the
Cascades through the remainder of the week...though the model details
do diverge a bit in the longer term. The models indicate another low
may set up off the West Coast by the early part of next week.
Aviation...first visible satellite images through 15z showed areas of
IFR along the coast...and pockets of IFR in some of the coastal
valleys. Otherwise...VFR clear everywhere else. North surface flow
should push the coastal IFR offshore by 18z...but expect it to return
between 03z and 06z Monday. VFR inland...but models indicate possible
MVFR ceilings pushing into the central Oregon Coast Range valley
overnight and might reach keug around sunrise Monday.
Kpdx and approaches...VFR clear through Monday morning. However...some
high-end IFR ceilings or low-end MVFR ceilings may push up the Columbia River
around dawn Monday...possibly reaching the terminal. Weishaar
Marine...main forecast change for the morning package was to extend
the Small Craft Advisory for wind into the northern waters valid 19z
through 03z Monday. Do not expect Small Craft Advisory wind speeds over
the entire northern waters...primarily from Cape Falcon to Cascade Head.
Although latest models maintain 15 to 20 knots over the northern waters this
afternoon...think persistence may be the way to go. Wind will
diminish over all waters later this evening and overnight. Next
threat of Small Craft Advisory level wind looks to be Monday night with
the approach of the next frontal system.
Seas should stay around 6 to 8 feet today...but could push close to 10
feet in the southern waters this afternoon with the stronger wind. Seas then
ease to 5 to 6 feet on Monday...but are expected to reach 10 feet Monday
Pz...Small Craft Advisory for winds from noon today to 8 PM PDT this
evening for coastal waters from Cape Shoalwater Washington to Cascade
Head or out 60 nm.
Small Craft Advisory for winds until 11 PM PDT this evening for
coastal waters from Cascade Head or to Florence or out 60 nm.
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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.