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Area forecast discussion 
National Weather Service Portland
841 PM PDT sun may 24 2015

Synopsis...an upper level low in southern b.C. This afternoon will
slowly move into eastern Washington Monday and into Idaho later
Tuesday and Wednesday. This will maintain the low level onshore flow
across southwest Washington and northwest Oregon for the next several
days with little or no clearing in the afternoons. It will also
provide the possibility of some night and morning drizzle over the
area...as well as a few late day showers or even a thunderstorm in
the higher Cascades. An upper level ridge will move into the Pacific
northwest during the second half of the week...weakening the onshore
flow and allowing more sunshine to break through with temperatures warming
above normal. Southwest flow aloft next weekend may bring a return to
the marine layer and possibly a few thunderstorms over the Cascades
again.
&&

Update...did not make any changes with the evening update as
forecast is on track with an upper low currently over b.C. Canada
slowly moving southeast tonight through Tuesday. A deepening marine
layer will continue marine clouds for the nights and mornings with
some patchy drizzle. Some showers and possibly mountain thunderstorms
may brush the southwest Washington Cascades in the afternoons. Tj

Previous short term discussion...we have seen a near repeat of the
last few days as cloudy conditions cover most of southwest Washington
and northwest Oregon this afternoon. There are some thinning spots
near the north Oregon Coast Range but suspect that any clearing this
afternoon and evening will be limited. Onshore flow continues to be
moderately strong as another low is in southern b.C. This low has
also turned the winds aloft to more west and northwesterly as well
which is maintaining the onshore flow. Some convection is trying to
develop along the east slopes of the Cascades this afternoon but is
drifting off to the east. Any threat this afternoon and evening will
be confined to near the crest.

Temperatures are running close to those Saturday...but the thin spots may
allow a few areas to reach the middle 60s this afternoon.

The southern b.C. Low will drop into central and eastern Washington
on Monday...reinforcing the strong onshore flow and the possibility
night and morning drizzle around the area. Believe the clouds will
have a tough time breaking up Monday...with temperatures staying in the low
to middle 60s. With the low centered in central and eastern
Washington...we may see our northern Cascades zones mainly near the
crest clipped by a few showers or thunderstorms in the flow on the
backside of the low.

The low is moving into Idaho by later Tuesday. The onshore flow
should continue...though we might see a few more sun breaks late in
the day and the evening vs Monday...and this may allow temperatures to get
closer to 70. The bit more sunshine in the higher Cascades combined
with the return flow on the back side of the low will maintain a
chance of late day showers or thunderstorms near the crest of our
northern Cascade zones.

On Wednesday the low is farther east...and 500 mb heights climb as
well. We should see more clearing inland in the afternoon...and
inland temperatures should have a chance to reach at least the middle 70s and
possibly higher. We still have a chance of some late day showers or
thunderstorms in the higher Cascades crest...aided by the increased
sunshine. Tolleson

Long term...Wednesday night through Sunday...a weak ridge builds
over the pacnw midweek...drying out the region. Continued weak
onshore flow will continue to bring marine clouds inland
overnight...but stratus should for the most part clear out by midday
allowing temperatures to reach into the upper 70s and maybe low 80s
inland. A thermal low develops in northern California on Thursday and
starts to push northward on Friday and Saturday...which could help
develop some afternoon showers and maybe a few thunderstorms along
the crest of the Cascades on Friday through Sunday. Background flow
will be out of the west-northwest though...which should keep these
storms from drifting westward into the valley. -McCoy
&&

Aviation...after a brief break and improvement in flight
conditions this evening...except increasing ceilings to lower once
again after 04z Monday. MVFR ceilings along the coast will drop to low
end MVFR after 08z Monday...with a brief period of IFR ceilings around
700-1000 feet between 11z-16z Monday. VFR ceilings inland will lower to
MVFR with ceilings around 1400-2100 feet through Monday am. Monday will
likely be a repeat of today with MVFR ceilings early and sun breaks
in the afternoon.

Kpdx and approaches...VFR ceilings around 4000 feet will gradually lower
to MVFR with ceilings around 1500-2000 feet after 08z-10z Monday. VFR ceilings
possible after 25/18z. /27

&&

Marine...high pressure continues to remain offshore with low pressure
over the inland Pacific northwest for the next several days. Expect mild
conditions over the waters with northwest winds generally below advisory
criteria. There may be brief periods of isolated advisory gusts
above 21 knots south of Newport once again Monday afternoon/evening.
However...do not think they will be widespread enough to warrant
issuing an advisory. Mixed seas will continue through most of this
week...with choppy conditions expected at times due to a fresh northwest
swell. /64

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...none.
&&
$$

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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.

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