Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
230 PM PDT Tuesday Sep 2 2014
Synopsis...a Pacific front will bring increasing clouds and a
chance for light rain showers to mainly southwest Washington and
extreme northwest Oregon late this afternoon and tonight. Cloudy
skies on Wednesday will keep temperatures near to slightly below
normal on Wednesday. Strengthening offshore winds will allow
temperatures to rise into the upper 80s and low 90s for many inland
locations late in the work week. There is a chance another storm
system could bring a chance for light rain showers to the area early
Short term...tonight through Friday... an upper level trough and
attendant Pacific front moving into the Washington coast this afternoon will
push southeastward across the northern half of the area this evening.
This will bring a few light showers across southwest Washington...and
the west facing slopes of the north Oregon Cascades and Coast Range.
Rainfall will be quite light with this system with only a few
hundredths to perhaps a couple of tenths of precipitation expected.
Cloud cover associated with the upper level trough will keep high
temperatures down to a couple degrees below normal to near normal
Offshore flow will develop Thursday as thermal low pressure builds
northward along the coast. This pattern will persist through
Saturday and will result in temperatures warming into the middle 80s to
around 90 in the Willamette Valley Thursday through Saturday.
Coastal temperatures will also rise into the seventies during this
period. No record temperatures are expected though as record temperatures
are in the upper 80s and 90s at the coast and upper 90s and even low
100s for the valley.
On the fire weather front...poor night time relative humidity
recoveries and modest east winds across the Cascades and Coast Range
will enhance fire weather concerns the latter part of the week. Tw
Long term...Saturday through Tuesday...a Rex-like weather pattern
with a ridge positioned north of a trough is forecast to move
onshore as the ridge breaks down on Saturday. The longwave trough
will be our next weather-maker when the ridge weakens enough to
allow the low-pressure system onshore...which the European model (ecmwf)...GFS...and
dgex agree should be sometime late Sunday night or early Monday
morning. Rainfall with this system will start at this time...and the
unstable northwesterly flow pattern that sets up behind this system
bring continued chances of rain through Tuesday with a lot of
uncertainty on timing and location. -McCoy
Aviation...generally VFR across the region this afternoon with
scattered-broken VFR deck between 3000-4000 feet....though MVFR ceilings are
approaching the S Washington/north Oregon coast near kast as a
weakening cold front moves towards the area. The front will pass
through the northern half of the forecast area with low VFR to
MVFR ceilings along the coast by 00z...and pushing into the interior
around 03z. Chance for some -ra north of a generally ktmk-ksle line.
With decent onshore flow...expect MVFR ceilings across much of the
interior 12z-18z Wednesday with general clearing thereafter. Lower MVFR
or perhaps IFR possible at the coast tonight through early Wednesday...but
expect stratus to push largely offshore around 19z or 20z Wednesday. Winds
along the Oregon coast may be gusty Wednesday evening...particularly
from konp to k6s2.
Kpdx and approaches...VFR conditions continue this afternoon with
scattered layer around 3000 feet. Ceilings gradually develop and lower
from 5000 feet into lower VFR to MVFR this evening with the
approaching weak front. Slight chance of a shower this evening to
early tonight. Continued onshore flow will bring MVFR ceilings by 12z
lasting until around 17z-18z...with VFR under mostly clear skies
Marine...a weakening cold front will move across the waters this
evening. Behind the front tonight...winds will shift north-northwest and
increase to near 20 knots...especially over the northern outer
waters. Cannot rule out occasional gusts to 23-25kt over this
area...but do not expect these to be widespread enough to issue a
Small Craft Advisory. A northwest swell may build seas in these areas
to near 9 feet tonight as well.
From Wednesday through the second half of the week...N-NW winds continue
as high pressure rebuilds in offshore and a thermal low pressure over
California extends north into Oregon. Gusts up to 20-25 knots are
possible over the central Oregon waters Wednesday...and appear likely
over all waters Thursday and Friday. Meanwhile...seas will build to 7 to
9 feet...dominated by a combination of wind wave and fresh swell.
Forecast models keep seas below 10 feet...but will need to monitor
trends as strongest winds may build seas into the 9 to 11 feet
range. Winds look to ease by sun as high pressure in NE Pacific weakens
and thermal low pressure shifts inland. Cullen
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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.