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Area forecast discussion 
National Weather Service Portland or
953 am PST Monday Jan 26 2015

Synopsis...a strong upper ridge will flatten and move away to the
east today and tonight...gradually weakening the low level inversion.
A very weak frontal system will approach the coast late
tonight...begin to spread inland in the north Tuesday...then linger
over northwest Oregon and southwest Washington through Wednesday.
Another upper ridge is expected to develop over the Pacific northwest
later in the week and continue into the weekend. The next chance for
rain is probably not until later next weekend or early next week.
&&

Short term...the fog is holding tough in most inland areas except
near Troutdale...so will continue the dense fog advisory this
morning. The fog and low stratus will again be slow to clear
anywhere...and probably persist in most inland areas except near the
Columbia River again today as light surface pressure gradients are
again the rule. The upper ridge that has been over the Pacific
northwest the last couple of days is moving east and weakening or
flattening which will weaken the temperature inversions aloft but not
enough to mix the fog vertically. Expect more to thicken and
redevelop tonight.

There is a frontal zone out near 130w that the models continue to
show will move into the coastal waters and close to the coast
tonight. This system will weaken a lot as it approaches. It will turn
the low level flow onshore near the coast...as evidenced by the bank
of low clouds just off the coast. Expect some light rain or drizzle
to reach especially the north coast tonight and early Tuesday. The
frontal band will slowly ease inland along the Columbia River
Tuesday...then hang up over the northern part of our forecast
area...say from about Salem northward...Tuesday night and Wednesday
before slowly drying out. Amounts will be light...but expect some
drizzly periods with some light rain Tuesday night into Wednesday.
Some drizzle should also reach the central Oregon coast...but expect
it to probably stay dry in the south Willamette Valley. Tolleson

Long term...no changes. Previous discussion follows...
Wednesday night through Sunday...a passing shortwave late Wednesday night
may provide enough lift for a few showers...mostly along the north
coast and Coast Range and into SW Washington. Models remain in good
agreement that a ridge of high pressure aloft rebuilds on Thursday and
persists through the end of the week. This will maintain dry
conditions but bring daily fog and low clouds to Interior Valley
locations. The ridge will likely break down and shift inland some
time next weekend...but still some uncertainty as to if this will be
Sat or sun. The 12z GFS is a bit more aggressive in bringing a moist
frontal system to the region by late Sat night...while the European model (ecmwf)
would keep things dry until Sun night. Increased probability of precipitation a bit for next
weekend...particularly for the northwest portion of the forecast
area...though restricted to lower end chance for Sat night through
sun given the support for the later timing from the European model (ecmwf) and
numerous gefs members. Cullen
&&

Aviation...high pressure and a strong inversion continue to maintain
LIFR/IFR fog across the interior taf sites this morning. Expect
little change at keug...ksle...and perhaps khio...through next 24
hours. If a few hours of clearing do develop after 21z today...expect
it to be short lived with a return to fog shortly after 01z Tuesday. East
winds pouring out of the Columbia River gorge are helping to keep
kttd mostly clear...and may aid in mixing at kpdx. The east winds
should subside later today...and IFR or lower conditions return to
kpdx tonight...but perhaps later and higher at kttd. Conditions
remain VFR and generally clear along the coast. A southerly wind
reversal will work its way up the coast bringing IFR onshore later
today or tonight...though confidence in timing this remains low given
large model uncertainty.

Kpdx and approaches...LIFR fog continues at the terminal this
morning. Light east winds through the Columbia River gorge continue
may aid in mixing a bit later this morning or early afternoon. Still
considerable uncertainty as to timing of improvement of fog/low
stratus...but suspect this may occur around 20-21z. A return to LIFR
fog appears likely again after 01z Tuesday. Cullen
&&

Marine...a southerly surge has begun across the southern and central
Oregon coast and will continue to push north over the waters and
coastline. Expect a brief period of gusts around 20 knots with a few
occasional gusts close to 25 knots possible through early this
afternoon. However...do not expect these to be frequent or widespread
enough for an advisory. Seas will hold around 8 or 9 feet today...but
may touch 10 feet at times especially where the wind wave component
increases with any stronger gusts later this morning.

A weak front will slide southeast across the waters for Tuesday bringing a wind
shift but no significant increase in speed. High pressure then
rebuilds by Wednesday or Thursday and will persist into at least the start of
the weekend. Seas will gradually subside slowly through the second
half of the week...remaining well below 10 feet. A more active pattern
may develop late in the weekend...with increasing seas and
winds...though some uncertainty remains in the timing and strength of
these systems. Cullen
&&

Pqr watches/warnings/advisories...
or...dense fog advisory until 11 am PST this morning for central
Willamette Valley-greater Portland metropolitan area-lower
Columbia-south Willamette Valley.

Washington...dense fog advisory until 11 am PST this morning for Greater
Vancouver area-I-5 corridor in Cowlitz County.

Pz...Small Craft Advisory for rough Columbia River bar until noon
PST today.

&&

$$

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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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