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Area forecast discussion 
National Weather Service Portland
234 am PDT Tuesday Apr 21 2015

Synopsis...an upper trough will replace the upper ridge today and
there will be a change to cooler and moister weather. Temperatures
will cool to the seasonal normals by Wednesday. A series of weak
fronts will result in periods of light rain this afternoon through
Friday. A more organized front will move across the region this
weekend.
&&

Short term...today through Friday...the upper ridge axis is
currently over the Cascades allowing an upper trough over the NE
Pacific to approach the coast. Marine clouds have filled in over the
central Oregon coast and Coast Range early this morning...but the
north Oregon and Washington coast are mostly clear. Satellite imagery
shows low clouds ahead of an approaching cold front quickly
approaching the Washington and north Oregon coast. This stratus will
merge with the stratus already established along the central Oregon
coast and fill in the entire coast by sunrise. The marine layer will
deepen as the upper trough approaches this morning...allowing the
marine clouds to push further inland...into parts of the Willamette
Valley after sunrise.

The cold front will weaken as it approaches the coast...but may hold
together enough to generate some drizzle along the coast this
morning...and some showers inland this afternoon. Cloudier skies and
cooling aloft will result in much cooler temperatures this afternoon
compared to the last few days...with high temperatures along the
coast in the middle to upper 50s and inland temperatures in the low to
middle 60s.

A chance of showers will continue tonight into Wednesday as the upper
trough swings over the area. Most of these showers will be limited to
the higher terrain. Snow levels will lower to around 3000 feet by
Wednesday morning...but the precipitation totals will be so light
that there is not a concern for snow at the passes. The precipitation
will decrease from south to north Wednesday afternoon and evening.

Another weak cold front will approach Wednesday night and possibly
bring some light rain and mountain snow to the region Thursday and
Thursday night. A weak upper level trough will maintain showers
Friday. Daytime temperatures will remain near normal...in the low 60s
inland and in the 50s for the coast Thursday and Friday. Tj

Long term...Friday night through Monday...models continue to
struggle on the details...but suggest a wetter frontal storm system
will slowly sag southward across the area late Saturday through
Sunday night. As a result...probability of precipitation were increased in the likely to
categorical range for the prime window of Saturday night and Sunday.
Both models suggest another storm system is likely next week...but
once again differ on timing so probability of precipitation were trended more into the high
end chance to low end likely range. /Neuman
&&

Aviation...VFR inland while areas of IFR on the coast. A weak
front will push another batch of stratus to the coast. This is
expected to bring ceilings around 1500-2500 feet to the coast through
at least this evening. Portions of the interior will probably see
MVFR ceilings between about 15z and 20z Tuesday. However westerly onshore
flow may retain occasional mountain obscuration through this
evening.

Kpdx and approaches...MVFR stratus around 2000 feet affecting the
terminal 16z to 20z Tuesday. VFR returns by 20z Tuesday with northwest winds
picking up middle afternoon into early evening. /Mh
&&

Marine...a weak front will bring a slight wind shift across the
waters this morning. The main effect however will be increase in
northwesterly winds into the Small Craft Advisory category which should
last into Wednesday morning.A much stronger front looks to move
across the waters over the weekend and will likely bring gusty
southerly winds and another round of building seas.

Seas will also increase with northwesterly swell exceeding 10 feet later this
afternoon and peak around 10 to 12 feet tonight before subsiding
below 10 feet Wednesday evening. This will lead to rough bar during the
next few ebbs. Will have to watch the tonight period if seas rise
a little higher than anticipated rough bar conditions may occur
during non Ebb time. /Mh
&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for winds from 7 am this morning to noon
PDT Wednesday for waters from Cape Shoalwater Washington to Florence
or from 10 to 60 nm.

Small Craft Advisory for hazardous seas from 4 PM this
afternoon to 6 PM PDT Wednesday for coastal waters from Cape
Shoalwater Washington to Florence or out 60 nm.

Small Craft Advisory for winds from 1 PM this afternoon to noon
PDT Wednesday for coastal waters from Cape Shoalwater Washington to
Florence or out 10 nm.

Small Craft Advisory for rough Columbia River bar from
6 am to 10 am PDT this morning.

Small Craft Advisory for rough Columbia River bar from
7 PM to 10 PM PDT this evening.

Small Craft Advisory for rough Columbia River bar from
5 am to 10 am 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.

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