Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
306 am PST Tuesday Feb 9 2016
Synopsis...high pressure over region maintaining dry and mild air mass.
Will continue to see areas of late night and morning fog for inland
areas away from the Columbia Gorge. High pressure weakens on Wednesday...with a
weak front pushing into region Thursday. Stronger front will arrive on
Friday...followed by cooler unsettled weather for the weekend.
Short term...today through Thursday...
main story is the strong high pressure ridge aloft that sits over the
western USA. This ridge is acting like a rock in a stream...forcing
the approaching fronts to weaken and shift around the high into SW
At surface...offshore flow continues to flow through the Columbia
Gorge. At 3 am...trans-Cascades pressure gradient over The Gorge was
holding steady around 10 mb...pointing offshore. This will keep
breezy east winds in the western gorge this am...with gusts 45 to 60
miles per hour for the most part...and 35 to 45 miles per hour over parts of eastern
Portland/Vancouver areas. But winds drop off quickly as move away
from that area. So much that fog and very low stratus have formed
over south Willamette Valley and over the Cowlitz valley. Web cams
in Eugene show very shallow layer of fog...generally 100 to 300 feet
deep. With visibilities in those areas running 1/8 to 1/4 mile...
will put up dense fog advisories for those two areas until 11 am.
Fog will form across parts of central Willamette Valley this am but
any dense fog would be quite isolated...and mainly between Salem
Otherwise...todays weather will be much like that of Monday...with
plenty of sunshine and mild conditions. Offshore flow will gradually
weaken today...and much more so tonight. So...coastal areas will be
tad cooler today...generally in the lower to middle 60s only.
Once sun GOES down this evening...air mass will cool rapidly again.
Will see another night of fog and low clouds...but they will spread
out farther across the inland valleys as winds will becoming light
and variable over most areas. Clouds will be slower to clear on Wednesday.
First of series of front will arrive later Wednesday/Wednesday night... but it
seems to be the sacrificial front as not much in way of moisture
reaching the region. Will keep 20 percent chance of rain for the
coastal areas...mainly north of Newport late Wednesday afternoon and evening.
Another front arrives later Thursday...but again most of its moisture will
be shunted to the north of the region. Still...will have plenty of clouds
and some chance of light rain on Thursday. Rockey.
Long term...Thursday night through Monday...the models are
indicating we will stay under southwest flow aloft early in the
period...then have a front come through Friday...with showery weather
into Saturday. The GFS the slowest and the European model (ecmwf) the fastest. The
models suggest we get back into west to southwest flow following that
system...with the GFS considerably drier than the European model (ecmwf) or Gem
models. Have trended toward climatology probability of precipitation for Sunday and Monday.
Aviation...VFR at the coast plus kttd and kpdx as offshore flow
will keep the lower atmosphere mixed. The offshore flow will
gradually weaken today but still remain rather gusty near the
mouth of The Gorge...including kttd. Keug quickly developed a
thin low deck and fog. Ksle and khio likely are not far off given
light winds and minimal temperature dew point spreads although
lamp guidance is an outlier among MOS information. Lamp does not bring
reductions in flight conditions to those two sites. Will lean on
persistence and probably keep some fog mention in place for ksle
and khio. All sites should see the fog dissipate by 21z with keug
holding on the latest. Continued persistence forecast will bring
fog back to keug around 08z tonight.
Kpdx and approaches...VFR conditions for the next 24 hours with
easterly winds. Gusty headwinds for the departures through 20z with
some speed shear under slightly veering winds through the lowest
2000 feet above ground level. /Jbonk
Marine...low pressure well offshore will move north today and
swing winds from southeasterly to more southerly this afternoon.
Feel there will be some localized 20 to 25 knots gusts but confidence
in a widespread Small Craft Advisory wind event is low. Have
better confidence for Wednesday afternoon when frontal remnants
will swing north across the waters with Small Craft Advisory level
wind gusts likely. But even then...notable discrepancy among the
models as to the strength with one keeping this as a non-event.
Will let things shake out a bit before issuing products yet.
Models in decent agreement in bringing possible gales Saturday
evening under a passing front but begin to stray again
Seas have remained a bit more stubborn and are hovering 10-11 feet
across the far northern waters. Have timed in the decay from buoy
5 and feel they will drop see the expected drop over the next
Seas have fallen to 8 to 10 feet
this evening and should generally continue to fall to around 7 feet
Tuesday. Will see seas build back up around 10 feet with the frontal
remnants Wednesday. While a longer period mixed swell arrives
Thursday solidly pushing seas into the lower teens. Active
pattern will keep seas above 10 feet through the weekend. /Jbonk
Or...dense fog advisory for southern Willamette Valley and the
lower I-5 corridor near until 11 am today.
Washington...dense fog advisory for the lower I-5 corridor and Cowlitz
valley until 11 am 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.