Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
308 am PDT Sat Aug 29 2015
Synopsis...an usually early and strong low pressure will produce
widespread rain and strong winds across northwest Oregon and
southwest Washington Saturday morning. A few thunderstorms embedded
in the rain may enhance the wind threat Saturday morning. A second
front pushing into the region from the northwest Saturday night will
produce another round of rain. Additional showers will follow on
Sunday. Expect cool weather and additional rounds of showers for much
of the upcoming work week.
Short term...a few prefrontal showers remain over the North Valley
this morning with the frontal rain band moving onshore. Radar already
showing some higher reflectivity activity over the Coast Range and
even a few lightning strikes. Although the rainband is narrow...its
currently moving much more north than east so it will be slow to
work its way inland and may drop 0.10 to 0.20 inches of rain per
hour with more in embedded stronger showers or thunderstorms. There
does not appear to be any rain immediately following this frontal
band...so there will be a definite break in rain from the time the
band moves through to when any showers begin to pop up this
afternoon with daytime heating. Have dried the forecast out quite a
bit 18z today to 00z Sunday but the rain may have moved through
earlier than even that.
The parent low continues to strengthen and is currently centered
about 200 miles west of North Bend according to latest infrared satellite
imagery. Based on comparison of buoy observation and 06z NAM and GFS
forecasts...looks like the low is around 989 mb right now...or
slightly stronger than the NAM and GFS have. The 08z hrrr shows a
stronger low currently and forecasts the pressure dropping to 985 mb
at its closest approach to the Oregon coast...about 100 miles
offshore. This would be 4 mb stronger than the NAM and GFS currently
show but with a similar track and seems fairly realistic. Hrrr also
shows rather impressive pressure rises behind the low late this
morning with 6+ mb in 3 hours over the extreme northwest Oregon and SW
Washington areas...mostly centered around Clatsop and Pacific
counties. Also seeing high pressure rises just offshore through the
central Oregon coast which should translate to high
winds...especially for the beaches and headlands. Will keep high wind
warnings in place as is. Pressure rises and pressure gradient through
the valley are less impressive with the north-south gradient peaking
only at about 4 to 5 mb but conditions will be noticeably windy
nonetheless. 850 mb winds in the valley are forecast to be 40 to 50
knots when this morning's rain comes through...so any convection could
mix down some good gusts. Will leave Wind Advisory as is since this
is an anomalously strong wind event for late August...even though
technical criteria may not be met.
Models have trended wetter with a second front dropping southeastward
out of the Gulf of Alaska that will push a healthy dose of rain
across the area Saturday night. Decent onshore flow and instability
should lead to more showers and possibly a wet thunderstorm or two
Below average temperatures and additional rounds of wet weather
appear likely next week. Bowen
Long term (previous discussion)...Monday night through Friday. The
Fall-like weather pattern will continue next week with primarily
cooler than normal temperatures...cloudy conditions...and chances for
additional rain. A trough will remain near the Pacific northwest
coast through most of the period with some rain at times...especially
during the early and middle parts of next week. Little change made to
the current forecast. Tolleson
Aviation...moist SW flow aloft over region...with rain increasing
this am. Will see front push north across the region later this am...
with increasing S winds. Strongest winds will be on coast and
over Coast Range...with S wind gusts of 40 to 55 knots for those
areas and lesser winds further inland. At 2 am...generally VFR
conditions. But will see increasing MVFR along the coast by 12z.
Rain will end by 18z for most areas...with ceilings breaking up a bit
as more unstable air mass with showers moves into region for the
afternoon. Would not be surprised to see transition back to VFR at
times after 20z. Ceilings will lower back to MVFR tonight as another
front and more rain pushes into the region.
Kpdx and approaches...VFR...with rain until 18z...then likely to
see a mixture of VFR with occasional MVFR as more showery pattern
pushes into region. Will have S winds gusting 25 to 35 knots between
17z and 23z. Winds ease after 00z...but will have increasing MVFR
as rain increases later Sat night. Rockey.
Marine...models continue to be well initialized with the strong
low pressure now sitting about 250 miles southwest of Newport this am.
This low will lift NE across the Oregon coastal waters and into
northwest Washington today. But appears the actual low is stronger than
the models indicate. The low will gradually weaken today as it
moves NE...strongest winds look to remain S of the Columbia River.
So...have issued Storm Warning for coastal waters S of the
Columbia River where there is moderate to high potential for 50 knots
gusts. Rest of the region will remain under a Gale Warning.
Seas will be building as winds increase. Still looks good for seas
to be running at 13 to 17 feet by middle to late this am. Seas hold in
that range through the day...but drop back this evening.
Winds will ease later this afternoon...likely below 25 knots by 8 PM.
Seas will also subside as winds ease...probably around 8 to 10 feet
later tonight into Sunday.
Unsettled weather continues into next week with seas staying 6 to
9 feet through at least midweek. Rockey.
or...Wind Advisory from 8 am this morning to 3 PM PDT this afternoon
for greater Portland metropolitan area-lower Columbia.
High Wind Warning until 1 PM PDT this afternoon for central
Wind Advisory until noon PDT today for central Willamette
Valley-south Willamette Valley.
High Wind Warning from 6 am this morning to 2 PM PDT this
afternoon for Coast Range of northwest Oregon-north Oregon
High Wind Warning from 5 am early this morning to noon PDT today
For central Coast Range of western Oregon.
Washington...Wind Advisory from 8 am this morning to 3 PM PDT this afternoon
for Greater Vancouver area-I-5 corridor in Cowlitz County.
High Wind Warning from 8 am this morning to 3 PM PDT this
afternoon for Willapa Hills.
High Wind Warning from 8 am this morning to 2 PM PDT this
afternoon for South Washington coast.
Pz...Storm Warning from 6 am to noon today on all coastal waters
south of the Columbia River.
Gale Warning until 8 PM today on all coastal waters through
8 PM today.
Small Craft Advisory for rough Columbia River bar conditions
today and this evening.
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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.