Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
945 am PDT Friday Oct 24 2014
Synopsis...a warm front will spread light rain into SW Oregon
this morning and across northwest Oregon and SW Washington in the
afternoon and evening. A cold front will push inland Saturday
afternoon and evening. An upper trough will continue showers
through Sunday. Another frontal system is expected early next week.
Short term...today through Monday...krtx Doppler radar showing some
light returns across the south part of the forecast area this
morning. 15z metars not indicating any precipitation...even at keug.
Suspect most of the echoes across Lane County are virga. Patchy fog
and low clouds in the interior valleys as well. Water vapor satellite
imagery clearly shows the developing low pressure area near 39n 133w at
15z. Warm front extending outward from the low center across southern
Oregon...which will continue to lift north during the day.
Offshore low-level flow will gradually strengthen during the day. 15z
kttd-kdls gradient was -2.6 mb...which is a bit stronger than the
3-hour 12z NAM forecast. Gradient likely to be around -3 to -3.5 mb by
00z Sat...enough to produce breezy conditions at the west end of the
Columbia Gorge. Areas north of ksle likely to remain dry through at least
Biggest short-term forecast challenge remains the developing low.
Models seem to be coming into better agreement with the depth and
track. By 12z Sat the 12z NAM...06z GFS and 00z European model (ecmwf) show an
elongated low...with the primary low center at 41n 129w. Some
differences start to crop up at the 18z time step...with the NAM
moving the low further north up the Oregon coast compared to the 06z GFS
and 00z European model (ecmwf). The 00z WRF-GFS backed off on the peak wind gusts for
the central Oregon coast. The 12z Thursday run showed a small core of
60-70 knots gusts early Sat afternoon between Newport and Florence. The
00z run knocked these down to around 50 knots. The 12z NAM does indicate
two separate low centers 18z Sat...with the tightest S gradient along
the north California coast. The NAM does show 50 knots 975 mb wind speeds
along the central coast 18z Sat...but by 21z the strongest 975 mb
wind speed is around 40 knots. Not convinced the coast will reach
high-wind criteria. Would not rule out isolated 55-60 miles per hour wind gusts
on The Headlands between 18z Sat and 00z sun. Have higher degree of
confidence for blustery conditions in the Willamette Valley. Expect a
brief period (2-4 hours) of windy conditions as the surface low moves
north-NE into SW Washington late Sat afternoon and early evening. 12z GFS shows a
+13 mb 6-hour pressure change bullseye near konp 06z sun...which is pretty
close to benchmark thresholds to get windy conditions in the valley.
Would like to see this isallobaric change in a more south-to-north
orientation. In any event...current forecast of 35-45 miles per hour gusts in
the Willamette Valley late Sat afternoon through early evening looks
good. Strongest gusts should be S of ksle.
A broad upper trough...similar to what was over the area
Thursday...settles over the region Sunday. This will bring additional
short-waves through the forecast area Sat night and Sunday. Snow
levels fall to near Cascade Pass levels Sunday. Weak ridging moves
over the area Sunday night and into Monday morning with rain chances
dwindling briefly before the next system approaches. Weishaar
Long term...no changes. Previous discussion follows...Monday night
through Thursday night...an active weather pattern continues through
next week. The next frontal system...including the remnants of
Hurricane Ana...will move through the region Monday night into
Tuesday bringing continued rain and winds along the coast. Shortwaves
will support rain through Wednesday. Beginning around Thursday the
models diverge significantly to the extent that climatology seems the best
bet at this point for the remaining 24 hours of the forecast period.
Aviation...VFR conditions with ligth rain...mainly
south...through around 19z. A few locations with MVFR or IFR ceilings
possible this morning. The warm front will lift north after 19z
and spread rain into northwest or. Expect conditions to remain mainly VFR
with occasional periods of MVFR ceilings. Rain decreasing after 08z as
the warm front moves into Washington.
Kpdx and approaches...mainly VFR conditions through tomorrow
morning. A few light showers are possible this morning before
more general rain develops...mainly after 19z as the warm front
lifts into northwest or. /64
Marine...mild conditions expected today with light east winds this
afternoon. A deepening low pressure system is expected to approach
from the SW and move through the coastal waters on Sat and inland
late Sat. The track and strength of the low is still uncertain at
the moment as models have the low becoming more elongated...but
still expecting southerly gales with this system. The gales will
bring steep seas with wave heights approaching 20 feet possible
through Sat night.
As the low moves inland winds will become west and decrease with seas
lowering to around 8 to 10 feet as high pressure builds into the Pacific northwest.
Another shortwave expected to approach the region early next week.
Pz...gale watch from late tonight through Saturday evening for
coastal waters from Cape Shoalwater Washington to Cascade Head or out
Gale watch from this evening through Saturday afternoon for
coastal waters from Cascade Head or to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar from 3 PM
this afternoon to 8 PM PDT this evening.
Small Craft Advisory for rough Columbia River bar from 3 am to
8 am PDT Saturday.
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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.