Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 222215
Area Forecast Discussion
National Weather Service Portland Oregon
315 PM PDT Sat Oct 22 2016
.SYNOPSIS..A developing low pressure off of the Oregon coast this
afternoon and evening will push a weak cold front onshore tonight.
Another surface low over the Gulf of Alaska will move south and
develop into a stronger and more complex low offshore of Oregon
Sunday. Disturbances moving around the low will bring a series of
fronts to the region through Tuesday. The low will weaken Tuesday
night and be replaced by another low on Wednesday.
.SHORT TERM...Tonight through Tuesday...A surface low is deepening
around 300 miles offshore of the Oregon/Washington border this
afternoon. As of 2 pm, the mean sea level pressure was estimated to
be around 1001 mb. The surface front associated with this low is
around 140 miles offshore the Oregon coast. Satellite imagery and
lightning detectors has been showing isolated thunderstorms along and
just behind this front, and have added a slight chance of
thunderstorms for the coastal waters 10 to 60 miles offshore.
The front will slowly move towards the coast as the low moves north,
with rain reaching the coast this evening and moving inland late
tonight. Expect around 0.25 inch of rain for the coast and coast
range this evening through sunrise Sunday with less than 0.20 inch
for the Willamette Valley and the Cascade foothills. The front will
weaken over the Cascades early Sunday afternoon with less than 0.10
inch and snow levels around 6500 ft.
The low will continue to deepen as it heads north tonight and bottom
out at around 990 mb just offshore the B.C. coast early Sunday
morning. The low will not move onshore then, but will instead move
back offshore and circulate around the next approaching low. This
second low is already forming near the gulf of ALaska and will evolve
into a complex low off of the Oregon coast Sunday night with multiple
disturbances moving around the low.
The models are still trying to come together on a forecast for this
system, but it is looking more likely that a broad low will remain
around 400 miles off of the Oregon coast Sunday through Tuesday.
Disturbances rotating around this larger low will generate surface
fronts, but there is some question of whether these fronts will
impact the Portland forecast area very much with the low remaining so
If the current model forecasts pan out then the coast, and coast
range may get some light rain and breezy winds Sunday night and
Monday but inland areas will be mostly dry and possibly even have
some sunshine. The second front on Monday is more likely to bring
rain inland, but the bulk of the moisture is headed to the south and
rainfall totals across nw Oregon and sw Washington should be fairly
light. The third front is expected Tuesday and may again just impact
the coast or even stay offshore. Snow levels will remain around 6000
feet and do not expect significant snow for the Cascades. Winds along
the coast will be breezy at times, but should stay well below warning
Think the largest impact of this low will occur over the waters and
along the beaches. Winds circulating around the quasi-stationary low
will likely create a large fetch leading to leading to large seas,
and possibly high surf along the beaches early next week. TJ
.LONG TERM...Tuesday night through Saturday...A surface low offshore
will weaken Tuesday evening and another low will approach from the
south. The models vary on the strength of this low, but it has the
potential to support strong gusty winds along the immediate coast as
it moves south to north offshore of the Oregon and Washington coast
on Wed. Modeled precipitable water values show that there could be
quite a bit of moisture available with this front and the Portland
forecast area may get a decent shot of rain with the warm front
Tuesday night into Wed morning, and again with the cold front
Wednesday night. There may be a break in wet weather Thursday before
another complex low sets up offshore Friday and Saturday. TJ
.AVIATION...VFR conditions across the area this afternoon. Have
some broken high level stratus up around 25,000 feet. Will see
clouds increasing and lowering to around 5,000 feet tonight,
starting along the Coast and pushing inland with the front moving
through. Will see some light rain with this system, but conditions
should remain predominantly VFR. Only wrench may be some patchy
fog around the area between 06Z tonight and 16Z Sunday.
KPDX AND APPROACHES...VFR conditions to continue this afternoon,
with increasing high clouds. Cigs should lower to around 5000 feet
by 09Z to 12Z Sunday with light rain. -McCoy
.MARINE...Active weather pattern ahead with a couple systems
bringing Gales to our waters through early next week. Seas are
hovering around 9 to 10 ft this afternoon with winds increasing ahead
of the weak front this afternoon. This front will bring small
craft advisory southerly winds, decreasing behind the front Sunday
morning. This front will also bring seas up to around 10 to 12 ft
with the winds ahead of the front.
On Sunday, we`ll see a bit of a break during the day as seas drop
to around 8 ft until Sunday evening, when an approaching Low
dropping out of the Gulf of Alaska dives south and starts to build
seas from the southwest and ramp up winds out of the south. This
Sunday night into Monday system is the first with a good chance
for gale-force winds across our waters ahead of a moderately
strong pacific front associated with this Low. Seas with this
system will build to around 12 to 15 ft Monday morning. The Low
associated with this front stalls and sits right off the Oregon
and Washington Coast, generating large swell that moves right into
our waters, continuing to build seas behind the Monday front.
Models show seas by Tuesday morning getting up to around 18 to 22
feet. Winds behind the front look to continue to be strong with
Gale-force gusts to 35 to 40 kt through Monday afternoon and
evening. Tuesday, this Low finally moves north up into Vancouver
Island, allowing winds to subside to small craft advisory criteria
and seas to fall to around 11 to 13 ft Tuesday night.
Another system moves up from the south along the Coast with the
potential for another round of Gales and large seas on Wednesday,
though there is some uncertainty in the track which will impact
how strong of winds we will see over the waters. Active pattern
looks to continue into next weekend. -McCoy
PZ...Gale Watch from Sunday evening through Monday morning for
Coastal Waters from Cape Shoalwater WA to Florence OR out
Small Craft Advisory for winds until 5 AM PDT Sunday for Waters
from Cape Shoalwater WA to Florence OR from 10 to 60 nm.
Small Craft Advisory for hazardous seas until 5 AM PDT Sunday
for Coastal Waters from Cape Shoalwater WA to Florence OR
out 60 nm.
Small Craft Advisory for winds from 8 PM this evening to 5 AM
PDT Sunday for Coastal Waters from Cape Shoalwater WA to
Florence OR out 10 nm.
Small Craft Advisory for Rough Columbia River Bar from 8 PM
this evening to 5 AM PDT Sunday.
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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.