NWS Portland, OR wrote its last area forecast discussion before the first event at Jan 20, 9:02 PM PST, 4 hours 48 minutes earlier.
Area forecast discussion, Jan 20, 9:02 PM PST, 4 hours 48 minutes before
Area Forecast Discussion
National Weather Service Portland Oregon
902 PM PST Fri Jan 20 2017
.SYNOPSIS...Broad low pressure continues to churn off the Pac NW
coast this evening, pushing bands of showers onshore into Oregon and
California. Another short wave or band of showers will swing through
the forecast area on Saturday for more rain and mountain and Gorge
snow. Another low will deepen to near 960 mb off the coast near buoy
5 Saturday night and Sunday for another round of rain and mountain
and Gorge snow changing to showers. The low will spread more showers
into the area into Monday before dropping south into California.
Expect drier weather for a good part of next week.
&&
.SHORT TERM...Tonight through Monday...Few changes were made to the
forecast this evening. KRTX radar continues to pick up some showers
moving northward through the Willamette Valley; these have been
bringing off and on rain to Portland and the I-5 corridor. Suspect
this activity will continue tonight, so we increased the PoPs a bit
to account for the more widespread showers. The next shortwave just
offshore from the OR/CA border will lift N-NE through the forecast
area late tonight/Sat...keeping showers in the forecast through
tomorrow.
Looking further offshore, cyclogenesis is beginning in the vicinity
of 35-40N and 145-150W. This is being fed by a strong 150 kt-plus jet
stream, and models are likely correct in developing this low rapidly
into a near-960 mb low over the next 24 hours. Latest runs of the
ECMWF, GFS, and NAM all suggest the bent-back occlusion associated
with this system will push onshore into western OR and possibly SW WA
early Sunday morning. Will need to keep an eye on this for S-SW wind
potential, but models seem to be unimpressed with the potential for
associated bent-back occlusion winds to reach the surface. Until
then, expect east winds to ramp up through the Gorge as this system
develops offshore. This will likely help keep much of the Gorge in
cold enough air to support snow as precip associated with the low's
occluding front moves through late Sat night/Sun. The easterly
component to the gradient may help keep some of the stronger winds
associated with the low a few miles offshore.
An important note about this developing low...while we expect its
central pressure to approach 960 mb, pressure is low in general
across the western U.S. at the moment. Therefore, the pressure
gradients are not quite as strong as they otherwise would be with a
sub-970 mb low inside 130W. This should help take the edge off winds
a bit as the low brushes by the Oregon Coast Sat night/Sun. That
said, this will still be a strong, rapidly developing low, so gusty
winds are likely for the coastal waters...and possibly along the
coast and locally inland. Weagle
Previous forecast discussion from 219 PM PST Fri Jan 20 2017 below...
A 975 mb low is centered just west of buoy 5, a little more than 300
miles west of Astoria. A frontal/deformation band was lifting north
through southwest Washington this afternoon. There are a lot of
showers offshore south of the low headed toward the west coast of the
U.S., but much of that area of showers will head south into
California. Some will spread onshore to the north into northwest
Oregon and southwest Washington overnight tonight, but coverage and
QPF will be less in our forecast area than to our south. Snow is
having trouble accumulating in the Gorge and will drop the Winter
Weather Advisory for tonight but not for later time periods.
A second decent short wave which is actually the wraparound band
currently on the west side of the offshore low will rotate around the
low and swing through our forecast area on Saturday for another surge
in showers or possibly a period of steadier precipitation. Look for
more rain and Gorge and Cascade snow with this band.
Our offshore low then weakens and moves north late Saturday as the
next system, currently out near 40N/155W, strongly develops out along
130W and deepens to 960 mb near the position of buoy 5 Saturday night
and Sunday. This low brings another period of rain and Gorge and
Cascade snow later Saturday night into early Sunday before turning to
decreasing showers.
Look for periods of brisk east winds through the Gorge that will keep
the Gorge cold enough for snow from these bands. The surface pressure
gradient even along the coast will stay easterly, which should
prevent high wind from developing near the coast from these 2 strong
lows off the coast.
When all is said and done, seems reasonable to see total snowfall for
today through Sunday of 2 to 6 inches in the Columbia Gorge (heaviest
east of Cascade Locks), and 4 to 8 inches in the Hood Valley, as well
as higher elevations of the Coast Range. For the Cascades, generally
12 to 15 inches of new snow on the passes, with up to 2 ft of new
snow on high peaks.
The low off the coast Sunday will drop south into California Sunday
night and Monday. This will bring decreasing east wind and drying to
the forecast area from the north. A few showers will linger on Monday
but decrease considerably, with the best chance in our southern
forecast zones. Tolleson
.LONG TERM...Monday night through Friday...As the low pressure
system drops south of our area Monday night, a large upper-level
ridge builds over the area. As the ridge moves in the GFS brings one
last shortwave across the area. The GFS is the only model with this
feature, so right now have left precipitation out of the forecast,
though I did increase cloud cover on Tuesday. Behind this, the ridge
builds over the area drying us out through Wednesday. GFS and ECMWF
differ on how long the dry spell will last. The ECMWF is quicker
with the next shortwave trough which it brings into our area on
Wednesday night. The GFS on the other hand doesn't bring our next
system in until Thursday night-Friday. Either way, expect easterly
winds to develop ahead of this next system. Right now snow levels
look to stay up above 3000 feet. -McCoy
&&
.AVIATION...Expect VFR conditions to persist through Sat morning
with occasional showers. A surface trough will bring increasing
steady rain and possibility of MVFR conditions between about 18Z
Sat and 00Z Sun.
PDX AND APPROACHES...Mainly VFR conditions but could see brief
MVFR in rejuvenated showers this evening. Increasing rain and
possible MVFR conditions move in between about 18Z Sat and 00Z
Sun. Easterly winds gusting 25 to 30 kt near the western end of
the gorge may impact easterly approaches. Bowen/Pyle
&&
.MARINE...A complicated weather pattern will impact the coastal
waters over the next several days. Expect south to southeast wind
gusts to 25 kt to continue tonight. A surface trough will move
through Saturday, turning winds to the southwest and increasing
speeds. Still think we will generally remain below gales, a few
gusts to 35 kt are possible.
Another low will approach from the southwest and is modeled to
strengthen rapidly as it nears the coast late Sat and Sun. The low
occludes off the Oregon coast on Sun and remains nearly stationary
into early next week. The models indicate that it will reach a
strength of 965 mb or even a bit lower by Sun morning. The timing
of system is now in pretty good agreement in the latest model
runs. However, there is still spread in the positioning of the
low. The GFS continues to model the low further offshore than the
NAM and ECMWF solutions, which would result in significantly less
wind for our coastal waters, likely only moderate gales. The
closer ECMWF and NAM solutions could support storm force winds
over at least a portion of our waters. For now, went with a blend
of these two scenarios and will continue the Gale Watch for all
of the waters from late Sat evening through Sun evening. The low
is modeled to finally move south down the CA coast late Mon into
Tue.
Seas at buoy 50 and buoy 29 (now about 15 miles west of Ocean
City, WA) are both showing seas of 11 ft this evening, which is
lower than anticipated. Given that, have slightly pushed back the
timing of the arriving significant long-period westerly swell. 00Z
NWPS run only shows seas peaking Saturday in the low 20 ft and
that combined with current lower seas creates some wariness
regarding how high seas will actually get tomorrow. Even the ENP
and GWW have backed off on Saturday's seas a little bit for the
northern waters. With that being said, still expect seas to build
into at least the low 20s early Sat morning.
The strong low over the weekend will produce significant swell as
well, but it looks like the bulk of it will head south to the
southern Oregon and northern California coast. However, think we
will another round of seas above 20 ft Sunday with possibly only a
small break Saturday night and early Sunday morning. Seas finally
subside early next week, likely dropping below 10 ft by Tue as
high pres settles over the waters. Bowen/Pyle
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 4 AM to 10 PM PST Saturday for Central
Oregon Coast-North Oregon Coast.
Winter Weather Advisory from 10 AM Saturday to 6 PM PST Sunday
for Central Columbia River Gorge-Upper Hood River Valley-
Western Columbia River Gorge.
WA...High Surf Advisory from 4 AM to 10 PM PST Saturday for South
Washington Coast.
Winter Weather Advisory from 10 AM Saturday to 6 PM PST Sunday
for Central Columbia River Gorge-Western Columbia River
Gorge.
PZ...Small Craft Advisory for Rough Columbia River Bar until 10 PM
PST Saturday.
Gale Watch from Saturday evening through Sunday evening for
Coastal Waters from Cape Shoalwater WA to Florence OR out
60 nm.
Small Craft Advisory for winds until 10 PM PST Saturday for
Coastal Waters from Cape Shoalwater WA to Florence OR out
60 nm.
Small Craft Advisory for hazardous seas until 10 PM PST
Saturday for Coastal Waters from Cape Shoalwater WA to
Florence OR out 60 nm.
&&
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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.