Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
425 am PST Monday Dec 29 2014
Synopsis...an Arctic cold front is presently moving southwest across
central Washington...and will blast through southwest Washington and
northwest Oregon later today and tonight with very gusty northeast
winds and a sharp drop in temperature. Ahead of the front showers
continue...with snow levels down to the lower foothill elevations.
Some wet snow may mix in with rain down to the valley floors in
heavier showers this morning...but any light accumulations should
remain relegated to the hills above 1000 feet. Much drier air will
arrive behind the Arctic cold front as strong high pressure builds
into the Pacific northwest. Temperatures and wind chills will be the
coldest we have seen this season for most areas. Wind chills in the
Cascades will be dangerously low...as low as 20 below zero for some
of the passes and ski resorts. Temperatures will be slow to moderate
in the valleys while faster moderation occurs for the coast and
higher terrain later this week. The next significant chance for
precipitation is not until the weekend.
Short term...today through Wednesday...apologies for the late
afd...we had to issue/adjust/cancel several advisories and warnings
earlier in the shift which delayed this issuance of this product.
The coldest air mass so far this season is still on track to arrive
later today and tonight...as an Arctic cold front is pushing
southwest across the state of Washington. Ahead of the front flow
remains onshore...with showers continuing this morning. Temperatures
remain in the middle 30s to lower 40s for most of The Lowlands...so
snow levels remain in the 1000 to 1500 foot range in the onshore
flow. Some lowering of the snow levels is expected this morning as
the Arctic boundary begins to push west across the Cascades and
through the Columbia Gorge. However the air mass will be drying out
fast due to the downslope flow off the Cascades. Therefore lowland
snow accumulations are not expected. May see a Novelty flake or two
mixed in with the rain in heavier showers.
Most of the accumulating snow is done in the South Washington
Cascades...though they may get another brief burst of snow with the
cold front this morning...and mainly in eastern Skamania County.
Therefore we cancelled the Snow Advisory a bit early for the South
Washington Cascades. The moisture axis appears more focused on northwest
Oregon ahead of the front...with numerous snow showers continuing in
the Cascades. So the Winter Weather Advisory looks good through midday...
though it may be able to be cancelled a couple hours early for the
north Oregon Cascades depending on how fast the front moves and how
fast the dry Arctic air clears the snow showers out of the Cascades.
By far the most widespread impact of this system will be the sharp
drop in temperature combined with blustery winds. With a very deep
and pronounced surge of cold air...gusty northeast winds will not be
confined to the Columbia Gorge. Confidence is a bit low on just how
strong the winds will be...though with these surges of cold air winds
often over-perform what would be suggested by the kttd-kdls
gradients. A better gradient to look at may be the broader scale
koth-kgeg gradient for this synoptic-scale northeasterly push tonight
That said...it appears it will be difficult to support widespread 60
miles per hour gusts in the Portland/Vancouver metropolitan area. 925 mb winds are in
the 50-60 knots range...but keep in mind there the mslp is expected to
rise to the 1035-1045 mb range as a very strong high builds down into
the Pacific northwest tonight and Tuesday. Therefore 925 mb will be
at a much higher elevation than it usually is...up around 3000 feet
above sea level. Time-height cross sections suggest the 50-60 knots
winds will be occurring in a very thin layer near this
elevation...with a significant amount of atmosphere to push through
if it tries to surface. Some mixing/surfacing will certainly
occur...but friction will likely limit gusts to 40-50 miles per hour for most
locations. Therefore we opted to convert the high wind watch to a
Wind Advisory for the Portland/Vancouver metropolitan area. Gusts up to 60
miles per hour are possible in the higher terrain...but should not be widespread
enough to warrant a warning.
The strong east-NE winds will also push wind chills to unusually low
values for our area. As temperatures push down through the single
digits in the Cascades...wind chills will be dangerously cold on the
order of -15 to -25 degree f at Cascade Pass level. We upgraded the wind
chill watch to a warning as everything looks to be on track for these
dangerously cold wind chills. Made a specific mention of concern for
folks who drive on high-elevation secondary roads...as getting stuck
or lost outside in the higher terrain will cause a life-threatening
situation tonight and Tuesday...as frostbite can occur in 15 to 30
minutes in such wind chills.
Wind chills in the lower elevations will likely push zero at times in
and near the Columbia Gorge and Portland/Vancouver metropolitan area... with
single digits and teens elsewhere in The Lowlands. Where winds
slacken off...nights will see temperatures falling into the teens and
lower 20s through Wednesday night. High temperatures are not expected to
make it above freezing Tuesday/Wednesday around Portland...and will only slowly
climb through the 30s and lower 40s along the coast.
Upper level ridging will strengthen valley inversions by midweek...
with the central and southern Willamette Valley likely remaining in
shallow cold air as the air mass warms aloft. This should eventually
cause fog or stratus to develop...which in turn will probably keep
Salem and Eugene in the 30s despite 850 mb temperatures pushing well above
zero degree c.
One last item of interest...as mentioned above models are suggesting
a very strong high pressure system will build into the Pacific northwest. Most
models are pushing mslp up toward 1050 mb or higher east of the
Cascades...with pressure reaching the 1035-1045 mb range west of the
Cascades. If this verifies...it will flirt with some of the highest
pressure readings ever recorded around our forecast area. The highest
pressure ever recorded at kpdx was 30.90 inches of Mercury...or
1046.4 mb...on Feb 1 1982. The highest pressure ever recorded at kpdx
during the month of December is 30.75 inches of Mercury...or 1041.3
mb...on Dec 1 2011. This pressure is forecast to be exceeded by most
models Tuesday morning...for example the 06z NAM and GFS have mslp
reaching 1045 mb...and 00z runs had similar values. Weagle
Long term...Wednesday night through Sunday...easterly winds start to
weaken Wednesday night into Thursday...allowing temperatures to warm
slightly during the day Thursday above valley inversions. Below
valley inversions...temperatures will struggle to get out of the 30s
all the way through the end of the week. The strong upper level ridge
responsible for the inversions is now expected to keep the storm
track well to our north through Fri/Sat...so we eliminated the
mention of any precipitation Friday. The possibility of a second
surge of cold air this weekend is finding less favor amongst
models...as the model consensus generally flattens the blocking ridge
over the NE Pacific allowing flow to become more zonal by early next
week. This should finally allow temperatures to crawl back to
seasonal norms and will also allow the chance of rain and mountain
snow to creep back into the picture this weekend and early next week.
Aviation...cool northwest flow aloft continues to maintain showers over
much of the region with generally VFR prevailing. However...some
pockets of local MVFR ceilings and visibilities in heavier showers. Expect
this pattern to continue with little change through about 18z Monday
and mountains will remain obscured. Snow levels lower to around
1000 feet by 18z behind a cold front...so some mixed rain/snow is
possible for the metropolitan Portland terminals but with no
accumulations expected. Showers will end along with rapid
improvement in ceilings/visibilities from north to S this afternoon weak surface
low moves south and strong offshore flow develops. Gusty east near
surface winds will develop across the northern terminals
18-20z...with gusty north-NE flow for ksle and keug developing 22-00z.
VFR under generally clear skies expected across the region by 00z
Tuesday except for overcast sky conditions near Cascade crest due to
upslope east flow.
Kpdx and approaches...generally VFR conditions through 21z Monday
though occasional periods of MVFR through this morning in showers.
Possibility for rain/snow shower mix 13-18z Monday but expect
primarily rain and no snow accumulations or impacts to runway
operations. Rapid clearing this afternoon as offshore low level
flow develops. Expect east wind at the terminal starting around
17-18z with increasing gusts into the evening and overnight. Gusts
30-40 knots probable near the terminal...with gusts 50-60 knots
developing at west end of gorge after 00z Tuesday. Cullen
Marine...gusty northwest winds will remain through this
morning...primarily over the northern waters. A weak surface low
pressure will slide S across the waters this morning...and expect some
gusty 25 knots winds over the outer waters on the western periphery
of this low. As the low slides south...winds may ease over the
remainder of the central Oregon waters. However...offshore low
level winds quickly develop over the waters as the low shifts
south. In this case...expect winds to not be confined to coastal
gaps but instead be more widespread as strong offshore surface
pressure gradient develops. Gusty winds to 30 knots look to continue
until at least Tuesday. Models then suggest a more north-NE wind develops
over the north with a coastal low pressure developing. This may keep
small craft winds ongoing a bit longer over the central Oregon
Wave heights remains in the 11 to 12 feet range as of 2 am...and
will continue to slowly subside. Given that they remain a foot or
so above guidance have elected to extend the advisory for
hazardous seas through 9 am. At this point...expect overall wave
heights to be around 9 feet and continuing to decline.
However...with no significant incoming swell...waves will become
increasingly wind wave dominated and rather choppy through at
least Tuesday. Cullen
or...Winter Weather Advisory until noon PST today for Cascades in
Lane County-northern Oregon Cascades.
Wind chill warning from 8 PM this evening to noon PST Tuesday
for northern Oregon Cascades.
Wind Advisory from 4 PM this afternoon to 6 PM PST Tuesday for
greater Portland metropolitan area.
Washington...wind chill warning from 8 PM this evening to noon PST Tuesday
for South Washington Cascades.
Winter Weather Advisory until 7 am PST this morning for South
Wind Advisory from 4 PM this afternoon to 6 PM PST Tuesday for
Greater Vancouver area.
Pz...Small Craft Advisory for winds until 10 am PST Tuesday for
coastal waters from Cape Shoalwater Washington to Cascade Head or out
Small Craft Advisory for hazardous seas until 9 am PST this
morning for coastal waters from Cape Shoalwater Washington to Florence
or out 60 nm.
Small Craft Advisory for winds until 4 PM PST Tuesday for
coastal waters from Cascade Head or 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.