Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
1000 PM PST Tuesday Dec 1 2015
Synopsis...a moist front will move onshore later today and tonight.
This will bring moderate rain to most of the region...with
significant ice accumulations expected tonight into Wednesday morning for
the Columbia Gorge...Hood River Valley...and other areas below 2500
feet in the S wash and north Oregon Cascades. A warm front will bring
more precipitation late Wednesday into Thursday...with gusty winds along the
coast and the potential for more freezing rain in The Gorge and Upper
Hood River Valley. The freezing rain threat should end on Thursday as this
system begins to scour out the cold air. A progressive weather
pattern continues through the weekend and into next week...with
numerous systems expected to bring rain...gusty winds...and periodic
Update...radar shows precipitation shield spreading east this
evening ahead of a cold front which is currently offshore. The main
forecast issue again tonight will be temperatures. The West Hills and
elevations above about 500 feet elsewhere within the Portland metropolitan
area appear to already be at or below freezing and we have received
multiple reports of freezing rain. However Road temperatures appear
to be above freezing...so ice is not sticking there yet. There are a
number of factors which will determine the areal extent and duration
of the freezing rain. First...temperatures should rise behind the
cold front as winds turn southerly which would limit the duration of
freezing rain...but there are still a number of hours before the
front passes. However the gradient through The Gorge will remain
strong with continued cold east winds which may limit warming.
Another factor is dewpoints...which have been trending slightly
higher as the rain rate increases. If dewpoints rise enough for wet
bulb temperatures to get or remain above freezing...freezing rain
should turn to rain. With all that being said...will stick with the
current Special Weather Statement regarding freezing rain for now but
if Road conditions worsen...may have to issue a relatively localized
Freezing Rain Advisory. Either way...people in and around Portland
are encouraged to check Road conditions before beginning their
Wednesday morning commutes.
For The Gorge...Ice Storm Warning looks good as most of the precipitation
will fall as freezing rain. With a wet bulb temperature of 31 degrees
at Troutdale and a current observation (04z) of freezing rain...the
warm nose may be a bit ahead of schedule and the western gorge may
see mostly freezing rain instead of sleet changing over to freezing
rain. If that is the case...ice accumulations in the western gorge
could be more than 0.50 inches. However impacts should remain
consistent with the current forecast.
In The Lowlands outside of the Portland metropolitan area and The
Gorge...temperatures remain warmer with upper 30s to low 40s in the
central and south Willamette Valley and around 50 degrees on the
coast...so there should not be any concerns with freezing rain in
those areas tonight.
Rain will begin to turn showery behind the front by morning and
temperatures are expected to warm well into the 40s for all but the
immediate gorge area so any ice which does accumulate outside of The
Gorge should melt. Due to continuing cold winds through The
Gorge...the outlook there is not as positive with ice likely
struggling to melt even in the middle of the day.
Short term...tonight through Friday...a moist frontal boundary is
nearing the Pacific northwest coast this afternoon...with a plume of greater
than 1 inch precipitable water values feeding into the front. The
latest National Weather Service Doppler radar imagery shows the leading edge of
precipitation with the front nearing the coast. Expect rain to
develop at the coast and Coast Range over the next couple of
hours...then spread inland later this afternoon and early evening.
Gusty southerly winds will develop along the coast later this
afternoon and evening as well. The gradient remains fairly
offshore...so the stronger winds should remain confined to the
beaches and coastal headlands...where gusts to 40 to 50 miles per hour are
Most areas will see all rain with this system. Snow levels will start
out somewhere around 4000 to 5000 feet...then gradually rise through
the night. Expect anywhere from 3 to 7 inches of snow at the highest
elevations of the S Washington and north or Cascade zones...mainly above 5000 feet.
However...the most significant impacts from this system are expected
in the area of the Columbia River gorge and Upper Hood River
Valley...as well as the lower elevation valleys in the S Washington and north or
Cascades. A cold pool with a depth of around 4000 feet remains in place
east of the Cascades today. The easterly gradient through The Gorge
has strengthened through the day...and the kttd-kdls gradient is now
at -9.2 mb. This is keeping temperatures well below freezing through The
Gorge and the Upper Hood River Valley. Based on the latest NAM and
uw WRF-GFS model...expect precipitation may start out as snow this
evening near Hood River and in the Hood River Valley. As warmer air
moves in aloft...precipitation should transition to sleet and then
freezing rain later in the evening and overnight. Precipitation
should taper off considerably by daybreak Wednesday. All told...expect
around 1-3 inches of snow for the central gorge and Upper Hood River
Valley zones ... ice accumulations 0.25 to 0.5 inches. The
western gorge and Cascade valleys should be mostly freezing rain and
sleet and are expected to accumulate around 0.5 inches of ice.
Travel through The Gorge will likely become difficult if not
impossible tonight into Wednesday morning.
There should be a relative lull in the weather through much of Wednesday.
There may some areas of light precipitation...but quantitative precipitation forecast should not
amount to much. However...a warm front will bring another round of
precipitation starting Wednesday evening and continuing into Thursday. The forecast
models keep the easterly gradient through The Gorge in place well
into Thursday...so expect we will see additional icing for The Gorge and
Upper Hood River Valley into Thursday am. Also...this system is expected
to pack more gusty southerly winds for the coast. There may be a
couple of bursts of winds associated with secondary lows forming
along the main frontal boundary. The first burst is expected Wednesday
evening...with another Thursday morning. Decided to issue a high wind
watch for the coast during this time period. There is also potential
for some gusty winds over the interior lowlands Thursday...depending on
the track this system takes.
A trailing cold front will move through the region Thursday. Expect this
may finally put an end to our freezing rain concerns near The
Gorge...as the forecast models are indicating the cold pool east of the
Cascades begins to scour out and we lose the easterly surface
gradient. Expect additional precipitation late Thursday with the
front...with Post frontal showers continuing into Friday. Snow levels
will come down considerably with the front. Expect we will see some
decent snowfall accumulations down to the Cascade passes on Friday.
Long term...Friday night through Monday...weather pattern remains
very active through the weekend. With little recovery time between
these systems...river flooding will be a concern over the weekend
into early next week. Thursday night into Friday could be somewhat
dry with only a few scattered showers mainly along the coast and in
the Cascades. Snow levels drop down to around pass levels for
Friday...so should see some significant accumulations with snow
showers down to the passes. Another system will bring more rain
Saturday afternoon into Sunday...and strong winds are possible again
along the coast Saturday night. Good news is that cold air should be
cleared out of The Gorge...so not expecting any freezing rain or
sleet in this area over the weekend. Snow levels also rise back up
above the passes with this system...and should stay up above 5000-
6000 feet through Monday. Storm track stays right over US...with
another system bringing more rain on Monday. -McCoy &&
Aviation...steady rain is moving across the forecast area this
evening as a cold front moves through. Strong easterly gorge winds
are affecting kpdx/kttd with 25-35 knots gusts and cold air. Temperatures
should remain above freezing at kpdx...but freezing rain is being reported
at kttd. Rain expected to taper to scattered -shra 09z coast...12z-14z
inland. Kttd ice sensor reporting significant accumulations...but
feel this is exaggerated due to the wind. Runway temperatures should
remain near/slightly above freezing with little to no runway
accumulations at kttd...but confidence in this is low to moderate.
As for cigs/vsbys...general MVFR across forecast area tonight
improving to VFR outside of showers Wednesday. More rain spreads up
from the south Wednesday afternoon/evening...but temperatures should be a bit
Kpdx and approaches...MVFR visibilities and occasional MVFR ceilings in rain
until about 12z. East winds 15 g 25 knots until 12z...then easing. Rain
tapers to -shra with mainly VFR after 12z. Temperatures presently 34 f at
kpdx...and expected to remain above freezing so no runway icing
issues anticipated. Weagle
Marine...latest klgx Doppler radar imagery suggest front has
moved through buoys 46029/46050...with any marginal gales coming
to an end for the outer waters. Expect front to move onshore by
midnight or 1 am...bringing an end to all gales for the time
being. Will see somewhat of a break in the winds early Wednesday
before increasing again as another set of low pressure ripples
swing north/northeast through the flow.
Models continue to waffle a bit about the details of the next
system. Confidence is high that a strong cold front will slowly
approach the coast Wednesday/Wednesday night...with waves of low pressure
developing along the front and bringing the potential for gales
through Thursday. Often times these systems have a couple brief periods
of strong S-southeast winds before the most consolidated low develops and
lifts north across the waters. This is the favored solution of the 00z
NAM and GFS models...with the strongest low moving north through our
waters Thursday. There is a decent chance of storm force winds with
this final low...but for now just extended the existing gale watch
out through Thursday. Seas may reach or exceed 20 feet depending on the
strength of this system.
Another break on Friday but another stalled front brings at least
gales back to the waters Friday night through Sunday. This one
also has potential for storm force gusts but will remain cautious
as long term model aggression has been tempered in the short term
for the last couple events.
Regardless of the details...seas will not be favorable for
smaller craft through the entire forecast. Seas primarily remain
strongly wind wave and fresh swell driven with an underlying west
swell around 10 feet. Weagle/jbonk
or...high wind watch from Wednesday evening through Thursday morning
for central Oregon coast-north Oregon coast.
Ice Storm Warning until 9 am PST Wednesday for central Columbia
River gorge-Upper Hood River Valley-western Columbia River
Washington...high wind watch from Wednesday evening through Thursday morning
for South Washington coast.
Ice Storm Warning until 9 am PST Wednesday for central Columbia
River gorge-South Washington Cascades-western Columbia
Pz...Small Craft Advisory for rough Columbia River bar until
4 PM PST Wednesday.
Gale watch from Wednesday morning through Thursday afternoon for
Coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Gale Warning until 11 PM PST this evening for waters from Cape
Shoalwater Washington to Cascade Head or from 10 to 60 nm-waters
from Cascade Head to Florence or from 10 to 60 nm.
Gale Warning until 3 am PST Wednesday for coastal waters from
Cape Shoalwater Washington to Cascade Head or out 10 nm-coastal
waters from Cascade Head to Florence or out 10 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.