NWS Portland, OR wrote its last area forecast discussion before the first event at Jan 15, 9:10 AM PST, 1 hour 50 minutes earlier.
Area forecast discussion, Jan 15, 9:10 AM PST, 1 hour 50 minutes before
Area Forecast Discussion...UPDATED
National Weather Service Portland OR
910 AM PST Wed Jan 15 2020
Updated Short Term
.SYNOPSIS...Strong low pressure is developing well off the Pac NW
coast this morning. This system will spread mainly light
precipitation onshore this morning, starting off as a brief period
of light snow for most of the forecast area. Precipitation will
increase across the forecast area later this afternoon and evening
as an associated occluding frontal system moves onshore.
The way it is looking now, most elevations below 500 feet appear
unlikely to see much more than a brief dusting of snow before
precipitation turns to rain, while more significant accumulations
occur in the Columbia Gorge and higher up in the hills. It should be
emphasized that the probability of impactful lowland snow/ice
accumulations has NOT decreased to zero for the greater Portland
metro area, but the chances of this are now down around 10 percent.
Meanwhile, winds will be quite strong today along the coast and near
the Columbia Gorge. Snow will gradually change to freezing rain from
west to east in the Gorge tonight and Thursday, and eventually rain
for the west Gorge. Additional frontal systems will bring periods of
rain and Cascades snow through early next week, possibly mixing with
snow or freezing rain in and near the Gorge.
&&
.SHORT TERM...Today through Thursday...UPDATED: At 17Z Infra-red and
water vapor imagery showed a surface low pressure area near 45N 129W.
The NAM indicated a central pressure around 978 mb. This is very
close to the 12Z model guidance. There is a well-defined cold front
along 125W and lightning detection has shows several lighting strikes
along the frontal boundary. Have updated the grids to indicate
isolated thunderstorm coverage for the waters inside 60 nm beginning
late this morning. Also put a slight chance along the coast by early
afternoon. Not as confident with the high wind warning for the
beaches and headlands as the 12Z WRF-GFS has trended slightly weaker
with its 10m wind gusts and the surface orientation remains more
southeast. The HRRR 80m wind product shows the strongest winds late
this morning through the afternoon over the inner coastal waters,
with slightly lower speeds along the immediate coastline. Will let
the high wind warning ride for now. Weishaar
REMAINDER OF THE EARLIER SHORT TERM FOLLOWS: An attendant warm
front is approaching the south Oregon Coast; this will be the initial
impetus for developing precipitation this morning. East winds will
increase ahead of the warm front, gobbling up a significant portion
of the warm frontal precip as virga east of the Coast Range due to
dry downslope flow off the Cascades. Once precipitation makes its way
down to the surface, it will probably fall as light snow ahead of the
warm front down to the lowest elevations, especially inland. That
said, this period is expected to be brief for much of the forecast
area, so it seems likely that most areas away from the Columbia Gorge
and below 500 feet will NOT see much more than a dusting to 1/2 inch
before precipitation changes over to rain.
Areas closer to the Columbia Gorge and SW Washington stand a better
chance to see accumulating snow due to the cold easterly Gorge
outflow. This will be quite strong, with gusts potentially reaching
35-40 mph around the PDX metro and up to 45-60 mph through the west
Gorge. This will push back on the warm front as it tries to lift
north and east across the forecast area, and in most cases would
allow precipitation to remain snow longer around the PDX/VUO metro.
However, this case is likely different, largely due to the 80-90 kt
southerly 850 mb winds approaching the coast later today. Guidance
continues to suggest this will be sufficient to scour out the cold
air east of the Cascade crest, despite the deep and strong easterly
flow starting out the day. Temperatures are likely to be above
freezing for the lowlands outside of the Columbia Gorge this
afternoon, so the Willamette Valley should just see wet conditions
once precipitation changes to rain aloft. We did issue a Winter
Weather Advisory for the Vancouver metro, Lower Columbia, and I-5
Corridor in Cowlitz County later this morning through tonight, with
mainly elevations above 700 feet in mind aside from typically cooler
spots like Battle Ground and Hockinson. These cooler valley spots may
also fall back below freezing tonight, allowing the potential for
refreezing of roads and possibly even some light freezing rain. We
also issued a High Wind Warning for the beaches and headlands for
today into this evening, largely due to the above-mentioned strong
winds aloft and a developing coastal jet. Another item to consider
is the potential for thunderstorms along the coast this afternoon
and early evening. 06z NAM/GFS soundings suggest a decent shear
environment during that period so any thunderstorms that do develop
could become strong. Heavy precipitation rates from any heavy
showers or thunderstorms along the coast this afternoon/evening
could briefly bring snow levels lower for a quick inch or two of
snow, but it is impossible to forecast where this would occur at
this time.
It will be a different story in the Columbia Gorge and Hood River
Valley, where up to 5 inches of snow may fall before precipitation
changes to freezing rain or rain tonight into Thursday. Subfreezing
temps will be stubborn Stevenson eastward, so there will likely be a
zone east of Multnomah Falls but west of Carson that receives
significant icing...perhaps as much a quarter to a third of an inch.
For now, we just have Winter Weather Advisories for the Gorge and
Hood River Valley, but will need to monitor closely for the potential
necessity to upgrade to a warning. Upper portions of the Hood River
Valley, eastern Skamania County, and elevations above 2000-2500 feet
elsewhere will see mainly snow from this system, so we have
advisories for all our hills for up to 6 inches of snow above these
elevations. Lane County Cascades will likely see the snow level rise
to 3000 feet today, but above that there could be up to a foot of
snow from this system by the time snow winds down Thursday afternoon.
Weagle
.LONG TERM...Thursday night through Tuesday...Models and their
ensembles remain in good agreement a shortwave trough sliding across
the region Thursday will be replaced by a shortwave ridge late
Thursday night and Friday. Colder air filtering into the region
initially Thursday evening will result in snow levels lowering to
less than 1000 ft across the region Thursday night. While showers
will be decreasing in intensity and coverage Thursday night, some
light snow accumulations are likely for some of the higher hills.
Cannot rule out a few spots on the valley floor receiving a
quick dusting, but temperatures appear will likely be just warm
enough to prevent widespread issues.
As the aforementioned ridge shifts eastward across the region late
Friday, isentropic lift will spread light, but steady valley rain
and mountain snow into the region Friday night. A trailing
occlusion will likely bring a reinforcing shot of precipitation
to the region Saturday. Snow levels should gradually rise to near
the Cascade passes by the end of Saturday with another round of
decent mountain snowfall expected. Many model scenarios suggest snow
levels will rise well above the Columbia River Gorge and upper Hood
River valley elevations as well, but given the pattern, cold air
will have a difficult time retreating in these areas so have held
snow levels closer to the valley floor in these zones for this time
period.
Otherwise, it looks like there should be another break in the
precipitation Sunday and/or Monday before another storm system
brings mountain snow and valley rain to the region early next week.
/Neuman
&&
.AVIATION...VFR with mostly clear conditions will persist
through the early morning as temperatures settle near or below
freezing. Winds will continue slowly shifting and becoming more
south/southeasterly. Expect winds to become gusty across the
forecast area around 15Z Wednesday as the pressure gradient along
the valley intensifies. Easterly winds through the gorge could
easily rise above 25 kt sustained, with gusts around 35kt or
higher around 18Z Wednesday.
A front will start to move through Wednesday morning bring MVFR
conditions to the forecast area. Timing is going to be difficult
but expect to see precipitation along the coast by 16Z
Wednesday, with inland locations seeing precipitation by 19Z
Wednesday. The precipitation will start as a snow/rain mix, but
will transition more towards rain through 20Z Wednesday. Cannot
rule out a period of freezing rain during the transition time.
Snow accumulations are expected to be minimal as ground
temperatures should be above freezing.
Overnight there were several reports from locations around KTMK,
and the central Willamette Valley have reported ice forming on
some wet surfaces so there is a potential for some slick surfaces
to persist until sunrise.
There is the potential for some thunder along the coast starting
00Z Thursday through 12Z Thursday.
KPDX AND APPROACHES...VFR through the morning, but a transition
to MVFR conditions with the frontal passage. Winds will begin to
become gusty after 16Z Wednesday as the pressure gradient
intensifies. Snow/rain mix with a transition more towards rain
through 20Z Wednesday. Cannot rule out a period of freezing rain
during the transition time. Snow accumulations are expected to
be minimal as ground temperatures should be above freezing. /42
&&
.MARINE...Things remain on track this morning for a deep low
pressure systems to move across the offshore waters. Based on this
morning's 12Z guidance decided to add the inner waters to the
current storm warning as the low tracks a little closer to shore.
Winds should subside later this afternoon over our southern waters
as the low lifts north, with the far northern waters hanging on to
the strongest winds through this evening. /64
Previous discussion (issued at 432 AM) follows...A 975mb low will
slowly lift north inside of 130W today before making landfall over
Vancouver Island on Thursday. Winds are expected to ramp up during
the early morning hours, with solid Gale force gusts in the inner
waters and Storm force gusts in the outer waters. Expect the
strongest winds to occur by late Wednesday morning and continue into
the afternoon over our southern waters and into at least the evening
over our northern waters. Expect a few hours of Gales across the
waters Wednesday evening/night with gusts to 40 kt possible.
Seas are also expected to quickly ramp up with this system as it
moves north becoming large and steep. Expect seas to build into
the lower 20s across the inner waters and zone PZZ275. Seas in
Zone PZZ270 building into the mid to upper 20s. Cannot rule out
the possibility for areas in the outer waters to experience seas
near 30 ft.
Winds briefly ease on Thursday as the low moves further north,
but seas will generally remain in the 13 to 18 ft range.
Guidance shows seas may briefly subside to around 8 to 10 ft late
Thursday/early Friday. A strong front will move across the
waters Friday afternoon bringing another round of Gale force gusts
to 50 kt with seas building into into the mid to upper teens.
Active weather continues through next week as additional
disturbances move across the waters. This will likely result in
periods of Small Craft Advisory winds with seas generally
remaining above 10 ft. /42
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 2 PM this afternoon to 4 PM PST
Thursday for Central Columbia River Gorge-Upper Hood River
Valley.
Winter Weather Advisory until 10 AM PST Thursday for Western
Columbia River Gorge.
Winter Weather Advisory from 1 PM this afternoon to 4 PM PST
Thursday for Northern Oregon Cascades.
Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
Thursday for Cascades in Lane County.
Winter Weather Advisory until 4 AM PST Thursday for Central
Coast Range of Western Oregon-Coast Range of Northwest
Oregon-Lower Columbia.
High Wind Warning until 6 PM PST this evening for Central
Oregon Coast.
High Wind Warning until 8 PM PST this evening for North Oregon
Coast.
WA...Winter Weather Advisory from 2 PM this afternoon to 4 PM PST
Thursday for Central Columbia River Gorge.
Winter Weather Advisory until 10 AM PST Thursday for Western
Columbia River Gorge.
Winter Weather Advisory until 4 AM PST Thursday for Greater
Vancouver Area-I-5 Corridor in Cowlitz County-South Washington
Cascade Foothills-South Washington Cascades-Willapa Hills.
High Wind Warning from noon today to 10 PM PST this evening for
South Washington Coast.
PZ...Storm Warning until 9 PM PST this evening for coastal waters
from Cape Shoalwater WA to Cascade Head OR out 60 NM.
Storm Warning until 3 PM PST this afternoon for coastal waters
from Cascade Head OR to Florence OR out 60 NM.
Small Craft Advisory until 9 AM PST Thursday for Columbia River
Bar.
&&
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This discussion is for Northwest oregon and Southwest Washington
from the Cascade crest to 60 nm offshore. This area is commonly
referred to as the forecast area.