Winter Weather in Oregon, January 26, 2021

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
852 AM PST Tue Jan 26 2021

Updated short term and aviation discussions.

.SYNOPSIS...The next frontal system is expected to reach the coast
around midday today and then migrate east during the afternoon and
evening. Low-level offshore flow will strengthen late this morning
and into the afternoon and then slowly weaken Tuesday evening through
Tuesday night. Snow levels will be around 500 ft across southwest
Washington and far northwest Oregon late Tuesday afternoon through
the evening, but closer to 1000 ft over the southern portion of the
forecast area. A residual upper level trough will linger over the
area Wednesday through early Friday. A return to a wetter and milder
pattern is expected this weekend.

&&

.SHORT TERM...Tuesday through Thursday...Morning update: Current
GOES-17 water vapor satellite shows an occluded front and associated
deep upper level low pressure pushing eastward this morning....
although they still remain well offshore. Radar returns are also
picking up on some shower activity along the far northern portions
of the forecast area. A weak meso-low has formed within the flow
along the front which has created a convective environment with
adequate lift for some isolated thunderstorms. With the cold air
pool aloft moving over the relatively warm waters, can't rule out
some thunderstorms moving over the waters this afternoon. Decided to
add the threat of scattered thunderstorms this afternoon through
early evening. Thunderstorms likely will not make it inland today.
Overnight, temperatures did not reach as low as was previously
forecast, mainly due to the presence of some lingering stratus.
This lack of cool air could dampen the chances for snow through the
Willamette Valley, but will need to continue to monitor the
temperatures around 850 mb.

Previous discussion follows: The primary focus period for the short
term is Tue afternoon through early Wed morning. The 06Z model runs
are pretty consistent advertising the aforementioned occluded front
reaching the coast around 18Z-20Z today and then pushing inland
during thereafter. The main forecast challenge will be precip type,
especially late this afternoon through the evening.

Offshore low-level flow will begin to strengthen late this morning
and continue to do through the afternoon. The 06Z NAM suggests a peak
KTTD-KDLS gradient around -5 mb early this afternoon. Applying the
typical 1-2 mb offset yields a more realistic peak gradient closer to
-6 or -7 mb. Normally, at this time of year, offshore flow through
the Gorge is one of the key ingredients for low-elevation snowfall.
However, the air mass in the Columbia Basin is not all that cold. At
10Z KDLS was in the mid 30s and KHRI close to freezing. MAV guidance
for KDLS indicates temps in the mid to upper 30s late this afternoon.
Using our 850 mb temp rules-of-thumb, the GFS shows 850 mb temps of
-6C in the central Gorge late this afternoon and -3C to -4C at KPDX.
By 06Z Wed the GFS 850 mb temps actually moderate slightly. Finally,
the 01Z model run of the NBM 1D viewer for KPDX shows about a 25%
chance of ptype as snow late this afternoon and early evening with a
snow level around 1000 ft. However, with offshore low-level flow
believe actual snow levels in the southwest Washington interior
lowlands and far northwest Oregon valleys will be closer to 500 ft.
There will be areas below 500 ft that may end up with some minor
accumulations, such as Banks, Forest Grove and Staley Junction.

The HREF 12-hr ensemble mean snowfall valid 12Z Wed indicates
advisory-level amounts for the Oregon Coast Range, south Washington,
north Oregon and central Oregon Cascades and foothills, much of the
Columbia Gorge and the Upper Hood River Valley. It also shows general
2-5 inch amounts across eastern Clark County. The 00Z 4km WRF-GFS
provides a similar solution, with the possibility of warning snowfall
amounts for the north Oregon Coast Range and maybe even portions of
the south Washington Cascade foothills. Will be hoisting a multitude
of winter weather advisories with this package. Will exclude the
Lower Columbia in Cowlitz County, the coastal zones and the central
and south Willamette Valley. These areas will be covered in an SPS
highlighting the potential for some snow above 500 to 1000 ft.

The low-level offshore flow weakens Tue night and models, including
the NBM, suggest slowly rising snow levels. Believe the most critical
period for low-elevation snow will be late Tue afternoon through
about 08Z Wed. The deterministic GFS, ECMWF and NAM do not indicate a
true frontal passage Tue night/early Wed morning. The low-level
offshore flow becomes nearly neutral by 12Z Wed, but then increases
to -3 to -4 mb Wed evening. The baroclinic zone virtually stretches
and weakens as it moves inland, with the main energy heading into
southwest Oregon and northern California. The GFS and ECMWF show a
developing 520 dm 500 mb low diving down the British Columbia coast
Wed and Wed night to reinforce the offshore upper level trough.
Eventually the entire upper level trough axis gets forced inland late
Thu, with the bulk of its energy heading into California.
-Weishaar/Muessle

.LONG TERM...Thursday night through Monday...Unsettled conditions
will persist Thu night and Fri as the upper level trough finally
pushes inland. The deterministic GFS and, to some degree, the ECMWF,
show a weak transitory 500 mb ridge moving across Fri. About 50
percent of the 12Z GFS ensemble members suggest this weak ridging.
The 500 mb cluster analysis valid 00Z Sat suggests minimal ridging
with a deep trough near the Great Basin. All in all, expect a gradual
drying trend early in the period, but likely not completely dry. The
deterministic runs and respective ensembles are in decent agreement
showing a return to a wetter and more progressive pattern over the
weekend. In fact, the GEFS Integrated Vapor Transport (IVT) forecast
valid Sun shows a possible weak atmospheric river (AR) event. The
more reliable EPS IVT ensemble mean forecast also shows a low-end
weak AR, but a little sooner (Sat night). Overall, it appears the
low-elevation snow threat comes to an end during the latter half of
the extended period. Deterministic and ensemble guidance does not
show any extended dry period.    -Weishaar

&&

.AVIATION...Dry at the moment, but mid and high clouds will be
increasing across the region through early afternoon. Front
offshore will spread light rain to the coast by 20Z, with mix of
rain and low elevations snow inland afterwards. Through early to
mid afternoon, will see the VFR lower to MVFR along the coast,
and a mix of MVFR/VFR farther inland. Stratiform precipitation
will become more showery in nature by 04Z, but expect MVFR with
spotty IFR CIGS to persist tonight.

Inland precipitation will be problematic later this afternoon
into the evening. At worst, would see a rain-snow mixture to
valley floor at times, with mostly just snow for areas above 500
to 1000 feet. But, could be brief period of just snow for a time,
but little if any accumulation on valley floors up and down the
I5 corridor. Accumulating snowfall stays in the mountains and
foothills, as well as the Columbia Gorge and Hood River Valley.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...VFR, but CIGS gradually lowering to low
VFR by 23Z, then a mix of low VFR and MVFR CIGS. Likely to
maintain MVFR after 02Z. East winds running 10 to 20 kt today,
but weakens a bit this evening. Main story is the snow potential
later today into this evening Air mass still cool enough aloft to
support snow falling. But, air mass near the surface will be
marginally cold, just enough to maintain a rain-snow mix from 23Z
to 04Z. May be a brief period of snow during that time, but not
expecting any accumulation, as ground should remain too warm.
/Rockey

&&

.MARINE...A low pressure system dropping south over the offshore
waters today and tonight will push a front east into the coastal
waters this afternoon and evening. Winds ahead of the front will
strengthen quickly today, especially over the outer waters. Will
continue to carry a gale warning for the outer waters into the
evening hours, when winds begin to slacken as the front weakens.
There is a chance for brief storm force gusts, mainly beyond 30
nm offshore, in the afternoon hours near the front. Winds over
the nearshore waters will not be quite as strong, and will be
slower to increase today. Expect peak gusts within 10 nm of the
coast to be just over gale force mainly from the late afternoon
to early evening hours. Seas through early afternoon today will
continue to be dominated by a west swell in the range of 12 to 15
ft. As the west swell subsides late in the day, locally driven
steep southerly seas will build, likely keeping combined seas in
the range of 10 to 14 ft inside of 30 nm through the evening
hours, and more in the range of 14 to 18 ft beyond 30 nm.

A rebuilding westerly swell late tonight are expected to keep seas a
little above 10 ft through Wednesday night. A broad area of low
pressure is expected to linger past mid week over the coastal
waters, and may bring some wind gusts up into the range of 20 to 25
kt Wednesday and Wednesday night. Another chance for small craft
level winds comes with a front Friday night.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 3 PM this afternoon to 7 AM PST
     Wednesday for Cascade Foothills in Lane County-Cascades in
     Lane County-Central Columbia River Gorge-Northern Oregon
     Cascade Foothills-Northern Oregon Cascades-Upper Hood
     River Valley.

     Winter Weather Advisory from 3 PM this afternoon to 4 AM PST
     Wednesday for Greater Portland Metro Area-Western Columbia
     River Gorge.

     Winter Weather Advisory from 1 PM this afternoon to 4 AM PST
     Wednesday for Central Coast Range of Western Oregon-Coast
     Range of Northwest Oregon.

WA...Winter Weather Advisory from 3 PM this afternoon to 7 AM PST
     Wednesday for Central Columbia River Gorge-South
     Washington Cascade Foothills-South Washington Cascades.

     Winter Weather Advisory from 3 PM this afternoon to 4 AM PST
     Wednesday for Greater Vancouver Area-Western Columbia
     River Gorge.

PZ...Gale Warning until 7 PM PST this evening for coastal waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory until 5 PM PST this afternoon for Columbia
     River Bar.

     Gale Warning from 1 PM this afternoon to 7 PM PST this evening
     for coastal waters from Cape Shoalwater WA to Florence OR
     out 10 NM.

&&

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