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FXUS66 KPQR 240524
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
840 PM PST Tue Jan 23 2018

.SYNOPSIS...A series of storm systems will bring periods of valley
rain and snow to the Coast Range and Cascades through the weekend.

&&


.SHORT TERM...Tonight through Friday...Water vapor satellite imagery
reveals a large low pressure system over the northeast Pacific. A
modest atmospheric river ahead of a front is currently streaming into
the Pacific Northwest. This resulted in widespread light to moderate
rain and mountain snow to much of the region today. Rainfall totals
have remained rather modest and should continue to do so. As a
result, do not anticipate much in the way of any flood issues.

A weak surface low pressure appears to have developed along the
aforementioned front and is moving northeastward towards the Olympic
Peninsula. This has temporarily allowed winds to strengthen along the
immediate coast with a few gusts of around 60 mph reported at Cape
Disappointment, Yaquina Bay Bridge and Yachats this evening. Extended
the High Wind Warning earlier this evening given this development,
but these winds should start to taper off towards midnight.
Meanwhile, the weak low pressure that developed offshore this evening
has helped cold air to persist east of the Cascades with a relatively
deep isothermal layer present. Snow appears to be sticking in the
upper Hood River valley above ~1500 feet so opted to extend the
Winter Wx Advisory through the overnight hours as pressure gradients
will be slow to relax overnight. Farther west, snow levels have risen
to around 4500-5000 feet where warmer air aloft has been able to
penetrate. Expect decent accumulations above this elevation in the
Cascades, but given the south to southwesterly flow, orographics will
be somewhat limited except across the volcanoes and Winter Storm
Warning amounts may be difficult to achieve.

Once the front pushes ashore later tonight and winds drop off along
the coast, the front will slow considerably and result in off and on
rain for much of northwest Oregon and southwest Washington on
Wednesday. Colder air aloft will work its way eastward into the area
on Wednesday and lead to precipitation turning more showery in
nature. This will also result in snow levels lowering to below the
Cascade passes Wednesday afternoon and evening. Fortunately,
precipitation in general should not be as persistent as it was today
so accumulations should remain at levels worthy of an advisory or
less.

Models are also suggesting at least two modest surface low pressures
will develop and move east to northeastward into the Pacific
Northwest including one Wednesday afternoon and another one on
Thursday. Expect each of these to bring a burst of winds to the
coast, but at this point winds are not anticipated to reach High Wind
criteria with either system.

Onshore flow and a cool and unstable airmass will result in
additional rounds of showers Thursday and Friday. Snow levels will
lower down to ~1500-2000' during this time with advisory level
accumulations possible across the Coast Range and Cascade foothills.
/Neuman



.LONG TERM...Friday night through Tuesday. The active weather
pattern will continue into the weekend. Models are in fairly good
agreement tracking a low pressure system across the northeastern
Pacific and across the forecast area. The trailing cold front
interacts with a fairly robust tap of moisture across the Pacific,
transported in southwesterly flow as the main 500 mb pattern becomes
more zonal across the far eastern Pacific and western CONUS. Snow
levels will increase as the air mass warms over the weekend through
early next week. However, confidence in the details of the forecast
remains a bit lower than normal, as the placement of the moisture
plume over the building ridge could either bring a period of heavy
rain to the region, or push north of the forecast area and keep our
region dry with the rain in northwestern Washington. However,
confidence is a little higher for the start of next week as there is
general agreement that the next upper trough will swing across the
Pacific Northwester and bring another round of enhanced rain chances
along with lowering snow levels for the start of next week. Cullen

&&

.AVIATION...Tough TAF forecast with ceilings continuing to bounce
between MVFR and VFR across the area. Looks like occasional
visibilities below 6 miles during heavier rain, but tough to time
these visibility differences in the TAF. Expect a mixture of
MVFR and VFR conditions across the area through the evening and
into the overnight hours, but keeping up with these changing
conditions in the TAF will be difficult. Conditions probably will
not change a lot on Wed.

PDX AND APPROACHES...Conditions expected to vary from VFR to
MVFR at times in cigs and vis. These conditions will probably
continue through much of Wed. pt


&&


.MARINE...Made some minor updates this evening, mainly to hold
onto the gales a little bit longer. A front is oriented SW-NE
across the waters and won't move much as a wave is currently
moving through, moving N of the waters after midnight tonight.
After that, winds will ease from N-S. Seas will peak in the
16-18 ft range overnight tonight.

After that winds drop off to around the lower end of the Small
Craft Advisory criteria. Seas fall to around 10 ft late tonight
and Wed morning. The models show a secondary front and possible
low moving through the waters Wed afternoon and evening that
could get winds close to gale force again.

One more low is expected late Wed night and Thu that could
produce another round of gales with seas back in the mid to upper
teens. The next strong system is expected Fri night and Sat with
more gales and seas in at least the mid teens. pt


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until midnight PST tonight for North Oregon
Coast.

Winter Storm Warning until 8 PM PST Wednesday for Northern
Oregon Cascades.

High Wind Warning until 2 AM PST Wednesday for Central Oregon
Coast.

WA...High Wind Warning until midnight PST tonight for South
Washington Coast.

Winter Storm Warning until 8 PM PST Wednesday for South
Washington Cascades.

PZ...Gale Warning until midnight PST tonight for Coastal Waters from
Cape Shoalwater WA to Cascade Head OR out 60 NM.

Gale Warning until 2 AM PST Wednesday for Coastal Waters from
Cascade Head OR to Florence OR out 60 NM.

Small Craft Advisory for Rough Columbia River Bar until 6 PM
PST Wednesday.

&&


$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.
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