Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Portland or
1216 am PST Wed Jan 24 2018
Update to headlines and marine section.
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
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.
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 Continental U.S.. 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...update - ended Gale Warning for all waters and will
have Small Craft Advisory winds through early morning. Seas
however will remain in the 16 to 19 ft range through early
morning then subide to near 12 ft by mid to late 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. This may be problematic in timing and
strength as the low is rapidly developing and fast moving system. /Mh
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
or...Winter Weather Advisory until 7 am PST this morning for Upper
Hood River Valley.
Winter Storm Warning until 8 PM PST this evening for northern
Washington...Winter Storm Warning until 8 PM PST this evening for South
Pz...Small Craft Advisory for winds until 4 am PST early this
morning for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for hazardous seas until 10 PM PST this
evening for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 6 PM
PST this evening.
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