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fxus66 kpqr 151146 cca 
afdpqr

Area forecast discussion
National Weather Service Portland or
303 am PST sun Dec 15 2019



Synopsis...fairly benign weather the next couple of days will likely
give way to a multi-day stretch of notably wet weather mid to late
week.



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Short term...today through Wednesday...water vapor satellite imagery
early this morning reveals a shortwave ridge offshore shifting
eastward towards the Pacific northwest. This has resulted in the
atmosphere stabilizing overnight. Light winds and lingering moisture
near the surface have allowed plenty of low clouds and some patchy
fog to develop across northwest Oregon and southwest Washington
early this morning. Given the weak pressure gradients expected
across the area today, low clouds and fog should be slow to clear
and many locations in the Willamette and lower Columbia River
valleys appear unlikely to clear. As a result, have lowered high
temperatures for inland valley locations a couple of degrees for
today.

Models are in good agreement a warm front will shift eastward
towards the region tonight. While operational models have come into
better agreement that it will fall apart enough that the area will
remain dry, there are enough ensemble members that bring at least
some light rain to our western zones that have maintained slight
chance to low chance pops for later tonight and early Monday.

Shortwave ridging will then shift more squarely over the region late
Monday into Monday night so expect more areas of fog and low clouds
to develop. The main exception to this will be across the northern
half of the Willamette Valley where increasing east winds near the
mouth of the Columbia River gorge will likely prevent widespread fog
and low clouds from developing.

Models are in good agreement the next shortwave trough approaching
the region will split and weaken, as most storms seemingly have the
last several weeks, considerably as it hits the Pacific northwest
later Tuesday. As a result, have kept pops in the slight chance to
chance categories for Tuesday into Tuesday night. With that said,
this storm system will likely be the sacrificial Lamb and open the
door for strong zonal flow to spread across the region Wednesday.
Models are in good agreement this will be in response to a broad
shortwave trough developing in the Gulf of Alaska. This will result
in a more solid shot of rain Wednesday as a cold front pushes
eastward across the area. /Neuman



Long term...Wednesday night through Sunday...models and their
ensembles are in general agreement a broad shortwave trough will move
into the Gulf of Alaska midweek before digging southeastward towards
the West Coast of the US next weekend. Concurrently, a shortwave
ridge will build northward into the Rocky Mountain states. As this
occurs, a strong zonal jet will take aim at the Pacific northwest
Wednesday night into early Thursday. There have been a few model
runs over the past couple of days that result in a surface low
pressure developing and moving northeastward towards the Pacific
northwest Wednesday night into early Thursday, but most model
solutions keep the low pressure either very weak or nonexistent.
Nonetheless, the pattern seems conducive for a surface low pressure
to develop and move close enough to the region to at least bring a
threat of high winds to the coast. Even though the latest
operational model runs show few signs of this occurring this will be
Worth monitoring.

Meanwhile, models are in better agreement that at least a moderate
strength atmospheric river will end up impacting the West Coast of
the US Thursday into Friday night. Given the digging nature of the
offshore trough, the atmospheric river could stall or at least waver
back and forth between northern California, Oregon and Washington
during this timeframe. While the operational GFS, and particularly
the ec are somewhat ominous and would likely result in at least some
flooding issues in our County Warning Area late in the week, the gefs and eps
suggest there is a lot of uncertainty in how long the steadier rain
will reside over our County Warning Area mid to late week...at least a lot more so
than just looking at the operational GFS and ec. Will wait for more
of a consensus among the models and their ensembles before issuing
any products highlighting potential impacts.

It should be noted there may be some winter precipitation type
issues in The Gorge when the initial warm front spreads into the
region late Wednesday night and Thursday morning, but given the
marginally cold temperatures east of the Cascades currently,
confidence in significant issues arising is low at this point. This
pattern does seem conducive for heavier snow to fall across the
higher terrain between Mt Hood and Mt Adams initially Wednesday
night into early Thursday regardless. Nonetheless, temperatures
should moderate enough late Thursday and Friday that snow levels
will likely climb above the Cascade passes during any heavier bouts
of precipitation late Thursday and Friday.

Expect precipitation to turn more showery and snow levels to lower
below the Cascade passes next weekend as the main upper level trough
spreads eastward over the area. Given the digging nature of the
shortwave trough as it hits the West Coast of the US and the bulk of
the energy appears likely to remain south of the region, low level
westerly flow should not be terribly strong across our County Warning Area. This
will likely limit snow totals across our Cascades next weekend
despite the colder temperatures and modest instability spreading
across the area. /Neuman



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Aviation...primarily a mixture of MVFR and IFR cigs have
settled in over the area, and are expected to keep any fog from
developing along the coast and within the Willamette Valley through
the morning. There could be some patchy LIFR cigs that develop along
the coast, but if they develop would expect them to dissipate by 19z
Sunday. There are a few areas within the Willamette Valley that
currently have a scattered cloud deck that might allow for some patchy fog
development, but overall areas should maintain VFR or MVFR
visibilities. Look for a slow improvement towards VFR conditions
starting around 19z Sunday for all locations. Expect increasing
precip chances starting after 00z Monday.

Kpdx and approaches...IFR cigs have developed over the area, and
are expected to persist through 19z Sunday as the cigs slowly
lift towards VFR. /42



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Marine...winds and seas will slowly build through the evening as
a front moves NE across our waters. This frontal passage will
bring Small Craft Advisory winds and seas to our outer waters
through Sunday night through at least Monday afternoon. A
stronger system is then expected to move into our waters by late
Monday evening with Small Craft Advisory winds and seas, that
could develop into marginal gales for the outer waters. However,
models are all over the place when looking at strength and
placement of this Tue/Wed system. Expect primarily southerly
winds to dominate through the middle of the week, along with a
primary westerly swell, and secondary weaker wind driven
southerly swell. All of these factors will keep seas around 9 to
10 feet for the early part of this week, and could push seas up
to 14 to 16 feet by Wednesday. /42



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Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory from 10 PM this evening to 4 PM PST Monday
for coastal waters from Cape Shoalwater Washington to Florence or
from 10 to 60 nm.

Small Craft Advisory until 9 am PST this morning for Columbia
River bar.

Small Craft Advisory from 4 PM this afternoon to 10 PM PST this
evening for Columbia River bar.

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