FXUS66 KPQR 180528 AAA

Area Forecast Discussion
National Weather Service Portland OR
928 PM PST Sun Feb 17 2019

Aviation discussion updated.

.SYNOPSIS...Plenty of clouds remain, but skies will gradually clear
through this evening for most of the forecast area. This will set the
stage for a chilly night with temperatures falling below freezing for
the vast majority of the area by Monday morning. Clouds will increase
for Monday as a very weak disturbance moves in from the Pacific. This
system may produce a few flurries or sprinkles Monday and Monday
night, but any precipitation should be very light. A better-organized
frontal system will spread more significant precipitation across the
forecast area Tuesday. Snow levels will remain low, but likely just
above the valley floors. Several more inches of snow are expected out
of this system for the Cascades.


.SHORT TERM...Tonight through Wednesday...Sunbreaks developed across
much of southwest Washington and northwest Oregon today, especially
along the coast where skies have turned partly to mostly sunny. KRTX
radar is quiet this afternoon, but based on visible satellite imagery
it appears a few showers linger under radar scans across portions of
Lane and Linn Counties. Snow levels remain low...so most of these
showers are probably falling as snow above 1000-1500 feet.
Accumulation appears doubtful due to the hit-or-miss nature of these
showers and marginal surface temps below the passes. Expect any
lingering showers to dissipate as the sun goes down.

The general clearing trend should continue this evening as high
pressure builds southward into Washington and Oregon. There does
appear to be a cold front at the head of this air mass; this front
has moved southward from the Okanagan across much of the Columbia
Basin. Precipitation with the front has been scant and should
generally remain so, though some easterly upslope flow may allow for
some flurries or light snow to develop for Hood River and eastern
Skamania Counties late tonight into Monday. This has been shown well
by the NAM, though it appears a little overdone on QPF. Again, this
should not amount to much, with any snow accumulation likely to
amount to 1 inch or less.

Models continue to hint toward very light precipitation developing
across much of the forecast area Monday as the remnants of the cold
front interact with a shortwave trough incoming from the Pacific.
Forcing is extremely weak over the CWA Monday and Monday night, but
there is some weak isentropic lift combining with shallow instability
(only up to 800-850 mb) per the 12z GFS. With seeder-feeder processes
possibly coming into play, decided to mention occasional flurries or
sprinkles for most of the forecast area Monday. Stronger isentropic
lift gradually develops as warm advection increases ahead of the next
low pressure system diving down from British Columbia Monday night
and Tuesday. This should cause precipitation to slowly increase
through that period. There may be a brief period of lowland snow or
rain/snow mix as precipitation develops early Tuesday, but models
suggest S-SW winds develop quickly as the low approaches and pressure
gradients tighten between the incoming low and our higher pressure
departing to the south. This should mitigate the threat of a
significant valley snow event, with snow levels rising to around
1000-1500 feet by the time most models bring the main batch of
precipitation associated with the incoming low/front Tuesday
afternoon. Robust onshore flow will likely rack up several more
inches of snow for the Cascades and foothills, where advisories may
be needed.

Valley rain and mountain/foothill snow showers should persist into
Wednesday as the low pressure system continues S-SE across western
Oregon and another lobe of low pressure tries to develop near the Pac
NW coast. However, the overall trend will be toward drying out later
Wednesday as the next cold and dry high pressure system builds
southward from British Columbia. Weagle

.LONG TERM...Wednesday night through Sunday... The 12z GFS and 12z
ECWMF have the area in a semi-moisture laden post-frontal
environment from Wednesday night into Thursday morning. Thursday the
pattern starts to shift as a ridge slowly becomes the dominant
feature through Friday morning. Friday afternoon a shortwave trough
will start to bring in the next slug of moisture. This is expected
to stay around as a deeper trough comes in from the Gulf of Alaska
to reinforce the all ready present moisture through Sunday. 850 mb
temperatures look to hold in between -6C to -5C and a steady onshore
flow pattern continue to signal minimal snow chances for the valley
floor for the later part of the week. However, the models are
showing the wind shifting into an offshore flow pattern towards the
start of next week. If this wind shift does happen it could provide
the needed push to bring flurries to lower elevations, but being
well into the extended this trend could easily change with new data.


.AVIATION...Almost exclusively VFR conditions this evening with some
CIGs around 4000 ft and some clear skies. It looks like enough low
VFR clouds will continue to move through the area to prevent
anything lower from developing overnight, but can't completely rule
out a couple of hours of MVFR CIG around daybreak. Light offshore
flow should keep coastal sites VFR, but may see some scattered light
precipitation after 12Z moving from northwest to southeast. Could
fall as rain or snow, depending on timing, but wouldn't expect much
of anything in the way of accumulation.

KPDX AND APPROACHES...VFR conditions should continue through much
of, if not all of, tonight. It's looking less likely now that MVFR
CIG will develop at the terminal, but if it does, it would be most
likely after 14Z. May see some very light precipitation Monday
afternoon after 20Z, but it would fall as light rain or sprinkles
with little to no accumulation. Bowen


.MARINE...Weak low pressure is moving from north to south through
the Oregon coastal waters today, keeping winds well below
advisory criteria across the waters. Weak high pressure begins
to build later today, which will persist through much of this
week. However, periodic low pressure systems are expected to move
through from the northwest. At this point, the most significant
of these looks to move through Tuesday night into Wednesday. This
will bring the potential for some higher-end advisory strengths
winds, or possibly gales, during this time period.

Seas will hold around 10 feet this afternoon, before gradually
dropping below 10 feet late this evening or overnight. Seas will
remain below 10 feet through the next several days, but seas will
build again more significantly with the system on late
Tuesday/early Wednesday. Expect seas to build to at least 15 feet
and possibly approach 20 feet around midweek. Cullen


PZ...Small Craft Advisory for hazardous seas until 10 PM PST this
evening for Coastal Waters from Cape Shoalwater WA to
Florence OR out 60 NM.

Small Craft Advisory for Rough Columbia River Bar until 4 PM
PST this afternoon.



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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 forecast area.
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