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Wx Forecast - Wx Discussion - Wx Aviation

339
FXUS66 KPQR 112010 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1210 PM PST Tue Feb 11 2025

Updated aviation discussion.

.SYNOPSIS...Overall mostly dry conditions are expected Tuesday and
Wednesday daytime hours as overnight temperatures continue to remain
quite cold through the workweek. The majority of our concerns through
this period will revolve around the abnormally cold overnight
conditions locally exacerbated by east winds through the Columbia
River Gorge and Cascade gaps. We continue to closely monitor the
arrival of large-scale winter weather event on Thursday which likely
brings a mix of snow, sleet, and freezing rain to the region through
at least Friday. Forecast confidence is only moderate Thursday onward
but should improve further the next few forecast cycles as additional
high resolution guidance becomes available.

&&

.SHORT TERM...Now through Wednesday night...Drier conditions are
expected through the short term until Wednesday night, with
increasingly offshore flow. Easterly flow through the Columbia River
Gorge is already beginning to increase, with gusts up to 25-30 mph
presently. Expect increasingly cold temperatures throughout the area
as winds continue to bring in the arctic airmass currently east of
the Cascades. Beginning Wednesday morning, the TTD-DLS pressure
gradient begins increasing up to -7 to -9 mb, allowing for much
stronger easterly gusts. Winds will peak Wednesday, with gusts in the
east Portland metro area up to 35-45 mph. Even higher gusts can be
expected at exposed areas along the Gorge, Potentially up to 55-65
mph Wednesday (and Thursday as well). Early indications are that
winds should begin to weaken on Friday, but uncertainty exists
regarding when exactly easterly flow will weaken. Possibility exists
that easterly flow remains throughout the weekend as well, with
continued probability of winter precipitation as a result (see the
long term discussion). While easterly winds continue, given the low
overnight temperatures and gusty winds, there will be an enhanced
risk for hypothermia and frostbite for those without adequate shelter
and winter clothing.

Temperatures remain depressed while easterly flow is ongoing as cold
air continues streaming in through the Columbia River Gorge. Low
temps Tuesday morning will be well into the 20s, with temperatures
already below 30 degrees throughout the Willamette Valley as of 3am
Tue. 60-90% chance of temperatures below 25 by the end of the night,
particularly in rural routes, inland valleys, and the foothills. Cold
Weather Advisory still on track through Tuesday morning as a result.
Tuesday night looks slightly colder still, with potentially
temperatures into the upper teens for cold pockets throughout the
Willamette Valley and Columbia lowlands. Chances of temperatures into
the upper teens (sub 20 degrees) are around 40-70% for the central
and southern Willamette Valley between Corvallis and Eugene, east
Portland metro, Coast Range, and Cascade foothills. These areas still
see potential for an Extreme Cold Warning, and the Extreme Cold Watch
remains in place as we evaluate the potential on Tuesday night. This
will mostly come down to how much wind chill will affect these areas,
which would make or break the difference between a Cold Weather
Advisory or an Extreme Cold Warning. /JLiu

.LONG TERM...Thursday through Monday...Continued agreement in a
winter precipitation event beginning Wednesday night, pushing south
to north and reaching the Portland metro area on Thursday morning.
Still some slight disagreement in the exact timing +/- around 6
hours, but generally expecting precipitation to begin somewhere
between Wednesday night to midday Thursday. Placement of the low
continues to show agreement that the low will arrive at the central
Oregon coast, allowing offshore flow to continue sustaining a cold
surface airmass. Combined with a warm nose overriding due to the warm
front, there is the right upper air setup for freezing rain and
sleet. Early precipitation late Wednesday night looks to be a mix of
snow and freezing rain as the warm nose won`t have had time to fully
set in, but this slowly transitions moreso over to freezing rain by
afternoon/evening on Thursday. The closer your location to the
Columbia River Gorge, the later this transition will occur due to a
constant stream of cold air attempting to fight the warming
temperatures. For spots like Eugene and Salem the switch to a
wintry mix should be rather quick, likely by Thursday afternoon
or early evening.

So with this in mind, the lowlands in general shouldn`t expect
to see too much snow accumulation partially due to a lack of
QPF coinciding with the cold temperatures both at the surface
and aloft; from about Salem northwards through SW Washington,
there is only a 20-40% chance of accumulations of 1 inch or more
from 4am Thu to 4am Sat. South of Salem, freezing rain looks to
be more prevalent, with snow being less of an issue. Obviously
with a greater depth of cold air in the Columbia River Gorge and
Upper Hood River Valley, there a better chance for snow
accumulation; 40-60% chance for 3 inches or more during that
same period. However, until we get closer to the event and
larger suite of high resolution guidance becomes available for
the Thursday-Friday time period take these probabilities with a
grain of salt since models struggle locally resolving warm air
overrunning scenarios. A northward shift in the storm-track
would allow for greater QPF availability and thus greater
impacts.

Freezing rain remains the primary impact, with the bulk of potential
accumulations expected Thursday afternoon onwards. The LREF
(comprising the raw ENS, GEFS, and GEPS) depicts a 40-80% chance of
accumulating freezing rain in the Willamette Valley and adjacent
foothills/valleys, with the best chance in the central
Willamette Valley between Eugene and Salem, and within the coast
range valleys. Should the worse case scenario occur, (LREF 90th
percentile) ice accumulation of a 0.25-0.60 inches would be
possible across a large chunk of the region, highest in the Coast
Range and central/southern Willamette Valley. A more reasonable
scenario (LREF mean) has closer to 0.1-0.2 inches throughout much of
the Willamette Valley, though once again slightly higher in the Coast
Range. Overall confidence in the timing and exact amounts/locations
is rather low, and we`ll be carefully monitoring model runs in the
next few days. Potential for winter precipitation through Friday
should be taken into consideration if making travel plans in the
middle of next week. Most ensemble members are showing a decrease in
precipitation near the end of next week, with possibility of more
moisture on Sunday or Monday. In some scenarios the low level flow
never truly turns onshore which means there is a
chance the Columbia River Gorge and Hood River Valley may be
dealing with freezing rain into early next week. -Schuldt/JLiu

&&

.AVIATION...High pressure will keep conditions VFR through the
next 24 hours. Offshore flow will persist with speeds generally
being driven by topography. In areas that are east-west channeled,
like the Columbia River Gorge, winds will be more elevated. Around
KPDX, KTTD, and K4S2 between 1000-3000 ft AGL, winds will range from
20-35 kt from the east. While speed shear will be present, due to a
lack of directional shear, did not include in the TAFs.
Through the lower elevation terminals in the Willamette Valley,
winds are more northerly at the surface and easterly aloft, but
speeds are not of concern.

Below freezing temperatures will return after 02-04Z Wednesday. Due
to conditions being so dry at the surface, no fog is expected in the
next 24 hours, but exposed surfaces will be quite cold.

PDX AND APPROACHES...VFR through the next 24 hours with localized
speed wind shear below 3000 ft. Winds from the east from the surface
to 3000 ft AGL but with a lack of directional shear, have omitted
from the TAF. Temperatures will fall below freezing  after 03-4Z
Wednesday. -Muessle/HEC

&&

.MARINE...Fairly benign weather through Wednesday as high
pressure lingers over the region. Expecting seas of 5-6 ft at 12
seconds, and wind gusts less than 20 kt through Wednesday
afternoon. Starting early Thursday, a strengthening low pressure
system will near the coast. Thus far, it appears to make landfall
around Newport. Will see an increased easterly wind with the
approach, especially around Tillamook and the Columbia River Bar.
Early Thursday morning through the early afternoon, guts up to 30
kt possible, and up to 35 kt around Clatsop Spit. The concern is
how widespread these wind speeds will be as they appear to be
focused in the coastal gaps. However, cannot rule out more
widespread gusty winds and potentially Small Craft Advisory
conditions in the outer waters. The easterly flow will aid in
combating the background westerly swell so not expecting
significant wave rises until early Friday behind the front. On
Friday, seas will build to 10-13 ft at 18 seconds before easing to
near 8 ft on Saturday.

As previously mentioned, the incoming low pressure system will
increase easterly flow which will cause temperatures to decrease.
Expecting freezing conditions along moorages overnight Wednesday,
Thursday, and Friday. With the added precipitation, cannot rule
out freezing rain, snow, or even freezing spray if the winds are
high enough from the east. Confidence is low as to whether any ice
will form.
  -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Cold Weather Advisory until noon PST today for ORZ101>128.

     Cold Weather Advisory from 7 PM this evening to noon PST
     Wednesday for ORZ101>103-105-107>111-114>118-126>128.

     Extreme Cold Watch from this evening through Wednesday morning
     for ORZ104-106-112-113-119>125.

WA...Cold Weather Advisory until noon PST today for WAZ201>211.

     Cold Weather Advisory from 7 PM this evening to noon PST
     Wednesday for WAZ201-202-204>206-211.

     Extreme Cold Watch from this evening through Wednesday morning
     for WAZ203-207>210.

PZ...None.

&&

$$

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