Small Craft Advisory 208, NWS Portland, OR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1010 AM PST Fri Dec 3 2021

Issued Update section and updated Synopsis and Aviation sections

.SYNOPSIS...Cooler today, but dry. Light rain spreads across region
early Sat, with snow levels around 2000 to 2500 feet across
southwest Washington. Brief break on Sunday. But, a rather moist
front will push into the region later Sunday night into Monday, with
rain and mountain snow. Cool and unsettled weather continues next
week, with snow levels running 2000 to 3500 ft.
&&

.UPDATE...Updated forecast this morning to reflect the current low
clouds covering the north and central Willamette Valley east of I-5
and much of the southern Valley. This these clouds could again be
fairly slow to burn off and dissipate for some areas. Attempted to
raise temps some for the clearer areas now and lower a couple
degrees where clouds may hang on all day, such as the south Valley.
/JBonk

&&

.SHORT TERM...Today through Sunday...Cooler, westerly flow aloft
continues, with variable high clouds this am. Overall, have not seen
much in way of fog this am. This mainly due to winds keeping up a bit
longer than had earlier expected. But, also, humidity has been much
slower to rise, as dew point temperature have been lowering all
night. This expected, as have air mass change from late Thu into the
overnight hours, from mild and moist to cooler and drier. So, will
keep any fog more patchy in nature this am. Still have some patchy
frost potential as well, as temperatures fall into the middle to
lower 30s towards pre-sunrise hours.

But, after a chilly start with temperatures in the 30s, will see a
dry day  with mix of sunshine and high clouds. But, will stay on
cooler side, with highs in the middle to upper 40s, which is where
our highs should be for early December.

Next system will be arriving later tonight into early Sat. Now,
models over the past several runs have increased their consistency in
location and movement of the surface low. As of this am, now looks
like the trend is to push a surface low eastward from well offshore
to near Astoria by mid to late Sat morning. But, still appears the
bulk of the lift and precipitation will be to north and northeast of
the low. As such, looks like will get decent shot of rain Sat morning
into the early afternoon, with transition to decreasing showers in
the afternoon. Areas to south of a Newport to Albany line will not
see much in way of rain, with less than a tenth of an inch. So, will
trend forecasts to reflect these trends.

High pressure quickly returns Saturday night with lingering post
frontal showers quickly dissipating.

Saturday night and Sunday will be relatively mild as the upper ridge
builds further while southerly flow and warm advection increases
ahead of the next system arriving Sunday night.      -Bumbonkey

&&

.LONG TERM...Monday through Thursday...The long term forecast can
generally be described in one word...uncertain. After an area of
high pressure forms over the Pacific on Sunday, Monday will be quite
an active day. The overall synoptic pattern is comprised of a
strengthening low pressure system, and an associated cold front
dropping down from the Gulf of Alaska. The general pattern and
timing is the cut off for consistency though which has made this low
challenging to track. As it stands there is a lot of variability in
timing, track and intensity. Within each model, ensembles only
emphasize this lack of consistency further.

In the long term the GFS is taking on a more robust deepening
pattern and is producing the most QPF. In this specific instance,
the low trajectory is headed straight towards the Columbia River.
The ECMWF and Canadian in contrast are much drier and take on a more
northerly track. As an example, within the GEFS (ensembles of the
GFS) around Astoria, the mean 24-hr QPF ending Monday evening shows
an accumulation of less than 0.50 inches of rain. The 90th
percentile though shows rainfall totals around 1 inch. However,
looking at meteographs, there are a few "bullseye" of accumulations
higher than that. Depending on the low track, if a southerly
progression occurs, more cool air will arrive in the area and snow
levels will drop. Thus, snow accumulations will likely be higher
along the south Washington and north Oregon Cascades.

Precipitation is not the only aspect of concern with this
approaching low; winds too will be elevated. As it stands, it looks
like this will most likely be a marine, and beaches and headlands
event. Again though, there is a large spread in wind gust speeds.
Ultimately, there is a lot of uncertainty to really narrow in on a
forecast so will continue to monitor as time goes on.

After Monday, weather settles as a typical fall pattern persists with
lingering light rain, clouds and a few shortwave disturbances passing
over the area. To pick out one of the more active days next week
(aside from Monday), Wednesday night into Thursday would be the best
bet. A stronger cold front looks to push over the region bringing a
swath of cooler air and precipitation. Snow levels will likely drop
to around 3000 ft or so so could see a decent snow event for the
Cascades. For some perspective, at 850 mb, temperatures look to reach
around -7C at this time.    -Muessle

&&

.AVIATION...18Z TAFs: Westerly flow aloft continues with variable
high clouds. Low stratus continues to spread through the
Willamette Valley, mainly east of I-5 from near KSLE and
northward, and especially throughout the southern valley. With
light northerly low level wind expect the low stratus will
persist through at least 19Z to 20Z, and perhaps longer from KCVO
to KEUG. Light offshore flow along the coast is expected to
maintain VFR through tonight. Flight conditions inland are likely
to become predominately VFR by 20Z today. A low pressure system
will approach the north coast early Saturday morning causing
surface winds to become more southerly. Expect light rain and
lowering cigs across the northern area to begin after 12Z
Saturday along the coast and after 15Z further inland. Latest
HREF guidance gives 60-80% chance of MVFR developing along the
northern coast by 14Z Saturday.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...Low stratus will keep IFR to low MVFR cigs
from KPDX to KTTD through 19Z to 20Z. VFR expected to prevail
this afternoon through tonight. There is about a 50-60% chance
for low stratus to develop after 15Z Saturday. /DH

&&

.MARINE...High pressure offshore continues to bring gusty
northerly winds across the waters, especially west of 10 NM. Seas
hovering just around 10 ft. Winds will ease through the day as
the high pressure offshore weakens. Seas will also subside later
today. Current Small Craft Advisories will continue through the
afternoon for the southern zones, while into the early evening
for the northern zones.

The weakening high pressure offshore will be replaced by a broad
low pressure system currently developing near 40N 145W. This low is
currently forecast to move east across the coastal waters on
Saturday. By early Saturday morning, winds will shift to the
southwest. There is still uncertainty in exactly where the low
will track, but model guidance has come into better agreement
with possible, but brief, gale force gusts south and east of the
low. Gusty northerly winds are then likely to follow behind
through Sat night. Conditions on Sunday appear to be more settled
with light offshore flow and seas subsiding to around 6 to 7 ft.

More active weather returns on Monday with a possibly strong low
moving into the region from the north. The GFS model continues to
be stronger with a particularly good fetch for seas near 17 ft,
while others are keeping seas closer to 10 ft. Because of this
drastic difference, have kept conditions more 'moderate' similar
to the ECMWF. /mh /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 10 PM PST this evening for coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 60
     nm.

     Small Craft Advisory until 4 PM PST this afternoon for coastal
     waters from Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory from 1 PM this afternoon to 6 PM PST this
     evening for Columbia River Bar.

&&

www.weather.gov/portland

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