Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 062214
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
214 PM PST Sat Mar 6 2021

.SYNOPSIS...Low pressure dropping south offshore will keep a threat
of showers over the region through the early part of the week. A
cold front moving ilnland will bring a few hours of more steady rain
to parts of the region late tonight. Upper level high pressure is
expected to return drier weather for the second half of the work
week.

&&

.SHORT TERM...Tonight through Tuesday...In the big picture, models
take the large closed upper low seen in water vapor pictures
centered near 51N 141W Saturday afternoon, and drop it steadily
south-southeast to near 40N 130W by Tuesday afternoon. Radar showed
scattered showers across much of the area increasing during the
early afternoon as low level air mass destabilizes with daytime
heating. A few reports of small hail may accompany some of these
shallow convective showers thanks in part to relatively low freezing
levels.

The first significant shortwave rotating around the low is
expected to swing across the northwest part of the forecast area
late tonight and early Sunday, pushing a cold front east through the
area late tonight. The main frontal precipitation band, seen on
satellite passing 132W early Saturday afternoon, is fairly narrow.
Model time height cross sections show good low and mid level
moisture as the best lift moves onshore after 06Z, so pops will
increase to categorical category by late tonight along the coast.
Moving inland the moisture profiles will not be quite as favorable,
and with the shortwave directing the best lift up into Washington
with time, will see pops drop off to likely category as the front
reaches the Cascades. As the upper heights come down some as the
upper trough nears and cold front pushes inland, will see snow
levels drop down into the Coast Range and lower Cascade Foothills by
morning. Precipitation turns more showery again Sunday behind the
front, and models suggest enough destabilization in the low to mid
levels to support a slight chance of thunderstorms for much of the
area in the afternoon.

as the upper low continues to drop south off the coast Sunday night
through Tuesday, the flow over the region backs from the southwest
to the south. The next significant shortwave rotating around the
parent is directed more towards northern California Sunday night,
bringing a temporary decrease to pops Sunday night. A couple of
shortwaves lifting north Monday will help to spread chances for
showers back north into the region Monday and Monday night, then by
Tuesday the best chances for showers will be across the south part
of the area as the upper low sinks down to 40N. Some model soundings
suggest some slim but sufficiently deep CAPE potentially developing
Monday and Tuesday in the afternoon hours, but lack eough confidence
in this occurring to make any sort of mention of thunder for the
time being. Without strong orographics or dynamics, expect mountain
snowfall potential will be limited to a few inches per each
day/night period.

.LONG TERM...Tuesday night through Saturday...Models continue
to show a trend towards a slow evolving positive tilt pattern
through the middle and latter parts of the week with a trough
lingering over roughly the southwest states, and an upper ridge
extending into the Pacific Northwest off the Pacific. Overall this
results in good confidence that drier weather returns mid week with
seasonable or slightly warmer daytime temperatures under an upper
level ridge. Late in the week however there is some uncertainty
regarding the disposition of the ridge, with some ensemble members
and the 12Z operational GFS suggesting the a slow southeastward
progression of the pattern that allows an upper trough and cold
front to sink south into the Pacific Northwest, while others hang
onto the ridge into Saturday. Clustering suggests the drier-ridgey
scenario the more likely outcome, but enough uncertainty exists to
necessitate a low chance for showers Friday into Saturday.

&&

.AVIATION...Broken VFR skies with increasing showers throughout
the area this afternoon. Satellite shows the advancing front in
the eastern Pacific but it will slowly make it`s way towards the
coast through the evening. Along the leading edge of this front
expecting breezy winds persisting through at least 14Z Sunday.
Likely going to see the breezier southerly winds along the coast
and through the central Willamette Valley, although other areas
with a more southerly orientation could experience these winds as
well. In general, expecting gusts up to 25 kt, but cannot rule
out isolated gusts up to 30 kt, especially with these passing
showers. Along with more elevated winds, MVFR cigs around 2500 ft
have accompanied the showers producing rain, pea sized hail and
sometimes slightly reduced visibility...especially when more
intense. Not expecting any thunderstorm activity with the showers
today. Showers should persist through the evening as they are
convectively lifted by the increased temperatures and clearer
skies.

Overnight the conditions become a bit more challenging. With the
added moisture from showers and broken skies during the day,
there is a slight chance for some slight fog formation. Any fog
that does form though will be isolated and not expected to be
long lasting. Too hard to pin down exactly where it is expected,
but can generously say that inland locations are going to be more
prone. Skies will fill in a bit more widely overnight and could
see a more consistent MVFR layer, especially along the coast
after 06Z Sun. In most locations though conditions will likely
fluctuate between MVFR and VFR. After the frontal passage,
expecting another round of widespread VFR cigs around 5000-8000
ft with spotty shower activity in the post-frontal environment.

Will mention that forecast soundings are showing a trend
consistent with thunderstorm activity between 15Z Sun and 00Z
Mon. The coast will be the first at risk area then will push east
through the day. With little confidence to the timing and
location, threat has been left out of the TAFs. But, with any
thunderstorms, there is a chance for some funnel cloud activity
and other convective weather.

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

KPDX AND APPROACHES...VFR through the next 24 hours with periods
of MVFR cigs around 3000 ft with passing showers. Could see
periods of light pea sized hail in some passing showers. The
front will move over the after after 06Z Sun and will quickly
push east overnight. Expecting a fluctuation between VFR and MVFR
cigs during that time. Return to convective but consistent VFR
conditions after 16Z Sun. Chance for afternoon thunderstorms
starting around 20Z Sun. Little confidence to timing so all were
left out of the TAF.  -Muessle

&&

.MARINE...Trends are still consistent with a series of disturbances
passing over the waters. Tonight the first, a weak front, will
push inland bringing rain and gusty winds along with it. Likely
will see southerly winds 20 to 25 kt with gusts up to 30 kt.
There is a chance that gusts 35 to 40 kt could occur in some
locations, especially the outer waters. However, with how fast
this front is moving, likely will only be for a few hours and
will be spotty. While temporarily meeting gale force warning
levels, there is quite a bit of uncertainty regarding the overall
intensity and location so decided to stay with a small craft
advisory as those winds are going to be the most consistent
conditions. Seas with this system will remain fairly stable
around 9 to 12 ft at 14 seconds through Sunday morning.

Sunday afternoon, another system will advance over the waters. At
this time, winds will ease slightly to 15 to 20 kt with gusts up
to 25 kt. Seas on the other hand will quickly build from the 9 to
12 ft to upwards of 17 ft by Sunday afternoon. There is still
quite a bit of difference between different model runs with some
suggesting combined seas of 16 to 18 ft, while others are higher
around 18 to 20 ft. Decided to stay around 18 ft for most of the
waters with nearshore seas slightly lower. Cannot dismiss more
elevated seas around 20 to 22 ft, however more guidance is
trending otherwise. Will continue to evaluate as the event nears.
With the elevated seas, there is a slight risk for high surf
conditions however they are falling just below thresholds so
decided not to issue a high surf advisory. If seas trend more
towards the unlikely 20 ft mark, high surf conditions are
possible.


By Monday morning, conditions will begin to ease over all of the
waters and more settled weather is expected through the next
week.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST Sunday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory from 8 PM this evening to 1 AM PST Sunday
     for Columbia River Bar.

&&

$$

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