Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 082233
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
233 PM PST Mon Mar 8 2021

.SYNOPSIS...Low pressure dropping south offshore will keep a threat
of showers over the region through Wednesday. Upper level high
pressure is expected to bring drier weather for much of Thursday
through at least early Saturday. A trough will drop south from
Alaska to bring precipitation chances back in the forecast at some
point this weekend, then cross the region through Monday.

&&

.SHORT TERM...Tonight through Thursday...A broad upper level trough
remains off the Pacific Northwest coast Monday afternoon. A
vertically stacked low pressure system centered around 500 miles
west of the Washington state coastline will continue to slowly move
southeastward as it gradually weakens, approaching the northern
California coast early Wednesday. Expect this low pressure system
will continue to keep a chance of convective showers across the
region through Tuesday.

A band of showers is developing this afternoon over central-western
Oregon as an upper level shortwave rotating around the low is
invigorating the atmosphere. Expect this wave to continue to lift
northward this afternoon moving into areas diurnally warmed by the
early March sun. CAMs have been indicating this line of convection
could move through the Portland metro area early this evening with
brief downpours, gusty winds and small hail possible. Currently
think any thunderstorm activity will remain limited to near the
central Oregon coast through tonight and sliding northward along the
Oregon coast into Tue morning. Satellite imagery this afternoon also
indicates a small meso low developing off the central coast and a
few models show this low quickly moving northward associated with
the wave lifting north. Could see a few higher wind gusts along the
coast associated with this meso low, likely up to 40 mph.

A second wave will likely produce a line of showers lifting north
early Tuesday, mainly along the coast. By Tuesday afternoon the
upper trough axis/upper low will shift closer to the OR coast as the
main surface low will likely be further south, just west of the
OR/CA coastal border. Colder air aloft Tuesday should allow for
increased instability, especially across the central and south
valley. The upper level trough will push inland Wednesday as surface
low become elongated northward as it approaches the northern CA
coast. Additionally, an upper trough with ties to the Canadian
arctic will be pushing southwest across Alberta. This will
destabilize the region again as it ties in to some of the lingering
moisture rotating north around the still nearby closed low. Given
multiple models have latched onto this, expanded the shower coverage
Wednesday. Again, showers likely dissipate near sunset.

Dry conditions will be likely Thursday as an upper level ridge
approaches the region. Temperatures will remain early spring-like
with morning lows in the mid-30s and afternoon highs around the low
to mid 50s. /DDH


.LONG TERM...Thursday night through Sunday...Ensemble guidance agrees
that weak ridging will build in behind the low making for dry
conditions Thursday night through Saturday morning. Low pressure to
the northwest will swing a cold front onshore Saturday afternoon
bringing rain across the area through the weekend. Heaviest rain will
be overnight Saturday but no impacts are expected. Snow levels will
drop to around 3500 ft Saturday night and continue to drop to around
1500 ft by Sunday night as cold air aloft moves onshore post-
frontal.

&&

.AVIATION...Inland TAF sites expected to remain mostly VFR over the
next 24 hours, although scattered showers may temporarily drop
conditions into MVFR category at any given time through 19Z Tuesday.
Along the coast expect more of a mix of VFR with occasionally MVFR
conditions over the next 24 hours, with KNOP the most likely site to
see temporary MVFR ceilings.

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

KPDX AND APPROACHES...Mostly VFR conditions tonight and Tuesday.
There is a chance of brief MVFR conditions associated with scattered
showers at any given time, although the most likely time would be
between 09Z and 18Z.

&&

.MARINE...A pretty quiet week is in store for the coastal waters.
Southerly winds gusting as high as 20 to 25 kt will continue through
Tuesday as a low pressure center slowly moves south beyond the
coastal waters. As the low moves inland over California and a ridge
of high pressure moves in over the waters, winds through most of the
rest of the week will tend to be limited to 20 kt or less. A cold
front is expected to arrive late Saturday or Sunday marking the next
chance for small craft advisory level winds.

Seas will continue to slowly subside through tonight, and should be
under 10 ft by Tuesday, then possibly dropping under 5 ft late in
the week. Seas are likely to rebuild Saturday behind a cold front,
likely building back to 10 ft or a little more by Saturday night.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 10 AM PST Tuesday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 3 AM PST Tuesday for Columbia River
     Bar.

&&


$$

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