Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 080511 AAA

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
911 PM PST Sun Mar 7 2021

Updated aviation discussion.

.SYNOPSIS...Low pressure dropping south offshore will keep a threat
of showers over the region through the early part of the week. Upper
level high pressure is expected to bring drier weather for Wednesday
through early Friday, and possibly extend into the weekend.


.SHORT TERM...Tonight through Wednesday...Radar and surface
observations continued to show scattered showers across the area
early Sunday afternoon. Water vapor pictures showed a shortwave that
grazed the northwest part of the forecast area this morning, lifting
north through western Washington. The primary upper low was still
centered out near 49N 138W, slowly moving south. As we lose daytime
heating, models continue to indicate air mass rapidly stabilizing
this evening across the region, with the focus of shower activity
shifting back offshore closer to the main upper trough. This will
lead to showers ending this evening, with the resulting low level
stable atmosphere conducive to some patchy fog development late
tonight into Monday morning.

An increasingly southerly upper level flow as the offshore low
pressure system continues to drift south will ultimately lead to a
return of deeper moisture Monday and Tuesday. A few weak shortwaves
will likely lift north through the forecast area in that time period
leading to chances for showers, but with the best energy associated
with the upper low directed to the south, the best chances for
showers will tend to remain across the south part of the forecast
area. Models soundings continue to hint at some minimal CAPE values
both days, but this appears to marginal at best, and so will
continue to omit any chances for thunder.

Tuesday night into Wednesday models show the closed low
consolidating and headed south to off the north California coast.
The loss of the southerly influx of moisture will tend to diminish
pops, with the primary threat of showers by Wednesday shifting to
the Cascades as a result of some wrap around moisture and
instability on the north side of the low.

.LONG TERM...Wednesday night through Sunday...GEFS 500 mb clusters
supported by EC operational and ensemble mean clearly signaling an
upper ridge over the Pacific Northwest from Wednesday night through
Friday, for a period of dry and seasonably mild weather. A majority
of these members suggest the dry weather continuing into Saturday,
but enough of the cluster members showing a breakdown of the ridge
to need to include some low chances for rain by Saturday. By Sunday
model solutions diverge considerably, with enough members showing
some form of a trough to modestly increase chances for rain for the
end of the weekend.


.AVIATION...Shower all but gone at 05Z with radar showing isolated
showers over the Cascades and coast range. VFR conditions prevailing
with mid and high level clouds.  Tonight would be a good night for
fog development with a cool air mass, recent rains and stabilizing
air mass.  But with mid and high clouds overhead radiational cooling
will be limited. However there are gaps in the cloud cover currently
over the north OR coast and coast range into sw WA.  Satellite
trends and HREF model cloud cover show this persisting through the
night, so may see some IFR fog develop in these areas. But for the
remainder of the area the cloud cover should remain thick enough to
limit fog considerably.

For detailed regional Pac NW aviation weather information, go
online to:

KPDX AND APPROACHES...Expect VFR overnight into Monday. As long as
the mid and high level clouds remain as forecast, fog threat will be
low. But if clouds become thin enough may see some fog or low


.MARINE...Tonight into Monday marks the last period of active
weather over the waters for much of the next week. A weak low
pressure and associated cold front are moving inland this
afternoon bringing gusty winds, showers, small hail and increased
wind waves. Winds are generally 15 to 20 kt with gusts to 25 kt,
but with showers, observing gusts to 30 kt in some locations. The
general background swell is around 10 to 13 ft but will rise to
16 to 18 ft at 16 seconds through the overnight hours. Most
model guidance is showing 18 ft really being the maximum combined
wave heights but there is a chance that the top 10% could reach
20 ft or so. These seas will ease as fast as they rise and should
reach 10 to 12 ft at 14 seconds by Monday afternoon.

Into next week, more settled weather expected with little
influencing systems approaching. Seas will remain 9 to 11 ft at
13 seconds Tuesday through Wednesday then ease even further on
Wednesday.   -Muessle


PZ...Small Craft Advisory until 4 AM PST Tuesday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 4 PM PST Monday for Columbia River



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