Tornado Warning 1, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
153 PM PDT Sun Sep 8 2019

.SYNOPSIS...Autumn-like weather will continue through Tuesday as an
upper low supports cool and showery weather. High pressure will
return more seasonal weather Wednesday and Thursday with temperatures
rebounding into the mid to upper 70s. A couple of fronts are expected
to bring more rain and slight cooling Friday into next weekend.

&&

.SHORT TERM...Tonight through Wednesday...An upper level low will
result in cool and showery weather through Tuesday. The showers will
be ramping up this afternoon as a shortwave trough moves over the
region. Models are consistent with forecasting enough instability to
generate thunderstorms in the interior this afternoon and evening,
and have added a slight chance for thunderstorms to the Willamette
Valley and Cascade Foothills. Am not super confident of the inland
thunderstorm threat, but the HREF 4-hour chance of thunder is
forecasting 30-40% across NW Oregon, and is a tool that has
forecasted local thunderstorms well in the recent past. With the cool
upper low lingering over the area through Tuesday have continued the
slight chance for thunderstorms along the coast through the night and
into Tuesday. Another shortwave trough is expected to swing by
Tuesday afternoon so have also added a slight chance for
thunderstorms inland Tuesday afternoon and evening. Expect more
breaks in the showers overnight and in the morning, with an increase
in the afternoons.

The upper trough will move east Tuesday evening, and showers will
rapidly end Tuesday night as an upper ridge builds. With the
lingering moisture, increased subsidence, and potential clearing
overnight, expect fog in the interior valleys and along the coast
Wednesday morning. The fog should clear late in the morning, with
gradual clearing in the early afternoon, and the interior
temperatures should warm into the mid 70s late Wednesday afternoon.
~TJ

.LONG TERM...Wednesday night through Sunday...On Wednesday night, a
thermal ridge builds in behind the trough that once sat over
northwest Oregon/southwest Washington. With this ridge building,
conditions will dry out and skies will partially clear through
Thursday. By Friday, a front approaches the coastal areas but there
is slight disagreement regarding the time of arrival. In general
though, the front will arrive on Friday morning bringing stratus
precipitation to the coast line. The marine push will maintain a
constant feed of moisture which will allow the rainfall to move
inland and persist through Sunday. Similar to earlier runs, there is
model disagreement regarding the overall synoptic pattern. The ECMWF
and Canadian are forming a deepening trough in the jet stream on
Sunday, while the GFS is more zonal. Because of the lack of
continuity, both with the synoptic features and with precipitation,
the quantity of rainfall as well as its stopping point cannot be
determined with high confidence. -Muessle
&&

.AVIATION...A series of weak fronts will move across the region
today. This will cause a mixture of VFR, MVFR and IFR conditions.
Coastal locations are a challenge at this time. KAST currently is
fluctuating between VFR and MVFR conditions, while KONP has solid
IFR conditions. Rain is expected along the coast through Monday
morning along with a slight chance for thunderstorms. Expect
flight conditions to briefly deteriorate with precipitation.

Inland locations are VFR, but expect periods of MVFR or lower
conditions under heavier showers. There is also a slight chance
for thunderstorms inland later today and through the early
evening hours.

Conditions should lower and become mainly MVFR with a mixture of
IFR along the coast this evening as things start to settle
behind the front. There could also be some LIFR conditions around
KONP into Monday morning. Inland areas are expected to lower to
MVFR this evening. Overall it will be very challenging to
forecast when certain flight conditions will happen.

KPDX AND APPROACHES...Predominately VFR. However,expect periods
of MVFR or lower conditions possible under heavier showers. There
is a slight chance for thunderstorms through this evening. MVFR
conditions expected to remain through Monday morning. /42

&&

.MARINE...A broad upper level trough will continue to linger over
the NE Pacific through Tuesday and send a series of weak fronts
across the waters. Winds will generally remain 20 kt or less
through the early part of this week, with periods of south to
southwesterly winds. Guidance continues to show a mild coastal
jet developing along the central coast through this evening. This
could bring the occasional gusts to 25 kt to nearshore waters
out to around 20 nm. Broad high pressure then rebuilds across the
waters towards the middle of the week, but winds should remain
15 kt or less.

There is a slight chance of thunderstorms for all of our waters
through Monday.

Expect seas to gradually increase to around 6 ft through the
first part of the week as a building westerly swell moves across
the waters. Seas look to then briefly subside to around 4 to 5
ft in the middle of the week before increasing again around
the end of the week.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.

&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.