Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 231009
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
310 AM PDT Sun Apr 23 2017

.SYNOPSIS...A weak upper level trough will move across the
Pacific NW today. A strong Pacific Jest stream will bring a couple
of unseasonably wet fronts to the area Monday and Tuesday with the
potential for some gusty winds, and accumulating snow in the
Cascades. Moist onshore flow will continue the threat of showers
Wednesday and Thursday.
&&

.SHORT TERM...Today through Tuesday...water vapor pictures early
this morning showed a broad trough from the coast west to 135W. This
trough will swing inland today, bringing an increase in chances for
showers to the region today.

Much deeper moisture moves in tonight, as the nose of a strong zonal
jet pokes into southwest OR. A surface wave moving east along a
baroclinic zone reaches the Oregon coast as a weak surface low on
the order of 1004 mb. Warm air advection ahead of the low results in
some mid-level moist isentropic lift as seen along model 300K
isentrope this evening, eventaully spreading down to low levels as
noted along the 290K isentrope late tonight into Mon morning. While
not exceptional, GFS precipitable water values do reach up to 0.8
inches along the coast by Mon morning resulting in a rather wet
period from Sun night into Mon morning. Cooler unstable air spreads
in behind the system Mon afternoon, changing the precipitation from
a stratiform rain to showers under the upper trough. Snow levels
expected to drop to near Cascade pass levels in the cooler air. A
west to nw flow developing behind the low Mon afternoon will add
some orographic enhancement to precipitation esp over the Oregon
Cascades. With the front and then orographics, expect to see a good
shot of snow for the Cascades, likely falling in the range covered
by an advisory Mon. Heaviest amounts appear likely to fall above
pass levels.

GFS and EC agree of some ridging following the upper trough Mon
night into early Tue for a temporary decrease in chances for
showers. On Tue mid level moisture spreading in during the morning
is accompanied by isentropic lift seen along the 300K isentrope,
which results in an increasing chance for rain Tue morning. By Tue
afternoon best moisture and isentropic lift shift down to lower
levels represented by the 290K isentrope, resulting categorical pops
again across most of the forecast area.

.LONG TERM...Tuesday night through Saturday...No changes. Previous
discussion follows. An occluded front is forecast to move through
southwest Washington and northwest Oregon Tuesday night for more
rain and higher elevation snow. Westerly flow will keep showers
going across the area Wednesday into Friday. Snow levels will tend
to hover around the Cascade passes. The models vary a bit with the
upcoming weekend have some low pops at time in the early part of the
weekend due to the uncertainty. Tolleson &&

.AVIATION...A short break in the rain overnight and this morning.
But the next batch of rain is already near coastal areas
bringing a mix of VFR and MVFR conditions. IFR is occurring at
KHQM so it may be possible at the coast later this morning. None
of the statistical guidance is showing IFR conditions so plan to
leave it out of the 12Z TAF package. Expect MVFR should predominate
today and tonight. However KAST may see VFR return as a surface
low pres approaches the central coast late tonight. As a result
of this low, KONP will become windy late tonight into early Mon.
Inland areas remain VFR through most of today and tonight.
However soundings and MOS guidance show CIGS near 3000 feet
between 18Z and 23Z. Will hold off on adding the lower
conditions to the forecast for now. More rain arrives this
evening into Monday.

KPDX AND APPROACHES...Predominately VFR through Sun night. Rain
returns after 14Z, with occasional periods of MVFR cigs possible
18Z to 22Z. /mh

&&

.MARINE...A weak frontal system will move across the waters
today. Winds will probably gust in the 20-25 kt range starting
this afternoon for the central Oregon coastal waters. Stronger
winds later tonight and Monday with a 1002-1005 mb low heading
west to east toward the coast. Current models project landfall by
mid morning Monday between Tillamook and Cascade Head. This is
further south than earlier forecasts.  There is a chance for
gales with this system, if the low is stronger (999 mb), while
the weaker solution would only generate high-end small craft
advisory winds late Sunday night through Monday morning. Beyond
Monday, we stay in an active pattern with systems every 24 to 48
hours through the end of the week. We may get a break going into
next weekend.

Seas will remain around 8 to 9 feet until the system Sunday night
into Monday which will bring seas generally south of Cape
Falcon up to 10 to 12 feet. After that system seas hover around 7
to 10 feet through early next week. Wednesday models suggest a
decent fetch developing with the low pressure system that moves
toward the coast of British Columbia which brings seas up into
the mid teens. If the ECMWF solution pans out, however, this low
will be weaker and move a bit faster which would result in a
smaller fetch and lower seas, so confidence isn`t very high at
this time. Later in the week if we can get a break in these
systems seas may drop down to 5 to 7 feet on Friday. /mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds from 11 PM this evening to 2 PM
     PDT Monday for Coastal Waters from Cape Shoalwater WA to
     Cascade Head OR out 60 nm.

     Small Craft Advisory for winds from 2 PM this afternoon to 2 PM
     PDT Monday for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

     Small Craft Advisory for hazardous seas from 11 PM this evening
     to 5 PM PDT Monday for Coastal Waters from Cascade Head OR
     to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 5 AM
     PDT early this morning.

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

&&
$$

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



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