-- Discussion containing changed information from previous version are highlighted. --
626
FXUS66 KPQR 220418 AAA
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
918 PM PDT Fri Sep 21 2018

Updated aviation and marine sections.


.SYNOPSIS...A weakening storm system will spread light rain showers
across the area tonight and Saturday before much warmer and
drier weather return next week.

&&

.UPDATE...Made a quick update to the forecast this evening to
adjust/refine the amount and timing of precipitation. Based on latest
radar imagery and observations, it appears the front is just about to
move onshore into Washington and should reach the north Oregon coast
around 9-10 PM this evening. With the best upper level dynamics
racing out ahead of the surface front, it appears likely that at
least some of the Willamette Valley will miss out on measurable
precip as the front moves through tonight, especially rain shadowed
areas west of I-5. Post-frontal showers will persist into Saturday,
tending to favor the higher terrain. These showers may end up being
more productive in terms of QPF for the inland valleys. The rest of
the forecast remains unchanged.  Weagle

.SHORT TERM...Today through Monday...Visible satellite imagery this
afternoon reveals a surface low pressure spinning off the Pacific
Northwest coast. A band of deeper clouds with embedded rain showers
are apparent via infrared satellite imagery and KLGX radar imagery
along a southwest to northeast oriented cold front. Models are in
good agreement this band of rain showers will drop southeastward and
gradually weaken as it moves into over the south Washington and north
Oregon coast between 4-7pm, the northern Willamette Valley towards
8-11pm and the southern Willamette Valley after midnight. This
initial band of precipitation should generally produce a tenth of an
inch or less of rain across even the wettest west facing slopes of
the Coast Range and Cascades with perhaps just a few sprinkles to a
couple hundredths of an inch in the Willamette Valley.

Low level instability behind the front will gradually deepen enough
to allow for additional showers to spread onto the coast and into the
higher terrain, which should produce a patchwork of heavier rain
totals overnight. As solar heating helps to destabilize interior
locations Saturday midday, expect shower coverage and intensity to
pickup Saturday afternoon and evening, particularly for areas north
of Salem. Storm total rainfall amounts look rather meager overall,
but a few of normally wetter locations in the Coast Range and
Cascades, particularly farther north, could see upwards of a
0.50-0.75" of rain. Expect shower coverage and intensity to decrease
Saturday night as the loss of solar heating helps to stabilize the
lower atmosphere. However, showers shouldn`t dissipate entirely as a
secondary shortwave trough drops southeastward across the Pacific
Northwest.

By Sunday, drier north to northwesterly flow will overspread the
region and result in decreasing clouds, but a rather pleasant
fall-like day. Shortwave ridging will begin to build eastward over
the Pacific Northwest Sunday night and Monday, which should result in
interior locations warming into the 70s. Breezy northerly winds may
develop along the coast and in the Willamette Valley as thermal low
pressure begins to strengthen over southwestern Oregon. /Neuman


.LONG TERM...Monday night through Friday...The long term part of
the forecast can be characterized as a brief return to summer or
first real taste of Indian Summer. The operational ECMWF, GFS and
CMC in good agreement through the period. In addition, the WPC
forecast is close to the GFS and its ensembles. A 590 dm 500 mb high
pressure center will be centered out near 45N 130-135W Tue and Wed.
North to Northeast 700 mb flow and a Northeast to East 850 mb wind
regime will result in a dry air mass over the area. Model forecast
850 mb temps Tue and Wed are 15 to 18C. These values would typically
produce mid to upper 80s for the interior valleys. However, with
shorter days and stronger night and morning inversions, full mixing
will likely not be realized. In any event, inland valleys should
still reach the lower 80s. The offshore low-level flow will aid in
warming, especially along the coast. A thermally-induced surface
trough is expected to reside near the coast Monday night then drift
to the interior valleys Tue. The offshore gradient Tue and Wed does
not appear to be all that impressive, but would still expect locally
breezy to windy conditions.

By Thu the CMC and GFS show the forecast area within a col area. The
500 mb high center closes off out near 140W with a closed low
directly to the south. Thus, would expect above-normal daytime
temperatures to continue Thu, but with weaker offshore low-level
flow. 12Z operational models then start to retrograde the strong
north 500 mb flow forecast to be present over the Northern Rockies.
This would bring cooler air through Eastern Washington and Northeast
Oregon. Some of this would slip through the Central Columbia River
Gorge and the Upper Hood River Valley. Bottom line: No precipitation
expected through the period and above-normal daytime temperatures.
Weishaar


&&


.AVIATION...A weak cold front will move through the region
tonight. Associated light rain and MVFR cigs have already
developed along the coast and will persist through most of the
night. There may be some local IFR as well. Breezy pre-frontal
southerly winds will persist through much of the night. Precipitation
will spread into the interior later this evening and overnight.
Mid- level clouds will increase for the interior TAF sites over
the next few hours, and then expect some MVFR cigs to develop
between 09Z-12Z Sat. Do not expect the interior MVFR to be
solid, more likely a mix of VFR and MVFR conditions. Conditions
should begin improving by mid to late morning in post-frontal
showery air mass. There may still be some occasional MVFR cigs
around tomorrow afternoon under heavier showers. Pyle

KPDX AND APPROACHES...VFR conditions through the evening. An
approaching cold front is expected to bring a mix of VFR and MVFR
cigs in overnight through mid-morning Sat, with occasional light
rain as well. Expect showers to increase Sat afternoon.
Conditions to be mainly VFR Sat afternoon and evening, but some
occasional MVFR possible with showers. Pyle

&&

.MARINE...A well developed low pressure continues to push
towards Vancouver Island. This low will keep southerly winds
gusting to around 25 kt over our northern waters through tonight.
Winds will be less over the central Oregon waters, but expect
occasional gusts over the outer central waters as well. Winds
will subside Sat morning as a cold front moves onshore. Winds
will gradually shift to W then NW through the day Sat. Then look
for northerly winds to return Sunday and continue through at
least the first half of next week. Gusts up to 30 kt will be
possible for the central Oregon waters late Monday through
Tuesday night.

Seas will build to 7 to 9 ft tonight. Expect the largest seas
north of Cascade Head and beyond 20 nm from shore. Seas will
become steep amd choppy tonight across the waters off the north
Oregon and south Washington coast. As a result there is a small
craft advisory for hazardous seas in effect now for the northern
waters. Think seas will remain just below small craft advisory
thresholds for the central Oregon waters. Seas will gradually
ease off during the day Sat, reaching 6 ft by Saturday night and
holding fairly steady through the start of next week. Pyle/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds until 8 AM PDT Saturday for
     Coastal Waters from Cape Shoalwater WA to Cascade Head OR
     out 60 NM.

     Small Craft Advisory for hazardous seas until 10 PM PDT
     Saturday for Coastal Waters from Cape Shoalwater WA to
     Cascade Head OR out 60 NM.

     Small Craft Advisory for winds until 5 AM PDT Saturday for
     Waters from Cascade Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar from 2 AM to
     6 AM PDT Saturday.

     Small Craft Advisory for Rough Columbia River Bar from 2 PM to
     6 PM PDT Saturday.

&&

$$

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland

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 CWA or forecast area.