Area Forecast Discussion
Issued by NWS Portland, OR

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066
FXUS66 KPQR 201024
AFDPQR

Area Forecast Discussion
National Service Portland OR
318 AM PDT Tue Aug 20 2019

.SYNOPSIS...A fall like storm system will likely bring widespread
rain on Wednesday before a return to more pleasant late summer
weather ensues late in the work week and over the weekend.


&&

.SHORT TERM...Today through Friday...GOES-17 low level water vapor
imagery shows a clearly defined low pressure spinning between 130W
and 140W off the Pacific Northwest coast. Models are in moderately
good agreement this low pressure will weaken as it moves towards the
northern tip of Vancouver Island over the next 24 hours while a
separate surface low pressure will simultaneously develop further to
the northeast and move towards Haida Gwaii later today and tonight.
The end result is that a trailing front will push eastward into the
region late tonight into Wednesday. This front will bring a good shot
of rain to the region.

Moisture will be plentiful with this front given models and their
ensembles are coming into agreement precipitable water values will
peak around 1.5-1.7". These values have a return interval of
occurring once every 10-30 years for the 3-week period centered on
August 21st. The bigger question remains whether or not there will be
enough forcing to generate significant precipitation. The GEFS and
ECS ensemble means suggest there will be a bit more than modeled 24
hours ago. As a result, anywhere between 0.25-0.50" of rain appears
most likely to fall in the Willamette Valley. With that said, both
the GEFS and ECS ensembles  suggest the range of possibilities are
quite a bit higher than that and range from just a few hundredths of
an inch to over an inch of rain falling along the coast and in the
Willamette Valley.

Given only 10-25 kts of southwesterly 850mb flow is forecast to
arrive with this event, orographic enhancement should not be huge,
but even then with the high moisture content, we could see rainfall
rates peak in the 1/4-1/3" per hour range for a couple hours in the
Coast Range and Cascades. Nonetheless, there are few, if any, hydro
concerns away from some nuisance ponding of water on area roads. In
fact, this storm system will likely be quite beneficial by
moistening fuels and at least temporarily reducing what limited fire
danger we currently have.

Models remain in agreement that any lingering showers across the area
late Wednesday night into Thursday morning should dissipate quickly.
A low level northwesterly flow pattern under mainly zonal flow aloft
should keep temperatures near average Thursday and Friday.
/Neuman


.LONG TERM...Friday night through Tuesday...Models are coming into
agreement another shortwave trough will slide eastward across
southwest Canada Friday night into Saturday. This appears likely to
drag a weakening front into our northern coastal zones during this
time. As a result, there is a chance for at least some brief light
rain along our north coast and Willapa Hills during this time, but
many model solutions weaken the front so much so before it arrives
that no precipitation is forecast to occur. Continued a blend of
model scenarios to account for the uncertainty.

The GFS and EC ensemble systems suggest another shortwave trough will
drop southeastward across British Columbia late in the weekend. This
should keep temperatures near average through the weekend with a
continuation of mainly dry weather across the region. Thereafter,
models continue to advertise a shortwave ridge will develop over the
eastern Pacific before shifting it eastward over the Pacific
Northwest midweek. It appears increasingly likely this will result
in a several day stretch of hotter weather that would potentially
begin next Tuesday. /Neuman


&&


.AVIATION...VFR conditions prevailed through the night and
continues early this morning. Satellite fog product indicating
fog/low stratus forming in deeper valleys of the coast range -
namely the Nehalem River valley. Fog product also showing possible
development of fog along the coastal strip where the air mass is
nearly saturated. As far as guidance goes, the HREF, NAMnest as
well as NBM show or infer low flight conditions developing along
the coast up to Lincoln City this morning. Later today is more of
a challenge as the flow turns southerly. Most times marine
stratus will fill in at the coast in the MVFR to IFR range. The
NBM is showing mostly VFR conditions later this morning with
MVFR cigs returning tonight with rain developing after 06Z Wed.

For inland areas fog/stratus will likely be limited to the KKLS
area and south of KEUG toward the foothills. Of note, the HREF
shows a ribbon of low cigs on the east side of the Willamette
Valley up to Clark Co. through 18Z. But that is the only model
showing that so will wait and see.

KPDX AND APPROACHES...VFR expect today and tonight. However can
not rule out some 3 to 5 SM fog, or SCT-BKN cloud layer at or
below 1000 ft in the area 12Z-17Z. Think chances are too low to
for it to happen so will not include in 12Z TAF package.
Otherwise expect increasing high clouds late today through
tonight. /mh


&&


.MARINE...Weak high pressure offshore is producing relatively
quiet conditions over the waters. Seas remain in the 4 to 6 ft
range, mainly from a NW swell. But that will change today as an
unseasonable front approaches the waters today and tonight, then
move through the waters Wednesday. Expect small craft winds to
develop this afternoon for the outer waters, then progress
to the inner waters by Wednesday afternoon. The strongest winds
are expected from tonight through Wednesday with gusts of 30 kt
in our outer waters, some isolated gusts higher than 30 kt could
be experienced.

Seas will generally start to build towards 6 to 8 feet with the
potential for some steep square seas too. Wind wave and swell
associated with this system could push seas towards 10 feet
Wednesday and possibly Thursday. The area that would have the
best chance of having 10 foot seas is our northern waters due to
larger swells and potentially stronger wind. /42


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 11 AM PDT
     Wednesday for Waters from Cape Shoalwater WA to Florence
     OR from 10 to 60 NM.

     Small Craft Advisory from 8 PM this evening to 5 PM PDT
     Wednesday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 10 NM.

&&


$$

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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.



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