Area Forecast Discussion
Issued by NWS Portland, OR

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177
FXUS66 KPQR 300336
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
834 PM PDT Wed Mar 29 2017

.SYNOPSIS...Showers will continue tonight then decrease later
Thursday as an upper level trough of low pressure moves east of the
region. Fri is expected to be dry as high pres moves across the
region, then chances for rain return for the weekend as
the next system moves in.

&&

.SHORT TERM...Tonight through Saturday...Showers on radar do not look
very impressive but there were some decent cells in ne Clark county
and western Skamania Co. around 5 pm.  Rainfall amounts over the last
6 hours in the 2 to 3 tenths of an inch in the mountains and less
then 2 tenths on the valleys.  There were some spots in the Lane Co.
Cascades with 5 to 7 tenths of an inch.  Satellite shows cold topped
clouds offshore that are tracking to the coast and is expected to
increase shower activity some around midnight. Once the feature
swings through the area will be on the back side of the upper trough
with flow turning more northwesterly. Showers will be deceasing
during the day but orographic lift will keep showers activity going
in the mountains into Thursday evening.


The trailing upper level trough is expected to move across Thu
morning, with moisture generally limited to low levels below about
8k feet. Once the upper trough passes, shower activity is expected
to become more reliant on orographic lift and thus will tend to
become more tied to the upwind side of the mountains into Thursday
evening. Thu night into Fri a ridge of high pres at the surface and
aloft pushes east into western Oregon and Washington. Subsidence will
stabilize the atmosphere, bringing an end to the showers. Low levels
will be slow to dry out, though, as offshore flow never really gets
going. Some fog Friday morning seems inevitable given all of the
moisture around, but will depend on low level flow either being very
light, which it should be, or being onshore, which is questionable.
It looks like there may be some offshore drift that prevents fog
along at least the central coast and possibly elsewhere.

The next system approaches Saturday and looks like it will produce
some light rain over the area, mainly in the northwestern portions of
the local forecast area with surface low pressure off Vancouver
Island. However moisture looks fairly limited, so not expecting any
impressive or terribly significant rain. Could even see some sun,
particularly during the morning hours. Bowen /mh

.LONG TERM...Saturday night through Wednesday...Models are in fairly
good agreement through about Monday with a weak upper trough moving
through the area for a continued low chance of showers. After Monday,
though, models diverge, with a wetter system moving into the area
sometime between Tuesday and Thursday. Overall, though, the extended
doesn`t look as wet as the trend has been. Bowen

&&

.AVIATION...Much improved flight conditions this evening compared
to earlier in the day. Mainly VFR in the interior valleys at 03Z,
especially from the est slopes of the Oregon Coast Range to the
center of the Willamette Valley. Likely more MVFR in the Cascade
Foothills due to orographic up-slope flow. Areas from the Oregon
Coast range to the beaches are mainly IFR. Expect primarily VFR
to prevail over the inland lowlands through about 10Z then shift
to more MVFR as the upper trough moves across the area. Coastal
sites likely to remain primarily MVFR, with areas of IFR through
10Z or so. Cascades and highest elevations of the Oregon Coast
Range and SW Washington Willapa Hills to remain obscured through
at least early Thu afternoon. All TAF sites expected to be VFR by
19Z Thu.


PDX AND APPROACHES...VFR conditions at the terminal and vicinity
as of 03Z. This to continue through at least 09Z. Upper trough
moves over the area around 10Z, which may enhance the chance of
MVFR conditions. VFR prevails by mid-morning and continues
through Thu evening. Weishaar

&&

.MARINE...Minimal changes to current forecast. Frontal passage
occurred shortly after 00Z Thu, resulting in wind shift to the
NW. Wind speeds generally 15 kts or less, but getting a few gusts
into the low 20s this evening. NW wind continues over the waters
through Thu night. The 18Z GFS was a little stronger with
boundary layer wind speeds compared to the NAM. Leaned toward the
NAM and NAMNest, which keeps gusts Thu below small craft advisory
thresholds. However, would not be surprised if 20 to 25 kt gusts
developed, especially Thu morning.

High pres settles over the waters Thu night and Fri, reducing
wind speeds to 10 kt or less. Small craft advisory wind likely to
return to the north outer waters Fri night.

Extended the small craft advisory for hazardous seas through 18Z
Thu. Seas subside to the 5 to 6 ft range by Fri afternoon. Next
threat of 10 ft seas looks to occur Sat night. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for hazardous seas until 11 AM PDT
     Thursday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 9 AM
     PDT Thursday.

     Small Craft Advisory for Rough Columbia River Bar from 4 PM to
     8 PM PDT Thursday.

&&

$$

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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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