Area Forecast Discussion
Issued by NWS Portland, OR

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034
FXUS66 KPQR 221043
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
243 AM PST Wed Feb 22 2017

.SYNOPSIS...A cold upper trough off the Pacific Northwest coast will
be spreading onshore today with snow levels lowering to around 1000
feet and some off and on shower activity into Friday night or early
Saturday. Snow may mix down to sea level from time to time. A brief
dry period is possible much of Saturday before more showers are
possible late in the weekend and early next week.
&&

.SHORT TERM...Today through Friday...Satellite imagery shows a cold
upper trough off the coast is already spreading onshore early this
morning, and will continue to be over the forecast area for the next
few days. The main short wave associated with this upper trough is
heading toward California. However, there is another short wave over
Douglas County that could spread some showers into Lane County at
times this morning before weakening and moving south this afternoon.
We are also seeing areas of fog developing due to the lull in showers
overnight, and will need to monitor the need for a dense fog advisory
as the morning progresses.

There is also a short wave off the Washington coast that is forecast
to drop south along the coast today and then move south tonight.
There will be a weak surface feature along the coast with this upper
level feature and produce some cyclonic onshore flow that will
increase the showers over the area during the second half of today
and early this evening. The models still show snow levels lowering to
1000 to 1500 feet today but QPF will be somewhat limited. There is
the possibility of an inch or two of snow accumulations today, but
mainly at elevations above 1000 to 1500 feet. Showers decrease
tonight as this feature moves south.

The main trough axis tries to come onshore Thursday. The lull in the
showers overnight tonight extends into early Thursday, but then we
should see a diurnal increase in showers as Thursday progresses. Snow
levels could drop to 500 feet or less early Thursday before rising up
to around 1000 feet in the afternoon.

On Friday the models show a short wave dropping south along the B.C.
coast and developing into a low along the Washington and north Oregon
coasts. The low stays off the coast and moves south of the area by
Saturday, but should result in some showers spreading onshore into
southwest Washington and northwest Oregon. Snow levels could be 500
feet or lower early Friday, rising to around 1000 feet again in the
afternoon. There could again be some local 1 to 2 inch accumulations,
mainly above 1000 to 1500 feet. Tolleson

.LONG TERM...No Changes. Previous discussion follows...
Friday night through Tuesday...Models and ensembles show good
agreement that below normal temperatures will persist through the
extended forecast period. The mean longwave upper trough axis moves
little through Monday, allowing additional reinforcing shots of cold
air to push down from Alaska. The associated shortwaves will keep our
weather unsettled through early next week, but none of these
disturbances look particularly strong. The 12z ECMWF holds 850 mb
temps generally in the -4 to -6 deg C range, which in onshore flow
would support some snow showers in the hills above around 1000 feet
in elevation. With snow levels this low, cannot rule out a few wet
snowflakes making it down to the valley floors in heavier showers.
However, accumulating snow appears unlikely this weekend or early
next week for elevations much below 1000 feet.

The longwave pattern appears to open up a bit toward Tuesday, with
most guidance ejecting our upper trough eastward into the Rockies.
However there are signs of yet another cool upper trough digging down
from the Gulf of Alaska mid- to late next week, which is likely the
primary reason CPC is showing high probabilities of below normal
temperatures across the Pac NW for the first week of March.  Weagle
&&

.AVIATION...Dense fog has started to develop across the area this
morning. With the high clouds overhead, it is hard to tell how
extensive the fog is, but based on current airport observations,
local observations, and tower cams, it appears fog is forming first
near area rivers and expanding from there. Currently have the dense
fog as a tempo group with 2 to 4 mile fog as the prevailing from
group, but may need to make dense fog the prevailing conditions
within the next hour or two if fog continues to expand.

KPDX AND APPROACHES... Dense fog will improve by mid morning with VFR
conditions expected afterwards. A few showers could impact the
terminal by the afternoon. /Bentley
&&

.MARINE...Quiet marine weather through the period with 6 to 8 foot
seas for the next 7 days and winds below small craft advisory
strength through at least Friday. There will be a chance for small
craft winds on Friday night/Saturday as a surface low tracks south
along the WA coast, but there is still some uncertainty how closely
this surface low will track to the OR coast which will impact the
winds on the waters. /Bentley
&&

.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. The area is
commonly referred to as the forecast area.



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