Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 221722
Area Forecast Discussion
National Weather Service Portland OR
920 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 through Thursday. Snow may
mix down to sea level from time to time. A surface low will drop down
along the coast Fri, bringing a better chance for more organized
precipitation. Fri morning will bring the best chance for any
accumulation snow to the lowest elevations. 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...Scattering clouds allowed for
areas of dense fog to form over the interior lowlands last night.
The fog is slow to clear this morning, and visibilities remain 1/4
mile or less from north of KSLE up through the I-5 corridor of SW
Washington. A dense fog advisory is in effect through noon. The fog
should gradually dissipate this afternoon.
Meanwhile, a cold upper level trough is dropping south from the Gulf
of Alaska today. This feature will bring increasing showers to SW
Washington and NW Oregon as the day progresses. Radar shows showers
increasing over the coastal waters and along the north Coast over the
past few hrs, aided by instability driven by the relatively warmer
ocean temperatures. The showers should begin to spread into the
interior this afternoon as a shortwave trough axis approaches. With
the warmer ocean temps, expect the showers will be most vigorous
along the coast today. Also have a slight chance of thunder at the
coast through this afternoon, with a mention of small hail as well.
With 850 mb temps around -6 C, snow flakes may mix in all the way
down to the lowest elevations during heavier showers. Think that
showers will not be heavy enough to bring any accumulations down to
the lowest levels, but could see a localized dusting to a half inch
of snow at elevations around 1000 ft and higher.
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. Pyle/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...Areas of fog or freezing fog persist in the inland
valleys this morning. The fog will slowly lift into a SCT-BKN
IFR/LIFR deck between 1830-2000z, then clear out by early
afternoon. Otherwise, VFR conditions prevail outside of
showers...which will increase throughout this afternoon. Showers
will be most frequent near the coast. Some of the heavier showers
could briefly put down a stray lightning strike or two, as well
as some brief small hail or graupel.
KPDX AND APPROACHES...Dense fog reducing vsby to <1/4 SM at times
through 18z, then slowly lifting to an IFR/LIFR stratus deck by
20z. VFR conditions likely for the afternoon, though a few
showers may briefly reduce cigs/vsbys in rain, hail, and/or
.MARINE...Quiet marine weather through the period with 6 to 8
foot seas for the next 7 days and winds likely to remain below
small craft advisory criteria. There will be a chance for small
craft advisory-level 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
OR...Dense Fog Advisory until noon PST today for Greater Portland
WA...Dense Fog Advisory until noon PST today for Greater Vancouver
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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.