Area Forecast Discussion
Issued by NWS Portland, OR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KPQR 251743 AAA
AFDPQR

FXUS66 KPQR 251107...UPDATED
AFDPQR

Area Forecast Discussion
National Weather Service
943 AM PST Sun Feb 25 2018

.SYNOPSIS...Rain and mountain snow will spread across the region this
morning as a strong cold front arrives. Showers later this afternoon,
with colder air spreading inland tonight. Lingering showers on
Monday, with snow levels rather low. But not much in way of
accumulations. Break in the weather Mon night, then the cool wet
weather resumes on Tue.
&&

Updated short term, aviation, and marine

.UPDATE...Precipitation with a cold front is moving across SW
Washington and NW Oregon this morning. The 12z KSLE sounding shows
near normal precipitation values (~0.5 inch), and a deep moisture
layer over the region with SW-W winds around 40 kt at 900mb, 850mb,
and 700mb. This will favor orographic lift and support enhanced
precipitation over the terrain. The Willamette Valley is rain
shadowed, and have reduced rain totals. SNow levels are still a
challenge for the foothills, but still think they will remain around
2500 feet for the Oregon Foothills, and slightly lower for the
Washington foothills.

Still on track for colder air to move in behind the front this
afternoon as the precipitation changes to showers. Open-cell Cumulus
on satellite imagery supports this. The colder air is just starting
to reach Quillayute, WA, and will be watching the obs north of the
Portland forecast area as the colder air nears us. Snow levels will
lower from north to south this afternoon through tonight, with
accumulations possible 500-700 feet and above. Precipitation decreases
as the colder air and any snow accumulations at these lower levels
will be light and spotty. Some locations may see close to a half of
an inch where as others a trace or nothing. The snow may mix down
below 500 feet at times, but will not accumulate or result in any
travel impacts as the road temperatures will stay above freezing. ~TJ


.PREVIOUS SHORT TERM from 305 AM...Today through
Tuesday...Precipitation with next front now spreading into western
Washington and far northwest Oregon. This in response to rather
vigorous low that is digging southeastward along the British Columbia
coast. The cold front will push onshore by late morning, then moves
across the region and over the Cascades this afternoon.

Decent shot of precipitation with the front, and to some degree the
post frontal showers. Generally, 0.50 to 0.75 inch for the Coast
Range/Willapa Hills, and 0.70 to 1.25 inch for the Cascades. But,
less for the interior lowlands, with 0.25 to 0.50 inch expected. This
mainly because the interior lowlands will experience another round of
rainshadowing off the coast mtns, such that showers will be less
intense/less in coverage later today into tonight.

Snow levels over the area sitting at 1500 to 2000 feet over the
Cascades, and 2000 to 2500 feet over the coastal areas. So while not
a lot of snow is expected on the coastal mountains, will see
significant snow in the Cascades, and to some degree, the foothills.
Will maintain the current Winter Storm Warning for the Cascades,
where will have gusty west to southwesterly winds today into the
evening, along with total snow accumulations of 15 to 30 inches
during that time. Foothills will be tricky, today into the evening,
as snow levels stay around 1500 to 2000 ft while the heaviest
precipitation is occurring, with the heavier precip over south
Washington at the optimal time. But, think snow levels tad too high
over the Oregon foothills, so will keep snow accumulations just below
advisory levels. By time snow levels lower to 1500 feet, the
precipitation will be waning.

Colder air will shift inland again tonight, with snow levels dropping
back down to 500 to 1000 feet. While precipitation is waning, is not
out of question to see mix of rain/snow showers again late tonight
into Mon am. But, think that areas below 500 to 800 feet will see
little if any accumulations. That said, will have to deal with
potential of icy spots on roads as temperatures drop into the lower
30s tonight, and even 20s closer to the Cascades foothills and Coast
Range/Willapa Hills.

Decreasing showers on Monday, as will see mix of sunshine and clouds.
Generally, best threat of showers will stay over the higher terrain.
Still expecting showers to end early Mon evening over most areas.
Cold and dry under partly cloudy skies Mon night, as region will be
between weather systems.

Next front will spread light rain across the region on Tue am. This
will not be all that strong, with 0.10 to 0.25 inch of rain. Snow
levels stay low at 1000 to 1500 feet. with several inches of new snow
for the higher terrain.
Rockey.

.LONG TERM...No Changes. Previous discussion follows...Tuesday night
through Saturday. A rather active weather pattern looks to continue
through the long term forecast period with a longwave trough
remaining across the western portion of the U.S. through the period.
Outside of the broad pattern, however, specific details remain
uncertain given large spread in the model guidance. While the 12z GFS
depicts a rather deep low pressure system just off the
Oregon/Washington coasts by Wednesday evening, there is considerable
spread in NAEFS members. It should be noted that the ECMWF does show
a somewhat similar low developing but with much less impressive
structure. Therefore, will maintain the forecast along a compromise
solution. Eventually, the large upper level trough becomes more
solidly centered over the far NE Pacific by late Thursday. This would
maintain an unseasonably cool air mass over the region with snow
levels again lowering to around 1000 feet. Will need to continue to
monitor the potential for some snow at least mixing in below 1000
feet early next weekend as the cold air mass looks to remain in place
with at least enough moisture to support showers. At this point,
however, expect that the extended period will feature continued
enhancement of the Cascade snowpack but not any additional low
elevation snowfall. Cullen
&&

.AVIATION...Frontal boundary draped through Western Washington to
near KONP at 1630Z. Widespread MVFR or worse conditions across
the area and this will be the rule into the afternoon. Expect
gradual improvement from north to south after 21Z. TAF sites
should all be VFR by 04Z Mon or so. West 850 mb wind up to 50 kt
will result in mountain obscurement, but conditions slowly
improve this evening.

PDX AND APPROACHES...MVFR will be the predominant category
through mid-afternoon. There may be some rain-shadow effects
beginning 20Z with the increase in southwest to west wind coming
off the coastal mountains. This may be enough to bring low VFR
or more VFR than MVFR conditions. Expect VFR by 03Z Mon, but
brief MVFR possible through 12Z in showers. There is also the
possibility of -SHSN around 12Z, but any snow that occurs will
not impact runways. Weishaar

&&

.MARINE...A cold front was moving through the waters this
morning. Peak wind gusts associated with the front have been
generally around 30 kt. Buoy 089 had a 35 kt gust at 16Z in the
post-frontal air mass. There may be additional isolated 35 kt
gusts, but not frequent enough to warrant a gale warning. Current
small craft advisories look in good shape, with wind speeds in
the inner waters falling below small craft advisory criteria by
06Z Mon, but holding on through 12Z for the outer waters. Wind
speeds continue to ease Mon as high pressure strengthens. Another
frontal system moves through the waters Tue, which could produce
20-25 kt gusts over the north zones.

Forecast confidence improving with regards to Wednesday through
the end of the week. The 12Z GFS, 00Z GEM and 00Z ECMWF are in
better agreement with the Wed night through Fri time period
compared to a few model runs ago. Part of the variance in
forecast models is how they are handling the cut-off low
northwest of Hawaii. The GFS has trended more in line with the
GEM and ECMWF, with a 980-985 mb low near Vancouver Island, B.C.
by Wednesday afternoon. Models somewhat similar in taking the
deep surface low near Vancouver Island 00Z Thu and dumb-belling it
west, then south Thu through Fri. 12Z GFS is much deeper and
tighter with the surface low circulation compared to the ECMWF.
The GFS then rotates the low inland northeast toward Cape
Disappointment by 12Z Sat and maintaining a tight pressure field.
This is much different than the ECMWF, which takes the weakening
low into Northern California.

Seas in the low teens this morning, but will build to the mid
and upper teens late today into Monday as the fetch remains
directed at WA & OR. Latest ENP guidance shows seas falling just
under 10 ft Tue morning. The latest ENP model is much different
than yesterday concerning the Thu-Fri time frame. The 00Z run now
shows a core of 25 ft seas well offshore 00Z Fri and then shifts
this feature due south Thu night and Fri. The 00Z ECMWF wave
model has a core of 30 ft seas way offshore 12Z Fri. In any
event, it looks like these high seas will stay well offshore.
Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 10 PM PST this evening for Cascades
     in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 4 PM PST this afternoon for South
     Washington Cascade Foothills.

     Winter Storm Warning until 7 PM PST this evening for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 10 PM PST this evening for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     10 NM.

     Small Craft Advisory for hazardous seas until 6 AM PST Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 NM.

     Small Craft Advisory for winds until 3 AM PST Monday for Waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 AM
     PST Monday.
&&
$$

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.


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.