Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 241734 CCA
Area Forecast Discussion
National Weather Service Portland OR
934 AM PST Fri Feb 24 2017
.SYNOPSIS...An upper level low will produce rain and snow showers
across the region today with snow levels between 500 and 1000 ft.
Snow will mix down to sea level at times through the morning. Showers
will decrease this evening, as high pressure moves over the area.
Saturday will be chilly but dry. Then another low will approach from
the north Saturday evening for another round of rain and snow showers
Saturday night and Sunday. Moist onshore flow will maintain showers
Sunday night through at least Wednesday.
.MORNING UPDATE...The low pres system dropping south out of Canada
is now centered about 200 miles west of the central WA coast. As
expected, the low and associated diffluence aloft are driving
enhanced shower activity over NW Oregon and SW Washington this
morning. The heaviest showers have been over the coast, Coast Range,
and Willapa Hills. To this point, the accumulating snow level appears
to be at elevations around 500 ft and above. Spotter reports and
webcams have confirmed that anywhere from 1 to 3 inches have fallen
in portions of the north and central OR Coast Range. Decided to
expand the Winter Weather Advisory to include the central Coast Range
zone this morning, with the advisory out through this evening. Have
also extended the advisory through this afternoon for the north OR
Coast Range and Willapa Hills. Webcams are also showing the snow
level briefly dropping to sea level along the central in heavier
showers, with spots appearing to pick up a dusting or maybe a little
more in some spots. However, this lower elevation snow is melting
quickly after it falls. The Special Weather Statement adequately
covers this. East of the Coast Range, surface temps are just a bit
too warm for any sticking snow today. There have been a few reports
of a dusting at higher elevations of the interior lowlands, generally
500-1000 ft, but otherwise most locations are seeing just a mix of
rain and snow, or just all rain, with no notable impacts.
As we get into this afternoon, showers will continue as the surface
low drifts south over the OR offshore waters. However, surface temps
will be warming, lifting the snow level to mainly above 1000 ft.
Flakes may still mix in at times at lower elevations, but do not
expect any additional impacts. Pyle
.PREVIOUS SHORT TERM DISCUSSION ISSUED AT 241 AM FRIDAY...
...Today through Monday...Radar imagery shows showers increasing
offshore and along the coast early this morning as an upper level
trough off of the B.C. coast moves south offshore of Washington. Web
cameras so far has shown mostly rain, although the ASOS stations at
the coast occasionally report unknown precipitation likely due to
graupel or sleet. The Mt Hebo Web camera near 3100 feet in the
Oregon coast range has been showing occasional light snow with the
Surface temperatures near the coast are just above freezing as are
the wet bulb temperatures. The wet bulb temperature at Astoria has
just dropped to freezing with the passing of a heavier shower and the
surface observation is reporting light snow. This suggests that there
is still a possibility for snow for the north coast and coast range
this morning. Temperatures are slightly warmer for the Central Oregon
coast range and coast, and although snow showers are possible there,
think it will take more for the temperatures to drop to freezing, and
any accumulations will be short lived and minimal.
The forecast confidence for snow accumulation in the Willamette
Valley is still on the low side. Not because the temperatures will
not support snow, but because the moisture may not make it much past
the coast range while the temperatures are still near or below
freezing. The best chance for snow accumulations for the Portland
metro area and the lower Columbia River is for the higher hills near
the coast range. Have decided not to make any changes to the winter
weather advisory for now, and will watch to see how the showers
behave the next few hours. .
Temperatures should rise during the late morning and afternoon hours
on Friday into the upper 30s and 40s for lowland locations so any
impacts from snow accumulations below 1000` should be relatively
short lived. Nonetheless, snow will likely continue to mix down to
near sea level through the day, but additional accumulations will
likely be hard to come by and very short lived...similar to the past
couple of days.
The showers will decrease from north to south this evening as a
shortwave ridge replaces the low, with dry weather tonight. Lingering
surface moisture combined with light east winds and clearing skies
will likely lead to radiation cooling and shallow fog late tonight
into Saturday morning. Expect widespread near to slightly below
freezing temperatures early Saturday morning. Saturday will be dry
but chilly, with daytime high temperatures peaking 5 to 7 degrees
below the seasonal normals (low 50s).
The next low will slide south from the B.C. coast Saturday evening
bringing a return of rain and snow to the region Saturday night and
Sunday. This low will move closer to shore than the other previous
lows, and may generate higher QPF. Snow levels will again be down to
around 1000 feet with snow mixing in with rain to around 500 feet or
possibly lower. The coast range and Cascade foothills will likely see
additional 2 to 4 inches of new snow Saturday night through Sunday
afternoon. The low will move inland across Oregon and Washington
Sunday night and Monday maintaining a threat for showers. ~TJ
.LONG TERM...Monday night through Thursday night...A series of weak
disturbances embedded within an upper trough near the gulf of alaska
will move over the Pacific NW early next week with moist onshore flow
continuing showers for SW Washington and NW Oregon through Wednesday
night. SNow levels will remain relatively low (1500 to 2500 feet). An
upper ridge over the NE Pacific and California will begin to
intensify Thursday and Friday for drier conditions for Oregon.
However, showers will likely continue across SW Washington through
Friday as the upper low to the north challenges the building ridge.
.AVIATION...Mixed rain/snow showers are becoming more widespread
this morning, as a weak low pressure system drops down the Pac NW
coast. Heavier showers may bring a quick coating of snow to
grassy surfaces, but significant runway accumulations are not
likely. Coastal terminals appear most likely to see accumulating
snow, and in the unlikely event runway snow accumulations do
occur, it will be very brief and slushy. VFR conditions with
MVFR cigs/vsbys in showers should persist through this afternoon
before tapering off this evening. Heavier showers bring the
possibility of small hail, graupel, or wet snow mixing in with
the rain showers. As skies clear this evening, fog/freezing fog
will become possible in the valleys.
KPDX AND APPROACHES...Generally VFR at KPDX this morning, though
some areas of MVFR persist around the PDX metro area in -RA.
Expect cigs to lift a bit as precip becomes more showery this
afternoon. Showers will bring an occasional mix of rain, small
hail, graupel, and wet snow through the day...but temps in the
mid 30s to lower 40s will likely keep runways too warm for any
snow accumulation. Some clearing is likely this evening, likely
allowing temps to fall below freezing tonight. Some fog is
possible late tonight...after 08z. Weagle
.MARINE...Weak low pressure just off the Oregon coast will
continue southward today and tonight, then into California later
Sat/Sat night. Marine impacts will be minimal other than some
squally showers. Next low pressure system from the Gulf of Alaska
will arrive Saturday night/Sunday as a surface low tracks south
down the Pacific NW shoreline. There will undoubtedly be small
craft winds west of this surface low, but there is a bit more
uncertainty regarding the strength of winds south of this surface
low and east of the surface low. A small craft advisory may be
needed, especially if the surface low tracks a bit further east.
Beyond Sunday, a lack of organized surface low pressure centers
will limit chances for small craft strength winds until later in
the week when a front approaches the area. Seas will follow this
same pattern, approaching near 10 feet on Sunday/Sunday night and
again later in the week ahead of the next front. Weagle/Bentley
OR...Winter Weather Advisory until 4 PM PST this afternoon for Coast
Range of Northwest Oregon.
Winter Weather Advisory until 10 PM PST this evening for
Central Coast Range of Western Oregon.
WA...Winter Weather Advisory until 4 PM PST this afternoon for
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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.