Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 231728
Area Forecast Discussion
National Weather Service Portland OR
928 AM PST Thu Feb 23 2017
.SYNOPSIS...A series of cool storm systems will bring cooler than
average temperatures, off and on showers and occasional snow to the
Coast Range and Cascade foothills through the weekend. The best
chance for snow to dip to sea level and the valley floors will be
Friday morning, but any low-elevation accumulations should be brief
and localized. A brief ridge will result in drier weather Friday
night and Saturday, but offshore winds will maintain cool
temperatures. A couple of cool storm system will return showers with
low snow levels Saturday night through Tuesday.
.SHORT TERM...Today through Sunday...As expected, today is shaping up
to be a near repeat of yesterday. A cold upper level trough over the
Pac NW is currently driving shower activity over the coastal waters
and along the coast. 850 mb temps are slightly colder this morning,
around -7C to -8C, which is around the point where we usually start
worrying about snow showers reaching the valley floor. Overnight lows
again dropped down to near freezing for most locations, which has
allowed some of the coastal showers to bring snow down to sea level.
Impacts are minimal, but there have been reports of some light
accumulations along the north Oregon coast. Meanwhile, it is a dry,
cloudy and, in some places, foggy start again over the interior
lowlands. Fog is not as dense as it was yesterday, and it should be
mostly gone by midday. Much like yesterday, expect shower coverage to
expand east of the Coast Range during the afternoon hrs as surface
temps warm and the showers gain a boost from diurnal surface heating.
Afternoon temps will rise into the 40s, which will mostly end the
threat for any light snow accumulations below 1000 ft. Any heavy
showers could still bring flakes down to the valley floor, but it
will quickly melt and not accumulate.
Showers will taper off this evening over the interior as surface
temps cool and we lose the diurnal component to the convection.
However, the relatively warmer ocean temps will keep scattered
showers going over the coastal waters. Then our attention turns to a
upper level shortwave and attendant surface low that are modeled to
push offshore out of B.C., then ride down along the coast late
tonight and Fri. The low circulation will pick up some additional
moisture, and diffluent flow aloft will enhance the shower activity.
Some of the higher resolution models, such as the 12Z WRF-NMM from
EMC show show fairly vigorous showers moving onshore after 12Z Fri
over the northern portions of the fcst area, continuing through the
day and becoming focused further south with the track of the low.
Think that the 12Z-18Z Fri window will be our best chance in this
extended cold showery period to get any sticking snow at the lowest
elevations. The best chance would appear to be along the south
Washington and north Oregon coast and Coast Range, where some
locations could pick up an inch or two of snow during this time.
Convective activity will be most vigorous over the waters and at the
coast due to the warmer surface temps. However, think some of the
showers will be strong enough to hold together east of the Coast
Range, bringing the potential for some light accumulations to the
northern interior lowlands. Do not expect more than a dusting to
maybe locally up to an inch, so the existing Special Weather
Statement should cover this threat. However, an advisory may
eventually be necessary for the coast. The timing is such that the
morning commute could be affected for some.
The focus of the heaviest showers will be moving over the southern
tier of the CWA Fri afternoon. However, surface temps will be warming
with the solar insolation into the 40s again. So this should limit
the potential for accumulating snow. Heavier showers will likely
still bring briefly sticking snow at times, but it should quickly
melt. The upper shortwave will move south of Oregon Friday night and
an upper level ridge begins to slide over the area for offshore winds
and drier conditions Friday night and Saturday.
Another upper shortwave, similar to the others will then slide south
from the B.C. Coast and bring a return of cool and showery weather
Saturday night and Sunday. Snow levels will again be low between 1000
and 1500 feet with brief snow possible down to the valley floor.
Models hint that there could be more moisture with this system, and
the coast range and Cascades may see 2 to 4 inches of snow with
locally higher amounts. Pyle/TJ
.LONG TERM...Sunday night through Thursday...The Pacific NW will be
between a broad upper level trough over most of the U.S. and an
Upper ridge over the NE Pacific. The trough will be more influential
to the local weather Sunday night through Tuesday with embedded
shortwaves bringing periods of rain and snow showers. The snow levels
will remain low through Monday, then begin to rise Tuesday. The upper
level ridge then moves over the area Tuesday night and Wednesday
holding through late in the week for drier and warmer weather. ~TJ
.AVIATION...Fog has been fairly widespread across the Willamette
Valley and Lower Columbia this morning. PIREPs from the PDX metro
area suggest tops around 700-1000 ft, which is fairly shallow.
Thus the fog in place now should clear shortly...likely by 19z.
Coastal areas are seeing mixed rain/snow showers this morning,
mainly KTMK southward. Slightly drier air pushing down the coast
will probably result in a decrease in showers along the coast
this afternoon...while increasing sunshine and resulting
instability cause an increase in showers inland. Showers will
likely end inland with the loss of solar heating this evening;
returning the focus of shower activity to the coast overnight and
Fri morning. The air mass may be just cool enough for some
accumulating snow along the coast Fri morning. Conditions will
generally be VFR outside of the above-mentioned valley fog or
KPDX AND APPROACHES...IFR stratus deck with tops around 1000 ft
will likely clear around 19z. VFR then expected through this
evening outside of some scattered afternoon showers. Weagle
.MARINE...Benign marine weather for for the next week with seas 5
to 8 feet and winds remaining below small craft advisory
criteria. There is a chance winds could briefly gust to 25 kt
Friday afternoon/overnight, but expect winds generally to remain
below small craft strength. There will be other similar
instances Sunday through Tuesday, but in each of these cases it
appears borderline and brief windows of stronger winds. While it
is possible we may need to issue a small craft advisory at some
point during this period, at this time do not anticipate one
being issued. Weagle/Bentley
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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.