Area Forecast Discussion
Issued by NWS Portland, OR

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535
FXUS66 KPQR 170526 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
926 PM PST Sat Feb 16 2019

Aviation discussion updated.

.SYNOPSIS...Cool and showery tonight, but drier weather will
spread across the region Sunday and continue into Monday. Next
weather system will arrive later Monday night, with rain or snow
spreading across the area on Tuesday. Snow levels stay low,
generally 500 to 1000 feet for Monday night through next week.
&&

.SHORT TERM...Tonight through Tuesday...Upper low pressure over the
region will maintain showers through the evening, but showers will
be gradually decreasing later tonight. Air mass still cool, with
snow levels running between 700 and 1000 feet. So, will see some
accumulations down that low, but generally 1 to 2 inches for most
areas through rest of the evening. But, heavier precipitation will
continue to fall just a bit higher.

Still have rather favorable flow for snow in the Cascades, with cool
westerly flow. So, will maintain higher snow accumulations in the
Cascades, generally at/above 1000 to 1500 feet, with another 3 to 7
inches expected through the evening into the overnight hours. But,
with showers decreasing later tonight, the bulk of the accumulations
being this evening. Current advisory expires at 11 pm, and seems to
be ok with that time. Even though snow showers continue overnight in
the Cascades, snow accumulations will become more spotty in nature,
with some spots getting another 1 to 3 inches after midnight.

As precipitation ends and skies start to clear, the potential for
temperatures to dip below freezing increases later tonight. This may
cause some black ice to develop on roads wet from today`s rainfall,
especially untreated secondary roads outside of the urban centers.
Additionally, patchy fog may form overnight, which may also
contribute to slick road surfaces where temperatures fall below
freezing by Sunday morning.

Morning fog and low clouds should give way to at least a partly sunny
day Sunday as high pressure builds across Washington and Oregon.
Latest guidance (such as 12z ECMWF and 18z NAM) appear to be a little
more bullish with cold air spreading southward from BC through the
Okanagan and into the Columbia Basin...this may come into play later
as our next organized low pressure system arrives Tuesday. For
example, the 18z NAM pushes 850 mb temps down as low as -12 deg C at
The Dalles by Monday morning. At the very least, this will likely
lead to a chilly night with low temps in the 20s Sunday night for
much of the forecast area. Models had been toying with the idea of
bringing a weak low pressure system southward along the coast Monday,
but the cold, dry air high pressure system will likely keep
precipitation mainly offshore.

Clouds likely increase Monday night in advance of our next low
pressure system digging southward along the coast from British
Columbia. Most guidance suggests southerly flow will develop ahead of
this system, keeping snow levels just above the valley floors as
precipitation develops Tuesday. Additionally, robust W-NW flow aloft
ahead of this system will serve to weaken whatever is left of the
cold pool east of the Cascades. However, should this system come in a
little weaker than presently modeled, weakening both the low-level
southerly flow and W-NW flow aloft ahead of it, there may be some
concern for snow down to the valley floors Tuesday and/or Tuesday
night. For now, we will be non-committal with precip type associated
with this system in the Willamette Valley, mentioning both rain and
snow as possibilities. At this point it appears the odds of a
widespread high-impact valley snow event Tue/Tue night are 10 percent
or less, but it still bears close watching.  Weagle

.LONG TERM...Tuesday night through Saturday... The 12z GFS and 12z
ECWMF have a cold front progressing across the area. This front
looks to bring a slug of moisture, this pool of moisture look to
hang around through the post-frontal environment. The surface low
associated with the front will slide across the area through
Wednesday. Thursday has a shortwave ridge starting to momentarily
dry the area out. However, a new cold front meanders into the area
Friday night into Saturday. Which looks to bring a new influx of
moisture to the area. Overall, for the extended period temperatures
look to remain slightly warmer than normal, with precipitation
chances remaining elevated, through brief dry periods intertwined
between cold fronts. 850 mb temperatures look to hold in between -6C
to -5C and a steady onshore flow pattern continue to signal minimal
snow chances for the valley floor. /42

&&

.AVIATION...Showery pattern continues into the overnight hours,
tapering off from north to south by daybreak. May see occasional
MVFR CIG and/or vis with showers as long as they last. Expect most
inland sites will settle into MVFR CIG after showers clear.
Conditions should then improve to VFR again by 20Z Sunday. There is
a small chance MVFR or even IFR fog develops instead of stratus and
if that happens, it might take longer to clear. Winds will be
generally light and variable.

KPDX AND APPROACHES...Showers continue on/off at the terminal
through about 09Z with CIG around 1000 ft, possibly occasionally
improving to VFR. Once showers clear, this low MVFR CIG is likely to
settle in for the rest of the night with conditions clearing and
returning to VFR by around 18Z Sunday. May see another drop to MVFR
CIG Sunday evening before 06Z Monday. Bowen

&&

.MARINE...Gusty NW surface continue across the waters this
afternoon. Gusts between 20 and 30 kt continue this afternoon,
but should gradually subside later this evening. Did extend the
advisory for winds across the northern zones through this
evening, as latest high res guidance shows more widespread 25 kt
gusts through the evening, especially across the outer waters.

Another low will drop south from Vancouver Island and move
directly over the waters tonight into early Sunday. This should
keep winds fairly light during this time. High pressure builds
over the waters early next week. However, weak lows will move
periodically through from the NW. The strongest system looks to
be later Tuesday into Wednesday. This one brings the potential
for small craft advisory winds, or potentially even gales.

Seas remain in the mid teens this afternoon and will gradually
subside tonight and Sunday, dropping below 10 ft by Sunday
afternoon. The Tuesday night/Wednesday system may bring the
potential for seas building into the upper teens, or potential
even above 20 feet. Cullen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 11 PM PST this evening for
     Cascade Foothills in Lane County-Cascades in Lane County-
     Northern Oregon Cascade Foothills-Northern Oregon Cascades.

WA...None.
PZ...Small Craft Advisory for winds until 11 PM PST this evening for
     Coastal Waters from Cape Shoalwater WA to Cascade Head OR
     out 60 NM.

     Small Craft Advisory for hazardous seas until 2 PM PST Sunday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 4 AM
     PST Sunday.

&&

$$

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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 forecast area.



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