Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 252224
AFDPQR

Area Forecast Discussion
National Weather Service
223 PM PST Sun Feb 25 2018

.SYNOPSIS...Post frontal showers will decrease tonight through Monday
morning with snow levels lowering to around 500 feet by Monday
morning. Cool dry conditions expected Monday afternoon and night. A
weaker front will bring a return of rain and snow Tuesday morning,
with snow levels initially low (around 500 ft). Snow levels will rise
Tuesday afternoon before a moister front produces additional
precipitation for the area on Wednesday.
&&

.SHORT TERM...Tonight through Wednesday. A cold front is moving
across NW Oregon this afternoon. The precipitation has been lighter
over the higher terrain with this feature than models indicated, but
have decided to not make any changes to the current winter weather
headlines until the new suite of ODOT snow observations come in later
this afternoon.

Colder air is moving in behind the front, and the Astoria Wind
profiler suggests the snow level has lowered to around 1500 feet
along the coast. This matches well with near by surface observations.
The current temperature at Naselle Ridge, WA (2000) feet is 32 degF,
and weather stations in the Willapa Hills are showing freezing
temperatures at and above 1500 feet. This aligns well with the
forecast, and expect the snow levels to continue to lower through the
night. Accumulating snow is expected down to 1000 feet and possibly
500 feet, but with the waning precipitation do not anticipate the
need for any additional winter weather headlines for the coast range,
foothills, or interior low lands. 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.

Showers will continue through Monday morning gradually decreasing in
intensity and frequency. Dry cool conditions expected Monday
afternoon and night as a shortwave ridge moves over the area.
Clearing of clouds should lead to widespread temperatures near or
below freezing early Tuesday morning as the next cold front moves
north to south across the area. Snow levels will initially start low,
around 500 feet as the precipitation arrives. This front is rather
weak, and precipitation should be light. Precipitation totals will be
highest for the north, and the south (Lane County) may not see much,
if any. Lower elevations (500-1000 ft) may see a up to an inch of
snow, with 2 to 3 inches possible for the Washington and North Oregon
Cascades and coast range. Snow levels rises to 1500-2000 feet Tuesday
afternoon as a warm front approaches and the cold front dissipates.


Models are in better agreement on the next weather feature and
forecast a deepening surface low will move across the NE Pacific
Tuesday night, and bomb out around 985mb just off of the British
Columbia Coast late Wednesday morning. This low will bring a moister
and more dynamic front across the region Wednesday afternoon and
night for another round of rain and mountain snow. Snow levels should
range between 2000 and 3000 feet with this front. ~TJ

.LONG TERM...Wednesday night through Sunday. Not too many changes to
the long term forecast period today as a rather active weather
pattern looks to continue for the second half of this week and into
next weekend. Broad model support for 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
with increasing variability in model solutions resulting in below
average forecast confidence by next weekend.

Today`s guidance has clustered a bit more to reflect a closed low
pressure system offshore and moving well north of the forecast area
and into central B.C. coast by Wednesday night with an associated
front pushing across the region through early Thursday. This may
bring some potentially windy conditions to the coast with this
front. Forecast models diverge a bit with respect to the development
of the upper trough at this point, but regardless of the details,
expect that a return of steadier precipitation and lowering snow
levels are possible. Though, it seems unlikely at this point to any
accumulating snow below around 1,000 feet and more uncertain snow
accumulation for the lower and middle foothill elevation bands. One
area of higher confidence is that snow will continue at and above
the Cascade passes as long as there is sufficient moisture to fall.

By the later part of next weekend, there is some indication that a
more zonal flow pattern will replace the rather persistent trough
across the Pacific NW, but until then the key message will remain a
continuation of cooler than normal temperatures and a fairly active
pattern with snow at the higher elevations and rain across the
lowlands.   Cullen
&&

.AVIATION...Frontal boundary sliding south through Oregon early
this afternoon. Flight categories, in general, have been a little
better than expected. Generally a mix of MVFR and VFR for the
coast and interior lowlands and this will continue through early
evening. Expect increasing VFR after 02Z Mon. TAF sites should
all be VFR by 04Z or so. West 850 mb wind up to 45 kt will
result in mountain obscurement, but conditions slowly improve
this evening as wind speeds aloft diminish. Model soundings for
KPDX and KSLE show fairly impressive speed shear this afternoon
through 03Z for potential turbulent conditions.

PDX AND APPROACHES...Appears the strong 850 mb west wind has
resulted in rain-shadow effects for the terminal. Flight
category has been more VFR than MVFR since 18Z and expect this to
continue through 03Z Mon. MVFR risk decreases after 03Z as
showers become less numerous. There is also the possibility of
-SHSN around 12Z, but any snow that occurs will not impact
runways. Weishaar

&&

.MARINE...Peak wind gusts through early afternoon have been
generally around 30 kt. Buoy 089 had a 35 kt gust at 16Z in the
post-frontal air mass and buoy 029 noted a 35 kt gust at 21Z.
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 11Z 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.

Improving forecast confidence with regard to the Wednesday
through the end of the week time frame. The 12Z GFS and ECMWF are
in rough agreement Wed through 12Z Thu, although the GFS
continues to show stronger wind speeds over the waters, even
approaching storm force over the south part of PZZ255 and PZZ275
00Z to 12Z Thu. Both models indicate roughly 7-8 mb KAST-KOTH
gradient 06Z Thu, but the GFS MSLP field is about 6-8 mb lower
compared to the ECMWF. Will stay with more of a compromise for
now, and go with gusts up to 40 kt 00Z to 06Z Thu. Models
somewhat similar in taking the deep surface low near Vancouver
Island 00Z Thu and dumbbelling it west, then south Thu through
Fri. 12Z GFS continues to be 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 compact MSLP 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 indicates a core of
26 ft seas out near 47N 134W 12Z Thu, but keeping 20 ft or
greater seas well beyond 60 nm Thu through 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 and
peaking in the upper teens over the coastal water forecast area.
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.
&&
$$

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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 CWA or forecast area.


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