Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 191810
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
1110 AM PDT Fri Oct 18 2019

Update and Aviation sections updated

.SYNOPSIS...An off and on rainy pattern can be expected to
continue through the weekend as a series of weather systems moves
through the Pacific Northwest. Snow will fall in the Cascades, with
snow levels lowering below pass levels Saturday. Periodic chances
for rain continue into the early part of next week as an upper
trough slowly pushes off to the east, then drier weather appears
likely in the second half of the week as a ridge of high pressure
builds back in.

&&

.UPDATE...
Strong short wave trof and associated surface cold front has cleared
the Portland CWA and Cascade crest as of mid-morning, with the
primary area of intense forcing for ascent now centered over eastern
Oregon/Washington as clearly supported by latest GOES-W band 10
water vapor imagery.  Large scale synoptic subsidence is actually
spreading across areas west of the Cascade crest, but very cold
temps aloft (-12C at 70mb/<-26C at 500 mb) and associated
instability will continue to result in scattered to widespread
showers into the evening across most of the Portland CWA.  Areal
coverage and frequency will, of course, be maximized in the upslope
areas of the coastal ranges and the Cascades with moderately
strong/moist onshore flow, and we will likely continue to see a few
flashes of lightning with the strongest convective echoes this
afternoon as we`ve seen several times already this morning. As such,
we have extended chances for thunder into this evening over the
coastal ranges and along the Columbia River.  The strongest of the
showers will also produce small hail given the cold temps aloft.

Webcams are revealing a dusting to perhaps a couple of inches of
snow at pass level in the Cascades, with the road surface of Oregon
58 completely snow-covered at Willamette Pass. While snow levels will
continue to lower through the afternoon as the cold temps aloft
within the main upper level trof sag southward, the precipitation
will continue to be showery in the post-frontal airmass, which
should act to keep additional snow accumulations at the high levels
of the Cascades from getting too out of control.  We have opted to
keep the winter storm warning going through 6 PM this evening for
the north Oregon and southern Washington Cascades for additional
accumulations between 4-8 inches, with conditions slowly improving
overnight tonight.

&&

.SHORT TERM...Today through Monday...Water vapor pictures early this
morning showed the nose of a strong jet digging in towards the north
Oregon coastline, likely aiding the development of a small surface
low off the Washington coast west of Hoquiam. A strong shortwave was
associated with the jet maxima, and appears on track to move across
the north part of the forecast area this morning, temporarily
enhancing precipitation rates across the northern half of the
forecast area over the next several hours. the passage of the
shortwave this morning will usher a surface cold front through the
forecast area with it, bringing an end to the stratiform
precipitation and changing it over to showers. Surface observations
suggested snow levels in the north Oregon Cascades on the order of
5500-6000 feet early this morning. Once the front passes, expect to
see snow levels lower during the day today, with models suggesting
snow levels down to about 4500 to 5000 ft by late afternoon. A cold
pool aloft will swing across the northern part of the forecast area
today with the upper trough, and model soundings suggest unstable
layer deep enough to support some thunder threat today across the
north. As precipitation rates are likely to peak yet early this
morning, then convective snow showers develop over the Cascades in
the northwest flow behind the shortwave, will hang on to the high
elevation winter storm warning through today. Expect to see snow
levels low enough in the afternoon to allow for the possibility of
several inches of snow down to the passes, but as the onshore flow
weakens quickly this evening leading to decreasing showers
overnight, do not see a need to expand any winter weather headlines
lower at this point.

From tonight through Monday, models remain in good agreement on
showing the upper heights slowly rising in response to the approach
of a rather flat-ish upper level ridge from off the coast. This
leaves the region under an upper level northwest flow, with
subsequent shortwaves tracking further north. A southeast tracking
surface wave along a baroclinic zone spreads moist isentropic lift
across most of the forecast area on Sunday in the mid to low levels,
best seen along the 290K isentrope. This system appears strong and
moist enough to support likely to categorical pops across most of
the forecast area. Behind that wave, showers will again decrease
Sunday night. Another wave riding along the baroclinic zone Monday,
but tracking further north towards Washington, again spreads a
chance for more rain due to isentropic lift. This system however is
most likely to impact the northern part of the forecast area, with
chances for rain tapering off to the south. Snow levels Sunday will
rise and should go above most of the passes in the afternoon. After
a little cooling behind the Sunday system on Sunday night, snow
levels are expected to rise again on Monday rising well above the
passes.

.LONG TERM...Monday night through Friday...No changes. Previous
discussion follows. Ridging starts building again offshore on
Tuesday with a ridge axis stretching to the British Columbia coast.
This will shift the storm track east, with a local drying trend for
the rest of the forecast time frame. Lowered PoPs some Tuesday and
relegated anything mentionable to the Cascades Wednesday, but things
will end up even drier than current forecast reflects if models
continue to trend as they have been. With high pressure taking hold
and plenty of moisture leftover from a week`s worth of rain, added
some fog to the forecast for Wednesday and Thursday mornings. Anyone
looking for a sun fix may get it during the afternoons in the second
half of next week. Bowen

&&

.AVIATION...Mostly VFR conditions across the forecast area this
morning, but fairly widespread showers and isolated thunderstorms
will continue through the day. These showers and storms will bring
brief MVFR conditions, and mountains of the Coast Range and Cascades
will frequently remain obscured in cloud and showers through the
day. SW wind gusts 30-40 kt along the coast and up to 30 kt over the
inland valleys through the afternoon. A break in showers late
tonight could allow more widespread MVFR conditions as the air mass
stabilizes a bit, but another front will provide for a focus of more
rain by midday Sunday.

KPDX AND APPROACHES...Expect VFR conditions most of the day, but
brief MVFR conditions in heavier showers or thunderstorms through
around 03z. West surface wind through the day with gusts to 30 kt
possible.

&&

.MARINE...Surface pressure gradient over the coastal waters
tightening early this morning. Water vapor satellite imagery hints
at as surface low north of buoy 46089 over the central Washington
waters at 09Z. Strongest gradient will be centered from about
Tillamook to Cascade Head through 12Z. The Newport Jetty has had
gusts around 40 kt earlier tonight with 30-35 kt noted at buoy
46050. Cannot rule out localized gale gusts over PZZ255 and PZZ275
as well as the south half of PZZ250 and PZZ270 through sunrise. West
wind behind the surface low may also briefly gust to 30 to 35 kt the
remainder of the morning, but do not expect the gusts to be frequent
or widespread enough to warrant a gale warning.

The latest model guidance is a little stronger with the Sun morning
system compared to the past couple of nights. The 06Z NAM shows
solid 25-30 kt boundary layer wind speeds Sun morning, while the 00Z
GFS is slightly higher. Will cap wind gusts at 30 kt for this
forecast, but next couple of shifts will need to evaluate the need
for a gale watch/warning. Yet another disturbance moves across the
waters Mon, which may lead to small craft advisory level gusts,
especially for the north waters. A switch to north wind occurs
around Tue and continues through much of next week.

Seas to remain in the 13 to 16 ft range today,then gradually subside
to 10 ft late Sunday afternoon. Model guidance indicates a slight
rise to around 12 ft over the north waters Mon associated with the
aforementioned frontal system. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 6 PM PDT this evening for Northern
     Oregon Cascades.

WA...Winter Storm Warning until 6 PM PDT this evening for South
     Washington Cascades.

PZ...Small Craft Advisory until 5 PM PDT Sunday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until 11 PM PDT this evening for Columbia
     River Bar.

&&

$$



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