Area Forecast Discussion
Issued by NWS Portland, OR

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029
FXUS66 KPQR 240947
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
247 AM PDT Mon Apr 24 2017

.SYNOPSIS...The first in quick moving series of Pacific weather
systems will move across the Pacific NW today bringing rain and
mountain snow. Another system arrives Tue and continues Tue night.
Moist onshore flow will continue the threat of showers Wed through
Fri.
&&

.SHORT TERM...Today through Wednesday...Water vapor imagery showed a
strong jet digging into SW OR early this morning. Satellite and
surface data suggested a low off the central OR coast, moving east
in line with model expectations. Isentropic lift has brought a wide
area of of rain to the region this morning. and should continue
until the surface low moves inland late morning and pushes a cold
front in, to be followed by the cool trough aloft. Moisture is deep
this morning with satellite derived precipitable water values on the
order of three quarters of an inch. As precipitation transitions to
showers in the afterrnoon, onshore flow behind the low will add
orographic lift to air in snow for the Cascades. Will continue the
winter weather advisory today for the Cascades, with snow levels
this morning suggesting heaviest amounts will generally be above the
main passes. 00z model soundings now suggest the potential for some
thunder with the showers late today with a narrow but sufficiently
deep unstable layer. Given the lack of apparent deep convection
offshore this morning, prefer to refrain from any mention of thunder
today at this time. With the surface low expected to track inland
east across the northern Willamette Valley, pressure gradients
peaking around 4 mb between KPDX and KEUG suggest a breezy day
mainly south of the low.

A bit of a break comes tonight with a shortwave ridge moving across.
This will diminish the chances for showers as warming aloft tends to
stabilize the air mass. The break though is short, as the next
system quickly arrives Tue thanks to the unseasonably strong zonal
jet over the east Pacific. Models all tend to move a wave along a
baroclinic zone across the region Tue night, which should be
sufficient for high pops. GFS however is the most vigorous with this
system, showing the most promising isentropic lift between the 290K
and 300K isentropes Tue afternoon and evening, thanks in part to
another surge of deeper moisture. Other models not quite so
promising as to the depth of moisture, so at this time will be a
little more conservative with precipitation potential for Tue night
through Wed morning time period than GFS would suggest. Showers
continue through Wed as cooler air slips in with a continued moist
low level onshore flow.

&&

.AVIATION...A 1000 to 1003 mb low will move ashore near Tillamook
around around 15Z and move toward Hood River by late morning. The
low will weaken as it moves inland. Gusty winds at the coast as
well as inland, south of KUAO can be expected this morning.
Inland gusts most likely to reach 30 kt or so between 14Z and
19Z. As far as flight conditions coastal areas are expected to
remain predominantly MVFR in cigs and vis as the low moves
ashore. Most guidance is holds MVFR at the coast through at least
06Z Tue, but there is some chances for improvement after that.
Inland, conditions will be a mix of VFR and MVFR to start the
next TAF cycle. MVFR cigs and vis should then become more
dominant and last through 21Z with trends toward VFR after that.
Precipitation becomes showers after midday as the low/front
shifts east of the area. Could see some heavier showers and
cumulus buildups as the lower level instability could favor
fairly deep cell growth.

KPDX AND APPROACHES...VFR to start out the 12Z TAF package but
occasional MVFR conditions are expected through midday then
trending toward more VFR. However heavier showers could briefly
bring MVFR cigs and vis this afternoon into early evening.  /mh

&&

.MARINE...A low pressure center approaching the central Oregon
coast. There has not made the turn to the northeast as much as
models are indicating. As of 2 am, the center of the low is
around 44.5N 126.5W per satellite and surface analysis. This
places the low center about 1 degree south of where models have
it. The low should eventually turn a little toward the north, but
may make landfall closer to Cascade Head than Tillamook as
earlier guidance indicated. Still though will keep Gale Warning
for the central Oregon coastal water early this morning with
Small Craft advisories elsewhere. Wind shifts to the west south
of the low, while areas north of the low will see a more dramatic
wind shift from south to southeast to northwest by afternoon.
Active weather pattern continues as there is another system on
Tuesday that will likely bring high-end small craft advisory wind
and possibly brief low-end gales. The active pattern continues
through the end of the week.

Seas are currently hovering around 8 to 10 ft. Seas south of
Cascade Head will build to around 10 to 12 ft as winds ramp up
with the approaching low. Seas start to come back down in the
afternoon as this system moves inland. Tuesday seas look to
remain around 7 to 9 ft. Next time seas get up above 10 ft looks
to be Thursday with a fetch developing from a Low off the Coast
of British Columbia. With this system models have backed off, now
only bringing seas up to 10 to 12 ft. Seas start to fall late
Friday and look to stay below 10 ft through next weekend.
/64/McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 8 PM PDT this evening for
     Cascades in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 8 PM PDT this evening for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 2 PM PDT this afternoon
     for Coastal Waters from Cape Shoalwater WA to Cascade Head
     OR out 60 nm.

     Gale Warning until 7 AM PDT this morning for Coastal Waters
     from Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 6 AM
     PDT early this morning.

     Small Craft Advisory for Rough Columbia River Bar from 2 PM
     this afternoon to 6 PM PDT this evening.

&&

$$

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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.



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