Area Forecast Discussion
Issued by NWS Portland, OR

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107
FXUS66 KPQR 231639
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
939 AM PDT Thu Mar 23 2017

.SYNOPSIS...Weak high pressure overhead will keep conditions mostly
dry this morning under cloudy skies. The next occluding Pacific
frontal system will spread rain across southwest Washington and
northwest Oregon later this afternoon into tonight, with showers and
perhaps a couple thunderstorms lingering into Friday. Higher pressure
will cause showers to taper off later Saturday, but the next frontal
system will likely bring more rain Sunday followed by showers Monday.
Occasionally wet weather is expected to linger through the middle of
next week, with temps fairly typical for late March. Snow levels will
occasionally dip below Cascade pass level, but the more significant
accumulations will likely stay up toward the higher ski resort
elevations.

&&

.SHORT TERM...Today through Saturday...Weak high pressure will
continue to move across southwest Washington and northwest Oregon
this morning bringing mostly dry conditions. Meanwhile, visible
satellite imagery reveals abundant mid and high clouds streaming over
the forecast area ahead of the next frontal system which looks to
arrive at the coast early this afternoon and push inland this
evening. No significant changes were made to the forecast this
morning as things remain on track. /64

Remainder of the previous discussion (issued at 316 AM Thursday)
follows...The approaching frontal system has a tap to subtropical
moisture just north of Hawaii, but the deepest moisture plume of
1.25-1.50 precipitable water values remains 35N southward. Models
continue to aim the axis of deepest moisture and heaviest QPF south
of our forecast area this evening, focusing instead on the OR/CA
border region as a triple point develops along the approaching front.
While the heaviest rain is expected to stay to our south, there will
probably be a decent period of light to moderate rain spreading
onshore around 3-4 PM. The 00z WRF-ARW fcst reflectivity then shows
rain spreading across the Willamette Valley just in time for this
evening`s rush hour...continuing all the way through this evening
before the main front pushes across the forecast area late tonight or
early Fri morning.

The 06z NAM shows a decent ribbon of 60-65 kt S-SW 850 mb winds with
the front this evening, but gradients will likely remain too offshore
for wind gusts much higher than 40-50 mph along the coast. As
mentioned above, a triple-point is expected to develop along the
front this evening near Cape Blanco, which would probably keep the
strongest wind south of Lane County. The wave of low pressure
associated with this triple point will ride NE along the front, which
is likely what will keep rain fairly steady throughout tonight.

The upper trough associated with this system will start to push
onshore Friday, with the 00z ECMWF swinging a decent vortmax through
during the midday and afternoon hours Fri. This shortwave will likely
be the leading edge of the cold pool aloft...with the 06z GFS showing
500 mb temps down to -27 deg C Fri afternoon. Given some sunbreaks,
this could be enough to support some deep but skinny CAPE and LI`s in
the 0 to -2 deg C range. 0-3 km shear profiles look fairly decent for
the coast and Willamette Valley Friday, so any cells that develop in
the 200-500 J/kg of CAPE Fri afternoon could be robust despite only
having tops around 20 kft. Based on all this, we decided to add a
slight chance of thunder for all but the Cascades Fri afternoon and
evening.

Higher pressure and warming temps aloft should cause convection to
become less robust Saturday, though there will probably still be
plenty of showers around. Expect showers to begin tapering off Sat
afternoon, then coming to an end Sat night as subsidence and a
stabilizing air mass take over.

Snow levels - on average - should be just above the Cascade passes
tonight and Friday. As the secondary shot of cold air comes in Fri
afternoon and evening, expect snow levels to lower to around 3000 ft
with some minor accumulation possible on the passes Fri night/Sat.
More significant accumulations are likely above 5000-5500 ft, where
around 3-6 inches of snow are expected.  Weagle

.LONG TERM...No Changes. Previous discussion follows...Saturday night
through Wednesday...Lingering showers Saturday night will come to an
end by early Sunday as shortwave ridging moves across the region in
between systems. However, the next round of moisture in the form of a
Pacific front is not far behind and will likely spread another of
round of steady rain later Sunday...with post-frontal showers
continuing into Monday. 00z GFS/ECMWF and most of their ensembles are
trending wetter for midweek, as the overall pattern remains
progressive and another frontal system pushes onshore Tue night or
Wed. It does appear this system will dive SE toward the Great Basin,
but the ridging behind it remains flat on both the 00z ECMWF and GFS.
This seems to indicate that models are becoming less bullish on
drying us out late next week...as the flat ridging would probably
lend to a continued progressive pattern across the NE Pac through the
end of next week. Overall, expect generally more of the same for the
next 7-10 days, with occasional rain and temperatures remaining
within a few degrees of typical late March values.  Weagle/Cullen

&&

.AVIATION...VFR conditions along the coast are likely to give way
to MVFR conditions after 01z this evening as a cold front spreads
rain in. In the interior, VFR conditions are expected to remain
dominant today into tonight. There is a chance though for areas of
MVFR conditions to develop mainly after 03z tonight as rain spreads
in.

KPDX AND APPROACHES...VFR conditions through 03z this evening. After
03z VFR remains the most likely condition overnight, but there is a
chance for seeing temporary MVFR conditions associated with rain
spreading inland this evening.

&&

.MARINE...A cold front will approach the waters today, and move e
across the waters this evening. South winds are expected to increase
to gale force over the outer waters today, peak this evening then
diminish late tonight. Nearshore waters will be a little slower to
increase to gale force, probably late this afternoon, then peak late
this evening. Steep seas will increase late today and this evening
in response to the increasing winds, generally building to a peak in
the 12 to 15 ft range, before subsiding again late tonight and early
Fri. Behind the front west winds gusting over 20 kt are likely to
continue into Fri. A ridge of high pres moves slowly east across the
coastal waters Fri night through Sat night for quieter weather.

Sun another cold front approaches from the west, increasing winds
again. Winds with this system expected to gusts as high as 30 to 35
kt late Sun.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning until 3 AM PDT Friday for Waters from Cape
     Shoalwater WA to Florence OR from 10 to 60 nm.

     Gale Warning from 5 PM this afternoon to 3 AM PDT Friday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     10 nm.

     Small Craft Advisory for Rough Columbia River Bar from 8 PM
     this evening to 5 AM PDT Friday.

&&

$$

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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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