Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 231017
Area Forecast Discussion
National Weather Service Portland OR
316 AM PDT Thu Mar 23 2017
.SYNOPSIS...Weak high pressure in between frontal systems is bringing
a break in the precipitation for most this morning, though a few
showers linger over SW Washington. The next occluding Pacific frontal
system will spread rain across the forecast area 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...Infrared satellite imagery
shows plenty of low clouds banking up along the Coast Range and
Cascades, but generally clearer skies over the Willamette Valley.
Weak high pressure is sliding across the Pac NW this morning, though
enough low-level moisture and shallow instability exist to keep a few
showers going...primarily over SW Washington where the air mass is
slightly cooler aloft. Expect these showers to decrease this morning,
but by then high clouds should already be increasing in advance of
our next Pacific frontal system. This system is presently crossing
130W, with the leading edge of the high cloud field just 75-100 miles
offshore. As a result, there probably will not be much in the way of
sunshine today...except perhaps during the first few hours of the
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
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...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...A low end VFR deck near 4-5kft has developed across
the region. It may lower at a few sites to around 2.5-3kft this
morning, but confidence is low in whether or not this will occur.
An area of rain associated with a cold front moving eastward
towards the region later today will likely produce a brief period
of MVFR conditions at some taf sites between 00-06z Friday.
Increasing low level instability behind the front should allow
conditions to return to VFR rather quickly.
KPDX AND APPROACHES...There is a chance the current 4-5kft deck
in the region may lower to 2.5-3kft and produce a period of MVFR
conditions this morning, but this appears increasingly unlikely.
An area of steady rain associated with a front may bring a very
short period of MVFR conditions between 00-06z Friday. Conditions
should return rather quickly to VFR after 06z Friday as the lower
atmosphere destabilizes behind the front. /Neuman
.MARINE...An approaching front will result in southerly winds
increasing into Gale Force criteria this afternoon and evening.
Seas should respond accordingly and climb into at least the mid
teens. The strongest winds will likely hold off across the inner
waters until this evening so the current start times for the
Gale Warning appear on track. Expect winds to decrease rather
quickly behind frontal passage, which should happen around
midnight or shortly thereafter. Seas should then subside and
should fall to near 10 ft towards daybreak on Friday. A secondary
weak front will move into the waters Friday and will likely keep
gusty 25 kt winds going across the waters with seas hovering
around 10 ft.
High pressure shifting over the waters Saturday should finally
produce a more substantial decrease in winds and a slight
decrease in seas Saturday and Saturday night. However, yet
another front appears likely to bring winds and high seas
Sunday. At this point, Gale Force winds appear the most probable
scenario, but it`s certainly far from guaranteed given the time
between now and then. The weather pattern looks to remain active
for much of next week with additional rounds of at least small
craft advisory level wind gusts of 25-30 kt. /Neuman
PZ...Gale Warning from 11 AM this morning to 3 AM PDT Friday for
Waters from Cape Shoalwater WA to Florence OR from 10 to
Gale Warning from 5 PM this afternoon to 3 AM PDT Friday for
Coastal Waters from Cape Shoalwater WA to Florence OR out
Small Craft Advisory for Rough Columbia River Bar until 4 AM
PDT early this morning.
Small Craft Advisory for Rough Columbia River Bar from noon
today 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.