Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 291544
Area Forecast Discussion
National Weather Service Portland OR
843 AM PDT Sun May 29 2016
.SYNOPSIS...An upper level low pressure system over southern British
Columbia will shift east today and tonight. Upper level ridging will
start to build tonight, then strengthen Memorial Day and Tuesday,
resulting in much warmer and drier conditions. The warmest days will
be Tuesday and Wednesday. Daytime temperatures will be slightly
cooler the latter half of next week, but remain well above normal.
.SHORT TERM...(Today through Tuesday)...Minimal changes to the
current forecast. Large-scale pattern consists of a broad low
pressure area of Southern British Columbia, an upper ridge centered
between 135W and 140W and a closed low over Southern Calfornia. Water
vapor imagery shows a distinct short-wave moving across Western
Washington and Northwest Oregon as of 15Z. This has resulted in some
spotty drizzle over SW Washington and extreme NW Oregon. 12Z KSLE
sounding showed the top of the marine layer around 6000 ft MSL, but
very dry above that level. As the shortwave moves east across the
Pacific NW today, will maintain some low pops for light rain or
drizzle this morning. Will also hang on to a slight chance of showers
across the north this afternoon. Air mass dries this evening and
tonight as the low-level flow turns more north to northeast.
Expect excellent radiational cooling conditions tonight, which will
lead to some patchy fog late tonight or around sunrise in the normal
fog-prone valleys. Memorial Day will be several degrees warmer than
today as GFS 850 mb temps go from a range of +5-10C today to +10-16C
Mon and the upper ridge axis moves closer to the coast. Subsidence
inversions become much more pronounced Monday night with a
well-defined thermally-induced surface trough along the coastline.
NAM model soundings suggest a sharp low-level inversion 12Z Tuesday.
Made significant edits to the Monday night temp grid to better
reflect the higher terrain subsidence inversion profile.
Latest guidance is a little quicker shifting the surface thermal
trough inland Tuesday. NAM and GFS hold it to the coastline through
about 18Z Tuesday then shift it to the interior valleys by the
afternoon. In any event, model 850 mb temps rise to around +16C over
the far north to +21C in the Lane County Cascades. NAM develops a
little offshore flow through the Columbia Gorge late Monday night and
Tuesday morning, with a peak KTTD-KDLS gradient around -3 to -4 mb.
.LONG TERM...Tuesday night through Saturday...The upper level ridge
will persist over the Pac NW through Wednesday keeping temps on the
warmer side of things. Models continue to show 850mb temps near 20C.
With the thermal trough shifting inland slightly on Wednesday this
should allow temps in the interior to warm significantly, with
widespread afternoon highs around 90F likely. SW flow aloft through
the end of the week likely to keep the region dry. Although still
warmer than normal through the end of the week under ridging aloft,
onshore flow should provide some moderation in temps late in the
.AVIATION...There are plenty of clouds around this morning in a
weak onshore flow pattern. Conditions are predominantly MVFR at
the coast. Most of the interior is VFR, but there is a patchy MVFR
deck around the PDX metro area. Cigs should lift over the next few
hours, with widespread VFR conditions by 18Z. Cloud cover to scatter
out this afternoon as drier northerly flow develops at the lower
levels. Patchy MVFR cigs may redevelop at the coast this evening
into the early overnight period, but they should improve to VFR
going into early Mon AM as the low level flow turns slightly
offshore. Think the interior will stay VFR tonight and Mon AM,
but cannot rule out some patchy shallow fog or low stratus around
daybreak Mon. Pyle
PDX AND APPROACHES...Cigs of 2500 to 3500 to persist through
around 18z, with a mix of VFR and MVFR expected during this time.
Cigs will lift later this morning, then scatter out this
afternoon. Expect VFR from this afternoon through Mon AM. There is
a slight chance of patchy fog or low stratus around the terminal
near daybreak Mon. Pyle
.MARINE...High pressure over the northeast Pacific will build
today and Monday. Meanwhile, thermal low pressure over NW
California will build north along SW Oregon coast. This will
bring typical early summer of gusty north to northwesterly winds,
with the strongest winds to south of Cascade Head. While may see a
few gusts 20 to 25 kt around Florence southward later today. But,
it appears strongest winds will be Mon and Tue afternoons/evenings,
where winds 15 to 25 kt to south of Cascade Head. To north of Cascade
head, may see a few gusts reach 25 kt, but think will be few and far
between. Northerly winds continue for Wed to Fri, but appears
gradients will be weaker, with winds only 10 to 20 kt.
Seas hovering at 3 to 5 ft trough Monday am, but as winds pick up
so will the short period wind waves. will see seas 5 to 8 ft for
late Mon through Wed, with higher seas south of Cascade Head.
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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.