Area Forecast Discussion
Issued by NWS Portland, OR

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144
FXUS66 KPQR 152151
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
246 PM PDT Wed Aug 15 2018

.SYNOPSIS...Finally, some relief from the hot conditions is on its
way. Onshore flow will increase later tonight and especially Thursday
as upper low approaches from the west. May see a few thunderstorms
over the Oregon Cascades into this evening, and again Thu
afternoon/evening. Otherwise, expect cooler marine air to spread into
the interior later Thu and Thu night, with rather refreshing Fri.
But, high pressure will gradually build again over the weekend, with
gradual warming trend for Sat into early next week.
&&

.SHORT TERM...Tonight through Saturday...Smoke and haze remain aloft
into Thursday, but at least low level smoke has been dissipating,
mainly in part to bit more in way of onshore flow through the coastal
mountains and up the Columbia River.

Upper low sits offshore, and is slowly drifting towards the Pac NW.
Flow aloft as now turned more south to southwesterly, allowing
instability and limited moisture to stream out of California into
southern  half of Oregon. Will see isolated afternoon/evening
thunderstorms over the Oregon Cascades from about Mt Jefferson
southward today, with a slight chance of showers/thunderstorms back
to west a bit over the foothills. Highest chance of
showers/thunderstorms will stay well southeast of our area through
this evening. Meanwhile, marine stratus will hug the coast early this
evening, then spread inland again overnight. Can not rule out patchy
drizzle along the coast again tonight.

Now, as upper low approaches on Thu, will see increasing onshore flow
especially in the afternoon. Likely to see highs inland struggle to
get out of the lower to middle 80s, but after past several weeks, who
would complain?  Instability will also increase over the Cascades,
and should be enough to trigger isolated showers and thunderstorms
from Mt Hood southward. With strengthening onshore flow and mid
levels trending more southwest towards late afternoon, the threat of
thunderstorms will shift closer to the Cascades crest towards
evening, and then east of the Cascades during the evening. The
combination of thunderstorms and dry forest fuels, will have a Fire
Weather watch for the high Cascades, which would include the Mt Hood
and Willamette Natl Forecasts for Thu afternoon/evening.

Will see much more in way of marine clouds later Thu night into Fri
am, thanks to deepening marine layer as onshore strengthens. Will
keep occasional drizzle for the coast and into west slopes of Coast
mtns during that time due to weak lift of the trough tugging and
lifting on the moist marine layer. Morning clouds on Fri, then will
gradually become partly to mostly sunny in the afternoon. Upper flow
behind the trough will turn more west to northwesterly. This will
bring much more in way of blue, yes real BLUE skies to region.
Temperatures will be in the upper 70s to lower 80s. So, should feel
like a really nice day for mid-August.

Back to warm temperatures on Sat. High pres will build behind the
departing trough Fri night and Sat, limiting the inland intrusion of
marine air. Expect inland temps back in the mid to upper 80s. But,
at least will not much haze or smoke in skies.Rockey.

.LONG TERM...Saturday night through Wednesday...The mid-range models
are in good agreement on a strong upper ridge building over the B.C.
coast Saturday night and Sunday as a large upper low deepens near
the Aleutian Islands. A weaker upper low will undercut the ridge and
move over the Pacific NW Sunday and Monday. This will continue
marine clouds along the coast in the nights and mornings with
clearing in the afternoons. The interior will remain dry with
afternoon temperatures warming a little each day. Light onshore flow
should prevent the temperatures from getting excessively hot. The
models vary on the departure of the upper trough, but all agree that
an upper ridge will dominate Tuesday or Wednesday. Increased
subsidence from this ridge will limit the inland impacts of the
marine air, and the interior afternoon temperatures will likely warm
into the low 90s Tuesday and Wednesday.    ~TJ

&&

.AVIATION...Low-level smoke continues to affect visibilities over
the northern interior lowlands this afternoon, from KSLE northward.
Conditions are generally MVFR in this area. An upper level trough
will approach the coast and move onshore tonight and tomorrow. This
will result in a deeper marine layer and increasing onshore flow,
which should improve the low level visibilities considerably. Expect
mainly VFR conditions inland by later this evening.

A mix of IFR and MVFR stratus at the coast may clear briefly over
the south Washington and far north Oregon coastal areas later this
afternoon. But the onshore flow should push it quickly back onshore
this evening. Expect IFR and low MVFR conditions to persist at the
coast through tonight and most of tomorrow. The marine layer should
also be deep enough to allow a more extensive inland stratus push
tonight. Expect MVFR cigs to affect KEUG and some of the northern
Willamette Valley TAF sites between 12Z- 15Z. Any inland cigs should
lift to VFR by midday.

KPDX AND APPROACHES...Low level smoke to bring a continued mix of
VFR and MVFR visibilities into early this evening. Conditions should
improve tonight. MVFR stratus will likely impact the terminal after
12Z Thu. Pyle

&&

.MARINE...High pressure will reside over the NE Pac through the
weekend and into early next week. Meanwhile, a thermally-induced
surface trough will remain over northern California and southern
Oregon. This will result in persistent northerly winds over the
coastal waters. Gusts of 20 to 25 kt are expected to develop over
the central Oregon waters during the afternoon and evening hours
each of the next several days. Think we may skate by without needing
a small craft advisory on Thursday, but it looks likely that we will
need them for PZZ255/PZZ275 on Friday and Saturday.
Winds look lighter late in the weekend and early next week.

Seas will remain around 5 to 8 ft over the next several days. The
seas will be dominated by fresh NW swell being generated by stronger
winds off of Vancouver Island. Dominant periods should be around 10
to 12 seconds. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT this evening for Willamette
     National Forest.

     Fire Weather Watch from Thursday afternoon through Thursday
     evening for Willamette National Forest.

WA...None.
PZ...None.
&&


$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.



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