Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 260436 AAA

Area Forecast Discussion
National Weather Service Portland OR
925 PM PDT Sun Jun 25 2017

.SYNOPSIS...High pressure over the Pacific Northwest will weaken over
northwestern Oregon and southwestern Washington. A southerly marine
surge will be working up the coast will then extend inland tonight
bringing much cooler temps on Monday. There is a slight chance for
some high-based thunderstorms over the portions of the forecast area
on Monday. Dry northwesterly flow sets up later this week across the
region, bringing more seasonable temperatures and a pattern of
morning clouds and afternoon sunshine.


.SHORT TERM...Tonight through Wednesday...Our final day of heat is
coming to a close. Records fell at Hillsboro and Vancouver and
Portland tied it`s record for the date. Please our record event
report, which will be posted on our homepage shortly.

An examination of surface observations reveals cooler marine
spreading into the Willamette Valley and Columbia River Gorge. The
AST-PDX gradient peaked at 6mb this evening and still remains >4mb so
we should continue to see westerly winds spread cooler marine air
through the Coast Range gaps for a good part of the night. It should
be noted that this morning`s low temperature was 75F at Troutdale,
which if it holds through 1AM would be the warmest minimum
temperature on record at Troutdale. However, we are not expecting
this to happen given the above mentioned cooler air filtering into
the region. Nonetheless, temperatures could be sneakily close to the
all time record of 71F even by 1AM so it`s worth monitoring.

Thunderstorms erupted over much of southwest Oregon earlier this
evening. A few weaker thunderstorms developed over Lane County and
moved northward into far eastern Linn and Marion Counties before
pushing east of the Cascade crest. This thunderstorm development has
been in response to daytime heating and the approach of an upper
level shortwave trough currently spinning off the California Coast
destabilizing the atmosphere. In general, most of this thunderstorm
activity should come to a close over the next few hours as the
boundary layer continues to cool, but cannot rule out an outflow from
ongoing convection kicking off another thunderstorm or two.

Nonetheless, mid level lapse rates of 8C+ continue to reside over
much of northwest Oregon and southwest Washington per the latest SPC
mesoanalysis. In addition, there has been an extensive altocumulus
castellanus field offshore per visible satellite imagery this
evening. The NAM and GFS models do pick up on this elevated
instability in the form of a negative thetae lapse rates between
700-500mb. These models weaken this instability slightly overnight,
but do spread it eastward into our CWA late tonight and Monday
morning. While the main vort max and synoptic scale lift will likely
slide south of our CWA, cannot rule out some isolated thunderstorms
developing over the CWA later tonight and Monday morning as this area
of instability and upper level diffluence currently offshore spreads
eastward over the region. As a result, extended the slight chance
mention of thunderstorms through 18z Monday for much of the
CWA...albeit the threat could linger for a few hours past this time
based on latest model guidance.

Considerably cooler temperatures on Monday for interior locations
look on track. A northwesterly flow pattern will bring more marine
cloud cover and even cooler temperatures Tuesday and Wednesday.
Clearing on both of these days could come later than earlier in the
afternoon with perhaps a few areas of morning drizzle possible along
the north coast and/or in the Cascade foothills.

Finally...KRTX radar is currently experiencing an outage. Electronic
technicians are at the site diagnosing the problem, but we are unsure
when it will return to service. /Neuman

.LONG TERM...Wednesday night through Sunday...No Changes...Previous
Discussion From Sunday Afternoon Follows...Models continue to
show an upper level ridge building over the Pac NW for the latter
half of the work week. This will bring slightly warmer temps to the
forecast area through the weekend, with periods of morning clouds.
The good news is that the ridge will not be as strong later this
week so temps will likely only reach into the 80s. Conditions are
expected to remain dry through the long term period, but there is a
hint that the upper level trough may swing south and impact the area
over the weekend. Will need to monitor this time period as there has
been a lot of run to run variability with the evolution of the
trough. /64


.AVIATION...A thermal trough will push east of the Cascades
tonight, allowing a surge of marine air and stratus clouds to
push into the interior. As the onshore flow strengthens and the
marine layer deepens tonight, the IFR stratus should gradually
lift to MVFR, but that may not happen until Monday morning. The
latest fcst models suggest that MVFR stratus will spread into the
southern Willamette Valley as early as 06Z Mon, then progress
through the rest of the interior lowlands overnight. Expect
widespread MVFR by 15Z. The interior clouds will gradually lift
and scatter during the day tomorrow, with VFR CIGs by late
morning. The coast may hang onto MVFR well into the afternoon.

An approaching upper level trough will bring a slight chance for
elevated thunderstorms late tonight and tomorrow morning, with
the best chances over the southern Willamette Valley and Lane
County Cascades.

KPDX AND APPROACHES...MVFR stratus likely to arrive 12-15Z Mon ,
but lift to VFR by about 18Z. Bowen/Pyle


.MARINE...A southerly wind reversal worked through the nearshore
waters today, while winds over the outer waters remained out of
the northwest. Expect little change through tonight. Weak high
pres will build in Mon and remain through most of next week. This
will bring a return to northerly winds. The winds may come close
to Small Craft Advisory threshold during the afternoon and
evening on Tue and Wed, but it looks very borderline. Models show
the high pres weakening late next week.

Seas to continue well below 10 ft for the next several days.
There is the potential for another uni-wave set-up Tue and Wed.
Overall, despite fairly low wave heights, the pre-dominant wind
wave component will make for choppy sea conditions. Bowen/Pyle





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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 forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.