Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 131545 CCA
AFDPQR

Area Forecast Discussion...CORRECTION
National Weather Service Portland OR
845 AM PDT Thu May 13 2021

Updated aviation discussion.


.SYNOPSIS...Summer-like weather will continue today, with above
normal temperatures inland and cooler, cloudier conditions near the
coast. Similar weather should prevail through the upcoming weekend,
though a weak disturbance may trigger a thunderstorm or two in the
southern Oregon Cascades Friday afternoon. The next substantial
chance for rain comes next Tuesday or Wednesday as a Pacific cold
front moves through the region.

&&

.SHORT TERM...Today through Saturday...Our dry spring continues today
as high pressure persists off the Pac NW coast. Overall, GOES-West
imagery and surface observations reflect a summer-like onshore flow
pattern with solid low clouds along the coast and mainly clear skies
inland. Marine stratus is encroaching on Philomath and Corvallis, and
another finger of marine stratus has pushed up the Columbia River to
near Kelso as of 3 AM. With KAST-KPDX MSLP gradients still decidedly
positive, suspect low clouds will reach at least eastern portions of
the PDX metro area. It will probably be a little more difficult for
stratus to push into central and southern portions of the Willamette
Valley, but cannot rule it out. Either way, most inland areas should
see low clouds clear by midday, leaving variable high clouds for the
rest of the day. With 850 mb temps similar to Wednesday, expect
similar high temperatures this afternoon, though perhaps a couple of
degrees cooler if high clouds are opaque enough.

Weak zonal flow eventually turns more cyclonic tonight and Friday as
some northerly jet stream energy carves out a weak upper trough over
the Pac NW. Some models suggest the resultant cooling aloft will
provide enough instability for a few showers and/or thunderstorms
over the southern Oregon Cascades Friday afternoon, possibly clipping
the Lane County Cascades with a cell or two. We went ahead and added
a slight chance of thunder to our Lane Cascades as a result.
Otherwise, Friday should be another day similar to Wednesday, with
high temps once again pushing 80 degrees for the inland valleys.
Upper level ridging re-establishes itself over the Pac NW on
Saturday, likely continuing our string of mostly dry and unseasonably
warm weather...especially inland.  Weagle

.LONG TERM...Saturday night through Wednesday...Minimal changes.
Previous discussion follows...Over the weekend models show the upper
ridge building again off the WA/OR coast as the aforementioned upper
trough continues to dig into California into the desert SW. This
should result in inland temperatures warming into the upper 70s to
mid 80s while onshore flow will keep the coast cool.

Confidence is increasing, but is not very high, that a more
significant shortwave trough will slide across the Pacific Northwest
next week. There are still substantial timing differences in the
deterministic models with the Canadian and GFS faster then the ECMWF.
The faster solutions indicating pcpn possible Monday or Tuesday while
the ECMWF slows the trough, then lifts it northward with a trailing
front moving ashore Wednesday. NCEP cluster analysis show most
clusters have QPF below the average of all models for Monday, so low
or no PoPs look quite reasonable. For Tue and Wed 2 of 4 clusters
have QPF above the average of all models, so high PoPs look
reasonable.

&&

.AVIATION...High pressure remains anchored over the Pacific for
the foreseeable future. Morning satellite imagery shows a layer
of high level VFR clouds with a lower IFR cloud deck along the
coast west of the Cascades. With weak northwesterly flow over the
area, marine stratus has filled in coastal terminals keeping
CIGs around 300-500 ft. With the high clouds in place atop of
this IFR deck and continued northwesterly, likely will be
challenging for them to lift and clear out through the day. The
only chance will be with the increase in winds through the
afternoon. These winds could help to mix out the low levels which
may assist in rising clouds to MVFR. Inland locations however
will maintain their VFR stratus becoming scattered at 25,000 ft
or clear through much of the day. Overnight could see some
filling as temperatures cool but overall should stay VFR through
the forecast period.

In the afternoon, the thermally induced trough over the Pacific
will intensify causing winds to increase slightly. This will
mainly impact the near coast sites as gusts up to 25 kt are
possible, but could see breezy conditions through the Willamette
Valley.

For detailed regional Pac NW aviation weather information, go
online to: https://weather.gov/zse

KPDX AND APPROACHES...No impacts expected. VFR cigs with
scattered to clear skies through much of the forecast. Could see
skies fill to broken but still remain VFR. Breezy north-northwest
winds possible in the afternoon.  -Muessle

&&

.MARINE...Minimal changes to the overall forecast through
Saturday as the overall pattern will show minimal change.
A thermally induced low pressure over southwest Oregon and
northern California continues to bring typical summertime
northerlies across the waters as high pressure persists offshore.

Expect afternoon and evening gusts around 25 to 30 kt, with the
highest gusts in the central waters and south of Tillamook Head
for the northern waters today. This afternoon a shortwave upper
level trough looks to pass parallel to the coast, as this
shortwave passes the surface pressure gradient will tighten up
causing the Small Craft Advisory winds to expand across the
northern marine zones this afternoon. Therefore have adjusted the
current Small Craft Advisories for all zones through Friday
morning.

Combined seas will largely be wind wave driven as incoming
distant swell trains will generally remain near 4 ft or lower.
Expect steep and choppy seas as wave periods bounce between 9 to
11 second swells and 7 to 9 second wind waves as general seas
will hover around 4 to 6 feet into the weekend. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM this morning to 11 AM PDT
     Friday for coastal waters from Cape Shoalwater WA to
     Cascade Head OR out 60 NM.

     Small Craft Advisory until 11 AM PDT Friday for coastal waters
     from Cascade Head OR to Florence OR out 60 NM.

&&


$$

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