Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
900 PM PDT Monday Jul 27 2015
Synopsis...high pressure aloft continues to build over the region
and strengthen through Thursday. This strong high pressure will
remain over the Pacific northwest into the weekend...providing for
another round of very warm conditions.
Update...skies have cleared this evening across northwestern Oregon and most of
southwestern Washington. Low level flow continues slightly onshore over northern half of
forecast area...such that morning stratus will return to coastal zone and
portions of the Columbia River valley to nearly pdx late tonight. By
15z...models Show Low level flow turning slightly offshore for sunny
skies and commencement of the next heat wave.
Upper ridge then builds into Pacific-northwest through Thursday. Under full sun
and offshore low level winds...temperatures reach the middle 90s Wednesday
and middle to upper 90s Thursday. Right now...heat peaks on Thursday as
model consensus shifts heat low east of the Cascades Friday. Airmass
still remains warm though...with high pressure aloft and maximum temperatures
near 90 over inland locations through the weekend. No updates to
existing forecast. Bright
Previous discussion... /issued 256 PM PDT Monday Jul 27 2015/
Short term...tonight through Thursday...a fairly abundant field of
stratocumulus has developed across the region today from the Coast
Range eastward. Meanwhile...high pressure aloft is building over the
region and will continue to strengthen tonight through the next few
days. These clouds inland are expected to gradually thin late this
evening and early tonight...enabling nearly ideal radiational cooling
conditions overnight. Models do suggest some marine stratus pushing
into the S Washington and north Oregon coasts after 06z and will probably
reach Kelso/Longview as well. Additionally...will likely see the
typical area of low clouds develop near the confluence of the
Willamette and Columbia rivers and stretching east into the far north
Oregon Cascade foothills around daybreak Tuesday. Low level winds
will turn to a dry north-NE flow by Tuesday morning which should enable
any clouds that do form to clear fairly quickly. The surface pressure
gradient down the Willamette Valley will increase to around 3-4 mb by
the afternoon so expect breezy conditions in the late
afternoon/evening...with gusts to 20-25 miles per hour from Salem south.
Temperatures will also be noticeably warmer for Tuesday as 800 mb
temperatures increase from around 5 to 6 degrees c today to ranging
from 15 degrees c in the north to 19 degrees c in the south. This
translates to temperatures in the middle to upper 80s across the
Willamette Valley...ten or more degrees f warmer than today.
Wednesday will be another day of significant warming as the upper
ridge further strengthens. Models suggest 500 mb heights rise to around
5890m Wednesday afternoon with 800 mb temperatures well into the lower
20 degree c range...which will support temperatures climbing well
into the 90s...ranging from the middle 90s in the north to near 100
south. Models continue to depict a less sharply defined thermal
trough through the week...but a slight NE component to the low level
flow may help the coast warm into the 70s...especially for the north
Oregon coast where temperatures may reach the upper 70s.
Temperatures climb slightly higher on Thursday...likely the hottest
day of the stretch for most locations...as models increase 500 mb heights
Above 5900 meters. Cullen
Long term...Friday through Sunday...the extended period of heat
continues into the weekend...but temperatures will likely cool a few
degrees over the weekend as the upper ridge flattens a little in
response to a weak shortwave passing through SW Canada.
Meanwhile...the forecast models suggest that some moisture may begin
to push north into Lane County Saturday and Sunday. And have
maintained the slight chance of thunder in the higher Cascades for
now. Thus...without a strong marine surge to bring about a sharp cool
down...temperatures will remain well above normal through the
Fire weather...the latest forecast models indicate that a thermal
trough will build north from California into Oregon on Tuesday and Tuesday
night...bringing drying northerly flow to the region. There will
also likely be some light easterly winds developing over the Lane
County Cascades and southern Willamette Valley Tuesday night into Wednesday
morning. The thermal trough then expands northward into the Oregon
Cascades and potentially into the Willamette Valley and adjacent
foothills on Wednesday and Thursday. This will bring enhanced instability and
mixing...that combined with the expected very hot and dry
conditions...may create critical fire weather conditions.
Also...although many locations received rain over the
weekend...fuels are forecasted to dry out rapidly during the midweek
period. A Fire Weather Watch has been issued starting Wednesday afternoon
and continuing through Thursday evening. The thermal trough is currently
expected to start pushing east of the Cascades on Thursday night and Friday.
Aviation...VFR conditions across the board this evening with
scattered cumulus around 6000 feet beginning to dissipate across inland
areas with the loss of daytime heating. MVFR marine stratus
returns to the SW Washington and northwest or coast between 04z-06z
Tuesday...pushing up the Columbia River a bit after 10z Tuesday. However
do not expect marine push to reach the pdx area. Marine clouds
should clear by late morning Tuesday for another VFR day. North
winds will increase Tuesday afternoon particularly along the central
coast and central/south Willamette Valley.
Kpdx and approaches...VFR next 24 hours. May see some scattered
clouds around 1500 to 2500 feet Tuesday morning but do not expect a
Marine...winds at buoy 50 are beginning to pick up with current
gusts to around 20 knots. Expect the increasing trend to continue
overnight and Small Craft Advisory for winds looks good. Although
difficult to reflect in the forecast grids...there is some chance
for square seas 7 to 9 feet Tuesday when wind wave may become
dominant with period around 8 seconds...so will also keep Small
Craft Advisory for seas in place as is beginning tomorrow. Best
chance for significant square seas will be over the central
Our typical summertime pattern continues the rest of this week.
High pressure will continue to build over the NE Pacific with thermal low
pressure strengthening over S Oregon. As the thermal low
strengthens...models show a few borderline gale force gusts
around 32-34 knots possible over the central waters Tuesday
afternoon...but the best chance for possible gale force gusts
looks to be Wednesday afternoon. However even then chances appear to be
fairly low. Bowen/64
or...Fire Weather Watch from Wednesday afternoon through Thursday
evening for zone 604-zone 605-zone 606-zone 607-zone 608.
Pz...Small Craft Advisory for winds from noon Tuesday to 5 am PDT
Thursday for coastal waters from Cape Shoalwater Washington to Cascade
Head or out 60 nm.
Small Craft Advisory for hazardous seas from noon Tuesday to 3
am PDT Wednesday for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for winds until 5 am PDT Thursday for
coastal waters from Cascade Head or to Florence or out 60 nm.
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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.