Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
339 am PDT Sat Jul 4 2015
Synopsis...little change in the overall weather pattern is expected
through the Holiday weekend and into early next week. A strong upper
level ridge will remain over the region to keep inland temperatures well
above normal with continued dry conditions. Early next week an upper
level low cutting off near the central California coast may spin
some moisture up toward the Lane County Cascades for a chance of
thunderstorms. Some cooling looks possible late in the week as the
upper level ridge gives way to a weak low pressure center aloft.
Short term...today through Monday...temperatures are a few degrees
cooler inland this morning...thanks in large part to a surge in
onshore pressure gradients Friday evening. This allowed a bit more
marine air to penetrate inland than the past several nights. Stratus
has been spotty along the coast thus far...but 11-3.9um satellite fog
product is starting to show some stratus developing up the Columbia
to near Kelso. Low clouds may make it as far inland as Clark County
toward sunrise...but are not expected to become widespread in the
The stronger onshore gradients are already starting to relax some as
an upper trough slides through British Columbia and into the Canadian
rockies. This is lowering surface pressure off Vancouver Island and
should result in weaker onshore gradients this afternoon and evening.
The upper trough is doing little to cool off the general air mass
across Oregon and SW Washington...850 mb temperatures are forecast to remain in
the +20 to +22 degree c range this afternoon. Slightly lower 500 mb
heights and 1000-500 mb thicknesses may shave a couple degrees off
high temperatures today versus yesterday...but it will still be hot inland
with temperatures well into the 90s. The lower Columbia/Kelso area will
probably be a bit warmer today due to the weaker onshore
gradients...back into the 90s after a brief reprieve Friday.
Tonight should be warmer again with near-record high minimum temperatures
expected again Sun morning. High pressure building behind the British
Columbia cold front will likely further weaken onshore
gradients...shoving thermal low pressure off the Washington/north or coast. This
will lead to another very hot day Sunday with little to no marine
influence except for the immediate coast. With 850 mb temperatures remaining
+20 to +22 degree c and little to no marine inversion...temperatures should
approach 100 degrees again for some of our inland valley locations.
With thermal low pressure being shoved offshore and an overall weak
synoptic flow pattern odds are good that a southerly wind reversal
will develop along the coast later sun/Sun night...and the 00z NAM
and 4km WRF-arw show this. With 500 mb heights remaining well into
the 580s it will be difficult for even a strong southerly surge to be
deep enough to bring relief from the heat into the Willamette Valley.
The best chance for this to occur Sun night/Monday is Eugene...but it
appears unlikely at this point. Overall air mass aloft changes little
Monday...so barring a deeper southwesterly marine push than
expected...Monday looks like another hot one with temperatures well into the
90s inland. Thought about extending the heat advisory into early next
week...but decided to hold off to get a better feel for the impact
the southerly surge will or will not have inland.
Meanwhile a cutoff upper low will linger off the California coast...but 00z
GFS/ECMWF/Gem all suggest it will be too far south through Monday to
bring thunder into our forecast area. This may change if the upper
low wobbles a little further north than currently expected. Weagle
Long term...Tuesday through Friday...GFS and European model (ecmwf) show the upper
col remaining over Oregon Tuesday and Wednesday for continued hot weather. This
also leaves the S Cascades open instability and moisture aloft
sneaking up from the S...so will keep a slight chance for diurnal
thunderstorms over the Lane County Cascades both Tuesday and Wednesday. Models
at this point are suggesting some relief to the extended heat wave
coming later in the week as the upper low cutoff off the California coast
moves inland over California potentially bringing a deeper SW push of marine
air inland. Models indicate the troughiness over the western states
is then reinforced Friday as another trough moves in off the North Pacific
for even cooler air. Will trend temperatures downward a little more for Thursday
and Friday given decent agreement in models with this type of
solution...but still hold some reservations in buying completely into
the change as models tend to struggle with cutoff lows especially this far
into the extended period.
Aviation...VFR next 24 hours under stable westerly flow aloft.
Expect local low MVFR to IFR stratus along the coast and lower
gaps leading inland this morning. So far the stratus has nearly
filled in kast to khqm...but is patchy for much the Oregon coast.
Do not expect ceilings to make it very far inland...maybe as far a
kkls. Afternoon breezes return this afternoon and early evening.
Kpdx and approaches...VFR will persist next 24 hours. There may be
some stratus reaching the kpdx area between 14z-17z and expect it
will be around 1000 feet if it forms. /26
Marine...high pressure over the NE Pacific and thermal low pressure inland
will continue to produce breezy northerly winds over the coastal
waters today. The stronger winds are further offshore but remain
below advisory criteria. The thermal low pressure will move westward
later tonight and Sunday...weakening the pressure gradient over the
waters. The forecast models are indicating that a southerly wind
reversal will occur sometime sun.
Expect periods of steep and choppy seas over the outer waters to
continue today with combined seas of 6 to 8 feet and a dominant
period around 8 seconds. Looks like hazardous sea conditions will
persist over the northern outer zone today primarily west of about
30 nm. Seas should gradually subside later this weekend as winds
slacken...reaching around 4 to 5 feet by early next week. Pyle/26
or...heat advisory until 6 PM PDT Sunday for Cascade foothills in
Lane County-central Coast Range of western Oregon-central
Columbia River gorge-central Willamette Valley-greater
Portland metropolitan area-lower Columbia-northern Oregon Cascade
foothills-south Willamette Valley-Upper Hood River Valley-
western Columbia River gorge.
Washington...heat advisory until 6 PM PDT Sunday for central Columbia River
gorge-Greater Vancouver area-I-5 corridor in Cowlitz County-
South Washington Cascade foothills-western Columbia River
Pz...Small Craft Advisory for hazardous seas until 5 PM PDT this
afternoon for waters from Cape Shoalwater Washington to Cascade
Head or from 10 to 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.