Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
831 PM PDT Sat Aug 1 2015
Synopsis...a strong upper level ridge persists over the area through
Sunday maintaining hot inland temperatures. An upper level low off
the California coast will steer some monsoonal moisture over western
Oregon into Monday. This will bring a threat of thunderstorms to the
Oregon Cascades...with some debris moisture possible west of the
Cascades later tonight into Monday. An upper level trough will move
over the area Monday through Thursday for gradual cooling.
Short term...we had another temperature record breaking day today
with five climate locations setting new daily high temperature
A low offshore of northern California is helping to steer monsoonal
moisture northward over southern Oregon. The low and the moisture has
resulted in quite a bit of thunderstorms just south of the Portland
forecast area this evening. A few of the thunderstorms and showers
brushed along the Lane County Cascades late this afternoon.
The storms are winding down with the loss of daytime heating...but
there may be some lingering showers tonight across the interior of
The upper ridge will shift inland Sunday as an upper level trough
approaches from the northwest. This will result in an increase of
onshore winds. The approaching trough will help steer the monsoonal
moisture and clouds from the south further northward over northwest Oregon
Sunday. This will increase the threat for thunderstorms over northwest
The increase of clouds and the onshore winds will result in slightly
cooler afternoon temperatures. Interior temperatures should peak in
the low to middle 90s rather than the triple digit temperatures of late.
Have therefore decided to replace the excessive heat warning for the
interior low lands with a heat advisory. There should be enough of a
decrease in temperatures Sunday to drop the heat advisory for the
Coast Range and Cascade foothills.
The onshore flow will continue to increase into Monday with possibly
some marine clouds reaching Portland and Eugene. This may be enough
to keep inland temperatures just around 90. Thunder after Monday
morning will become more confined to the higher Cascades and in the
south as the upper flow starts to turn more southwesterly Monday
afternoon and evening.
The models indicate a better marine intrusion inland on Tuesday
morning and went with more morning clouds in the North Valley then.
Temperatures will be in the lower to middle 80s in the North Valley and
middle to upper 80s in the South Valley. The best chance of any thunder
on Tuesday afternoon will be the central Oregon Cascades...but light
southwest flow aloft makes that not certain either. Tj/Tolleson
Long term...no changes. Previous discussion follows...the general
trend in the extended period will be towards cooler weather. Models
continue to show an upper level trough brushing by the region late
Tuesday...but keeps most of the energy to our north. Will keep a
slight chance of thunderstorms over the Lane County Cascades for late
Tuesday due to some lingering instability. Temperatures return
towards seasonal normals by Wednesday...but will increase slightly
for the latter half of the week as a weak upper level ridge builds
into the Pacific northwest. Models diverge significantly as we go
into next weekend. As such...have used a broad brush approach for
next weekend and introduced a slight chance of showers over the South
Washington Cascades and a slight chance of thunderstorms for the
Oregon Cascades. /64
Aviation...VFR inland. An area of stratus continues to stay close
to the coastline with pockets of IFR...especially along the
central Oregon coast. Expect stratus and IFR ceilings to move into
north coast later tonight. Middle and high clouds from convective
debris over SW Oregon/nocal will continue to stream northward
overnight into Sunday. There will be some convective potential and
cumulus build ups Sunday afternoon/evening in the Eugene and
perhaps as far north as Salem.
Kpdx and approaches...VFR conditions tonight and Sunday.
Marine...no significant change in the large scale pressure
pattern. Will continue to see northerly winds of 10 to 20 kts with
occasional higher gusts over the waters for the next several
days. Gusty winds around 25 kts will develop Sunday afternoon and
evening...then easing Monday.
The persistent northerly winds is producing choppy seas of 6 to 7
feet with short periods making it rough for boats venturing out for
Fire weather...red flag warning remains in effect through 05z for
middle and high-level Haines 5-6. Most unstable conditions will be over
the north zones...north of a line from around Benton County east to
Montana. Jefferson. Middle and high-level moisture filtering into the south
zones will likely produce high-level Haines 4 values...helping to
reduce potential column vertical development. One other near-term
concern is lightning in the far southeast part of the area. At 22z a few
strikes noted along the Cascade crest near Willamette Pass. Feel any
convection this evening should be limited to zone 608 and perhaps
the east half of zone 606. Convective threat spreads north and west
Sunday as south flow aloft further north and 500 mb ridge axis
gradually shifts north as well. Have spread mention of thunderstorms
to the S Washington Cascades sun evening through Monday morning. Will need to
consider potential highlights for sun...but for now feel like
coverage will not be enough for red flag. Also...any storms that
develop over the S half of the area Sunday will tend to become
wetter due to the longer-duration moist south flow. Convective
threat gets pushed east of the Cascades Monday night and Tuesday...with any
threat Tuesday near the crest of zone 608. Weishaar
or...red flag warning until 10 PM PDT this evening for central
Oregon Cascade foothills-east slopes of the central Oregon
Coast Range-Mt.Hood National Forest west of Cascade crest-north
Oregon Cascade foothills-north Oregon Coast Range-
Willamette National Forest-Willamette Valley.
Heat advisory until 9 PM PDT Sunday for central Columbia River
gorge-central Willamette Valley-greater Portland metropolitan area-
lower Columbia-south Willamette Valley-Upper Hood River
Valley-western Columbia River gorge.
Washington...red flag warning until 10 PM PDT this evening for Clark County
lowlands-east Willapa Hills-extreme South Washington
Cascades and foothills.
Heat advisory until 9 PM PDT Sunday for central Columbia River
gorge-Greater Vancouver area-I-5 corridor in Cowlitz County-
western Columbia River gorge.
Pz...Small Craft Advisory for winds until 10 am PDT Sunday for waters
From Cape Shoalwater Washington to Cascade Head or from 10 to 60 nm-
waters from Cascade Head to Florence or from 10 to 60 nm.
Small Craft Advisory for winds from 3 PM Sunday to midnight PDT
Sunday night for coastal waters from Cape Shoalwater Washington 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.