Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
255 PM PDT Thursday Jul 2 2015
Synopsis...strong high pressure aloft will persist over the
forecast area through the weekend. The warmest day of the current
hot spell appears likely to be Sunday. Slight cooling looks possible
early next week as the ridge weakens some...but daytime temperatures
are likely to remain above normal.
Short term...northwest to southeast oriented ridge axis expected to remain just
to the S of the forecast area through the weekend. Models continue
to show some weakening of the ridge over the region resulting in the
development of a col. Overall this results in a continued rather
stagnant weather pattern.
The low level onshore flow is expected to strengthen a bit Friday and
Sat. Upper heights coming down a little...aided in part by a
shortwave rotating around the west Canada upper low...suggest daytime
highs to come down a few degrees Friday and Sat. Model 800 mb temperatures however
remain in the 20 to 22 degree c range...so expect high temperatures likely to
remain above 90 through Sat in the inland valleys. As the SW Canada
shortwave moves on...upper heights rebound and 800 mb temperatures return to
near thursdays numbers. With the quick return to very hot weather
sun...will extend the current heat advisory out through sun. Marine
clouds through this period will remain rather limited. With the
strengthened onshore flow expect to see more clouds pushed up the
lower Columbia by Sat morning...then less clouds again Sun morning.
Long term...Sunday night through Thursday...the extended period
begins with high amplitude 500 mb Rex block over the eastern Pacific
and or/Washington in weak trough or col separating southwestern U.S. Ridge and eastern
Pacific/Gulf of Alaska ridge. Despite the weak troughiness...air mass remains
warm with above average heights and northerly low level flow. Little
change occurs in overall pattern into Tuesday. By Wednesday...block
begins to break down with ridging to the north and Pacific cutoff
making slow eastward progress toward central California cstln. Deterministic models
and gefs members in good agreement through Tuesday...with
differences thereafter mainly in amplitude of ridging over or/WA/wrn
Canada and position of eastern Pacific cutoff. Despite the increasing
uncertainty middle week...warm temperatures nearly certain /10-12 degree
above normal/ but influence of eastern Pacific cutoff provides a chance
of inland/mountain thunderstorms Tuesday-Thursday. Bright
Aviation...VFR conditions will persist at inland taf site for the
next 24 hours under stable westerly flow aloft. However...marine
stratus looks to push inland up the Columbia River tonight and
could bring periods of MVFR ceilings to kkls and possibly kspb. The
stratus does not look like it will reach the Portland metropolitan
Marine stratus along the coast has generally cleared leaving
widespread VFR conditions. Expect VFR conditions to continue
through around 05z. Widespread IFR/LIFR conditions are expected to
return to the coast tonight after 05z. Expect fog/stratus to fill
in from south to north...reaching kast after 09z.
Kpdx and approaches...VFR will persist for the next 24 hours.
Marine...high pressure over the NE Pacific and thermal low pressure inland
will continue to produce gusty northerly winds into the weekend.
The strongest winds are expected to continue over the outer waters
so decided to extend the Small Craft Advisory till Sat morning.
Advisory level winds over the inner waters will continue though
late this evening before falling below 20 knots. Thermal low pressure
will move offshore this weekend with winds across all waters
remaining below 20 knots into next week.
Expect periods of steep and choppy seas over the outer waters to
continue into this weekend with combined seas of 7 to 8 feet and a
dominate period around 8 seconds. As such...decided to extend the
Small Craft Advisory till Sat morning for square seas. Strong
winds off of Vancouver Island will produce a building northwest swell
this weekend that could push seas above 10 feet. Seas will subside
early next week to around 4 to 5 feet.
or...heat advisory until 6 PM PDT Sunday for northwest Oregon interior
lowlands...Cascade foothills...and central Oregon Coast
Washington...heat advisory until 6 PM PDT Sunday for lower Columbia and
SW Washington interior lowlands.
Pz...Small Craft Advisory for winds and hazardous seas until 7 am
PDT Saturday for waters from Cape Shoalwater to Florence
from 10 to 60 nm.
Small Craft Advisory for winds until 11 PM PDT this evening
for coastal waters from Cape Shoalwater to Florence out
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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.