Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
326 am PDT Friday Jul 3 2015
Synopsis...little change in the overall weather pattern is expected
throughout this Holiday weekend...with a strong upper level ridge
resulting in persistent inland heat. The ridge is keeping the marine
layer shallow...with its cooling influence generally limited to the
coast and adjacent coastal valleys. Inland valleys remain well above
normal with temperatures in the 90s through at least Sunday...
possibly longer. An upper level low cutting off near the central
California coast may spin some moisture up toward the Lane County
Cascades...but the best chance of thunder will be south toward the
or/California border. Some cooling is possible later next week as the upper
ridge loosens its grip...but well above normal temperatures appear
likely through at least the middle of next week.
Short term...today through Sunday...hot weather continues for all
but the coast of southwest Washington and northwest Oregon as a
strong and sprawling upper level ridge extends from Texas through
Oregon and into NE Pacific. This ridge will likely keep the marine
layer too shallow for any appreciable cooling in the inland
valleys...at least through Sunday and probably longer.
Model 850 mb temperatures remain in the +20 to +22 degree c range...yielding
temperatures well into the 90s for the inland valleys given full daytime
sunshine through Sunday. Surface pressure gradients remain onshore...
keeping the coast and adjacent coastal valleys much cooler with areas
of night and morning low clouds clearing each afternoon Fri/Sat.
A cold front moving across western Canada Sat/Sat night will likely
be too far north and east to have much of an impact on the marine
layer. In fact...high pressure building into interior British Columbia may shove
part of the thermal low offshore...weakening onshore pressure
gradients and causing Sunday to be nearly as hot as yesterday.
Pushing the thermal low offshore may invite a southerly wind
reversal/surge to push up the coast...but at this point it appears
doubtful the resulting surge of marine air will be deep enough to
have a large impact on inland temperatures. If it does...the best chance for
a cooldown Monday would be near Eugene. The heat advisory remains in
place for most inland areas through Sunday...will hold on to that
though it may need to be extended into the night hours as relief will
be slow to come inland.
Will also need to watch an upper low forming off the California
coast...which is expected to remain nearly stationary the next few
days. This system may trigger afternoon and evening thunder as far
north as the south Oregon Cascades this weekend...with some debris
cloudiness clipping Lane County. Unless the upper low wobbles a
little further north than presently expected...instability will be
lacking for thunder in our Cascades this weekend. Kept mention of
thunder out of the forecast through Sunday. Weagle
Long term...no changes. Previous discussion follows...
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...widespread VFR next 24 hours at inland taf sites under
stable westerly flow aloft. IFR stratus returned to the coast
with ceilings below 1000 feet and occasional visible down to 2 sm. Should
still see some marine stratus 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
Kpdx and approaches...VFR will persist for the next 24 hours. /26
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
where a Small Craft Advisory continues until Sat morning.
Advisory level winds over the inner waters have ended in the north
but has persisted south of Cascade Head so will keep the Small
Craft Advisory up through this evening. Thermal low pressure will
move offshore this weekend with winds across all waters remaining
below 20 knots into next week. Some models are showing a wind
reversal occurring late sun at that looks reasonable.
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. Strong winds off of Vancouver
Island will produce a building northwest swell this weekend that could
push seas to near 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 until 7 am PDT Saturday 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
Small Craft Advisory for hazardous seas until 7 am PDT Saturday
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 until 11 PM PDT this evening for
coastal waters from Cascade Head to Florence or out 10 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.