Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
916 am PDT sun Aug 2 2015
Synopsis...as a strong positively tilted upper level ridge slowly
moves east today...an upper level low off the California coast will
continue steering more moisture and instability northward over
western Oregon and into southwest Washington today into Monday... for
a threat of some showers or thunderstorms. An upper level trough
along the b.C. Coast will begin moving south toward the Pacific
northwest later tonight and Monday...and have an increasing impact
over southwest Washington and northwest Oregon during the middle week
period for cooler temperatures and more night and morning clouds.
Short term...as suspected Saturday...a considerable amount of
convective cloudiness and debris have already spread over northwest
Oregon and southwest Washington overnight and this morning...and even
over the coastal waters. Lightning late Saturday was mainly near
Willamette Pass...then shifted to over the coastal waters overnight
and this morning.
The area of moisture over the coastal waters and over the north
coastal mountains into the South Washington Cascades with some
showers. Have not seen any lightning with this area in a while. This
area of moisture will move north into west central and northwest
Washington during the middle day period and slowly out of our forecast
area. The models this morning showed a weak upper jet segment with
this moisture that moves north out of our area later today.
Another area of moisture is moving north from southwest Oregon into
Lane County this morning...and did produce some lightning an hour or
two ago as it was crossing into Lane County from Douglas County. This
moisture will continue moving north through northwest Oregon and
eventually into southwest Washington through the afternoon and
tonight. The models show another jet segment associated with a short
wave pivoting through our forecast area from the south tonight into
Monday morning. This takes some of the moisture back to the coast and
into the coastal waters again. Expect a few showers to move across
the area as this moisture continues to feed north. Cannot really rule
out thunder anywhere as well. There will be some chance of thunder in
the south part of our area this morning spreading north in the
afternoon and evening.
The short wave and moisture area will still be over the forecast area
early Monday before moving east in the afternoon.
By Monday an upper trough along the b.C. Coast will be sagging south
along the coast toward the Pacific northwest. This will help turn the
upper flow Monday afternoon to more southwesterly...and limit the
thunderstorm potential later Monday to our southern Cascade areas and
especially near the crest. The upper trough will continue to sag
south with more energy in Washington by Tuesday...with more westerly
flow aloft over southwest Washington and northwest Oregon.
Again...the main threat of thunder for our area will be near
Willamette Pass later Tuesday.
The marine layer is again shallow and near the coast this morning.
Will continue the heat advisory for the inland valleys today...
though the clouds may affect how warm we get today. Humidities will
be slowly rising today as well. The marine layer will begin pushing
into the coastal gaps tonight and Monday morning...but inland
coverage will be spotty. Inland temperatures will be lower Monday due to the
gradual cooling of the air mass aloft. Expect better inland coverage
of low clouds Tuesday morning...especially north. Temperatures Tuesday will
be closer to normal especially north. 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...previous discussion...VFR inland this morning but with
some middle/high clouds from convection over SW Oregon will drift over
the region. Slight chance of seeing rain/thunder over the southern
half of the forecast area 10z-17z but chance appears small enough to
preclude mention in any particular taf. At the coast...shallow IFR
stratus will continue along the central coast and likely develops
along the north coast by 11z. This stratus will retreat offshore
around 18z for VFR conditions along the coast. Expect developing
cumulus Sun afternoon/evening with the possibility of thunderstorms
expanding over much of the area east of the Coast Range 03z-10z
Tuesday...but again insufficient confidence to include in any particular
taf at this time.
Kpdx and approaches...VFR conditions tonight and Sunday. Some
residual middle/high clouds from convection over SW Oregon will drift
overhead this morning. Likely see cumulus development this afternoon
with the potential for a thunderstorm. Chances best S and east of
terminal and in Cascades...but cannot rule out one impacting the
terminal during 04z-10z. Cullen
Marine...previous discussion...a few thunderstorms offshore are
drifting from S to north into the outer central Oregon coastal waters
early this morning. Potential threats are lightning and gusty outflow
winds...with the threat area expanding to cover much of the coastal
waters this morning and again in the evening hours.
Otherwise...little change to the overall pattern for Sunday with north
winds over the waters. An overnight ascat pass revealed winds still
around 20 knots over the outer waters...so will allow the advisory for
winds to continue through 10 am as areas of marginally advisory
strength winds may still exist. Another Small Craft Advisory for winds in effect for
this afternoon/evening as more widespread gusts to 25 knots expected to
Meanwhile...the persistent northerly winds are maintaining steep seas
over the waters of 6 to 7 feet at 6 to 8 seconds. Therefore...with
conditions fluctuating above or very near our criteria for hazardous
seas...the advisory for hazardous seas looks on track for today.
or...heat advisory until 9 PM PDT this evening 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...heat advisory until 9 PM PDT this evening 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 this morning 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 winds from 3 PM this afternoon to
midnight PDT tonight for coastal waters from Cape Shoalwater
Washington to Florence or out 60 nm.
Small Craft Advisory for hazardous seas until 4 PM PDT this
afternoon 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.