Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
904 PM PDT Sat Jul 30 2016
Synopsis...an upper level trough will move across the area through
the weekend, which will result in near normal daytime temperatures.
The marine layer will deepen tonight and Sunday as a weak disturbance
associated with the upper trough slides across the region. Weak high
pressure returns Monday, but a deeper upper trough is expected to
track across the Pacific northwest Monday night through Tuesday
night. Seasonal conditions return for the middle part of next week,
followed by another upper trough late in the week.
Short term...(tonight through tuesday)...
a great evening across the region, with temperatures in the upper
60s to lower 70s inland, and 60s along the coast. Not much change in
the overall forecasts and grids, as will only make minor tweaks to
cloud cover across the interior later tonight into Sunday am.
Water vapor imagery shows one upper trough over southern British
Columbia, with an attendant short-wave near the trough base. This is
the feature the NAM and GFS show sliding across Washington and
northern Oregon tonight. Another upper low was near 53n 153w.
Low clouds continue to increase over the inland coastal areas of the
South Washington and far north Oregon coast. With the short-wave
passing across the region later tonight, will see marine layer
deepen. NAM soundings for kpdx and ksle valid 14z Sunday nearly
double the marine layer depth compared to this morning. Soundings
imply tops to near 4000 ft msl, but only 2500 ft mean sea level over keug. The
higher resolution models, such as the hi-res arw, project the marine
layer to reach the north Oregon Cascade crest and possibly make it
into the Mt. Adams area valleys. Not sure if will get much in way of
clouds to south of Albany, but will keep patchy clouds for that area
early Sunday am. Models suggest later clearing inland, likely not
until early afternoon for valley areas north of ksle. This also means
slightly cooler daytime temperatures compared to today.
A weak transitory 500 mb ridge moves over the forecast area late
Sunday night and Monday morning. This feature gets kicked east Monday
afternoon ahead of the next upper low. The GFS continues to be
slightly more south with the low center versus the GFS, with the NAM
closer to the European model (ecmwf) solution. Models show a fairly strong 300 mb jet
associated with this low, on the order of 100 kt. The marine layer
Monday morning will likely end up a little less extensive than Sunday
morning and have warmed Monday Max temperatures slightly to account
for this trend. The upper low center moves to near Vancouver Island
06z Tue, with the GFS maintaining a slightly more south track. The
best dynamics with this trough will be north of the Portland forecast
area. However, 12z model runs indicate some spotty light quantitative precipitation forecast over the
far north zones Monday night and have increased probability of precipitation into the chance
category for the South Washington coast and Willapa Hills. The bulk
of the energy moves across Washington and extreme northern Oregon
late Monday night and Tuesday morning. The 12z GFS hints at a little
surface-based cape skirting the South Washington Cascades and
foothills in northern Cowlitz and Skamania counties late Tuesday
morning into the early afternoon. Have not included any thunder
mention, but is something future shifts will need to consider.
Long term...(tuesday night through saturday)...upper level ridging
will start to build into the pac northwest late Tuesday. Low level onshore
flow will bring the typical pattern of morning clouds and afternoon
sun. Expect a slight warming trend through Thursday, with temps near
Models continue to hint at the potential for an upper level trough
to impact the region late next week. However, the general trend has
been to keep these systems well to our north, so sensible weather
impacts to our forecast area looks to remain minimal. The good news
is that temps will cool slightly late next week, but generally
remain near seasonal normals. /64
Aviation...primarily VFR conditions across the area with clear
skies. Coastal stratus has started to increase, bringing MVFR
stratus to kast and the north or/South Washington coast. Will see cigs
lower to around 1500 ft later tonight and spread down the coast to
impact konp around 05z-06z. Will see stratus spread up the
Columbia River again tonight. Models indicate a stronger push of
marine stratus inland overnight, so expect MVFR cigs to cover more
of the north interior Sunday morning compared to this morning.
Interior clouds will become scattered around 18z. Coastal areas may
not completely clear, so expect sct-bkn MVFR layer through Sunday
afternoon and increase in coverage again Sunday evening.
Kpdx and approaches...VFR through 12z then increasing chances for
MVFR cigs around 2500 feet. MVFR ceilings most likely between 13z and
17z sun, then VFR conditions through Sunday afternoon and evening.
Marine...cancelled Small Craft Advisory for winds early for our
waters north of Cascade Head within 10 nm of the coast. Winds have
weakened to below 20 kt. Further south, wind observations along
the coast seem to indicate that we are still getting gusts to 20
to 25 kt within 10 nm of the coast. Will generally see winds
continue to weaken over the waters tonight as high pressure
weakens over the waters. General trend is for the surface high
over the NE Pacific to weaken somewhat, but remain in place
through the weekend. Advisory strength northerly winds may
continue over the central Oregon waters on Sunday, but should ease
below criteria further north. By Monday, winds should be below
advisory criteria over all our waters, with weaker northerly winds
until around Wednesday when strengthening high pressure may bring
a return of gusty northerly winds.
Seas are starting to subside, though remain very steep with wave
heights around 10 feet and dominant wave periods around 10
seconds. Will see wave heights in the near-shore waters drop below
10 ft as soon as after midnight tonight, while the outer waters
probably will continue to be around 10 ft until later Sunday
Pz...Small Craft Advisory for winds until 2 am PDT Sunday for
coastal waters from Cascade Head to Florence or out 10 nm-
waters from Cape Shoalwater Washington to Florence or from 10 to
Small Craft Advisory for hazardous seas until 5 am PDT Sunday
for coastal waters from Cape Shoalwater Washington to Florence or
out 60 nm.
Small Craft Advisory for rough Columbia River bar from 1 am to
4 am PDT Sunday.
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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.