Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
837 PM PDT Sat Apr 18 2015
Synopsis...sunny and warm weather is expected through Monday as high
pressure dominates the Pacific northwest. Temperatures will warm up a
bit each day...likely reaching 80 degrees for some inland areas by
Monday. An upper trough may bring a few showers as early as
Tuesday...with unsettled weather and cooler temperatures expected for
the rest of the week.
Short term...tonight through Tuesday...few changes needed to the
on-going forecast. 00z ksle sounding yielded an 850 mb temperature of
12... higher than model forecasts from yesterday. Inland
valley maximum temperatures were in the middle to upper 70s today. European model (ecmwf) shows 850
temperatures 13-14c sun...with the NAM and GFS just a touch lower. Biggest
change Sunday will be the wind. The north-S surface gradient should relax
a bit. This would also allow the southern Willamette Valley to see more
warming Sunday than the rest of the valley. Nudged maximum temperatures up a
couple degrees in some locations...putting a few Interior Valley
spots at 80 degrees. NAM and GFS show the thermally-induced surface
low along the coastline 12z sun then drifting it somewhere between
the Oregon Coast Range and Willamette Valley during the afternoon.
The 00z NAM has the upper ridge centered over the forecast area Monday
afternoon...with the thermally-induced surface low axis over the
Willamette Valley. Thus...would expect a couple degrees of additional
warming inland Monday. Models actually show 850 mb temperatures starting to
cool Monday afternoon...but with a slightly warmer start would expect
that to counteract the 850mb cooling trend. A couple other things to
consider for Monday is how much drift smoke aloft will be over the area
and how the reduced low-level mixing attributed to the diminished
surface wind. Stratus likely to return along the coast Monday as the
thermal trough shifts inland. Typically...the models tend to shift
the thermal trough east too soon. The 00z NAM shows it along the
Cascades 00z Tuesday...but would not be surprised if it slipped east of
the crest a little later.
The marine layer and associated stratus will continue to deepen
Monday night...pushing inland on onshore flow later in the night and
Tuesday morning. 12z/18z NAM suggest a southwesterly marine push Monday
night/Tuesday morning...which seems reasonable considering SW flow aloft
and the thermal low crossing the Cascades into the Columbia Basin.
This should result in a significant cooldown Tuesday...bringing temperatures
closer to April normals. Some areas of drizzle may develop in the
stratus along the coast and in the hills. Model quantitative precipitation forecast also seems to
indicate the deep marine layer producing a couple showers Tuesday
afternoon as the boundary layer mixes and turns over. The marine
layer should severely limit the potential for deep convection across
SW Washington and northwest Oregon except right along the Cascade crest...
where surface parcels will start above the marine inversion. Decided
to carry a slight chance of thunder right along the crest Tuesday
afternoon as a result.Weishaar/weagle
Long term...Tuesday night through Saturday...unsettled weather
expected into the weekend as a longwave trough remains situated over
the northern Pacific and multiple shortwaves develop off the parent
trough. As the first of these waves moves through the area on
Wednesday...expect showers through the day. When this front moves
off to the east...there could be a dry period Wednesday night
through midday Thursday as high pressure briefly builds ahead of the
next front forecast to move through Thursday night into Friday.
Have left enough probability of precipitation for a slight chance during the day Thursday as
there is some uncertainty on the timing of these frontal systems
moving through. The next front will bring another round of showers
for Thursday night through Saturday...with even more uncertainty on
the timing of this front moving through. -McCoy
Aviation...VFR conditions through 20/06z as high pressure aloft
remains in control. Some patchy ground fog may form near the
coastal terminals but have too low of confidence there will be any
significant impact to visibilities. A surface thermal low will remain
over SW Oregon and extend up the north Oregon coast. Lower
pressure gradients will result in lighter winds Sunday but still a
bit gusty along the coast late afternoon and early evening.
Kpdx and approaches...no restrictions expected. /Jbonk
Marine...high pressure remains in place through Monday. Surface
thermal low pressure over northern California will continue to
exert influence along the Oregon coast. Pressure gradients will
weaken Sunday but still provide enough wind gust speeds to
warrant a Small Craft Advisory for winds through Monday morning.
The strongest winds will be over the central Oregon waters south
of Cascade Head...where winds of 25 to 30 knots will continue. Winds
will be lighter north of Cascade Head...closer to 20 to 25 knots. The
winds will be strongest during the afternoon and evening hours.
Winds then diminish somewhat and shift out beyond 20 nm for the
overnight and early morning hours.
Slightly weaker wind wave components with a lessening northwest swell
should keep seas at 9 feet or below through Monday...although they
may touch on 10 feet from about Florence and south Sunday afternoon
The next front will move through the waters on Tuesday. Then the longer
range shows high pressure over the NE Pacific strengthening during the
middle of next week..with northwest winds gusting 20 to 30 knots at times.
Also expect increasing northwest swell to push seas back into the low to
middle teens starting Wednesday. Jbonk/pyle
Pz...Small Craft Advisory for winds until 11 am PDT Monday for
coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar from 4 am to
8 am 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.