Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
159 am PDT Friday may 29 2015
Synopsis...high pressure aloft will be over the forecast area
through Saturday. Meanwhile...a closed upper level low will move into
British Columbia. Despite west flow aloft...there will be enough
residual instability over the higher Cascades today for a continued
threat of late-day thunderstorms. The upper ridge starts to flatten
today...but is expected to strengthen a little Saturday. Any
thunderstorm threat Saturday should be confined to the southern Lane
County Cascades. The next upper low swings toward the Oregon coast
Sunday...resulting in a little more cooling but a better chance of
Cascade thunderstorms. A much deeper marine layer is expected over
the area early next week.
Short term...today through Sunday...the large-scale pattern remains
virtually unchanged from 24 hours ago. Water vapor loop shows 500 mb
high pressure ridge axis extending from 30n 125w to a 577 dm high center
near 50n 140w. Also of note is a closed upper low along the central
British Columbia coast and another one out around 40n 150w. This
latter low is the one that will become a player for our weather late
in the weekend.
Today looks to be a near repeat of yesterday. The marine layer has
not changed all that much. Kkls reported overcast conditions around
07z...similar to 24 hours ago. The NAM and hi-res arw suggest marine
stratus will come down the Columbia River and fill in western Clark
County in SW Washington by 12z. The kast-kdpx gradient is slightly stronger
at 08z...2.1 mb...compared to 08z Thursday. This is likely due to the
warmer interior resulting in slightly lower surface pressure. Model forecast
850 mb temperatures are similar to Thursday...maybe up a degree inland.
Thus...have nudged maximum temperatures up a degree or two...based on Thursday
values. Today is likely to be the warmest day of 2015 at kpdx...if we
hit the forecast high of 85.
The forecast problem continues to be the thunderstorm threat for the
Cascades. Models did a pretty good job pin-pointing the convective
threat for Thursday...along the Cascade crest. 06z model soundings for a
point near Santiam Pass continue to show fairly robust convective
parameters late this afternoon. NAM sounding gives an impressive 2800
j/kg surface-based cape...with a ml cape near 1450 j/kg. Again...
believe the NAM is over-doing the surface dew points. The GFS
has a ml cape of 500 j/kg. GFS 700 mb Theta-E ridge is centered along
the Oregon Cascade crest 00z Sat. A little lower Theta-E values in
the S Washington Cascades compared to Thursday. Decided to expand the thunderstorm
threat just a bit more west...but still keeping it in the higher
Cascades and not the foothills.
For Sat the 500 mb heights rise just a bit as the upper low in the
central Pacific gradually migrates east. The main impact will be a more
stable air mass with deeper westerly flow aloft. GFS still indicates some
instability over the southern Lane County Cascades Sat afternoon and have
left the mention of thunderstorms for that area. Mav and met guidance
indicate 2-3 degrees cooling Sat...but not sure to believe that.
Model forecast 850 mb temperatures ease about a degree...but with a slightly
warmer start am expecting maximum temperatures to be close to Friday values.
The flow aloft backs to the SW late Sat and Sat night. The 500 mb
ridge axis is expected to be centered over the forecast area Sat
night...then slowly shift east sun. NAM cross-section from kast
through The Gorge shows increasing middle and high level moisture Sun
afternoon. Reduced probability of precipitation sun as models not showing deeper moisture
developing until late sun and sun evening. Also raised inland maximum
temperatures 2-3 degrees. Pattern recognition would suggest there could be
nocturnal thunderstorms Sun night...but model soundings do not
support convection anywhere west of the Cascades. Weishaar
Long term...Sunday night through Thursday...the first and strongest
upper level trough will slide north-NE into the region late Sunday and
continue to enhance shower chances through the early part of next
week. The associated front will cross the region overnight providing
for much cooler high temperatures on Monday given the abundant cloud cover
and models suggesting 800 mb temperatures decreasing around 10 degree c by Monday
morning. While models suggest some instability...particularly
Tuesday as the cold core aloft moves across the area...think much
cooler surface temperatures will largely limit thunder chances.
However...the timing of shortwave impulses may enhance this
potential at times but confidence in this scenario remains low
enough to exclude any thunderstorms in the extended period for now
with the exception of maintaining a chance for Monday late
afternoon/evening in the Cascades south of around Montana Hood.
Temperatures likely rebound a little...closer to seasonal
norms...for the second half of the week as showers end and weak
shortwave ridging providing for some afternoon sun after morning
Aviation...dry SW flow aloft...with variable high clouds over
the region today. IFR stratus will persist along the coast. Like
on Thursday am... will have areas of stratus along the Columbia River
upstream to kpdx/kttd this am. Also...will have patchy fog or low
clouds from ksle through keug to k61s...and in Cascades until 17z.
Kpdx and approaches...VFR under variable high clouds. Will have
IFR stratus arriving from the north between 12z and 13z this am. Like
on Thursday am...this stratus will burn out 16z to 17z. Rockey.
Marine...benign conditions continue into next week...with west to northwest
winds 15 knots or less and seas running 4 to 6 feet. Rockey.
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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.