Scientific Forecaster Discussion
fxus66 kpqr 192124
Area forecast discussion
National Weather Service Portland or
224 PM PDT Thu Apr 19 2018
Synopsis...dry and mild weather is expected today and Friday. A weak
frontal system will move across the region Friday night and Saturday
for light rain and Cascade snow. Dry and mild weather returns Sunday
and lasts possibly through all of next week.
Short term...this afternoon through Monday...not many changes to the
forecast this afternoon. A thick cumulus field has developed over the
Cascades and central Coast Range, but it looks like the clouds are
actually thick enough to prevent further heating and any shower
development, so will keep the forecast dry for this afternoon and
evening. The upper level ridge building over the Pacific northwest today
holds through Friday for dry weather, mostly sunny skies, and warming
temperatures. Some high clouds will likely move over the region
Friday as a frontal system approaches the coast.
A warm front will move well north of the area and have little impact
Friday morning, but the cold front will bring a chance of rain Friday
evening through Saturday morning with showers possibly lingering
across SW Washington Saturday afternoon. Snow levels will remain
above the Cascade passes most of Friday then lower to around
3500 feet Saturday morning as the rain decreases and turns to
showers. Rain and Cascade snow will be on the light side with the
highest 24 hour (fri afternoon-Sat afternoon) totals around 0.20
inch along the north coast, north coastal mountains, and the South
Washington Cascades. The greatest chances for precipitation will be
along the coast late Friday night and over the Cascades Saturday
morning. The Willamette Valley, especially south of Salem, may not
see much rain at all.
Dry weather is expected Sunday with surface high pressure over the
area and low-level offshore winds. However, a shortwave upper trough
moving over the area will likely keep some clouds around and somewhat
limit heating, although still expect highs to be slightly above
seasonal norms on Sunday. High pressure continues to take hold Monday
and skies should be sunny behind the shortwave. Still expect
temperatures to hit 70 in the valley on Monday.
Long term...Monday night through Thursday...models have come into
better agreement regarding the weather next week, now showing
continuing high pressure and dry, warm conditions. Even the 12z GFS
is on board with continued warming Tuesday and Wednesday with mex
guidance showing temperatures as warm as the upper 70s in the valley
on Wednesday. Haven't gone that warm with the forecast, but do have
70s inland Monday through at least Wednesday.
Where the models do still diverge is with the track of an upper low
pressure system which moves south off the West Coast through midweek.
After that, the GFS continues to spin the low well off the California
coast through the end of the week while the European model (ecmwf) brings the low into
Southern California. Any shower or thunderstorm chances in the
Cascades next week will depend on the exact track of the low, but
have kept mentionable chances out of the forecast for now. This means
that after Saturday evening, there are currently no mentionable rain
chances anywhere in the local forecast area through the rest of the
7-day forecast. Vampires beware. Here comes the sun.
Aviation...high pressure is resulting in mainly clear skies across
the district, aside from a few cumulus buildups and perhaps a shower
or two over the Cascades. Otherwise, VFR will persist well into
tonight across much of the district. Will need to watch for
potential fog and low clouds along the coast, but marine inversions
will likely remain too weak for stratus development. High clouds
associated with an incoming cold front will gradually increase from
the north and west tonight, but should remain too high to impact
approaches. If skies remain clear enough for long enough tonight,
some patchy fog may develop again in some of our southern valleys
including around keug. Any fog should be shallow enough to burn off
quickly Friday morning.
Kpdx and approaches...VFR to persist tonight with light west-northwest
flow through this evening. Weagle
Marine...high pressure will remain across the waters through
Friday, then give way to an approaching cold front Friday night.
Expect low-end Small Craft Advisory winds and seas associated
with this system Friday night and Saturday morning...mainly
across the northern waters. The front will be a fast mover, and
high pressure will already be rebuilding across the waters
Saturday. High pressure will then dominate the pac northwest and
adjacent waters through at least Monday. However, thermal low
pressure building up the Oregon coast from northwest California will
bring increasing northerlies to the Oregon waters Sunday and
Pz...Small Craft Advisory for rough Columbia River bar from 5 am to
10 am PDT Friday.
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