Scientific Forecaster Discussion
fxus66 kpqr 231015
Area forecast discussion
National Weather Service Portland or
315 am PDT Mon Oct 23 2017
Synopsis...strong high pressure will generally prevail throughout
the upcoming week, resulting in mostly clear and mild conditions
outside of night and morning valley fog. A weak system passing by to
the north Wednesday will be the exception, bringing a few showers to
our northern zones before skies clear again by Thursday. East winds
will be gusty at times through The Gorge, particularly Tuesday and
again late in the week.
Short term...today through Wednesday...most of the smaller rivers
have crested this morning, with all rivers below flood stage and
expected to remain that way. The Luckiamute near Suver is still
rising, but is expected to crest below flood stage later this
morning. Slow rises are still expected on the lower portions of
larger rivers such as the Willamette and Tualatin, but these will
generally remain below bankfull and well below flood stage.
Locally dense fog is forming in the valleys this morning as mid and
high clouds clear out of the area. Fog appears most dense near the
rivers and in the Coast Range valleys per observations, spotter
reports, and area webcams. The fog and low clouds will probably be
slow to clear this morning, with weak flow under the building high
pressure system. Thermal low pressure is expected to strengthen near
Brookings today, increasing the northerly flow a bit. This should
clear out the northern and central Willamette Valley around midday,
with low clouds likely lingering the longest around Eugene as the
northerly flow stacks up the low level moisture against the south end
of the Willamette Valley. The air mass is warming up quickly as the
upper ridge strengthens, so temps today should be a few degrees above
normal with 60s fairly widespread across the forecast area.
Offshore flow is expected to increase through the Columbia Gorge
tonight and Tuesday, which should result in less fog across eastern
portions of the Portland Metro area. With 850 mb temps climbing into
the upper teens celsius, the foothills should easily reach the 70s
and perhaps approach 80 degrees Tuesday afternoon. With 500 mb
heights exceeding 590 dm, there will be inversions which will likely
keep the valleys a bit cooler.
The upper ridge flattens Tuesday night then a weak system drops
across the Pacific northwest Wednesday and Wednesday night. This will
turn the flow onshore for more clouds, with some showers possible
later Wednesday and Wednesday night. Used a model blend for the
timing of the precipitation, but the trend is toward a bit later
arrival of any precipitation as compared to model runs from Saturday.
As a result, we may see some morning fog or low clouds again on
Wednesday morning before the weak system arrives. Weagle/Tolleson
Long term...Wednesday night through Sunday...after the weak system
moves through Wednesday night, the upper ridge rebounds and rebuilds
strongly off the coast Thursday and spreads onshore Friday and
Saturday for more dry weather and some more offshore flow. The models
try to break down the upper ridge early next week but anything that
tries move through is weak at this point. Tolleson
Aviation...high pressure building over the area today. Seeing a
mix of fog (with LIFR conditions) and low stratus (with MVFR/
IFR conditions) across the area, which will continue until around
18z when clouds start to really break up. Light offshore winds
should keep the coast VFR through tonight. After around 18z,
expect VFR conditions across the area through the rest of today.
Kpdx and approaches...fog will keep LIFR conditions at the
Airport through much of this morning. Expect fog to start
breaking up around 18z, with improvement to VFR by 19z-20z.
Marine...high pressure building in over the waters. An inverted
surface trough along the coast strengthens Monday night and
Tuesday, which will produce northerly wind gusts around 20-25 kt.
This will be short lived as a weak cold front drops south into
our waters on Wednesday, bringing northerlies around 20 kt behind
this feature. The inverted surface trough under high pressure
returns late in the week with 25 kt wind gusts possible again
Thursday and Friday.
Seas gradually falling today, dropping below 10 ft late this
evening. Seas continue to gradually fall to around 6 ft by
Wednesday, and stay around 5 to 7 ft through Friday. -Lm
Pz...Small Craft Advisory for hazardous seas until 11 PM PDT this
evening for coastal waters from Cape Shoalwater Washington to
Cascade Head or out 10 nm-coastal waters from Cascade Head
to Florence or out 10 nm-waters from Cape Shoalwater Washington to
Cascade Head or from 10 to 60 nm-waters from Cascade Head
to Florence or from 10 to 60 nm.
Small Craft Advisory for rough Columbia River bar until 9 PM
PDT this evening.
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