Wildfire in Washington, November 1 to 13, 2020

Area Forecast Discussion
National Weather Service Portland OR
850 PM PDT Sat Oct 31 2020

.SYNOPSIS...High pressure will build across the Pacific NW Sat night
and hold through Monday. This will maintain mild days and cool night
under mostly clear skies. Typical early November weather arrives
next week, with clouds and rain returning by Tue and continuing
through next weekend.

&&

.SHORT TERM...Tonight through Monday Night...Water vapor satellite
imagery this afternoon shows an upper level ridge building over the
eastern Pacific. High pressure will continue to build into the
interior Pacific Northwest this weekend with dry weather and
variable high clouds expected through Monday. Expect a pleasant
Halloween evening as mostly clear skies will allow for great views
of the full moon.

At the surface thermal low pressure builds northward along the coast
tonight. This will increase the pressure gradient through the
Columbia Gorge which would support easterly winds of 15 to 25 mph
from the west Columbia Gorge in to the far east Portland metro area
tonight into Sunday. The winds overnight will likely maintain more
mild conditions while locations away from the windy conditions will
see cooler temps as skies remain mostly clear overnight. Early
morning fog Sunday will likely be limited to the south Willamette
Valley and parts of southwest Washington as drier air is expected.

High pressure will continue to shift east Sunday through Monday.
Expect dry weather as November begins with temperatures possibly
warming into the low to mid 60s. Will see increasing clouds later
Monday.

.LONG TERM...Tuesday through Friday...Expect a change in the weather
pattern arriving Tuesday with active wet weather likely through the
end of next week. A broad upper level trough over the Gulf of Alaska
will allow the jet stream to dive further south next week. The first
front out ahead of the upper level jet will weaken as it approaches
the region Tuesday. Rainfall amounts up to 0.5" will be possible
along the north Oregon coast and parts of southwest Washington, but
elsewhere light rain will be likely by Tue afternoon. Models show
the upper level jet shifted a bit further north with the 150-160 kt
core directed towards Vancouver Island Wed evening into Thursday.
This will direct a series of surface lows along the southern Gulf of
Alaska towards British Columbia. The associated fronts will likely
bring occasional rain and showers to our area through the middle of
next week.

Late Thursday models are in decent agreement that the longwave upper
trough drops out of the Gulf of Alaska south in the eastern Pacific.
By Friday the trough digs inland bringing colder air aloft. Snow
levels are likely to drop below 3000 ft late Friday. Surface
temperatures should also begin to cool late next week into the
weekend. With each frontal passage will also likely have windy
conditions at the coast and breezy southerly winds inland. /ddh

&&

.AVIATION...VFR conditions prevail early this evening under a
dry and stable air mass. Nocturnal inversions will strengthen in
some areas, allowing for areas of fog to form in southeastern
portions of the Willamette Valley and perhaps the I-5 corridor
Kelso northward. That said, increasing NE-E low-level flow will
likely keep coastal areas and inland areas between KKLS-KSLE
mainly fog-free through Sunday morning. There will be a period
of LLWS over portions of the PDX metro area (except KTTD) later
tonight into Sunday morning as E-NE winds above 500-1000 feet AGL
struggle to reach the surface due to the nocturnal inversions.
Any LLWS should ease Sunday as winds aloft weaken and mix down to
the surface.

With offshore winds struggling to affect areas south of KSLE
Sunday, any fog which develops near KEUG may persist into the
afternoon hours due to established inversions and poor mixing.
Otherwise, expect VFR area-wide by midday.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...VFR conditions this evening. Expect
increasing LLWS overnight as nocturnal inversions keep winds
calm near KPDX while E-NE winds increase above 500 to 1000 ft
AGL. If surface winds remain calm into Sunday morning, there is
a chance shallow fog may develop by 12z. Any fog Sunday morning
would likely clear by 17z-18z as E-NE winds increase.  Weagle

&&

.MARINE...High pressure building into interior WA/OR remains
fairly strong this evening, while a thermal trough of low
pressure lingers along the Oregon Coast. This is driving locally
breezy NE winds across mainly our northern and outer waters this
evening. Will allow the SCA to expire for the southern outer
waters as gusts have generally dipped below 20 kt. For the
northern waters, gusts will remain on the cusp of SCA criteria
overnight, with persistent gusts near 20 kt especially across
PZZ270 and below gaps in the coastal terrain. This will need to
be monitored overnight, but we opted for no SCA for the time
being as most guidance shows slight weakening of MSLP gradients
overnight. Winds in general will ease Sunday, with seas mostly in
the 3 to 6 ft range tonight through Sunday.

Confidence is high a more active weather pattern will take hold
beginning late Monday into Tuesday. A series of low pressures and
fronts will likely bring rounds of Small Craft Advisory to Gale
Force level wind gusts of 25 to 40 kt to the waters every 12-36
hours beginning Monday night and continuing for much of the work
week next week. Expect seas to pick up accordingly and to
generally hover closer to 10 ft on average next week. Seas will
likely spike higher at times during stronger bouts of winds and
could approach 20 ft. Confidence in timing stronger bouts of
winds and higher seas beyond Tuesday night is relatively low at
this point.  Weagle/Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.

&&

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