Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 211031
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
224 AM PDT Mon Sep 21 2020


.SYNOPSIS...Expect fairly seasonable and mainly dry weather through
Tuesday before a more active storm track brings a period of wet
weather beginning Wednesday and continuing through at least the end
of the work week.


&&


.SHORT TERM...Today through Wednesday...Water vapor satellite imagery
reveals an impressive low pressure, at least relative to what models
initialized, inside 130W spinning towards the OR/WA coast. This mid
level circulation is forecast to track northeastward across our
northern zones today bringing with it a chance for brief light rain
across our more northern zones. Meanwhile, a weak surface trough is
also located off the coast and models agree it will push onto our
coast late this afternoon and evening. This may provide just enough
low level convergence for a few areas of light rain and drizzle to
develop along the coast before eventually pushing inland,
particularly across the Cascade foothills.

Expect increasing mid and high level clouds on Tuesday with
temperatures possibly struggling to reach 70F due to limited
sunshine. Models are in good agreement our first fall front of the
season will drop southeastward across the area on Wednesday. Rain
appears likely to begin on the north coast late Tuesday night before
pushing into the Portland metro Wednesday morning. Rain should
arrive into much of Lane County by late Wednesday afternoon or
evening. The GEFS and EPS systems are in good agreement IVT values
ahead of the front will likely top out in the 750-1000 kg/ms range.
The front accompanying the atmospheric river will weaken as it
shifts southeastward across the area and fortunately will be rather
transient so no major hydrology issues are anticipated. Nonetheless,
NBM guidance suggests the IQR for 24 hour QPF amounts for Astoria is
between 0.60-1.20", 0.75-1.40" for Portland and 0.40-0.90" for
Eugene ending Wednesday night. 24 hour rainfall amounts will likely
top out in the 1-4" range across the higher terrain of the Coast
Range and Cascades.

Finally, gusty southerly winds will develop ahead of the front
Tuesday night into Wednesday morning across the area. This will
likely create quite windy conditions along the coast and breezy
conditions inland. Pressure gradients are not terribly impressive
with this front, but modeled 925-850mb winds would support our most
exposed beaches and headlands receiving wind gusts of 45 to 55 mph
with possibly a few spots reaching higher late Tuesday night into
Wednesday morning. /Neuman

&&

.LONG TERM...Wednesday night through Sunday...Models are in good
agreement the broad shortwave trough responsible for pushing the
aforementioned front across the area on Wednesday will shift
eastward and skirt the Pacific Northwest. This will bring a
continuation of showers along the coast and across the higher
terrain Wednesday night into Thursday. Some deeper instability could
result in a few thunderstorms across our waters and along the coast
Wednesday night into Thursday. A reinforcing shortwave trough will
likely result in another round of more widespread showers either
Thursday night or Friday before shortwave ridging gradually shifts
eastward across the Pacific Northwest over the weekend. This will
result in showers decreasing Friday night into Saturday across the
area. Sunday looks more definitively to be dry with high
temperatures climbing back into the 70s. /Neuman

A large westerly swell looks to move across the waters later this
week, which will bring a high threat for sneaker waters
Thursday into Friday. /64

&&


.AVIATION...A few hours of patchy fog/low stratus is possible
through around 18Z, especially in coastal valleys. Otherwise,
conditions remain VFR through tonight with passing mid/high
clouds. A weak disturbance will brush by the area today and
could bring some isolated showers, but chances are low.

Refer to http://weather.gov/zse for detailed regional aviation
weather and hazard information.

KPDX AND APPROACHES...A few hours of patchy fog/low stratus is
possible between 13-17Z. Otherwise, VFR conditions continue with
passing mid/high clouds. /64


&&


.MARINE...A weak disturbance will brush by the waters today, but
winds should remain below 20 kt with seas around 4-5 ft. Guidance
continues to show a strong frontal system approaching the offshore
waters Tuesday and moving across the coastal waters Tuesday night
into Wednesday. Gales appear likely with this front with boundary
layer winds around 40 kt possible late Tuesday and Wednesday so
will hoist a Gale Watch with the morning forecast. Expect seas to
build to around 7-9 ft Tuesday, then into the mid teens as the
front moves across the waters Wednesday. Winds subside behind the
front Wednesday night, but marginal advisory gusts are possible
into Thursday. A large westerly swell looks to then move across
the waters later this week which will keep seas in the mid teens
and bring a high threat for sneaker waters Thursday/Friday. /64


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Watch from Tuesday evening through Wednesday evening for
     coastal waters from Cape Shoalwater WA to Florence OR from
     10 to 60 NM.

     Gale Watch from late Tuesday night through Wednesday evening
     for coastal waters from Cape Shoalwater WA to Florence OR
     out 10 NM.

&&


$$

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