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000
FXUS66 KPQR 211044
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
330 AM PDT Sun Apr 21 2024

.SYNOPSIS...Cooler on Sunday with mainly dry weather and seasonable
temperatures. Widespread frost is expected Sunday night over the
Upper Hood River Valley, north Clark County lowlands, south WA/north
OR Cascade foothills, and north OR Coast Range/Willapa Hills.
Becoming warmer with more dry weather Monday into Tuesday as
low-level offshore flow develops. Cool and showery conditions are
likely to return late in the week as an upper level trough settles
over the area.

&&

.SHORT TERM...SUNDAY THROUGH TUESDAY NIGHT...Radar observations from
early Sunday morning depicted isolated post-frontal showers occurring
over southwest WA, the north Oregon coast, and north Coast Range.
Elsewhere, conditions were dry with partly to mostly clear skies,
calm winds, and temperatures in the 40s. Expect isolated showers to
continue in the aforementioned areas through Sunday afternoon before
diminishing in the evening. Conditions will otherwise stay dry with
seasonable temperatures; the deterministic NBM suggests afternoon
high temps in the low 60s, except 50s at the coast.

Clear skies and light winds should allow for frost to develop in some
areas Sunday night. It appears temps will stay a bit too warm for
frost along the I-5 corridor from Kelso to Eugene. However,
widespread frost is expected over the coldest zones, which includes
the Upper Hood River Valley, north OR Coast Range, Willapa Hills, and
south WA/north OR Cascade foothills. These zones have a 60-90% chance
for overnight lows below 35 degrees. Given these probabilities and
the high confidence involved, decided to issue a Frost Advisory for
the aforementioned areas as temps are likely to fall to around
32-35F. Also decided to include the north Clark County lowlands for
temps right around 34-35F in the Battle Ground area. Cannot
completely rule out areas of frost in the Hillsboro area as well, but
the current forecast is slightly too warm to justify the headline.

Conditions will warm up quickly Monday into Tuesday as an upper level
shortwave ridge moves over the region and low-level offshore flow
develops. Models suggest a mix of sun and high clouds both days.
Expect temperatures to warm to 66-70F Monday afternoon and 68-74F on
Tuesday across the interior lowlands and low to mid 60s at the coast.
The probability for high temps above 70 degrees peaks on Tuesday over
the Portland/Vancouver metro and Willamette Valley at 60-80%. -TK

.LONG TERM...WEDNESDAY THROUGH SATURDAY NIGHT...The GEFS/ENS/GEPS
ensemble mean all show 500 mb flow veering to the west on Wednesday
in response to a weak upper level trough brushing the forecast area
to the north. Models and their ensembles continue to suggest the bulk
of precipitation with this system will fall over British Columbia and
far northwest WA, with very little to no rain at all across southwest
WA and northwest OR. NBM PoPs remain at 10-20% chance in the lowlands
and 20-35% in the mountains. Temperatures are shaping up to be right
around normal for this time of year Wednesday afternoon.

Conditions will become noticeably cooler on Thursday as highs fall
into the 50s across the area. This is in response to a cool
upper level trough that is set to move overhead late in the week,
bringing higher chances for rain. While NBM 6-hr PoPs only peak
around 60-70% in the lowlands, 24-hr PoPs peak around 80-85%. This is
due to model timing differences. Only a very small number of
ensemble members are showing no rain at all, suggesting rain is
likely to occur at some point on Thursday and/or Friday. According to
the NBM, the probability of receiving no measurable rain at all on
Thursday and Friday is around 10%, which seems reasonable based on
ensemble guidance. While confidence is high at least some measurable
rain will fall late in the week, confidence is low regarding exact
rain amounts as QPF plumes from the ENS/GEFS/GEPS continue to show a
large degree of model spread as some members are at zero while other
members are around or even slightly over 1 inch. The NBM is showing a
20-40% chance for 24-hr rain amounts in excess of 0.25" across all of
northwest OR and southwest WA, except 40-60% in the mountains. -TK

&&

.AVIATION...Widespread VFR under generally clear skies with
lingering clouds along the Coast and Cascades which could obscure
the ranges at times. There is a 10-15% probability of IFR fog
roughly from KUAO to K77S through 17Z Sun per HREF guidance. Also
a weak disturbance moving into WA may bring an increase in showers
over southwest WA this afternoon into early evening.

Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission. As it is not
maintained locally by the National Weather Service, there is no
estimated time of repair.

PDX AND APPROACHES...VFR expected next 24 hours. There is up to 10%
probability for fog 12-16Z Sun.
/mh

&&

.MARINE...Winds winding down as high pressure resides over the
waters. Seas rather choppy at 6 to 9 ft at 7 to 9 seconds.
Guidance indicating the period will increase after 8 AM this
morning so will have a Small Craft Advisory until then.
The next surge of winds will arrive late this afternoon where we
will likely see marginal Small Craft Advisory winds again as a
thermal trough strengthens along the south Oregon coast. Will
initially start a Small Craft Advisory in the south zones
PZZ253,273; then north to PZZ272 later this evening with wind
gusts 25-30 kt and steep, choppy seas. May eventually need to
expand the Small Craft Advisory further north Monday, but marginal
case at this time. Later Monday into Tuesday, the thermal trough
begins to weaken as will the winds. However various guidance a bit
different in timing of this change.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory from 11 PM this evening to 8 AM PDT Monday for
     ORZ104-106-121-123.

WA...Frost Advisory from 11 PM this evening to 8 AM PDT Monday for
     WAZ202-203-205-208.

PZ...Small Craft Advisory until 8 AM PDT this morning for PZZ251>253-
     271>273.

     Small Craft Advisory from 5 PM this afternoon to 11 PM PDT
     Monday for PZZ253-273.

     Small Craft Advisory from 10 PM this evening to 11 PM PDT
     Monday for PZZ272.

&&

$$

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