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000
FXUS66 KPQR 212154
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
254 PM PDT Sun Apr 21 2024

&&

.SYNOPSIS...Mix of clouds and sun with mostly dry conditions through
Tuesday. After a chilly start on Monday, temperatures expected to
warm into the upper 60s to mid 70s through 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...Tonight through Tuesday night...Broad shortwave trough
over western Canada maintaining onshore flow this afternoon gets
shunted east as upper shortwave ridge approaches tonight. Low level
flow turns more north to northeasterly by Monday morning allowing
for continued dry conditions. Clearing skies and light wind in
sheltered rural areas should allow for frost to develop tonight into
early Monday morning. Temperatures are likely to fall to around 32-
35F in these areas where a Frost Advisory is in place, including the
Tualatin Valley, Upper Hood River Valley, north OR Coast Range,
Willapa Hills, portions of northern Clark County and south WA
Cascade foothills, as well as portions of the north  OR Cascade
foothills. Other areas will either stay too warm, or see wind
increasing overnight, especially in the southern part of the
forecast area, including the southern Willamette Valley west of the
I-5 corridor and central OR Coast Range, where northerly winds could
gust up to 20-30 mph per the HREF.

High pressure offshore with low pressure over the Great Basin will
maintain breezy north winds on Monday. Dry conditions persist with
temperatures warming into the upper 60s to lower 70s on Monday.
Increasing clouds overnight will likely bump minimum temperatures 5-
10 warmer than tonight`s lows. A mix of sun and clouds again on
Tuesday with warming 850mb temps should allow for another warm day
on Tuesday with highs in the lower to mid 70s, except 55-60F along
the coast.  /DH

.LONG TERM...Wednesday through Saturday...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 15-30% chance in the lowlands and 20-40% 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. Northerly wind shift through the day will
cause winds to increase along coastal terminals. Some
orographically lifted showers and cumulus in the north of KUAO and
along the Coast Range. Inland locations susceptible to northerly
winds (KSLE, KUAO, KEUG, KCVO, etc) will also see gustier winds.
There is a weak disturbance that will pass near the area late in
the period which will cause a resurgence of wind. Some models are
winds above 1500 ft gusting to around 25-30 kt after 06Z Monday
while surface winds are less than 10 kt. Because winds will be
northerly aloft, have not included LLWS in the TAFs. However,
approaches may be impacted by these stronger north- northeasterly
winds.

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 with west northwesterly winds. High level
cumulus above 15,000 ft AGL. Some eddies between 500-2000 ft.
Northwesterly winds up to 20 kt above 1000 ft in the evening but
LLWS not expected. -Muessle


&&

.MARINE...Northerly wind reversal today which are amplifying along
the coast. Thus far, small craft winds have not yet manifested in
the inner waters, but with the thermal trough forming just to the
south, the pressure gradient is increasing enough that the outer
waters are likely experiencing gusts up to 25 kt. There are
staggered small craft advisories in place due to the increasing
pressure difference between the ridge over the Pacific Northwest
and low to the south. A weak disturbance will pass over the region
late Monday which will bring the highest wind speeds with gusts
up to 30 kt. Seas will react very little as the predominate wind
wave is from the north and the background swell from the
northwest. Gales not expected, and chances are quite low. -Muessle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory from 11 PM this evening to 8 AM PDT Monday for
     ORZ104-106-109-121-123.

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

PZ...Small Craft Advisory from 3 AM to 11 PM PDT Monday for PZZ251-
     271.

     Small Craft Advisory from 8 PM this evening to 11 PM PDT Monday
     for PZZ252-272.

     Small Craft Advisory until 7 AM PDT Tuesday for PZZ253-273.

&&

$$

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