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FXUS66 KPQR 032134
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
234 PM PDT Fri May 3 2024

.SYNOPSIS...
Cool and wet this weekend as an upper low moves into
western OR and northern CA, bringing a prolonged period of light to
occasionally moderate stratiform rain late Friday through Saturday.
Remaining cool Sunday through Tuesday with showers at times. Upper
level ridging will then bring a quick warmup mid to late week, with
high temperatures likely rising above 80 degrees across the interior
lowlands by Friday.

&&

.SHORT TERM...Friday through Sunday night...Surface weather
analysis from 1pm Friday revealed a warm front/cold front pair
over the coastal waters moving east towards the coast. This
frontal system was associated with a broad area of low pressure
centered around 400-500 miles west of Vancouver Island. A wave
of moisture ahead of the aforementioned warm front has already
resulted in light rain at the coast. This moisture will continue
spreading inland Friday afternoon as the system`s warm front
lifts over the area. Once rain begins, expect a prolonged period
of light to occasionally moderate stratiform rain across all of
northwest OR and southwest WA through the day on Saturday as
the trailing cold front pushes through. Precip will transition
to showers on Sunday within the post-frontal environment. This
system will have an abundant amount of moisture as IVT values
max out around 300-400 kg/ms this evening. Models still show
precipitable water values around 1 inch, which is about two
standard deviations above normal for this time of year.

There have been minimal changes the total QPF forecast with
this system. Still expect around 1-1.5 inches for the interior
lowlands, and 1.5-2 inches for the coast and mountains from late
morning today to tomorrow (Saturday) afternoon. No impactful
winter weather is expected for the Cascade passes today as snow
levels will rise above 6500 to 7000 feet. Despite the impressive
rain amounts for early May, no flooding concerns are expected as
hourly rain rates look to generally be under 0.2 inches/hr, except
under heavier showers on Sunday. HEFS probabilistic guidance
continues to show a less than 5% chance of reaching action stage for
the majority of rivers. Two rivers with the highest chances of
reaching action stage are Marys River at Philomath (5-10% chance) and
Tualatin River near Dilley (15% chance).

As the low pressure system progresses southward toward California
Saturday, cold air aloft will return and snow levels will fall
back down to around 3500 to 4000 feet. Given the precip expected this
weekend, another round of winter weather will impact the Cascades. A
Winter Weather Advisory has been issued above 3500 feet for the North
Oregon Cascades to the Lane County Cascades from 5 AM Saturday to 11
AM Sunday morning. These locations are forecast to receive around 4
to 10 inches of snow, except up to 16 inches for the highest peaks
above 5000 feet. Those traveling through the passes this weekend
should prepare for winter weather conditions. Meanwhile, the lowlands
will continue to see rain showers through the weekend.-TK/Alviz


.LONG TERM...Monday through Friday...The long term forecast is
highlighted by a rapid change in the weather pattern from cool and
showery conditions Monday through Tuesday to much warmer and drier
conditions late in the week. Models and their ensembles continue to
show a shortwave trough moving across Washington and Oregon Monday
into Tuesday, bringing a shift from westerly flow aloft to
northwesterly behind the trough axis. This trough will bring the
continuation of cool and showery weather with high temps mainly in
the 50s and snow levels around 4000 ft on Monday falling to 3000 ft
on Tuesday.

Beyond Tuesday, confidence is high for a rapid shift in the weather
pattern from cool and wet to warm and dry. This abrupt shift is the
result of an upper level ridge that is set to build over the region,
and all four clusters shown in WPC`s cluster analysis depict this
ridge with varying amplitudes. The deterministic NBM currently
suggests highs in the 60s Wednesday, 70s Thursday (except 60s at the
coast), and 80s Friday (except low 70s at the coast). The coolest
model solutions are similar to the NBM 10th percentile, showing
inland high temps only peaking in the mid 70s on Friday. Meanwhile,
the warmest model solutions show highs near 90 degrees from Portland
to Eugene. Either way, temps will be running above normal for this
time of year. The probability for high temps at or above 80 degrees
on Friday is already at 70-80% across the interior lowlands, which is
quite high for a forecast that is seven days out. Overall, the
warmest temps of the year so far are likely to occur next weekend.
This may draw some people to area rivers and/or lakes, but keep in
mind water temps are still very cold and river currents will be swift
with mountain snowmelt. We plan on actively messaging cold water
safety tips late next week in preparation for the warm weather ahead,
as cold water shock and hypothermia will be a very real risk for
anyone who decides to take a swim.  -TK

&&

.AVIATION...Satellite and radar observations this afternoon show
rainfall quickly spreading across western Oregon as our next
weather disturbances races into the Pacific Northwest. Confidence
is high after precipitation initially begins 21-23z cigs drop to
MVFR with model guidance indicating a 70-95% chance for less than
3kft CIGS after 03-06z for all sites. KONP likely continues to
fluctuate between MVFR and IFR conditions the rest of the
afternoon before trending solely to IFR overnight and through
Saturday. Well also have to watch for a period of IFR CIGS at
inland sites Saturday morning between roughly 10-16z, although
confidence in the timing of any such category change is low to
moderate currently. Given the deep moisture, expect mountains to
become obscured. Wet weather persists through the rest of Saturday
accompanied by a mix of MVFR to temporarily IFR conditions through
at least 00z.

Winds remain fairly variable and light (<10 kt), but once the
front nears, winds will take on a stronger southerly character.
Winds at coastal terminals will begin to gust to around 20-25 kt
from the south around ~22z Fri to 03z Saturday, then gradually
veer more southwesterly and weaken through Saturday morning
before increasing again to the 15-20 knot range Saturday
afternoon.

PDX AND APPROACHES...VFR conditions under increasing high clouds
will soon transition to rain(~2130-2200z) with gradually lowering
CIGS the rest of the afternoon into the early evening hours.
Chances for CIGS less than 3kft increase to around 80-90% by 06Z
Sat. Winds generally east through about 00Z Sat then more south to
southeasterly. Expect periods of rain and low-end(<2kft) MVFR
conditions to continue early Saturday morning through the rest of
the forecast period. -Schuldt

&&

.MARINE...A cold front associated with a low dropping out of the
Gulf of Alaska continues to spread rainfall across the region this
afternoon. Winds associated with this feature have been gusting
around 20-35 knots based on the very limited bouy observations
available but at least this lines up well with the latest high-
resolution guidance. The current Small Craft Advisory was
expanded to include the inner waters south of Cape Falcon where
gusts up to 25 knots are expected to occur with more regularity
into the evening - although that`s not to say isolated gusts of
this magnitude won`t be found further north. Winds ease this
evening into Saturday morning, but expect to see a period of
marginally steep seas (6 to 8 ft at 8 to 9 seconds) Saturday
afternoon and evening over both the inner and outer waters.
Otherwise, anticipate another burst of SW wind gusts near 20-30
knots Sunday into Monday morning before we begin a transition to a
calmer weather pattern and eventual shift to N-NW winds Tuesday
and Wednesday. -Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 5 AM Saturday to 11 AM PDT Sunday
     for ORZ126>128.
WA...None.
PZ...Small Craft Advisory until 8 PM PDT this evening for PZZ252-
     253-271>273.
&&

$$

www.weather.gov/portland

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