329
FXUS66 KPQR 042133
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
233 PM PDT Sat May 4 2024

.SYNOPSIS...Steady light to moderate rain will continue across the
lowlands of SW Washington and NW Oregon through Sunday with
light to moderate snow over the Cascades above 4000 feet.
Travel impacts in the Cascades will largely occur tonight into
Sunday morning when road temperatures become cool enough for
snow to stick. Another round of rain and Cascades snow is
expected Monday as the next system moves onshore, with post-
frontal showers lingering into Tuesday. A significant warming
and drying trend is expected later in the week as high pressure
builds over the region.


&&

.SHORT TERM...Saturday Afternoon through Tuesday...Radar
imagery shows showers continuing across NW Oregon and SW
Washington this afternoon. The surface front has pushed into
eastern Oregon through the morning with satellite imagery
indicating the associated upper closed low pressure system
approaching the Oregon/California border. Ensemble guidance
remains in solid agreement that the closed low will move onshore
along the Oregon/California border this evening, moving slowly
east through northern California into Nevada tonight through
Sunday morning. NW Oregon and SW Washington is currently
situated on the northeastern side of the closed low and will
remain on the north side of this closed low through Sunday.
Enough moisture will wrap around this low to allow precipitation
to continue across the region through Sunday. The heaviest
bought of precipitation for most locations should taper off
tonight and become lighter as the region transitions from the
northeast to north side of the low except for over the Cascades
where orographics will keep precipitation more steady. The
following precipitation amounts have been recorded since rain
began Friday: 1.25 to 2.5 inches along the coast and Coast
Range, 0.75 to 2 inches in the lowland valleys, and 0.4 to 1.5
inches for the Cascades and adjacent western foothills. Highest
amounts have been roughly in and south of Lincoln County.
Additional QPF amounts through 5pm Monday are as follows: 0.5 to
2 inches along the coast and Coast Range, 0.5 to 1.25 inches in
the interior lowlands, and 1 to 3 inches along the Cascades and
adjacent foothills.

Since this is a colder weather system, snow levels have lowered
to around 4000 feet this morning and will lower to around
3000-4000 feet tonight through Sunday. Webcams show snow did
begin right around 10am this morning over mountain passes.
However, as of 2pm, a limited amount of snow seems to be
sticking to roads due to warm road temperatures lingering. Only
in the past hour do webcams show slushy snow beginning to stick
along Willamette Pass. As temperatures remain cool this
afternoon and lower overnight, expect more snow to begin
accumulating along roadways. Despite the higher QPF for the
Cascade passes, snow accumulations should remain fairly low for
the passes due to the marginal thermal profile and resulting low
snow-to- liquid ratios. By far, the most significant deviations
our forecast makes from NBM were in the snow-to- liquid ratios
(SLRs). Our SLRs were largely based on the more reasonable WPC
guidance, though we lowered the SLRs 10-15% from even the WPC
guidance (and 30-40% below NBM guidance). This was necessary to
get the SLRs below 10:1 for the passes, where NBM was showing
SLRs in the 13:1 to 16:1 for the passes and even higher than
that for higher elevations. Such SLRs would be difficult to
achieve for the passes even in the middle of winter, much less
the middle of spring. Even then, a Winter Weather Advisory
remains in effect through 6 pm Sunday as at least 4 to 8 inches
of snow are expected at pass level. NBM and HREF (SLR of 10:1)
indicate a 60-90% probability of at least 8 inches above 4000
feet generally along and south of Highway 20 through 5pm Sunday.
Since the snow ratios on both these are both a bit high, would
expect 8 inches to be the higher end of snow accumulation at the
passes, with up to 14 inches above 5000 feet.

A relative lull in precipitation is possible Sunday night except
for lingering Cascades showers as the upper low and its
deformation zone shift into the Rockies. However, ensembles are
in good agreement that a shortwave trough and reinforcing cold
front will move onshore Monday morning, bringing another round
of widespread precipitation Monday night through Monday
afternoon with post- frontal showers persisting through Tuesday.
Upper level flow will be westerly with this system which will
lead to quite a bit less accumulation in the lowlands (0.5-0.25
inch) than the Cascades (0.5-1 inch). Snow levels will remain
around 3500-4000 feet with lower SLRs, so a few more inches of
snow is possible over passes. Temperatures will remain below
seasonal normal, with lowland highs stuck in the 50s through
Tuesday. -HEC/Weagle

.LONG TERM...Wednesday through Friday...Ensemble guidance
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 500 mb cluster analysis
depict this ridge with varying amplitudes. The deterministic NBM
currently suggests lowland 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 temperatures
peaking in the mid to upper 70s on Friday. Meanwhile, the
warmest model solutions show highs near 90 degrees for the
inland valleys. Either way, temperatures will be running above
normal for this time of year by Thursday. The probability for
high temperatures at or above 80 degrees on Friday has raised to
75-90% across the interior lowlands, which is quite high for a
forecast that is six days out. Overall, the warmest temperatures
of the year so far are likely to occur late this week high
confidence this pattern will continue into at least Saturday.
This may draw some people to area rivers and/or lakes, but keep
in mind water temperatures are still very cold and river
currents will be swift with mountain snowmelt. If you make
plans near area waterways, be very vigilant and wear a life
vest as cold water shock and hypothermia will be a very real
risk for anyone who goes into the water. -TK/HEC

&&

.AVIATION...Widespread bands of rainfall persist across western
Oregon and SW Washington this afternoon. Current observations show
flight conditions remain quite variable ranging from VFR to IFR
across the terminals regionally, although most locations
currently sit at MVFR. At least there is still high confidence
(65-95% chance) based on the latest guidance for mainly MVFR
conditions persist this evening through Sunday morning both the
coastal and inland areas. It`s worth noting there are
chances(20-30%) for a period of IFR cigs at most inland sites
Sunday morning between 12-18z; much higher chances (40-60%)
comparatively along the coast during this same period. Wind
generally stay less than 10 kts inland but coastal areas appear a
little breezier with westerly winds 10-20 kt.

PDX AND APPROACHES...A mix of VFR/MVFR conditions have quickly
trended back to MVFR as rainfall rates have increased over the
Portland/Vancouver metro area early this afternoon. It`s
certainly possible if these rainfall rates ease again after the
band overhead passes we could see temporary windows of VFR cigs
return roughly ~23-03z but in general anticipate MVFR conditions
through the period. It`s worth noting guidance gives around a
15-25% chance for IFR after 10z through 16z Sunday. Thus brief
periods of IFR cigs will still need to be monitored. -Schuldt

&&

.MARINE...The active weather pattern continues into early next
week before a shift to generally calmer conditions and a more
summer-like pattern. A broad upper level low off the far southern
Oregon coast will continue to progress inland into northern
California tonight leading to a decrease in 15 to 25 knot gusts
over the waters (highest beyond 10nm) and subsiding seas. After a
quick break the next frontal system arrives Sunday bringing
another round of gusts up to 25 kt as well as steep seas across
most of the waters Sunday morning through Monday morning.
Following this front high pressure over the east-northeast Pacific
begins build over the waters and bring breezy west to northwest
winds (gusting 20-25 kt) across the waters later Monday into
Tuesday. Also expect the arrival of a pounced WNW 9-11 ft swell
at ~12 seconds Monday night. The high pressure shifts closer to
the waters later next week followed by a larger offshore component
to the wind in the Friday/Saturday timeframe. -Schuldt

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 6 PM PDT Sunday for ORZ126>128.
WA...None.
PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ210-
     251>253-271>273.
     Small Craft Advisory from 11 AM Sunday to 8 AM PDT Monday for
     PZZ251-271.
     Small Craft Advisory from 5 AM Sunday to 8 AM PDT Monday for
     PZZ252-253-272-273.
&&

$$

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