997
FXUS66 KPQR 050439
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
939 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...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...An upper level trough across southern ORegon/northern
California continues to support a surface low now located across
eastern Oregon. Precipitation has lightened up but will
continue as wrap-around moisture continues to move into
northwest Oregon and and southwest Washington. Conditions range
from VFR to IFR and guidance suggests sub-VFR conditions
developing or persisting at all terminals by 12Z and persisting
into early Sunday afternoon. There is a small (10-30%) chance of
IFR CIGs developing between 12-20Z but confidence is low at
this time. Winds are generally westerly across the coast and
southerly along the interior valley at 5-10 knots and will
maintain these relative directions and speeds through Sunday.

PDX AND APPROACHES...A mix of VFR/MVFR conditions will drop to all
MVFR as the night continues. There is a 40-80% chance of MVFR CIGs
developing by 12Z. Light rain is expected to continue but
shouldn`t bring VIS below VFR. S/SW winds between 5-10 knots will
continue into Sunday afternoon. -Batz

&&

.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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