587
FXUS66 KPQR 041740
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1040 AM PDT Sat May 4 2024

.UPDATED AVIATION DISCUSSION...

.SYNOPSIS...
Steady light to moderate rain will continue across the
lowlands of SW Washington and NW Oregon, while snow levels lower to
just below the Cascade passes later this morning then remain there
through Sunday. Low pressure off the Pac NW coast will move onshore
near the OR/CA border this evening, with wraparound precip
persisting through Sunday for much of the forecast area. Travel
impacts in the Cascades will largely occur tonight into Sunday
morning. 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
aloft over the region.

&&

.SHORT TERM...Now through Monday Night...Latest GOES-West
infrared and water vapor imagery reveal broad low pressure aloft
just off the Pac NW coast. An associated frontal band has
tapped into some subtropical moisture with blended total
precipitable water (TPW) values of around 1 inch associated with
the front. The frontal zone will more or less pivot over
northern Oregon as the upper low drifts SE and moves onshore
near the OR/CA coast through this evening. As a result, expect
fairly steady light to moderate precipitation through this
evening. This may continue all the way through Sunday if most of
the deterministic models are correct in maintaining deformation
precipitation over much of the CWA. NBM median QPF shows
weekend precip totals of 0.4 to 0.8 inch over the lowlands with
1-2 inches still to come over the Cascades.

Snow levels have been lowering a bit slower than expected overnight,
so we postponed the start time of the Winter Wx Advisory for our
Oregon Cascades until 10 AM. The temperature at Timberline Lodge
actually rose to 38 deg F at 3 AM, indicating there is still a ways
to go before snow levels lower to the Cascade passes. This delay
could be very beneficial for those with plans to travel across the
Cascades today - if the transition to snow occurs later this morning
or during the midday hours, the roads may just remain wet or
occasionally slushy due to the strong May solar energy (even with
cloud cover). 09z RAP analysis suggests wet bulb zero heights below
5000 ft remain offshore, adding confidence to the delay in the
lowering of snow levels.

Despite 1-2 inches of QPF for the Cascade passes through Monday
morning, snow accumulations should only amount to a few inches 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.

A relative lull in precipitation is possible Sunday night as the
upper low and its deformation zone shift into the Rockies. However
the next system and reinforcing cold front are likely to move onshore
Monday morning, with post-frontal showers persisting through Tuesday.
Temperatures and snow levels will remain below seasonal normal, with
lowland highs stuck in the 50s through Tuesday.  Weagle

.LONG TERM...Tuesday 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...Widespread rain continues across the area today into
tonight. Flight conditions remain quite variable ranging from
VFR to IFR, although most locations currently sit at MVFR. At
least there is high confidence (65-95% chance) for mainly MVFR
conditions through the overnight hours for both the coastal and
inland areas. If there is any trend, later this afternoon will
transition to more westerly flow with chances for MVFR/IFR
slightly decreasing and chances for VFR increasing, but probably
not enough to see prevailing VFR conditions at any point during
the TAF period.

Wind generally less than 5-10 kts inland but coastal areas a
little breezier with westerly winds 10-15 kt.

PDX AND APPROACHES...MVFR conditions expected to prevail through
the rest of the day and overnight, but with around 20-30% chance
for IFR conditions into the midday hours and again after ~09z
through Sunday morning. Thus brief periods of IFR cigs will need
to be monitored but chances for IFR appear too low to include as
prevailing in the 18z TAF package. Winds generally light,
variable early becoming southwest less then 10-15 kt.
-Schuldt/mh

&&

.MARINE...
Active weather pattern continues into early next week
with series of troughs moving through the waters. Currently in
between systems with weak high pressure over the waters. However
with relatively short wave period and seas around 6 to 8 ft will
have steep seas later this morning through this evening. So have
issued a Small Craft Advisory for hazardous seas. Next front
arrives Sunday and bring gusts to 25 kt as well as steep seas
across most of the waters Sunday morning through Sunday night.
Following this front high pressure over the northeast Pacific will
build over the waters and bring breezy west to northwest winds
(gusting 20-25 kt) across the waters later Monday into Tuesday.
The high pressure shifts closer to the waters later next week. /mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 6 PM PDT Sunday for ORZ126>128.
WA...None.
PZ...Small Craft Advisory until 10 PM PDT this evening for
     PZZ251>253-271>273.
&&

$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland