966
FXUS66 KPQR 300505
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
1004 PM PDT Mon Apr 29 2024

.UPDATE...We just issued a Winter Weather Advisory for the S WA and N
OR Cascades as far south as Linn County, including Santiam and
Tombstone Passes along US Highways 20/22. High-res forecast models
look increasingly impressive with QPF as our next upper level trough
swings through the area. With many areas now expected to receive
0.50" or more of QPF (HREF now showing 80-100% chances of this for
the Cascades Santiam Pass northward), and snow ratios expected to be
around 10:1 for the Cascade passes and above, it now appears there
will be enough snow to warrant a Winter Wx Advisory for up to 8
inches of snow for our Cascades Linn County northward. The advisory
runs 3 AM-6 PM, though it may need to be extended if convection over
the Willamette Valley persists into the evening.  Weagle

&&

.SYNOPSIS...A relatively cold weather system continues to move
through the region today, with another arriving tomorrow
morning. Tomorrow, expect lowland rain showers, Cascade snow
showers, and potential for thunderstorms. With any
thunderstorms, potential hazards include lightning, small hail,
heavy rain, and gusty winds. Warming and dry conditions briefly
come Wednesday, followed by another storm early Thursday
morning. Potential unsettled weather may continue into the
weekend.

&&

.SHORT TERM...Now through Wednesday... Showery activity continues
across northwest Oregon and southwest Washington as a cold
surface front pushes over the Cascades, leaving post frontal
showers. Snow levels will remain between 2500-3500 feet for the
rest of today, bringing 1-2 inches more snow to the Cascades.
As for thunderstorms, while the front moving east over the
Cascades has created cloud breaks for warming, the front exiting
the region has lessened the overall forcing, reducing chances
of thunderstorms. However, thunderstorms still remain possible
(15%) until this evening, especially with post frontal showers
and warming present. With models showing 500 mb temperatures
around -30 to -35 deg C and steepening lapse rates, small hail
is possible for areas north of Lincoln City/Salem. Sure enough,
as of 2PM, thunderstorms have started to develop along with some
small hail, so expect thunderstorms to last until this evening.
Thunderstorms within the Cascades could persist this evening as
the system exits east. Otherwise, for today expect around 0.25
inch more rain accumulation in the Coast Range, and under 0.10
inch more in the lowlands, with showers decreasing this evening.

Next low pressure system moves southeast into the north Oregon
Coast tonight/tomorrow morning, bringing more thunderstorms,
lowland rainfall, and Cascade snow. Similar to today, there is
a 15-25% chance of thunderstorms across the region. Expect rain
accumulations around 0.25-0.50 inch along the coast and inland
valleys. The Cascades will see another round of snowfall, with
snow levels around 2000 feet and 2 to 6 inches of snow within 24
hours ending at 11 PM Tuesday.

Tuesday night to Wednesday, conditions begin to dry up as the
second system pushes east and upper level high pressure builds.
We could see some clearing Tuesday night, which could lead to
radiational cooling and the potential for frost development.
Currently, it looks like the highest frost potential will be in
the Coast Range, Willapa Hills, and Upper Hood River Valley.
Based on the latest probabilistic guidance, these locations have
a greater than 70-90% chance of Wednesday morning lows dropping
below 35 degrees. For the Willamette Valley, that probability
is around 30-40%. Warming temperatures will likely return
Wednesday, with a 60-80% chance of highs exceeding 58 degrees
for inland valleys.
-JH

&&

.LONG TERM...Thursday through Sunday...WPC cluster
analyses have around 65% of ensemble members suggesting an
additional trough pushing into the Pacific Northwest Thursday.
In this case, we would see another round of light precipitation.
The other 35% of ensembles members suggest ridging, which would
maintain drier conditions. Current NBM probabilities for 24 hour
QPF exceeding 0.25 inch ending 5 PM Thursday is less than 20%
for inland valleys and 50-60% for the Coast Range and Cascades.

Friday looks relatively drier as the majority of WPC clusters
suggest transient ridging; however, the ridging doesn`t look
particularly strong. This may allow for some shortwave
troughing to push through and bring some precipitation. The
majority of clusters still show precipitation on Friday, despite
them also showing ridging. With this uncertainty, NBM PoPs of
25-50% across the region Friday look reasonable.

Saturday to Sunday, the majority of the clusters are showing a
troughing pattern returning. However there is uncertainty with
the strength of the next trough. About half of the ensemble
members show weak troughing moving into the Pacific Northwest,
while the other half show a deeper trough developing over the NE
Pacific. The members that show the deeper trough have it
progressing southward toward southern Oregon and northern
California by Sunday, and becoming negatively tilted. In either
scenario, unsettled weather with showery activity would
continue for our area.
-JH/Alviz

&&


.AVIATION...Conditions mostly improving towards VFR as of 22z Mon
as showers start to decrease in coverage, but a few intermittent
bouts of MVFR are still occurring in passing showers. Showers
will continue to produce mountain obscurations into the
evening. A few breaks of sunshine have helped to develop enough
instability for a few lightning strikes near KSLE in the past
hour. There remains a 10-20 percent chance of thunderstorms
across the area this afternoon, but thunder chances diminish after
00z. Winds out of the W-NW at around 10 kt through the afternoon.
Another disturbance arrives after 06z Tue, bringing increased
shower activity and a return of MVFR conditions 08-10z Tue as
winds shift southerly through the end of the period.

Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission, but a tentative work
around has been implemented. There is no estimated time of
full repair and restoration for either of these locations.

PDX AND APPROACHES...Mainly VFR with cigs above FL040 through this
evening, with a few intermittent dips below FL040 possible in
passing showers. Can`t rule out a thunderstorm near the terminal
through around 00z Tue, but chances are small enough to not
include in the TAF. Conditions trend back towards MVFR after 10z
Tue as another approaching disturbance brings increased shower
activity. W-SW winds 5-10 kt this afternoon, diminishing in the
evening and then turning southerly and increasing back to 10 kt
after 10z Tue. /CB

&&

.MARINE...Blustery conditions characterized by gusty west winds
and scattered showers continue across the coastal waters in the
post frontal environment this afternoon, maintaining low end
Small Craft Advisories as winds occasionally gust to 25 kt and
seas linger around 8 to 10 ft per recent observations. Expect an
uptick in these conditions as the next system arrives late this
evening, with winds turning a bit more southwesterly overnight in
response to a weak surface low cross the northern waters. Winds
will shift northwesterly and start to diminish Tuesday morning as
the low moves inland near Cape Falcon, but a building westerly
swell will maintain seas around 10 ft and keep the current small
craft headlines in place through Tuesday evening. Weak high
pressure will bring a brief lull in conditions Tuesday night into
Wednesday morning. More active weather then returns later
Wednesday into Thursday as another low approaches the waters. /CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory from 3 AM to 6 PM PDT Tuesday for
     ORZ126-127.

WA...Winter Weather Advisory from 3 AM to 6 PM PDT Tuesday for
     WAZ211.

PZ...Small Craft Advisory until 11 PM PDT Tuesday for PZZ210-251>253-
     271>273.

&&

$$

www.weather.gov/portland

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