635
FXUS66 KPQR 301836
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1136 AM PDT Tue Apr 30 2024

.UPDATED AVIATION...

.SYNOPSIS...
Another relatively cold weather system will move
through the region today, bringing rain showers, Cascade snow
showers, and a 15-30% chance for thunderstorms. Potential
hazards with any thunderstorm development include lightning,
small hail, heavy rain, and gusty winds. Very brief drying and
warming arrive on Wednesday before the next system arrives on
Wednesday night to Thursday. Unsettled weather likely continues
this weekend into early next week.

&&

.SHORT TERM...Now through Wednesday night...An upper level
trough is making its way toward the Pacific Northwest this (Tue)
morning. Satellite imagery as of 3 AM PDT shows the associated
surface low centered around 47.14N 127.39W, just off the coast
of Washington. This system is already returning shower activity
across northwest Oregon and southwest Washington this morning.
Expect rain showers, Cascade snow showers, and a 15-30% chance
of thunderstorms today as the low progresses southeast and makes
landfall along the north/central Oregon coast. Snow levels in
the Cascades will be around 3000 to 3500 feet.

Any thunderstorms that do develop today could bring moments of
lightning, small hail, heavy rain, and/or gusty winds. We`ll see
an unstable environment today as the aforementioned upper level
trough advects positive vorticity and cold air into the region.
This is often associated with upward motion (lifting) of air. NBM
and HREF guidance shows CAPE values ranging between 150-400 J/kg
across the area today. Models also show 500 mb temperatures as
low as -30 to -35 deg C with lapse rates of 7.5-8.5 deg C/km.
This will help with the formation of small hail.

The latest CAMs show the heaviest showers generally north of
Lane County. 24 hr QPF amounts ending 5 AM Wednesday are
forecast around 0.20-0.50 inch for the coast, Coast Range, and
inland valleys. Meanwhile, forecast QPF in the Cascades will be
between 0.75-1 inch, especially north of Lane County. The
current Winter Weather Advisory for the Cascades between the
South Washington Cascades and Marion/Linn County Cascades
remains in effect through this evening, as convection may bring
heavy snow showers at times. With snow ratios around 10:1,
forecast snow amounts will generally be between 2-8 inches above
3000 ft, with the heaviest amounts above 3500 ft. Those traveling
through the passes (highways 20/22/26) today should prepare for
winter weather conditions.

Tonight into tomorrow (Wednesday), conditions dry as this
system pushes east and upper level high pressure builds. We
could see some clearing or cloud breaks Tuesday night, which
would help with radiational cooling and thus frost development.
NBM/HREF probabilities for Wednesday morning lows below 36
degrees are around 60-80% in the Coast Range, Willapa Hills, and
Upper Hood River Valley, and 20-40% in the Willamette Valley.
However, there remains uncertainty with frost development since
most model guidance is not showing complete clearing. In addition,
winds Tuesday night don`t go completely calm, so that could
inhibit cooling as well. For now, a slight chance (15-24%) for
patchy frost was kept in the forecast Tuesday night.

Temperatures warm up slightly on Wednesday, with highs in the
upper 50s along the coast and Coast Range, low 60s for inland
valleys, and 40s in the Cascades. The next system looks to
arrive Wednesday evening into Thursday, bringing another round
of precipitation, including Cascade snow.          -Alviz

.LONG TERM...Thursday through Monday...Most ensemble guidance
suggests another trough will push through the region Thursday,
bringing additional lowland rain and Cascade snow. The latest
NBM shows a 30-40% chance that inland valleys receive 0.25 inch
or greater of 24 hr QPF ending 5 PM Thursday. For the coast,
Coast Range, and Cascades, this probability is around 40-60%.
Advisory-level snow is currently not expected in the Cascades,
as NBM shows less than 5% chance of 4 inches of snow or more
during this time frame.

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 (at least 75%) still show precipitation on
Friday, despite them also showing ridging.

Unsettled weather continues Saturday to Monday as WPC cluster
analyses show a troughing pattern over the weekend and into
early next week. There is some uncertainty with the weekend
trough - Currently, about 65% of ensemble members show the
trough pushing through the Pacific Northwest. In this scenario,
we would receive another decent around of precipitation.
Meanwhile, 25% of members show the weekend trough dipping south
toward the central coast of California. If this pans out, we
would remain relatively drier. Finally, 10% of members depict a
middle-ground scenario with the trough axis taking aim toward
southern Oregon and northern California. We`ll see how this
shapes up later in the week. In general, the long term forecast
does not depict any overly impactful weather. -Alviz

&&

.AVIATION...Post-frontal conditions are producing generally MVFR
thresholds inland and VFR at the coast, where the strongest
precipitation has already ended. Cloud cover is sufficiently thick
that northern terminals won`t see enough instability for strong
thunderstorms until mid/late afternoon. A short-lived thunderstorm
has around a 25% chance of occurring at some point between 21z
Tue to 02z Wed at northern terminals, but with how spotty
precipitation will be, this will be difficult to predict.

Probabilities drop considerably going southward, dropping to <10%
somewhere between KSLE and KEUG. Temperatures aloft are warmer
further south, leading to less instability and lower threat for
thunderstorms.

Precipitation ends at all terminals by 04z Wed, and the rest of
the TAF period sees dry conditions due to weak high pressure
building in. Southerly winds at southern terminals are currently
gusting to around 20-24 kts, but will decrease rapidly once the
area begins to dry out. In the latter half of Tuesday night, high
resolution model guidance points to a 30-40% chance of IFR
conditions due to fog or low clouds forming. However, other
statistical model guidance has considerably lower chances. For
now, guessing that chances will be around 25-30% chance of IFR
conditions across inland terminals at that time. /JLiu

Notes: The ASOS at KTTD is only available via the dial in option.
There is no estimated time of full repair and restoration for
this location.

PDX AND APPROACHES...Current MVFR conditions will give way to
predominantly VFR around 22z Tue as the main band of rain passes
to the east. Afterwards there will be a period of a few stray
showers, but these will be isolated. With increasing instability,
threat of a stray mid/late afternoon thunderstorm exists with
these passing bands of moisture, with probabilities around 25-30%.
All precipitation ends 04z Wed, and VFR conditions with weak
variable winds are expected afterwards. High resolution model
guidance points towards fog or low stratus being a possibility
11-16z Wed, with around a 25-30% chance of IFR thresholds being
met at that time. /JLiu

&&

.MARINE...
Gusty west/northwest winds and scattered showers
continue across the coastal waters through this afternoon,
maintaining low end Small Craft Advisories as winds occasionally
gust to 25 kt and seas linger around 8 to 10 ft with a dominant
period of 10 to 12 seconds through this evening. Weak high
pressure will bring a brief lull in conditions tonight into
Wednesday morning. More active weather then returns later
Wednesday into Thursday as another low approaches the waters. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 6 PM PDT this evening for ORZ126-
     127.
WA...Winter Weather Advisory until 6 PM PDT this evening for WAZ211.
PZ...Small Craft Advisory until 11 PM PDT this evening for PZZ210-
     251>253-271>273.
&&

$$

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