508
FXUS66 KPQR 301030
AFDPQR

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

.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...Generally VFR across the airspace, with a mixture of
low-end VFR to high-end MVFR through at least 03Z Wednesday as
showers and thunderstorms move across the airspace. Showers will
continue to produce mountain obscurations through at least 03Z
Wednesday. 15-20% probability for thunderstorms through 03Z
Wednesday as showers diminish in a west to east direction. Any
thunderstorms or heavier showers could result in small hail. After
03Z Wednesday expect improvement towards predominately VFR which
should persist through the remainder of the TAF period as weak
high pressure builds across the airspace.

Generally westerly winds with gusts up to 25 kt along the coast
and southerly winds inland with gusts up to 20 kt. Expect a
windshift around 18Z Tuesday to a more northerly wind pattern
along the coast and more westerly inland around 21Z Tuesday along
with gusts diminishing.

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...Generally VFR, with a mixture of low-end VFR
to high-end MVFR through at least 03Z Wednesday as showers and
thunderstorms move across the airspace. 15-20% probability for
thunderstorms through 03Z Wednesday. After 03Z Wednesday expect
improvement towards predominately VFR. Southerly winds inland
with gusts up to 20 kt, with winds becoming more westerly around
21Z Tuesday. /42

&&

.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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