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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 HRRR 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/HRRR 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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Vancouver
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Tue -- 12:50 AM PDT     1.29 feet High Tide
Tue -- 02:34 AM PDT     Moonrise
Tue -- 04:03 AM PDT     0.85 feet Low Tide
Tue -- 05:59 AM PDT     Sunrise
Tue -- 06:37 AM PDT     1.04 feet High Tide
Tue -- 08:29 AM PDT     0.95 feet Low Tide
Tue -- 11:00 AM PDT     Moonset
Tue -- 12:27 PM PDT     1.80 feet High Tide
Tue -- 08:17 PM PDT     Sunset
Tue -- 09:13 PM PDT     0.05 feet Low Tide
Tide / Current data from XTide NOT FOR NAVIGATION

Vancouver, Columbia River, Washington (dubious accuracy), Tide feet

Knappa
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Tue -- 01:01 AM PDT     3.19 feet Low Tide
Tue -- 02:40 AM PDT     Moonrise
Tue -- 06:01 AM PDT     Sunrise
Tue -- 06:11 AM PDT     7.76 feet High Tide
Tue -- 11:02 AM PDT     Moonset
Tue -- 02:13 PM PDT     0.09 feet Low Tide
Tue -- 08:22 PM PDT     Sunset
Tue -- 08:48 PM PDT     6.13 feet High Tide
Tide / Current data from XTide NOT FOR NAVIGATION

Knappa, Knappa Slough, Columbia River, Oregon, Tide feet

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