textproduct: Portland

SYNOPSIS

Widespread showers expected through tonight with a 10-25% chance of strong showers or thunderstorms. Cascade snow continues through tonight with a Winter Weather Advisory in effect through 4 AM Thursday. A strong ridge develops over the northeast Pacific into Saturday with daytime temperatures warming into the 50s by Friday. However, scattered shower chances continue through early next week as moisture rides over the ridge and into the Pacific Northwest. Colder air returns late Monday into Tuesday, bringing a low chance of snow to the low elevations.

DISCUSSION...Wednesday afternoon through Tuesday

Widespread precipitation moved through NW Oregon and SW Washington this morning associated with the frontal system impacting the PacNW. Conditions have become showery this afternoon as the front has moved into eastern OR and WA. Wind gusts up to 25-30 mph were observed over the area with localized gusts up to 40-45 mph over the terrain and coast as the front passed this morning. Expect winds to slowly decrease this afternoon everywhere except for the terrain where gusts continue into Thursday. The post-frontal environment is becoming convective as visible satellite imagery shows periods of clearing followed by cumulus cloud development. Model soundings still suggest skinny CAPE soundings with CAPE values up to 100-200 J/kg through the early evening hours today, leading to a 10-25% chance of isolated thunderstorms. With CAPE this low, not expecting widespread thunderstorms, but periodic stronger showers that could produce brief gusty winds, small hail, or heavy rain/mountain snow with one or two becoming strong enough to produce lightning. Snow continues over the Cascades above 3500-4000 ft into early Thursday morning. Heaviest snow expected through this evening above 4000 ft. An additional 6-11 inches of snow is possible through 4 AM Thursday at Santiam and Willamette passes, though with the showery nature of precipitation amounts could fall above or below this range depending on where individual showers set up. Due to the convective nature, heavier showers could also produce snowfall rates up to 1 inch per hour at times through around 9 PM tonight.

High pressure builds over the eastern Pacific Thursday into the early weekend, though the PacNW remains on the eastern periphery of the flow. This will allow moisture to continue streaming into the area from the north. Scattered showers continue Thursday into Friday with fairly benign accumulations expected. Snow levels continue falling late tonight into Thursday to around 2500-3000 ft, though accumulation is expected to be very limited below 3500 ft. As the high pressure builds, so do the snow levels rise to above pass level on Friday. Expect daytime temperatures for the interior lowlands to peak in the upper 40s on Thursday, warming a few degrees to the low 50s by Friday.

Ensemble guidance suggests high pressure will spread inland on Saturday, allowing temperatures to warm even more into the mid to upper 50s. Dry weather is likely for the interior valley as well as the Coast Range and Cascades near and south of Highway 20. However, dry weather will be short lived as a shortwave moving east through British Columbia causes the flow to become zonal over the PacNW on Sunday, allowing the return of moisture and therefore scattered showers Sunday into Monday. Daytime temperatures on Sunday will be similar to Saturday, but latest guidance suggests a 5-10 degree drop on Monday as colder air from Canada begins pushing south into SW WA and NW OR. Uncertainty remains on timing of this push and how far south the cold air will make it, but latest guidance indicates that at least 75% of the GEFS, EPS, and GEPS members bring 850mb temperatures below -5 deg C by late Monday night/early Tuesday morning to the Portland area. Latest NBM guidance has a 50-70% chance for low temperatures to fall to or below freezing for Tuesday morning's low temperature across the lowlands. With temperatures this low along with shower chances continuing into Tuesday, NBM is indicating a 30-45% chance of at least a rain/snow mix Tuesday morning down to the valley floor for the Portland/Vancouver metro area and north through the Cowlitz valley. Looking at probabilities for at least 1 inch of snow, this drops to around 5-15% for locations below 1000 ft elevation, with around 40-60% chance for elevations between 1000-2000 ft elevation. This is definitely a pattern we will be keeping our eye on as the forecast unfolds. -03

AVIATION

Showery precipitation continues behind the cold front with shower chances slowly decreasing throughout the TAF period. West/northwest winds along the coast with south/southwest winds inland, all less than 10 kt. These conditions will result in a mixture of VFR/MVFR conditions with a 15-40% chance of MVFR conditions at any time through the TAF period across the airspace. There is also a a 5-10% chance for isolated pockets of IFR conditions, with KONP, KHIO and KTMK having the highest probability of these IFR conditions.

KPDX AND APPROACHES...VFR/MVFR conditions expected with a 10-40% chance of MVFR conditions at any through the TAF period. Shower activity continues overnight but slowly decrease. Southwesterly winds less than 10 kt. /42

MARINE

Breezy southwest winds will gradually turn more northwesterly through the evening with wind gusts up to 25 kt expected through 4 AM. Winds are expected to decrease Thursday morning. Seas around 7-9 feet at 7-9 seconds will increase to around 10-11 feet at 10-12 seconds overnight, leading to steep seas and continued hazardous conditions for Small Craft through Thursday. High confidence for calmer marine conditions behind this system at the end of the week and into the weekend, with seas falling below 10 feet and winds turning more westerly and remaining under 15 kt by Thursday afternoon. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...Winter Weather Advisory until 4 AM PST Thursday for ORZ127-128.

WA...None.

PZ...Small Craft Advisory until 4 PM PST Thursday for PZZ210-251>253- 271>273.