textproduct: Portland

SYNOPSIS

Scattered showers continue today as northwesterly flow sets up over the region. A weak front will bring more widespread showers tonight into Friday. Slightly warmer and drier conditions return on Saturday with scattered shower chances continuing 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 for snow in the lowlands tuesday morning. Widespread precipitation returns Wednesday.

DISCUSSION...Thursday afternoon through Wednesday

High pressure builds over the eastern Pacific today 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. Light, scattered showers continue today with more widespread precipitation tonight into Friday as a weak front brings moisture in from the northwest. Low rain accumulations expected through Friday. As high pressure builds, snow levels also gradually rise to above pass level on Friday and ends the snow for the Cascade passes. Expect daytime temperatures for the lowlands to peak in the low 50s today and Friday, which is generally near or slightly below normal for this time of year.

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 south of the Portland metro area as well as the Coast Range and Cascades near and south of Highway 20. Shower chances continue elsewhere. However, dry weather will be short lived as a trough moving into British Columbia from the Gulf of Alaska 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 near or slightly cooler than Saturday, but guidance continues to 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 at least 80% of the GEFS, EPS, and GEPS members bring 850mb temperatures below -5 deg C by late Monday night/early Tuesday morning to the area. Latest NBM guidance has a 50-70% chance for low temperatures falling at or below 32 degrees for Tuesday morning's low temperature across the interior lowlands, with lower chances (10-30%) along the coast. With temperatures this cold along with shower chances continuing into Tuesday, there is a 30-45% chance of at least a rain/snow mix down to the valley floor from the Cowlitz Valley to the greater Portland/Vancouver Metro Area. Looking at probabilities for at least 1 inch of snow, this drops to around 5-15% for locations below 1000 ft elevation, with around 50-70% chance for elevations between 1000-2000 ft elevation. If there are any accumulations in the lowlands, it would be short-lived as temperatures are marginally freezing due to onshore flow and daytime highs are expected to climb in to the upper 40s. By Wednesday, a little over 50% of ensemble members suggest 500 mb heights will climb and bring relatively warmer temperatures, reducing the chances for lowland snow.

A stronger push of moisture over the eastern Pacific ridge into the PacNW is forecast on Wednesday, bringing another round of widespread precipitation to the region. This far out, uncertainty remains in specific details, but confidence is high that there will be no flooding impacts. Additionally, there's an increased chance of breezy winds on Wednesday with a 20-40% chance of max wind gusts over 35 mph for inland valleys, and a 30-50% chance of max wind gusts over 40 mph for coastal communities and the terrain. -03/10

AVIATION

Widespread VFR conditions across the airspace. However, a weak frontal passage starting around 08Z-12Z Friday will bring a return of light precipitation along with predominately MVFR conditions. Inland locations have a 60-80% chance for MVFR conditions starting around 10Z-12Z Friday and a 40-60% chance for IFR conditions developing between 15Z-18Z Friday. Along the coast, there is a 75-90% chance for MVFR conditions and a 40-60% chance for IFR conditions developing between 13Z-15Z Friday. Overall, expect predominately MVFR conditions after 08Z-10Z Friday with an increasing chance of IFR conditions to develop between 10Z-15Z Friday. Afterwards, any IFR conditions are expected to improve towards MVFR, which is expected to persist through the remainder of the TAF period. Winds generally westerly to southwesterly and under 10 kt, becoming more southerly around 10Z-12Z Friday.

KPDX AND APPROACHES...VFR conditions will become MVFR starting around 10Z-12Z Friday as a frontal passage will bring increasing clouds and light precipitation. There is a 30-50% chance of IFR conditions to develop from 15Z-18Z Friday. Afterwards, any IFR conditions are expected to improve towards MVFR, which is expected to persist through the remainder of the TAF period. Southwesterly winds under 10 kt, turning more southerly around 11Z Friday. /42

MARINE

Seas and winds continue to trend down today. Seas drop below 10 feet to around 6 to 8 feet at 10 to 11 seconds through Saturday. The next system approaches the region on Sunday and will bring west to northwest gusts of 15 to 20 kt. There is a low 10-30% chance of Small Craft gusts Sunday evening and overnight with slightly higher over the northern outer waters up to 50%. Holding off on any headlines given the low confidence at this time. Next week looks like it could bring another round of Small Crafts in the middle of next week but confidence is also low. -19

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.