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 early Friday morning. Slightly warmer and drier conditions return on Saturday, 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 for snow in the lowlands overnight.

DISCUSSION...Today through Wednesday

Radar imagery as of early Thursday morning depicts light scattered showers across the area as northwesterly flow sets up over the region. The Winter Weather Advisory for the Marion, Linn, and Lane County Cascades has been allowed to expire as the threat for heavy snow along Santiam and Willamette Pass has ended. Snow showers will continue across the Cascades today with light accumulations.

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 morning as a weak front brings moisture in from the northwest. Light rain accumulations expected through the remainder of the week. 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 as well as the Coast Range and Cascades near and south of Highway 20. However, dry weather will be short lived as a trough moving into through 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 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 80% of the GEFS, EPS, and GEPS members bringing 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. -10/03

AVIATION

Radar imagery as of early Thursday morning depicts scattered showers mainly confined over the Cascades as northwesterly flow sets up over the region. Expect predominately VFR CIGs with occasional drops to MVFR today as light showers continue. Guidance suggests a 30-50% chance for MVFR CIGs at any given hour across the Willamette Valley and a 60-70% chance along the coast today, with the highest chances between 18z Thu-00z Fri. Tonight, a weak front will bring more widespread showers and increased chances for MVFR CIGs at all terminals after 09-12z Fri. Winds generally westerly to southwesterly and under 10 kt, turning more southerly tonight with the weak front.

KPDX AND APPROACHES...Predominately VFR conditions with light showers and a 30-50% chance for MVFR CIGs (2-3 kft) through this evening. Southwesterly winds today under 10 kt, turning more southerly after 06z Fri. -10

MARINE

Northwesterly winds gradually ease across the waters today as pressure gradients ease. Seas remain around 9-11 feet at 11 seconds as a westerly swell moves through; therefore, the Small Craft Advisory remains in effect through 10 PM tonight for all waters including the Columbia River Bar. High confidence for calmer marine conditions at the end of the week and into the weekend, with seas falling below 10 feet by early Friday morning and winds turning more westerly and remaining under 15 kt. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ210- 251>253-271>273.