textproduct: Portland

SYNOPSIS

Yet another day of unsettled conditions is expected to start the weekend with continued shower and isolated thunderstorm activity streaming south to north overhead. We'll transition to slightly cooler conditions Sunday into early next week as flow turns more west-northwesterly but showers chances persist. Come the middle of next week confidence continues to increase in a potent cold frontal system ushering in widespread precipitation while shifting impacts to Cascade pass snowfall and cooler overnight temperatures for the end of next week as well.

LONG TERM...Monday through Friday

Ensemble guidance indicates broad west-northwest to northwest flow aloft for early next week helping to steer a weak embedded shortwave into the region on Monday. As a result showers chances (50-90%) will be maintained. Beyond this shortwave feature shower activity decreases Monday night into Tuesday as a transient ridge briefly builds overhead. Our attention then turns to the next Pacific frontal system arriving from the Gulf of Alaska late Tuesday into early Wednesday morning. Confidence is high cooler temperatures aloft associated with this system will lower snow levels to near or below Cascade pass elevations, raising the concern for potential travel impacts returning. A stark change from the rather warm conditions we've seen the last couple of weeks. The latest NBM probabilistic guidance indicates a 60-90% chance of 6+ inches of snowfall at the Cascade passes (Gov Camp, Santiam, Willamette Pass) over the 48 hours between 5pm Tuesday and 5pm Thursday. As for 12+ inches of snowfall, there is a 40-60% chance during this same time period. Should these probabilities and current forecast snowfall amounts hold, snowfall related highlights would be needed for our Oregon and Washington Cascade zones.

Additionally, cooler and potentially clearer conditions behind the system may introduce renewed frost concerns across portions of the interior valleys by mid to late week. Wednesday night into Thursday morning is the first of these such nights with NBM guidance suggesting a 30-70% chance for areas frost (<36F with partial clearing) across most inland locations and a 60-80% chance for freezing temperatures in the Upper-Hood River Valley. A similar set-up is anticipated Thursday night into Friday morning as well. At least the trend is to moderate back towards normal temperatures for the following weekend. -99/12

AVIATION

The region is under a mix of flight conditions this morning as an upper level low pressure systems off the northern California coastline continues to be the main driver of the weather pattern today. Moist, southerly flow continues to support waves of shower activity when overhead has pushed CIGs/VIS to MVFR and even IFR at several terminals like KHIO. Expect these variable flight conditions to continue although the morning goes on they'll trend back to primarily VFR. This afternoon expect another chance for thunderstorms, highest in the central and southern Willamette Valley at 25-30% around KEUG and 15-20% elsewhere. Any thunderstorm activity likely comes to an end around 3-5z Sunday with just showers the remainder of the TAF period. Outside of any convective outflow, winds generally remain under 5-10 knots.

KPDX AND APPROACHES...A mix of VFR to MVFR conditions continue this morning through at least 15-16z before trending back to VFR. From there expect generally VFR but any showers passing overhead may degrade CIGs/VIS temporarily. Low 15-20% chance for thunderstorms Saturday afternoon with the 23z Sat to 04z Sunday the time period of focus. However, confidence in timing and placement remains only low to moderate. -99

MARINE

Headed through the weekend conditions remain rather benign as far as the sea state and winds are concerned with significant wave heights holding in the 4 to 7 foot range with winds gusts below 20 knots across the inner and outer waters. We'll likely see south/southwest winds slowly become more north/northwest today into this evening before persisting through at least Monday night. Looking towards the middle of next week in the Tuesday and Wednesday time period a decently robust frontal boundary will likely (75-85% chance) result in Small Craft Advisory wind gusts and seas building to around 7 to 10 feet. Wave heights and winds then slowly decrease into the end of the week. -99

PQR WATCHES/WARNINGS/ADVISORIES

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