textproduct: Portland

SYNOPSIS

Zonal flow through the weekend will transition into a long wave trough with a closed low over northern California. Will see a steady cool down but not a cold snap. Rain and mountain snow expected. Another round of cold and moist air occurs on Thursday through Friday though conditions have warmed slightly since previous forecasts.

LONG TERM...Sunday through Friday

The ridging will begin to break down on Sunday as a long wave trough associated with a broad low pressure system to the north advects southward. The main impacts from this system will occur on Monday through Tuesday. Unfortunately this trough has yet to come into good agreement and that is due to it's split nature. Some models are suggesting a closed low forming near the base of the trough while others are keeping it as one "solid piece". Let's look at each possible solution and what the impacts would be.

If the closed low theory takes hold, the most likely track is again over northern California. The low itself would be active and would advect colder air over the region. The colder air and more active pattern would promote heavier precipitation and likely lower snow levels. If a more elongated low manifests then the cold air intrusion will be less robust and thus snow levels slightly lower. While the pattern may have much to be desired for consistency, the sensible weather has not be effected significantly. Over the last few forecasts snow levels have continued to rise with lower and lower probability for advisory level snow (6-12"). On Monday through Tuesday there is less than a 10% chance of pass level advisory level snow, and less than a 20% chance of volcano heavy snow accumulation.

On Wednesday, an inverted trough forms as the upper level low moves into the desert southwest. Fast forward to Thursday and Friday. This is when yet another fall system will move in. In current ensembles there is basically no agreement in specifics and anomaly is high in the 500 mb heights. There continues to be a low attempting to form just off the coast but once again, poorly realized. Currently the ECMWF is the coldest of the modeled ensembles while the GEPS is much warmer. With that in mind, probabilistically we are all over the place. For example, at Willamette Pass there is around a 25% chance of 6" or more of snow. There is high consensus in overall temperatures lowering on Wednesday and Thursday. -27

AVIATION

Late this afternoon a frontal system continues to slowly lumber into the region with a mix of IFR/MVFR conditions at the coast to MVFR to VFR CIGs inland. As the evening and overnight hours progress flight conditions are expected to degrade (mainly inland) due to the progression of the aforementioned front. Expect light rain already intermittently occurring in the Portland metro through the north Oregon coast to spread northwest to southeast across the rest of the inland and central Oregon coastal terminals. Most likely onset times of steadier light rain 04-06z Sat Portland metro area and northern Willamette Valley followed by the central Willamette Valley and KONP 06-12z. However, light rain as a prevailing condition may fail to materialize at KEUG - moderate confidence the bulk of the light rainfall get hung up to their north. Precipitation gradually decreases later Saturday morning first at inland sites before finally ending around KAST near the end of the TAF period. Winds stay breezy at most terminals tonight with occasional gusts up to 15 to 25 knots out of the south, highest at coastal sites.

KPDX AND APPROACHES...Light rain will increase this evening (04-06z) with CIGs dropping to a prevailing MVFR the latter half of the night. Could still see the occasional BKN025 before this occurs however, similar to what was observed earlier this afternoon. Light rain slowly ends 18-21z Sat before dry weather returns although there is a ~60% chance MVFR CIGs last through he latter half of the TAF period. -99

MARINE

A slow moving frontal system continues to progress through the waters this evening accompanied by a lingering westerly swell of 7 to 10 ft at ~12 seconds. Small Craft conditions today have been rather persistent but bouy observations are finally starting to show a gradual decrease in regards to wave height. Moderate confidence Small Craft level waves/winds drop off across the inner and outer waters south of Cape Foulweather late this afternoon followed by the rest of the waters overnight into early Saturday morning. Otherwise, seas eventually subside to 6-8 ft for Saturday and Saturday night with gusts holding below 10-20 knots.

Them come the second half of Sunday into Monday another frontal system will traverse the waters bringing increased southwesterly winds with gusts to 15-21 knots before turning out of the northwest. A westerly swell helps to increase wave heights to 12-14 ft by Monday morning as well, before falling below 10 ft again on Tuesday. Should this forecast remain on track, an additional round of Small Craft Advisories will be needed for the inner and outer waters. Expect a fairly active and progressive weather pattern to continue the rest of the week as yet another weather system may bring hazardous winds and seas later in the next week. Models show a 20-30% chance for wave heights to meet or exceed 15ft late Thursday through Friday. -99

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 3 AM PST Saturday for PZZ210-251- 252-271-272. Small Craft Advisory until 4 PM PST this afternoon for PZZ253- 273.