textproduct: Portland

SYNOPSIS

A brief ridge will bring a period of drier weather from late Tuesday into early Wednesday before a weakened front arrives late Wednesday, introducing lowland rain and mountain snow through early Thursday. Another short-lived ridge builds in on Friday, followed by a return to showery conditions through the weekend.

DISCUSSION...Now through Sunday

Cooler and drier air spreads across the region today as a shortwave ridge moves overhead, promoting clearing skies later this evening. Patchy fog will develop overnight into early Tuesday, although increasing cloud cover moving in from the northwest late tonight introduces some uncertainty regarding fog development, especially for southwest Washington. Another weak shortwave passes Tuesday, bringing mostly clouds and light precipitation potential, with the best chances along the north Oregon coast from Tillamook County northward and across southwest Washington. Rainfall totals through Tuesday evening have a 20-40% probability of remaining below one-tenth of an inch.

Snow levels, currently around 3500 to 5500 feet this afternoon, will fall to approximately 3000 to 3500 feet by late Tuesday morning before rising again to above 4500 feet by Wednesday afternoon. Precipitation remains limited for meaningful Cascade snow through Tuesday. Clearing returns Tuesday night, with an increasing likelihood of fog; there is a 40-60% chance for freezing fog in the southern Willamette Valley. Areas of frost are also possible in the coldest valley locations including the Hood River Valley, sheltered valleys of the northern Coast Range, and parts of the southern Willamette Valley. Expect overnight lows in the 30s, with mid to upper 30s along the coast and within the Portland/Vancouver metro, and mid to low 30s elsewhere.

Dry conditions hold through Wednesday afternoon before the next frontal system moves in late Wednesday into Thursday. Forecast uncertainty persists regarding whether the approaching trough will drop south towards California - resulting in weaker precipitation and a milder cooling trend, or will approach around the Washington and Oregon border - resulting in a trough that could bring stronger forcing, colder air aloft, and broader precipitation coverage. While there is more model agreement on a solution, location and strength still remains uncertain. Significant snowfall at the Cascade passes (six inches or more) remains unlikely at 10% or less, while the probability for at least one inch of snow is around 25-35% at Santiam and Willamette Passes and near 10% at Government Camp. Thursday onward, temperatures remain near or slightly above seasonal normals, with highs in the low to mid 50s and lows in the upper 30s to mid 40s through the rest of the week and into the weekend. A shortwave trough arriving from the northwest on Saturday brings a renewed chance of lowland rain and mountain snow, setting up a showery pattern through the weekend. At the moment, there is only a 5-10% chance for 6 inches or more of total snow on Sunday. Could see higher chances for 6 inches or more of total snow at the start of next week. ~12

AVIATION

As of 04Z Tuesday, satellite and surface weather observations continue to show VFR flight conditions across the airspace, with north to northwest winds less than 10 kt. Given the clearing skies, moist air and light winds, there is a probability of IFR/LIFR conditions developing within the Willamette Valley from 12Z-15Z Tuesday with a general 5-10% probability for most inland locations. However, given the moist northerly flow expected for KEUG and KHIO, there is a higher probability (25-40% probability) for IFR/LIFR conditions to develop. Otherwise, VFR conditions expected to be the predominant flight category. Any IFR/LIFR conditions that manifest, should improve to VFR around 17Z-19Z Tuesday. After 19Z Tuesday, VFR conditions expected to dominate through the remainder of the TAF period.

KPDX AND APPROACHES...Expect predominately VFR conditions continue through the TAF period. However, there is a less than 10% probability of IFR/LIFR conditions developing from 12Z-15Z Tuesday. Any IFR/LIFR conditions that manifest, should improve to VFR around 17Z-19Z Tuesday. /42

MARINE

Buoy observations from early Monday afternoon showed the continuation of north to northwest winds around 10 to 20 kt, with seas around 9 to 11 ft at 11 to 13 seconds. Expect seas to fall below 10 ft across all waters by midnight. Until then, small craft advisories remain in effect.

A brief reprieve from the active weather is expected Tuesday and Wednesday as seas subside to 6 to 8 ft at 11-14 seconds. Winds will also ease while remaining out of the northwest through Tuesday evening. Winds become northeasterly Tuesday night and then easterly for a brief period of time Wednesday morning as offshore flow develops. East winds should generally stay under 10 kt at that time, but may be locally stronger downwind of gaps in the coastal terrain.

Winds veer to the south Wednesday afternoon and evening as another frontal system approaches the waters. Wind speeds will also increase at that time, likely peaking around 15 to 20 kt with gusts up to 25-30 kt, strongest over the outer waters beyond 10 NM offshore. Southerly winds should then decrease through the day on Thursday. That said, a westerly swell will build over the waters on Thursday despite weakening winds. Seas will most likely become steep and hazardous Thursday afternoon through Thursday night with significant wave heights peaking around 14-15 ft with a dominant wave period around 14-15 seconds. Although this is currently the most likely outcome, there is still a large degree of uncertainty. Some wave model guidance shows seas only peaking around 10 ft or less, while other guidance shows seas peaking near 25 ft. For now, there is a 50% chance seas peak near 14-15 ft, and a 10% chance seas peak near 20 ft or higher. Given the low forecast confidence in place on Thursday, have decided to hold off on issuing a Hazardous Seas Watch for now. -23

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until midnight PST tonight for PZZ210- 251>253-271>273.