textproduct: Portland

SYNOPSIS

Calm weather conditions today with fog/frost again overnight tonight into Saturday morning. This weekend, a weather system moving into northern California will brush the region and return chances for precipitation, mainly from Salem southward. Dry weather returns early next week before another system arrives and brings widespread precipitation Tuesday into Wednesday.

DISCUSSION...Friday afternoon through Thursday

Mostly clear skies are being observed on visible satellite across NW OR and SW WA early Friday afternoon as upper level flow remains fairly zonal over the region. Temperature observations are on the way to seeing peak temperatures in the mid 50s across much of the region outside of cloud cover today. Another round of patchy fog and frost is expected tonight into Saturday morning across the interior lowlands. Overnight temperatures aren't expected to be quite as chilly as this morning with most locations falling to around 32-35 degrees, which is still plenty chilly for frost. However, high cloud cover from a weather system to our south could begin streaming over the region by early tomorrow morning, and depending on the timing of this, could impede the cooling needed for frost and/or fog.

Ensemble guidance remains in excellent agreement that a cut-off low pressure system will swing through northern California from the northeast Pacific Saturday through early Monday morning. Ensemble guidance has locked in that the low track will be far enough north that wrap-around precipitation from the south will reach at least the southern-most parts of the CWA Saturday afternoon into Sunday, mainly from Lincoln/Polk/Marion Counties southward. For locations north of this, there's about a 25-30% chance of rain all the way north to the Columbia River on Sunday, including the Portland/Vancouver metro area, and about a 10-15% chance north of this. Not much rain nor impacts are expected from this system. In 48 hours ending 10 AM Monday, general QPF is 0.05-0.45 inch south of Salem, heaviest in southern Lane Co for the valley and the Cascades. Additionally, guidance currently suggests that the wettest scenario (10% chance) is around 0.25-0.8 inch for the interior lowlands. Snow levels will also be around 6000-7000 feet, keeping snowfall well-above the Cascade passes.

Transient ridging returns to the PacNW for Monday, leading to a return of completely dry weather and warmer daytime temperatures in the upper 50s. The next weather system approaches the region Tuesday with uncertainty continuing for when precipitation will begin. Many ensemble members have been pushing the start time a touch earlier into Tuesday, though the majority of ensemble guidance still (~75%) favors the dry weather coming to an end by Tuesday evening. At this point, no wind impacts are expected with this system, though could see some gusts up to 25 mph, locally up to 35-40 mph over the terrain, as the front passes on Wednesday. Snow could return to the Cascades as snow levels drop to around 3500-4500 feet on Wednesday. Chances for 6+ inches of snow in a 48 hour period from 4 AM Tuesday through 4 AM Thursday are around 50-70% along the Santiam and Willamette Passes and 20% for Highway 26 at Government Camp, with the most snow falling late Tuesday into Wednesday. On Thursday, rain and mountain snow decrease and transition into showers. -03/10

AVIATION

Currently VFR conditions throughout the airspace, with high pressure keeping skies mostly sunny. Expect conditions to stay VFR through at least 06Z Saturday. By 06-12Z Saturday, fog and low stratus may redevelop around KHIO, KSLE, and KEUG, possibly bringing a return of IFR/LIFR conditions (20-40% chance of LIFR conditions). Along the coast, there continues to be a 20-30% chance of MVFR conditions after 15Z Saturday. For all other terminals, there is around a 10-25% chance of MVFR conditions between 12-18Z Saturday. Otherwise, expect mostly northerly winds around 4-8 kt through the TAF period.

KPDX AND APPROACHES...Current VFR conditions with mostly clear skies will continue through the TAF period. Northwesterly winds around 5 kt or less continue. ~12

MARINE

A northwesterly swell today continues to bring seas around 9 to 12 ft with wave periods around 14 to 16 seconds. While the swell will decrease slightly tonight into Saturday, north to northeasterly winds will increase, bringing gusts up to 25 kt over the outer waters and up to 15-20 kt over the inner waters during Saturday morning and afternoon. As a result, a Small Craft Advisory will remain in effect for all the waters until 10 PM Friday, then for just the outer waters south of Cape Falcon until 4 PM Saturday. Into Sunday, winds weaken and shift easterly, while seas decrease to around 5 to 6 ft.

Seas and winds will likely decrease further heading into Monday, then increasing towards Small Craft conditions Tuesday into late next week as a frontal system approaches. Although seas will likely stay under 10 ft, guidance suggests a 40-50% chance for wind gusts over 21 kt Tuesday through Wednesday morning. ~12

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 10 PM PST this evening for PZZ251>253- 271.

Small Craft Advisory until 4 PM PST Saturday for PZZ272-273.