textproduct: Portland

SYNOPSIS

Chances for fog and frost have lowered this morning due to increased cloud cover from the next approaching weather system. This system will mainly be directed toward northern California but return chances for precipitation, mainly from Salem southward. Dry weather returns Monday before another system arrives and brings widespread rain and pass-level Cascade snow Tuesday into Wednesday. Rain and mountain snow transition to showers Thursday to Friday.

DISCUSSION...Today through Friday

Satellite imagery as of early Saturday morning show increasing high clouds over northwest Oregon and southwest Washington as a cut-off low pressure system approaches the region from the Pacific. Because of the increasing cloud cover, fog and frost may be harder to develop this morning. However, some places are still clear and/or have thin high clouds, so there is still low to moderate confidence (30-50% chance) for fog development this morning for interior valleys. Temperatures are also dropping into the low to mid 30s this morning, so that's still cold enough to support frost development over grasses and metal surfaces. However, locations with complete cloud cover may end up warmer and thus go frost/fog-free this morning. Any frost or fog that develops this morning should dissipate by 10 AM-12 PM.

Ensemble guidance remains in excellent agreement that the aforementioned low pressure system will swing through northern California from the northeast Pacific this afternoon 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 northwest Oregon Saturday afternoon into Sunday, mainly from Lincoln/Polk/Marion Counties southward. For locations north of these counties, there's about a 25-40% chance of rain all the way north to the Columbia River on Sunday, including the Portland/Vancouver Metro Area, and about a 15-30% chance north of this. Not much rain nor impacts are expected from this system. There is still some uncertainty with exact precipitation amounts, but guidance suggests high confidence (>75% chance) that 48 hour rainfall amounts ending 4 PM Monday remain under 0.50 inch across interior lowland valleys. An exception are the Linn and Lane County Cascades, where there is a 60-70% chance that precipitation amounts exceed 0.50 inch due to orographic enhancement. Additionally, guidance currently suggests that the wettest scenario (10% chance) is around 0.50-0.85 inch for locations south of Tillamook and Salem. North of these areas, the wettest scenario is between 0.20-0.35 inch. Snow levels will also be around 6000-7000 feet, keeping snowfall well-above the Cascade passes.

Transient ridging returns to the Pacific Northwest 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 on Tuesday, with the majority of ensemble members (70-80%) suggesting precipitation beginning along the coast Tuesday morning and spreading inland by the afternoon. Meanwhile, the other 20-30% of ensemble members have precipitation starting late Monday night into early Tuesday morning. In either scenario, dry weather will come to an end by Tuesday evening. More substantial rain arrives Wednesday, with the chances for 48 hour liquid precipitation exceeding 1 inch being around 10-20% along the I-5 corridor from Cowlitz to Lane County, 30-50% along the coast, and 60-80% for the Coast Range and Cascades.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 60-80% along the Santiam and Willamette Passes and 25% for Highway 26 at Government Camp, with the most snow falling late Tuesday into Wednesday. Thursday to Friday, rain and mountain snow decrease and transition into showers. -10/03

AVIATION

Satellite imagery as of early Saturday morning depicts increasing high clouds as the next system approaches the region. Due to the increased cloud cover, there is low to moderate confidence (30-50% chance) for LIFR conditions from fog development this morning. The highest chances for fog would be terminals where there are cloud breaks. Otherwise, fog will be difficult to form. In addition, the lack of precipitation the last couple days has further dried out the surface. Will note though that temperatures falling to the low to mid 30s this morning will still be favorable for frost development over metal surfaces. However, the cloud cover will also make this more difficult and some terminals may end up staying warmer. Fog/frost threat should end by around 18-19z Sat. Afterwards, VFR conditions prevail with CIGs lowering to low-end VFR over KEUG and KONP as the system moves into northern California and brings light rain from the south. Most of the rain is expected south of Salem.

KPDX AND APPROACHES...Predominately VFR conditions with increasing high clouds. Cloud cover will result in low confidence (15-25% chance) for LIFR VIS/CIGs due to fog between 12-18z Sat. VFR conditions prevail 18z Sat through the end of the TAF period, with a 15-25% chance for rain between 06-12z Sun. -10

MARINE

A northwesterly swell continues today with seas gradually subsiding to 7-8 ft at 11-12 sec by this afternoon. Surface high pressure offshore and a thermal trough along the coast will result in breezy northeasterly winds today with gusts up to 25 kt, mainly for the outer waters beyond 10 NM offshore. Weaker winds around 10-15 kt expected for the inner waters from the shore to 10 NM. The Small Craft Advisory remains in effect for the outer waters through 4 PM this afternoon. Sunday to Monday, winds further weaken and seas continue to subside to around 3-5 ft. Tuesday into mid-week, southerly winds return ahead of the next system with a 40-60% chance for frequent and widespread small craft wind gusts of 22 kt or greater. There is also a 10-20% chance that seas exceed 10 ft by mid to late next week. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 PM PST this afternoon for PZZ271>273.