textproduct: Portland

SYNOPSIS

Unsettled conditions continue through today with scattered showers and isolated thunderstorms developing this afternoon and evening, especially across the Cascades and adjacent foothills. Conditions turn cooler Sunday into early next week as flow becomes more west to northwesterly, though showers persist. Confidence continues to increase in a stronger system arriving midweek, bringing widespread precipitation and a shift toward Cascade snow impacts and cooler overnight temperatures.

SHORT TERM...Now through Sunday night

As of mid-afternoon, a few scattered showers continue across northwest Oregon and southwest Washington as a shortwave lifts north through Washington. Coverage has become more intermittent compared to this morning, particularly across portions of the Willamette Valley into southwest Washington, though additional development is expected through the afternoon.

Attention remains on thunderstorm potential this afternoon and evening. The broader pattern remains favorable, with a closed low offshore maintaining moist southerly flow and precipitable water values around 0.7 to 0.9 inches. This setup continues to support at least isolated thunderstorm development, particularly over elevated terrain. Instability appears somewhat reduced compared to yesterday, with guidance suggesting peak MUCAPE values in the 300 to 600 J/kg range this afternoon. Effective shear remains modest around 20 to 25 kt. While this will likely support convection, limited organization is expected. The primary uncertainty continues to be the degree of cloud cover and resulting surface heating. Areas that have seen more breaks in the cloud cover will be more favorable for thunderstorm development, while more persistent cloudiness may limit instability and keep activity primarily as showers. Thunderstorm chances this afternoon and evening generally range from 15-35% along the I-5 corridor and in elevated terrain, with the highest probabilities over the Cascades, Coast Range, and adjacent foothills where orographic lift will aid initiation. Storms that develop will be capable of producing lightning, small hail, brief heavy rainfall, and localized gusty or erratic winds. As of 2 PM this afternoon, thunderstorms have begun to develop in southwest Oregon, often weakening as a few of them drift northeastward towards the Lane County Cascades and adjacent foothills.

On Sunday, the upper-level low shifts inland, transitioning to more westerly flow and effectively ending most thunderstorm potential outside of a slight chance (around 10-15%) over the Oregon Cascades crest. Showers will persist under wrap-around moisture, with precipitation probabilities of 60-95% over most of northwest Oregon and elevated areas of southwest Washington. North of Tillamook into the Washington coast is the main exception, with just a 30-50% chance of precipitation. Temperatures cool into the 50s to low 60s inland, closer to seasonal normals (warmest to the north).

LONG TERM...Monday through Friday

Ensemble guidance indicates a transition to broad west-northwest to northwest flow aloft early next week. A weak embedded shortwave is expected to move through Monday, maintaining a 60-90% chance of showers, highest across the Cascades and foothills, with 24 hour rainfall totals ranging from 0.1 to 0.3 inches in the lowlands and 0.3 to 0.7 inches in the Cascades and foothills. Temperatures remain near to slightly below seasonal averages.

Shower activity decreases Monday night into Tuesday as a transient ridge briefly builds overhead. Attention then turns to a stronger Pacific frontal system expected between late Tuesday and early Wednesday. Confidence continues to increase that this system will bring widespread precipitation across the region. 24 hour rainfall totals still remain uncertain from Tuesday to Wednesday evening, with totals around 0.2 to 1.0 inches in the lowlands and 0.5 to 2.5 inches in the mountains.

Cool temperatures associated with this system will lower snow levels to near or below Cascade pass elevations, raising the potential for travel impacts. Current probabilistic guidance indicates a 85-95% chance of 6+ inches of snowfall at the Cascade passes between Tuesday and Thursday evening (48 hours), with a 40-70% chance of 12 inches or more. If these trends continue, winter weather highlights may become necessary for portions of the Cascades. In addition, cooler and potentially clearer conditions behind the system may introduce renewed frost concerns across portions of the interior valleys by mid to late week. The first opportunity appears Wednesday night into Thursday, with a 30-70% chance of frost across much of the interior lowlands (lowest around the Portland-Vancouver metro) and a 50-90% chance of freezing temperatures in the Upper Hood River Valley (chances from Odell to Parkdale). Similar conditions may develop again Thursday night into Friday morning, but with slightly lower chances. ~12

AVIATION

South to southeast flow aloft as a low pressure system off the northern California coast moves onshore tonight. Predominately VFR conditions expected this evening, with periods of MVFR conditions occurring with moderate rainfall associated with a band of showers lifting north across NW Oregon. Chances for thunderstorms have decreased to below 10% for the remainder of the evening. Rainfall is also expected to decrease this evening with mostly dry conditions between 06-12z Sunday. Chances for widespread MVFR begin to increase around 09-12z Sunday as stratus develops and CIGs lower. Lower flight conditions of IFR to MVFR are likely to persist through Sunday morning. Another round of showers is expected to spread across the area on Sunday.

KPDX AND APPROACHES...VFR conditions expected to continue through at least 10z Sunday. Light rain showers possible through 06z this evening with brief period of MVFR. Chances for MVFR CIGs increase to around 40% after 12z Sunday, followed by another round of widespread showers through Sun afternoon. Variable winds become mainly northwest through Sunday morning. /DH

MARINE

Rather benign conditions expected through the weekend 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. South/southwest winds slowly become more north/northwest this evening with northerly winds persisting through at least Monday night. The next period of concern is with a decently robust frontal boundary Tuesday into Wednesday which will likely (75-85% chance) result in Small Craft Advisory wind gusts over 21 kts and seas building to around 7 to 10 feet. Wave heights and winds then slowly decrease into the end of the week. -03/99

PQR WATCHES/WARNINGS/ADVISORIES

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