textproduct: Portland

SYNOPSIS

Showers return this morning to the northern coast, spreading inland this afternoon as a weak frontal system moves across the area. Low pressure over western Washington and upper level troughing will maintain showers with seasonably cool temperatures through Saturday, as well as a 10-20% chance of afternoon thunderstorms, generally north of Salem. Cooler temps will also bring snow levels down to 3500-4000 ft, allowing for light snow along the Cascade passes tonight through Saturday night. Drier and warmer weather returns Sunday through next week as high pressure re- builds.

DISCUSSION

Water vapor satellite imagery early this morning shows the leading edge of an upper level jet stream over the northern Pacific pushing toward the PacNW. This will push another relatively weak frontal system across the area, with the highest chances for precipitation along the coast and northern portions of the forecast area through this evening. This system will also bring breezy southwesterly winds with gusts up to 25 mph across the area later this afternoon.

Broad upper level troughing dropping out of the Gulf of Alaska will slide across the region tonight cooling temperatures aloft, with ensembles suggesting 850 mb temps dropping down to around -2 C. This will allow snow levels to fall to around 3500-4000 ft (lowest across the SW Washington Cascades). Weak surface low pressure will linger near the Puget Sound tonight through Saturday while shortwave disturbances maintain shower activity overnight through at least Saturday afternoon. Orographics of the Coast Range and Cascades will likely enhance precipitation as west to northwesterly flow persists. This may lead to some accumulating snow, especially at higher elevations. Latest guidance suggests there is anywhere from a 20-60% chance of exceeding 6 inches of snow (in a 24-hr period) at Cascade passes, highest near Government Camp. If planning on traveling to the Cascades, make sure to check the latest forecast, refer to ODOT/WSDOT for the latest road conditions, and pack an emergency supply kit. Considering many of the forest roads are free of snow from the early melt-out and access to unmaintained roads is growing, the lower snow levels may catch people off guard if they are unprepared for cold, wet, and even snowy conditions.

While rainfall amounts in the lowlands are expected to be light and non-impactful, there is also a 15-20% chance for short-lived thunderstorms Saturday afternoon, mainly from Lincoln City/Salem northward. Colder air aloft filtering in from the passing trough will gradually increase atmospheric instability, especially in the afternoon, though only about 100-300 J/kg of skinny CAPE. Any thunderstorms that do develop may produce lightning, brief heavy downpours, small hail, and erratic winds.

After seasonably cool temperatures today and Saturday, expect a warming trend to return Sunday through early next week. As the upper level trough shifts east, expect high pressure to return back over the region. Ensemble guidance is is general agreement that upper level ridging builds into the NE Pacific. But, there remains uncertainty with how far east the ridge progresses toward the PacNW. The farther east the ridge does shift over the Pacific NW, temperatures are more likely to see higher end amounts, mid to upper 70s by the middle of next week. If the region stays under more northwesterly flow aloft, afternoon temps would be more likely to remain in the upper 60s to lower 70s. The majority of ensemble guidance does return to mostly dry conditions for next week. /DH

AVIATION

Increasing westerly flow aloft this morning as a broad upper trough approaches the region. Predominately VFR conditions continue through at least 15z this morning, likely later for inland terminals. Another front will push across the northern OR coast later this morning with chances for MVFR CIGs increasing to over 50% by 17-19z at KAST. Rain is expected to spread south and east through the day, with chances for MVFR conditions developing at KONP increasing to around 30-50% by 21z. Chances for MVFR conditions inland generally remain around 30% or less after 00z Saturday, highest near KPDX/KTTD and northward. Southwesterly winds are also expected to increase this afternoon, with gusts up to 25 kt at the coast and to around 20 kt inland.

KPDX AND APPROACHES...Predominately VFR conditions are expected through the period, though chances for MVFR increase this afternoon into this evening to around 20-30% as rain showers increase across the area. Southwest winds are expected to increase to around 9-13 kt with gusts up to 20 kt later this afternoon. /DH

MARINE

Onshore winds continue today into the weekend as a weak frontal passage will bring another round of elevated westerly winds with gusts up to 25 kt later this afternoon. A mid-period west-northwesterly swell will also help build seas to around 10 to 12 ft tonight into Saturday morning. Therefore, Small Craft Advisories remain in effect this afternoon through late Saturday. Additionally, very strong ebb currents across the Columbia River Bar early Friday and Saturday mornings will shorten the wave period and result in steeper seas.

Northerly to northwesterly winds return Sunday into early next week while seas ease to around 4 to 8 ft as high pressure builds offshore. Strong currents during morning ebb tides will continue daily through Thursday, potentially yielding steep and hazardous seas for small craft crossing area bars through much of the week. /DH

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 7 AM PDT this morning for PZZ210.

Small Craft Advisory from 2 PM this afternoon to 2 AM PDT Sunday for PZZ210-251-271-272.

Small Craft Advisory from 5 AM Saturday to 2 AM PDT Sunday for PZZ252-253-273.