textproduct: Portland

SYNOPSIS

Typical fall rainy weather through Sunday with minimal breaks and seasonable temperatures. While rainy, no hydrological impacts expected. The next pattern of concern arrives early next week when we have the chance to see a cool down with Cascade Pass snow.

LONG TERM...Saturday Night through Wednesday

Conditions will remain showery on Saturday night into Sunday morning but then conditions will quickly shift Sunday night. A cold air wrapped low aloft will move over British Columbia extending a long wave trough over the region. This trough has a band of enhanced vorticity advection, moisture, and potentially breezy winds. Accumulating rain will be less on Sunday but will ramp up through Monday as the trough intensifies and a closed low forms at the base. This forming low will wrap even more cold air behind it causing temperatures to drop. At 850 mb, temperatures will reach freezing by late Monday morning and continue to lower through late Tuesday night. At the surface this trend will be coupled causing snow levels to fall to around 3500 ft. With this current pattern there will be pass level snow. However, if the low advects further north then the colder air may not filter in as far south and thus we would miss enough of the cold effect for snow to form.

While snow in the passes is one component to look at, surface temperatures too will cool on overnight into Tuesday morning. While freezing temperatures (32 deg F or 0 deg C) are not expected to be widespread in the lowland, the probability is not zero. In the outskirts of Eugene/Corvallis there is around a 20% chance of freezing temperatures less than a 10% chance for the northern Willamette Valley and less than a 5% chance for the coast. In the Upper Hood River Valley these probabilities are closer to 60%. Similar chances are observed through Wednesday morning, though the Willamette Valley north of Albany will see slightly lower chances. Will say though that the pattern in the long term is very messy and therefore there is ample room for forecast change - similar to what we have seen with this upcoming cooler air mass. -27

AVIATION

At 22z Tue, a warm front is pushing north into Lane and Lincoln Counties in central OR. This front will continue moving north over the next few hours with -SHRA or -DZ accompanying it, moving into central WA by 03-05z Wed. As the front moves over terminals still reporting MVFR/IFR CIGs and VIS due to stratus, it will cause enough mixing to improve conditions to VFR high mid/high level clouds. Periods of rain will continue behind the front through Thursday. Heavier showers could lower conditions to MVFR/IFR CIGs or VIS, though these conditions are not expected to become more widespread until after 21z Thu-00z Fri. Winds generally light and variable across the region for most of the TAF period, except for brief southerly winds along the coast through 00-03z Thu with gusts up to 20-25 kts as the front passes through. Also, east winds are gusty through the Columbia River Gorge, bringing gusts up to 20-25 kts at KTTD through 00-03z Thu then again after 09z Thu.

KPDX AND APPROACHES...VFR with light and variable winds under 5 kt expected through the period. Rain showers begin after 00-03z Thu. -03

MARINE

A front associated with a low pressure system in the NE Pacific is moving north through the waters Wednesday afternoon and evening. South winds 15-20 kts with gusts up to 20-25 kts are expected with the frontal passage. Seas will build to 9-10 feet at 13-15 seconds by early Thursday morning. Winds briefly turn easterly and ease to around 20 kt or less Wednesday night, though winds will quickly increase and turn southerly again on Thursday as the low deepens and swings another front through the waters. Will note that occasional gale-force wind gusts of 34 kt or greater are possible between 10 AM and 4 PM Thursday, mainly south of Cape Falcon and beyond 20-30 NM. There's also a 10-20% chance of storm force wind gusts of 55 kts or greater in this area. This area will also have a 40-50% chance for seas building above 15 ft. The Small Craft Advisory for all marine zones remains in effect through 4 AM Friday.

Another frontal system will swing through British Columbia and far northwest Washington on Friday, maintaining relative breezy southwesterly winds over our coastal waters. There's a 60-75% chance of wind gusts stronger than 21 kt with this front. -10/03

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 4 AM PST Friday for PZZ210-251>253- 271>273.