textproduct: Portland

SYNOPSIS

Low pressure offshore will maintain rain across northwest Oregon and southwest Washington today. A showery pattern continues through the weekend with seasonable temperatures. Cooler temperatures early next week will bring snow levels down and return chances for snow through the Cascade passes.

SHORT TERM...Now through Saturday night

A lifting warm front associated with a low pressure system centered offshore of northern California is spreading rain from south to north across our area. Ensemble guidance remains in agreement that this low will progress inland toward southern Oregon and northern California. Therefore, the majority of impacts from this system is favored further south of our area. However, we'll still get a decent amount of rain out of this system. Over the next 24 hours (4 AM Thursday to 4 AM Friday), chances for 0.50" of rain or greater is around 25-35% across the Willamette Valley and Portland-Vancouver Metro Area, 40-60% across the Lower Columbia, Cowlitz Valley, and Hood River Valley, and 60-80% along the coast, Coast Range, Willapa Hills, and Cascades. The 90th percentile (10% chance), shows around 1.2-2" along the coast and Coast Range, and around 0.70-0.90" inland. Snow will not be a concern today as the warmer air ushered in from the south from this low will maintain snow levels well-above the passes (9000 ft+).

Winds are also non-impactful today, but easterly winds remain breezy through the Columbia River Gorge and eastern Portland Metro through this afternoon with gusts of 20-25 mph. Easterly winds are also present along the coast with similar wind gusts. Pressure gradients between Troutdale and The Dalles (KTTD-KDLS) remain around -3 to -4 mb this morning, but will gradually ease this afternoon, thus weakening easterly winds. Winds will generally turn more southerly/southwesterly this afternoon as a weak occluded front moves northward from the low, breeziest along the coast with gusts up to 20 mph. Elsewhere, winds remain light.

Rain will persist through Friday as the low shifts southward. Another frontal system will swing through British Columbia and far northwest Washington Friday evening into Saturday, maintaining >60% chances for precipitation along the coast and southwest Washington. Warm air will linger over the area but a brief area of cooler air settles in. This airmass has warmed though over the last few forecast cycles so, while snow was previously expected at pass level, that is no longer the case. Snow levels will fall to 5500-6000 ft, so impactful snow is no longer expected through the Cascade passes. Chances for precipitation fall (but remain non-zero) especially inland by Saturday evening as the region is in between systems and weak ridging builds over the region. Overall, expect pretty typical fall weather these next few days. -10/27

LONG TERM...Sunday through Wednesday

Conditions will remain showery on Saturday night into Sunday morning but then conditions will quickly shift Sunday night. Another 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 likely 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. Current NBM guidance suggests a 20-30% chance for 6 inches of snow or greater from 4 AM Sunday to 4 AM Tuesday through Highway 26 near Government Camp and the Santiam and Willamette Passes.

While snow in the passes is one component to look at, surface temperatures too will cool Monday night into Tuesday morning. While freezing temperatures (32 deg F or 0 deg C) are not expected to be widespread in the lowlands, the chances are non- zero. In the southern Willamette Valley and outskirts of the central Willamette Valley, there is around a 30-50% chance of freezing temperatures, in the Portland-Vancouver Metro there is a 5-15% chance, and a less than a 5% chance along the coast. In the Upper Hood River Valley these chances are 60-90%, with the highest chances near Parkdale. Similar chances are observed through Wednesday morning. Heading into mid-week, there is uncertainty among ensembles on the overall weather pattern. -10/27

AVIATION

A lifting warm front is spreading rain from south to north across the region as of early Thursday morning. CIGs remain VFR around 6-10 kft for most terminals, and is expected to remain VFR through the majority of the TAF period. An exception would be the southern Willamette Valley (KEUG) where a light north- northwest wind is maintaining LIFR/IFR VIS/CIGs. Conditions should improve to MVFR/low-end VFR at KEUG by 19-21z Thu. The coast also has a 30-60% chance for MVFR CIGs today through 00z Fri, with the highest chances along the north Oregon coast including KAST. Any moderate to heavy rain today may lead to brief VIS reductions to IFR/MVFR.

Easterly winds along the coast will turn more southerly with gusts up to 20 kt this afternoon as another front swings through. Otherwise, winds remain light and variable elsewhere except for breezy easterly winds through the Columbia River Gorge with gusts up to 20 kt near KTTD through the early afternoon. Winds ease in the evening.

KPDX AND APPROACHES...VFR with mid to high level clouds and rain through the TAF period. Easterly winds around 5 kt or less turning southerly this evening. -10

MARINE

As of early Thursday morning, a front associated with a low pressure system centered offshore of northern California is bringing east-southeasterly winds 10-15 kt with gusts up to 20 kt across the waters. Later this afternoon, the low will deepen and swing another front through the waters. This will return breezy southerly winds 15-25 kt with gusts up to 30 kt. Seas build towards 10-13 feet at 12-15 seconds by early Thursday afternoon for all waters. Isolated gale-force gusts up to 40 kt are possible Thursday afternoon/evening, mainly south of Cape Falcon and beyond 20 NM off the coast. This area will also have a 50-70% chance for seas building towards 13-15 ft for the same time frame. Therefore, the current Small Craft Advisory for all marine zones remain in effect through at least 7 AM Friday. Seas gradually subside to 8-9 ft at 13 seconds Friday afternoon as the low pressure system weakens and progresses southward.

Another frontal system will swing through British Columbia and far northwest Washington Friday afternoon, maintaining relative breezy southwesterly winds over our coastal waters. There is a 30-50% chance for southwesterly winds with frequent gusts of 21 kt or stronger across the waters north of Cape Falcon including the Columbia River Bar by late Friday afternoon and evening. South of Cape Falcon, these chances drop to 15-30%. Winds ease on Saturday in between systems. -10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Small Craft Advisory until 7 AM PST Friday for PZZ210-251>253.

Small Craft Advisory until 1 PM PST Friday for PZZ271>273.