textproduct: Portland

SYNOPSIS

Low pressure sits just offshore which will be the main weather maker through Friday. Gusty southerly winds along the coast and within the Willamette Valley are amplifying, with easterly winds through the Columbia River Gorge. Another round of rain will arrive overnight with continued breezy winds. Trending drier through the weekend with weak shortwaves embedded in the flow. Cool down remains on Monday with lows dipping into the mid 30s Monday morning. Rain returns mid-week.

LONG TERM...Sunday Night through Thursday

The high pressure ridge remains in place with relatively high confidence given the distance into the future. Nearly all 500 mb ensembles show this ridge being in place. Therefore, we continue to come into better agreement on the Sunday night into Monday forecast. With the ridging skies will clear. The clearing skies, especially overnight into Monday morning, in combination with cooler air filtering in from the east will cause overnight lows to drop. These lows have the potential to be the coolest observed thus far this year. In Eugene for example, the 25th to 75th percentile (most likely range) shows a spread of 26-32 degrees F, while there is a 10% chance of 24 degrees F. The thing to remember is that these are the minimum temperature for the night and may only be one instantaneous observation. There is a higher probability that that temperatures will be above freezing for most of the night into Monday morning. With that said, light winds, clear skies, and these temperatures will lead to widespread frost.

Another warm frontal system moves in on Tuesday with even more stratiform rain. At this time, no major impacts expected as snow levels will be above pass level. The 10th percentile snow level (the lower elevation snow levels that would occur if the colder solutions manifest) is still above 3000 ft. So even if the warm frontal system loses some momentum, snow is not in the forecast for quite a while. Light rain persists through mid-week with minimal change in the overall forecast. -27

AVIATION

Shallow, light rain showers continue behind the departing frontal boundary. Largely MVFR cigs with unrestricted vis are favored through the mid-afternoon. Intermittent rain showers may briefly yield MVFR/high-end IFR vis if showers move over area terminals, however precise timing of these potential impacts is low confidence. Otherwise, increasing southerly winds are expected across the region as an area of low pressure nears the coast, reaching around 20 kt with gusts to 30-35 kt along the coast and 15 kt with gusts to 25-30 kt inland into this evening. One exception to this will be west of the Columbia River Gorge, where offshore flow will continue until the low moves onshore, yielding continued gusty east winds at KTTD through 00z Fri. These easterly or southeasterly gusts will also reach to KPDX, however the flow will ultimately turn out of the south by midday.

As steadier rain arrives by 21-24z Thu, a period of marginal VFR cigs may be expected inland, however there will remain a 40-50% chance cigs remain below 3 kft. Vis restrictions to MVFR levels within light rain are most likely along the coast, 40-60% chances, and less so inland, 20-40% chances, through the evening and into tonight as cigs trend back down to MVFR levels at 1-2 kft. Widespread IFR cigs late in the period remain relatively unlikely, less than 20% chances across the region. Behind this additional frontal boundary, wind gusts will weaken below 20 kt as winds turn out of the southwest to west after 06-12z Fri.

KPDX AND APPROACHES...Largely MVFR conditions expected to continue into this afternoon, however brief periods of IFR vis within intermittent rain showers remains possible. After 22-24z Thu, steadier rainfall may aid in lifting cigs above 3 kft, but there is a 40% chance conditions instead remain MVFR. Chances for further MVFR cigs increase late in the period, after 06-09z Fri. Initial easterly/southeasterly winds will turn out of the south by 20-22z Thu, and increase to around 15 kt with gusts of 25-30 kt. These gusts are expected to continue through at least 06z Fri before easing behind the frontal boundary. -36

MARINE

An area of surface low pressure visible on satellite imagery about 100 miles west of the mouth of the Columbia River early this afternoon will continue to weaken as it nears the coast this evening. Despite the weakening trend, a tight pressure gradient over the waters will support persistent gusty winds. Regional buoys have thus far observed south winds around 25 kt with gusts near 35 kt, conditions which will continue to degrade as sustained winds further rise through this afternoon to near 30 kt with gusts of 35-40 kt as the low tracks toward the south Washington coast, before easing as winds veer out of the west and ultimately out of the northwest behind the low through tonight. Meanwhile, buoys are additionally observing 10-12 ft seas across the inner waters, while 14-16 ft seas in the outer waters are expected to increase to 15-19 ft across the waters this evening. There is still a 15-20% chances seas exceed 20 ft beyond 30 NM and south of Cape Falcon, and lesser chances elsewhere. Conditions will improve through tonight as seas fall below 10 ft and winds below 10 kt by Friday afternoon.

These conditions support Gale Warnings in place across the waters through this evening, specifically through 8 PM beyond 10 NM and across the Columbia River Bar, and through 10 PM across the inner waters where coastal wind enhancements may maintain elevated winds a few hours later. Hazardous Seas Warnings will then be in effect across all coastal waters through 2 AM early Friday morning, then Small Craft Advisories from 2 AM through 11 AM Friday as winds and seas continue to ease.

As high pressure builds through the weekend, offshore flow Saturday will turn northerly by Monday, generally at 10 kt or less, although areas downstream of gaps in coastal terrain may see locally stronger east winds. North to northwest winds at 10-15 kt and seas of 5-8 ft are then favored through next week. -36

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...Gale Warning until 6 PM PST this evening for PZZ210-271>273. Hazardous Seas Warning from 6 PM this evening to 2 AM PST Friday for PZZ210-271>273. Small Craft Advisory from 2 AM to 10 AM PST Friday for PZZ210- 251>253-271>273. Gale Warning until 8 PM PST this evening for PZZ251>253. Hazardous Seas Warning from 8 PM this evening to 2 AM PST Friday for PZZ251>253.