textproduct: Portland

SYNOPSIS

Decreasing showers across southwest Washington this evening, with dry conditions elsewhere. High pressure ridge builds over the Pacific Northwest on Tuesday, returning dry weather through Friday. Clear skies and calm winds will promote fog and frost potential across interior lowland valleys this week. Some interior valleys may also experience air stagnation. Easterly winds through the Columbia River Gorge will help prevent air stagnation in the Portland/Vancouver Metro Area and West-Central Willamette Valley.

DISCUSSION...Today through Sunday

Radar imagery and surface observations as of early Monday afternoon depict rain across southwest Washington as a weakening warm front lifts across western Washington. Since this front is directed further north, the majority of northwest Oregon will remain dry. High resolution guidance suggests that rain will taper off into light, scattered showers through this afternoon with conditions eventually drying out by tonight as high pressure re-builds. Mid to high level cloud cover will begin to scatter out, favoring fog and/or low stratus development tonight across the Willamette Valley where winds are expected to be calm. Some light easterly winds through the Columbia River Gorge may prevent fog development, particularly in the far eastern Portland/Vancouver Metro Area. Will note that there is also some uncertainty with exactly where low stratus may develop. Some model soundings are showing east-southeasterly winds aloft around 1000-2000 ft which would prevent stratus, while surface winds remain calm or northerly under 5 mph. In this case, locations with this set up would favor fog development than low stratus. This would be more likely for areas not receiving the easterly surface winds from the Gorge. If you will be commuting through any fog tomorrow morning, make sure to slow down, use low beam headlights, and leave plenty of distance between you and other vehicles.

Beginning tomorrow (Tuesday), deterministic and ensemble guidance remains in strong agreement that upper-level ridging becomes firmly established over the Pacific Northwest. This pattern will support an extended period of dry weather and above-normal temperatures through at least Friday. High temperatures are forecast in the upper 50s to low 60s, which is around 10 degrees above normal for this time of year. Mid to high level clouds will still linger over our area through Tuesday afternoon as a system in the NE Pacific rides the northwest periphery of the ridge and brings precipitation to western British Columbia. However, clouds should clear out and return sunny skies Wednesday to Thursday.

Winds will generally be light throughout this period of dry weather, with the exception of the western Columbia River Gorge and surrounding terrain. Tightening pressure gradients will lead to breezy easterly gap winds, mainly on Wednesday and Thursday. There is still some uncertainty with how tight the pressure gradients between Troutdale and The Dalles will get, however deterministic and ensemble guidance suggests pressure gradients between these two places will be around -5 to -6 mb, potentially up to -7 to -8 mb. Chances for maximum wind gusts exceeding 35 mph are around 20-40% across the far eastern Portland/Vancouver Metro on Wednesday and Thursday, with higher chances of 70-90% through the western Gorge and surrounding terrain. There is also 30-50% chance that exposed ridgetops such as Three Corner Rock and Crown Point experience wind gusts exceeding 50 mph.

For areas away from the Gorge, light winds and persistent ridging will result in stagnant air conditions which could affect air quality. An Air Stagnation Advisory has been issued from 4 AM Tuesday to 4 PM Friday across the Southern Willamette Valley, East Central Willamette Valley, North and Central Oregon Cascade Foothills, North Clark County lowlands, and the Lower Columbia River/Cowlitz River Valleys. Based on soundings and model guidance, mixing heights in these locations are forecast below 2000 feet with transport winds around 5 kt or less. Decided to leave out the Portland/Vancouver Metro Area and West Central Willamette Valley from the advisory as these areas will remain better mixed from the increasing easterly winds this week (these winds turn more northerly as it gets into West Central Valley). Other than air stagnation, high pressure will maintain chances for fog/frost this week across all interior valleys.

Chances for precipitation return this weekend, however there is still uncertainty with the exact timing and precipitation amounts. About 65% of ensemble members show precipitation returning by Saturday afternoon, while the rest show precipitation returning by Sunday afternoon. Most ensemble members also show 500 mb heights decreasing this weekend, which suggests that temperatures will also cool down. More details will be ironed out this week, but either way it's looking like a wetter and cooler start to next week. -10/12

AVIATION

Skies are trending clearer from south to north behind an earlier frontal system which has since lifted to the north. Sct-bkn low clouds around 5-6 kft continue across the northern Willamette and Lower Columbia Valleys, while IFR cigs at KAST are also favored to improve at least to MVFR levels, with about a 50% chance in scattering out by 10-12z Tue. Inland, clearing skies will support the development of valley fog, most likely in the Tualatin and southern Willamette Valleys (50-70% chance of IFR vis within fog), and slightly less so elsewhere along the Willamette (40-60% chance of IFR vis from KSLE to KUAO). Vis restrictions at Portland-area terminals may hinge on the wind direction overnight; in the east metro near KTTD, persistent east winds will limit fog chances to less than 10%, while chances at KPDX are 30-40% depending on whether winds shift out of the north or easterly flow is maintained. Any fog will linger into the morning hours, dissipating around 18z Tue. Winds remain around 5 kt or less for much of the period, aside from gusty east flow at KTTD developing on Tuesday.

KPDX AND APPROACHES...VFR conditions expected to continue into tonight, with increasing chances for fog development at KPDX after 08-10z Tue. Overall, chances for IFR vis are 30-40%, but may vary substantially based on the wind direction. Winds turning out of the north represent higher chances of fog at the terminal, while even light east winds would act to inhibit fog formation. Light and variable winds could see intermittent terminal impacts from nearby areas of fog. Any vis restrictions will ease after 15-17z Tue, with VFR conditions then prevailing through the remainder of the period. -36

MARINE

South winds with gusts 20 to 28 kt will slowly decrease this afternoon behind this morning's weak front. Expect wind gusts to fall below 20 kt by 5 PM PST. Expect wave heights of 10 to 13 feet at 12 to 15 seconds through this evening. Due to a combination of winds and seas, a Small Craft Advisory is in effect through 1 AM PST on Tuesday. High pressure builds over the waters Tuesday with southerly winds less than 10-15 kt and seas less than 10 ft. Winds remain mainly south to southwest less than 10-15 kt through the rest of the week. By Wednesday, an increasing westerly swell is forecast to push into the waters, increasing seas to around 10-12 ft around 12-16 seconds through the end of the week. -12/03

BEACH HAZARDS

A long period westerly swell is continuing to pose a high threat for sneaker waves into tonight, with a Beach Hazards Statement is now in effect through 1 AM PST Tuesday for the coasts of northern/central Oregon and southern Washington. Waves will be able to run significantly farther up on a beach than normal, including over rocks and jetties. Sneaker waves can suddenly knock people off of their feet and quickly pull them into the frigid ocean which may lead to serious injury or drowning. Caution should be used when in or near the water, especially those who will be razor clamming this week. -42/36

PQR WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory from 4 AM Tuesday to 4 PM PST Friday for ORZ108-115>118-123>125.

WA...Air Stagnation Advisory from 4 AM Tuesday to 4 PM PST Friday for WAZ204-205.

PZ...Small Craft Advisory until 1 AM PST Tuesday for PZZ210-251>253- 271>273.