textproduct: Portland

SYNOPSIS

Low pressure brings chances for showers Friday evening into Saturday across northwest Oregon and southwest Washington, with a 15-20% chance for thunderstorms over the Cascades and 5-10% chance elsewhere. Winds out of the north/northwest, will shifts towards the west/southwest on Saturday. Seasonable weather returns Sunday through the middle of next week.

DISCUSSION...Tonight through Wednesday

Tonight into Friday morning, low temperatures around 60-65 degrees across the Portland/Vancouver Metro, Columbia River Gorge, and Cascade foothills will result in limited overnight relief from the heat. Chances for low temperatures remaining above 65 degrees are around 15-25% for the Portland/Vancouver Metro and 60-80% along the Columbia Gorge.

The potential for limited overnight relief in addition to the afternoon heat will result in Moderate HeatRisk today along portions of the I-5 corridor and Columbia Gorge. This means that this level of heat will affect those sensitive to heat, especially those without adequate cooling or hydration.

On Friday, an upper-level trough over the northeast Pacific will progress toward the Pacific Northwest, bringing slightly cooler temperatures, increased cloud cover, and increasing chances of precipitation. Guidance has this trough swinging inland by Friday night into Saturday. The southerly flow in the upper-level pattern will bring an increase to for thunderstorm chances, primarily for the Cascades. The southerly flow aloft will bring increased moisture and instability, vorticity from the upper-level trough will also provide the necessary forcing for convection. Thus resulting in a 15-20% chance for thunderstorms along the Cascades starting early Friday evening through Saturday morning.

The latest guidance has precipitation chances beginning over the Cascades after 5 PM Friday at 20-35% chances, with chances of precipitation spreading west of the Cascades after 11 PM Friday. Precipitation chances continue into Saturday morning, mainly along and north of the OR/WA border. It will be important to monitor the weather if you have outdoor plans Friday afternoon.

Ensemble guidance continues to show the upper-level trough moving further into the Pacific Northwest Saturday. As the associated surface front moves through, will see winds shift and come out of the west/southwest. Winds will increase through west to east aligned mountain gaps, especially through the Central Columbia River Gorge into Hood River Valley where winds could gust approaching 35 mph. There is also a 40-60% chance for at least isolated gusts exceeding 40 mph for these areas, especially for higher terrain and exposed ridgetops. Stronger onshore flow across the region will support much cooler temperatures on Saturday with afternoon highs peaking in the upper 70s.

Sunday into early next week, most ensemble members suggest a return of zonal flow as the trough moves northeast into eastern Washington and Canada. As a result, expect temperatures to gradually cool into the low to mid 80s. Guidance also indicates another upper-level trough dropping down from the Gulf of Alaska and approaching British Columbia early next week, potentially impacting the Pacific NW by mid-week. At this point, there still remains high uncertainty with the exact magnitude and placement of the trough next week. However, this would suggest that onshore flow continues across the region resulting in seasonable temperatures for late August.

AVIATION...Through 00Z Saturday

Marine stratus continues to flirt with the coastline, bringing conditions bouncing between MVFR/IFR to the coastal terminals, most notable at KONP. Confidence for any persistent flight restriction remains low. Regardless, by 00-03Z Fri, marine stratus will push back into the coast and bring LIFR/IFR conditions to the coastal terminals (60-70% chance of LIFR and greater than 80% for IFR conditions). These deteriorated conditions will persist until at least 18Z Fri and then possibly improve to IFR/MVFR thereafter. As for the Willamette Valley, VFR conditions and clear skies will prevail through the TAF period (with the caveat of a 10-20% chance of MVFR CIGs between 10-19Z Fri). Otherwise, current west/northwest winds under 10 kt will shift westerly to southwesterly between 03-12Z Fri (earliest to latest from coast to inland).

KPDX AND APPROACHES...VFR and clear skies will continue through the TAF period, with a 10-15% chance of MVFR CIGs between 12-18Z Fri. Northwest winds around 3-6 kt will shift westerly by 12Z Fri. ~12

MARINE

High pressure continues to support northerly winds across the waters today with gusts remaining well below 20 kt. Winds over the waters then shift south to southwest on Friday as a low pressure system moves over the northeast Pacific. As the surface front continues to move through Saturday, there is a 40-70% chance for maximum wind gusts to exceed 21 kt for the waters north of Cape Foulweather, including the Columbia River Bar. For the rest of the waters, chances are 15-30%.

High pressure and northerly winds return by Sunday, with a 30-50% chance for northerly wind gusts to exceed 21 kts in the late afternoon and evening hours, with the highest chances south of Cape Falcon. Seas of 4-6 ft persist through the period. -12/10

PQR WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None. PZ...None.