textproduct: Portland

SYNOPSIS

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

DISCUSSION...Today through Wednesday

Dry and very warm weather continues today as an upper-level ridge amplifies over the Rockies and Great Basin. Locations with morning clouds should gradually clear out by the afternoon as daytime heating progresses. Given the stronger high pressure today, afternoon temperatures across inland valleys are forecast to peak in the low 90s, which is slightly warmer than yesterday. Along the coast, onshore flow will maintain mild conditions with highs in the upper 60s to low 70s. 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. This upper-level pattern looks like a typical set-up for thunderstorm chances, at least over the Cascades. Southerly flow aloft will bring increased moisture and instability into the mid-levels of the atmosphere (5-10 kft). Vorticity from the upper-level trough will also provide the necessary forcing for convection. The latest guidance has precipitation chances beginning over the Cascades after 5 PM Friday at 20-30% chances, with 25-40% 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. Have maintained a 15% chance of thunderstorms across the Cascades, but uncertainty is too high for those chances to extend west. PWATs and RH increase significantly with this moisture, so if thunderstorms do form, fire concerns are on the lower side. Of course, any lightning that strikes outside of areas that have received rain could cause fire starts. It will be important to monitor the weather if you have outdoor plans Friday afternoon.

Ensemble guidance shows the upper-level trough moving further into the Pacific Northwest on 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 to 25-35 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 rebound 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 midweek. 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. -10/03

AVIATION...Through 18Z Friday

Marine stratus that pushed into the coast early this morning has begun to burn off. As a result, expect stratus to break out at KAST and improve to VFR around 20-21Z Thu. There is less confidence for improvement to VFR at KONP, with guidance suggesting a 30-50% chance of MVFR CIGs through the afternoon. Regardless, LIFR/IFR stratus will return to coastal terminals around 00-03Z Fri and persist overnight. As for the Willamette Valley, VFR conditions and clear skies will prevail through the TAF period. Otherwise, expect west/northwest winds under 10 kt to shift westerly to southwesterly around 06-12Z Fri.

KPDX AND APPROACHES...Predominately VFR with clear skies through the TAF period. Northwest winds 3-6 kt shifting westerly around 12Z Fri.~12

MARINE

High pressure will continue to support northerly winds across the waters today with gusts remaining below 20 kt. Friday to Saturday, low pressure moving over the northeast Pacific will shift winds out of the south to southwest across the waters. As the surface front moves through Saturday morning, there is a 50-80% chance for maximum wind gusts between 21 to 25 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 40-50% chance for northerly wind gusts exceeding 21 kts in the afternoon and evening hours with the highest chances south of Cape Falcon. Seas of 4-6 ft persist through the period. -10

PQR WATCHES/WARNINGS/ADVISORIES

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