textproduct: Portland

SYNOPSIS

Warm and dry conditions through Thursday. Low pressure brings chances for showers Friday late afternoon into Saturday across NW OR and SW WA, with a 15% chance for thunderstorms over the Cascades and 5-10% chance elsewhere. Winds out of the north/northwest through Thursday, shifting to the west/southwest on Friday. Seasonable weather returns Sunday into early next week.

DISCUSSION...Wednesday afternoon through Tuesday

Visible satellite imagery early Wednesday afternoon shows mostly clear skies across NW OR and SW WA. However, the marine layer will once again inundate the coast this evening into tomorrow morning with marine stratus making a run down the Lower Columbia River. Dry and warm weather continues today and tomorrow as an upper- level ridge amplifies over the Rockies and Great Basin. Expect high temperatures near or slightly above average for this time of year, with mid 80s this afternoon across inland valleys and 60s along the coast. Highs for inland valleys increase slightly on Thursday into the upper 80s as the upper ridge reaches its peak strength. Onshore flow continues as broad upper-level troughing sits over the northeast Pacific with light winds from the north/northwest.

On Friday, the aforementioned trough over the northeast Pacific will progress toward the Pacific Northwest, bringing slightly cooler temperatures, increased cloud cover, and increasing chances of precipitation. Quite a bit of uncertainty remains with the exact location of the trough as it moves toward the area, leading to uncertainty in timing and location of precipitation over NW OR and SW WA. One note is that this upper- level pattern on Friday 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. However, due to the previously mentioned uncertainty, the timing and location of where vorticity sets up will really determine if thunderstorms actually occur. The latest guidance has shifted the precipitation chances to begin over the Cascades late on Friday afternoon at 25-45% chances, with 20-30% 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 included 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 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.

Sunday into early next week, the trough moves northeast into eastern WA and Canada, and most ensemble members suggest a return of zonal flow. This will allow for temperatures to rise back toward near normal, in the low to mid 80s for inland valleys, with the continuation of onshore flow. Guidance indicates another upper- level trough dropping down from the Gulf of Alaska and approaching British Columbia early in the 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. -03/10

AVIATION...Through 06z Friday

A typical pattern of marine stratus pushing inland overnight is expected. Along the coast, satellite imagery and terminal observations already show widespread low clouds resulting in IFR/LIFR cigs, which will, with high confidence, continue through at least 18z Fri before any marginal improvements to MVFR/VFR. Confidence is lower in the inland penetration of low clouds overnight. Terminals along the Lower Columbia River (KKLS, KSPB) and into the Portland area (KPDX, KTTD) will have the highest chances, 40-60%, of low-end MVFR cigs below 2 kft between 12-18z Fri. Chances for IFR cigs are not much lower: 20-40%. Within the Willamette Valley, the likelihood of MVFR cigs is lower, 20-40%, with prevailing VFR conditions favored. Any low clouds will tend to mix out by 16-18z Fri. Diurnal winds will rise out of the north to northwest at 5-10 kt by Friday afternoon.

KPDX AND APPROACHES...VFR conditions favored through at least 12z Fri before chances for MVFR/IFR cigs increase as marine stratus penetrates inland along the Columbia River. Any cigs will likely be at 2 kft or lower, with nearly 50% chances for MVFR cigs and 20% for IFR. Any low clouds will tend to mix out by 16-18z Fri. Northwest winds ease below 3 kt tonight, then rise to around 5 kt Friday afternoon. -36

MARINE

High pressure will continue to support northerly winds across the waters through Thursday with gusts remaining below 20 kt. Friday to Saturday, weak 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 30-50% chance for maximum wind gusts between 21 to 25 kt for the waters north of Cape Falcon, including the Columbia River Bar. For the rest of the waters, chances are around 5-10%.

High pressure and northerly winds return by Sunday with a 50-70% 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. Periods around 12 sec today may result in a moderate risk for sneaker waves along the coast. Caution should be used when in or near the water. -03/10

PQR WATCHES/WARNINGS/ADVISORIES

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