textproduct: Portland

SYNOPSIS

High pressure will maintain dry and warm conditions through early next week. A weak shortwave trough will bring increased cloud cover and slightly cooler temperatures tomorrow. Temperatures rebound on Monday and Tuesday as high pressure re- builds. Precipitation chances return on Wednesday as a trough approaches the West Coast. Another trough from the Gulf of Alaska will maintain precipitation chances and a trend toward cooler temperatures by the end of next week.

DISCUSSION...Today through Friday

Satellite imagery and surface observations as of early Saturday afternoon depicts mostly clear skies with a few high clouds across northwest Oregon and southwest Washington as an upper-level ridge moves overhead. Temperatures remain seasonably warm today with afternoon highs forecast in the upper 70s across interior valleys and 60s along the coast. Tonight, onshore flow and high pressure aloft will support re-development of marine stratus. We could also see some stratus form across interior valleys early Sunday morning as the low level atmosphere remains moist.

Tomorrow (Sunday), a weak shortwave trough will brush the Pacific Northwest and bring mid to high level clouds over the forecast area. Since this shortwave is tracking further north, the bulk of the precipitation will be directed towards western British Columbia while conditions remain dry across northwest Oregon and southwest Washington. Because of the increased cloud cover tomorrow, high temperatures will be a few degrees cooler than today but remain seasonably warm across interior valleys with afternoon highs forecast in the low 70s while coastal areas remain in the upper 50s to low 60s.

Heading into early next week, the majority of ensemble guidance are in agreement with upper-level ridging re-building over the Pacific Northwest. We'll see a return of sunny skies while dry weather prevails. There is high confidence that temperatures will rebound into the upper 70s to near 80 degrees by Monday across interior valleys, however there is still some uncertainty with how warm it will get on Tuesday. Deterministic guidance suggests that Tuesday would be the warmest day of the week with temperatures peaking in the low to mid 80s; however, ensemble guidance is still uncertain with the exact timing of the trough that follows the ridge. While the majority of ensemble members are in agreement with the upper ridge axis being generally over the Intermountain West, about half of the ensemble members have the next trough tracking close to the Pacific Northwest while the other half keep the trough further offshore in the northeast Pacific. If the trough ends up closer to shore, we could see increased cloud cover and slightly cooler temperatures in the upper 70s to low 80s. If not, then temperatures could get as warm as the upper 80s.

The next chances for precipitation return on Wednesday as the aforementioned trough approaches the West Coast. The majority (75%) of ensemble members show this trough tracking south of our area, either toward the California/Oregon border or further south into California. As a result, chances for precipitation are currently 15-25% west of the Cascades and 25-40% across the Cascades. In addition, southerly flow from this pattern would bring increased instability and thus a 15-25% chance for thunderstorms across the Linn and Lane County Cascades. The other scenario depicted by 25% of ensemble members has the trough swinging directly through the Pacific Northwest; in this case, we would see increased moisture and chances for precipitation.

There is greater uncertainty in the pattern Thursday-Friday, however, most ensemble members are showing another trough dipping down from the Gulf of Alaska by the end of the week. This will maintain chances for precipitation across the area and cool temperatures down to seasonal normals, however, the exact track of this system, timing, and precipitation amounts are still uncertain. -10

AVIATION

Southwest flow aloft as upper level ridge shifts east with scattered to broken high clouds streaming across the region. High pressure will maintain predominately VFR conditions through the period. An exception to this will be marine stratus pushing onshore with IFR to lower MVFR conditions are expected at the coast through Sunday morning. Chances for LIFR CIGs at KONP increase to around 40-50% between 12-16z Sunday, while IFR conditions are around 40% at KAST after 12z Sunday.

For the Willamette Valley from KSLE southward, some models are hinting at lower stratus along the Cascade foothills that may backbuild to KSLE and KEUG. However, probs for MVFR remain low (at around 20-30%), so included a scattered deck.

KPDX AND APPROACHES...VFR with high clouds streaming across the area. Diurnally driven northwest winds increasing in the afternoon. /DH

MARINE

Southerly winds will become more northerly this evening as high pressure develops over the region. Overall, will see winds of 6 to 10 kt with gusts up to 20 kt along the coast. The highest winds are expected in zones PZZ253 and PZZ273 with less than a 10% chance of isolated gusts up 25 kt. Seas 5 to 7 ft at 10 to 12 seconds. These conditions are expected to persist through Sunday.

There is a shift on Monday as there is the potential for a low pressure system to move into the northeastern Pacific. At this time, we are expecting the northerly winds to persist and intensify on Monday with a trend towards Small Craft Advisory level speeds through Tuesday. There is greater than an 80% chance of gusts up to 25 kt on Monday and around a 40-50% chance on Tuesday though would be in the morning. -27

PQR WATCHES/WARNINGS/ADVISORIES

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