textproduct: Pendleton

DISCUSSION

Mostly quiet conditions across the forecast area tonight as a ridge of high pressure sits overhead. Of note, however, is an area of stratus meandering across portions of Crook and Deschutes counties, and the first hint of fog beginning to develop along the Columbia River north of the Tri-Cities. In fact, traffic cameras and the Richland airport indicate patchy dense fog has developed in portions of the western Tri-Cities already. Through the remainder of the morning, the expectation is that fog/low stratus will continue to develop across portions of the WA Columbia Basin, as well as in other sheltered areas in the lower elevations.

Today through early Friday: Upper level ridging will continue to sit over the forecast area throughout today and into early Friday. Dry conditions and light winds will prevail under the high pressure aloft, however, this will also allow for patchy areas of fog/low stratus to develop across portions of central OR, the eastern Gorge, the lower Columbia Basin, and adjacent foothills/valleys.

Friday through Sunday: Friday will mark the beginning of a period of active weather across the the forecast area that will persist into next week. By Friday afternoon, ensemble cluster solutions show a trough dropping out of the Gulf of Alaska with the main low circulation passing the PacNW to the west. This trough will continue its journey south and set up offshore central CA by late Sunday afternoon, with the trough becoming a closed low (confidence 55-75%). Trends over the past several forecast cycles have been for this trough to remain offshore during its weekend transit, with minimal moisture support for the PacNW. This has resulted in a downtrend in snow amounts across the mountains through the weekend, with mod-high confidence (55-80%) that snow amounts over the three day period will be 2 to 6 inches, with low confidence (20%) in 72-hr snow accumulation amounts exceeding 6 inches in any of our passes. Meanwhile, the lower elevations are expected to see their best chance of precipitation Saturday, which will consist of mostly rain during the day (confidence 60-75%), while a light rain/snow mix may develop over central OR Saturday night (confidence 50-60%). Otherwise, conditions will dry out for the lower elevations Sunday as the low dips further south (confidence 50-70%).

Monday through Wednesday: Ensemble guidance starts in good agreement Monday with a second upper trough diving south out of the Gulf of Alaska and setting up between 40N and 50N Monday into Tuesday. However, disagreement arises between the ensemble members on the position, timing, and evolution of the trough the latter half of Monday through Tuesday. The two main solutions presented by ensemble cluster solutions are a trough that stays offshore through Tuesday, resulting in light to moderate snow accumulations across the mountains; or a trough that crosses inland during this period, resulting in moderate to locally heavy snow accumulations in the Cascades and northern Blues. The former solution is currently favored by a majority of ensemble members at this time, and would still result in impactful winter weather conditions through the passes. Other than winter weather concerns, the NBM shows a return of breezy winds (gusts 25-40mph) to the region, with a 40-65% chance of gusts greater than 35mph developing in the lower elevations Tuesday.

By Wednesday, ensemble members agree in the PacNW continuing to be impacted by an upper trough, however disagreement remains in timing of impacts as well as the strength/position of the trough. That said, confidence is at least moderate (35-55%) in mountain snow and light rain and/or rain/snow mix across the lower elevation zones Wednesday. Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/

VLIFR conditions have been observed at PSC this morning due to dense freezing fog. Confidence is high (70 percent) that this fog will continue to slosh around the Columbia and Snake rivers and gradually dissipate by 19-21Z today. Confidence is medium (40 percent) that fog and/or low stratus will redevelop at PSC tonight, but excluded mention from the 18Z TAF. If confidence increases, will include fog and/or low stratus in the 00Z TAF. Elsewhere, VFR conditions and winds of 10 kts or less are forecast for the next 24 hours.

Preliminary Point Temps/PoPs

PDT 33 50 36 48 / 0 0 20 30 ALW 35 49 39 49 / 0 0 30 40 PSC 31 50 36 51 / 0 0 10 10 YKM 30 46 31 47 / 0 0 10 10 HRI 31 50 36 50 / 0 0 20 20 ELN 30 43 30 42 / 0 10 20 10 RDM 24 49 29 45 / 0 0 0 30 LGD 29 49 34 48 / 0 0 30 60 GCD 28 51 33 48 / 0 0 10 50 DLS 35 48 37 48 / 0 10 40 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until 10 AM PST this morning for WAZ028. OR...None.