textproduct: Pendleton

KEY MESSAGES

- Areas of fog this morning

- Weak frontal systems through Monday

- High pressure Tuesday through Friday

DISCUSSION

Water vapor imagery reveals a broad closed low spinning offshore in the Pacific. Ensemble NWP guidance indicates the low will gradually lift northeastward into the Gulf of Alaska and British Columbia through Monday. Multiple weak shortwaves and attendant fronts will facilitate periods of light precipitation, mainly for the Cascades, through Monday. Areas east of the Cascades will remain mostly dry with current NBM guidance showing PoPs in the 5-30 percent range with each frontal passage.

The first in the series of fronts, a weak warm front, is crossing NW OR and SW WA early this morning, and is bringing light rain showers to the region, primarily along the Cascade crest. Areas of fog, locally dense, have developed in the Columbia Basin and surrounding lowlands. While 00Z HREF and REFS guidance indicate medium-high (40-80 percent) chances of fog and mist expanding to fill the majority of the lower Columbia Basin of OR/WA, the Yakima and Kittitas valleys, and portions of the eastern Columbia Gorge and foothills of the Blue Mountains, current observations indicate dense fog is predominantly confined to the Washington side of the lower Columbia Basin and the Yakima and Kittitas valleys. Have issued Dense Fog Advisories for the aforementioned zones, valid until 10 AM PST this morning.

Looking ahead to Sunday, a shortwave impulse will wrap around the base of the parent upper low, with an attendant weak cold front passing from west to east across the forecast area Sunday morning through Sunday afternoon. High precipitation chances (70-95 percent) will be largely confined to the Cascades, with low chances (5-25 percent) for the lowlands and Blue Mountains. The front will bring lowering snow levels, in the 4000-5000 ft range, but precipitation is expected to remain light enough to preclude any winter weather highlights.

Monday, another weak warm frontal passage will cause snow levels to rise above mountain pass levels again. Little to no precipitation is forecast east of the Cascades.

Tuesday through Friday, ensemble clusters all show some flavor of an upper-level ridge over the Pacific Northwest. This pattern favors air stagnation, lowland stratus and fog, and warm mountain temperatures.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/

Stratus and fog will remain the primary aviation hazards through the period. 00Z HREF and REFS guidance indicate medium-high (40-80 percent) chances of fog and mist impinging on DLS/YKM/PSC through morning, with lower chances (10-20 percent) for ALW/PDT. Precipitation will be predominantly confined to the Cascades this morning, but there are low (5-20 percent) chances of light rain for DLS/YKM/PSC/ALW/PDT. Anticipating the return of fog and stratus again tonight into Sunday morning, but confidence is low in timing/magnitude of CIG/VSBY reductions.

Preliminary Point Temps/PoPs

PDT 34 53 35 55 / 0 10 0 10 ALW 37 53 39 54 / 0 10 10 20 PSC 33 54 35 53 / 0 10 0 10 YKM 35 50 33 50 / 0 20 0 20 HRI 34 54 35 54 / 0 10 0 10 ELN 33 45 32 44 / 0 40 10 40 RDM 33 53 29 51 / 0 10 0 0 LGD 37 53 32 50 / 0 10 0 10 GCD 36 54 31 51 / 0 10 0 10 DLS 39 53 40 52 / 0 40 10 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until 10 AM PST this morning for WAZ026>028. OR...None.