textproduct: Pendleton

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Fog/mist will continue for KDLS/KYKM/KALW with VSBYs at 1/4-1/2SM. While VSBYs are expected to slowly improve through early this afternoon, if not sooner, the fog may ebb and flow for the rest of this morning for KALW and possibly KDLS. Otherwise, KDLS/KALW will return to VFR this afternoon with KYKM being MVFR due to lingering low CIGs. However, fog/mist could also return later this evening for KYKM with low CIGs (BKN at 300ft or lower). KPSC Will be IFR with CIGs at 700ft, but will improve to VFR with the aforementioned sites this afternoon before dropping to LIFR due to potential fog. The remaining sites will be VFR. Feaster/97

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.

Preliminary Point Temps/PoPs

PDT 34 51 35 55 / 0 20 0 10 ALW 36 50 38 55 / 0 30 10 10 PSC 34 52 34 51 / 0 20 0 10 YKM 34 49 32 49 / 0 40 0 20 HRI 34 52 34 53 / 0 10 0 10 ELN 33 44 31 44 / 0 60 10 30 RDM 33 52 28 54 / 0 20 0 0 LGD 38 52 32 51 / 0 10 10 10 GCD 36 52 31 51 / 0 10 0 0 DLS 39 52 39 51 / 0 60 10 20

PDT WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until noon PST today for WAZ024-026-027. OR...Dense Fog Advisory until noon PST today for ORZ041.