textproduct: Pendleton

KEY MESSAGES

- Areas of dense fog this morning

- Weak frontal systems through Monday

- High pressure Tuesday through Friday

UPDATE

Areas of dense fog have expanded into north-central Oregon, with patchy dense fog also observed in portions of the Yakima Valley, lower Columbia Basin, and northern Blue Mountain foothills of Washington. Have issued additional products to highlight the fog, valid until 10 AM PST today.

DISCUSSION

Water vapor imagery shows a large area of low pressure churning in the Gulf of Alaska. Meanwhile, a shortwave wrapping around the base of the aforementioned low is approaching the OR/CA border. An attendant weak cold front will pass from west to east across the forecast area later this morning through the 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. However, cold air advection (CAA) aloft will induce sufficiently steep lapse rates this afternoon such that confidence is high (80 percent) that enough mixing will occur to result in partial to full scouring of the existing fog and low stratus within the Columbia Basin and surrounding lowlands.

Monday, another weak warm frontal passage will cause snow levels to rise above mountain pass levels through the day. Prior to snow levels rising, light snow (trace to 2 inches) is forecast for the Washington Cascades. Little to no precipitation is anticipated east of the Cascades.

Tuesday through Friday, ensemble clusters all show some flavor of an anomalously strong upper-level ridge over the Pacific Northwest. This pattern favors air stagnation, lowland stratus and fog, and warm mountain temperatures. Have refrained from putting fog in the gridded forecast during this period due to the long lead time, but confidence is high (80 percent) that there will be at least periods with areas of fog in the Columbia Basin and surrounding lowlands. Also worth a mention, should widespread fog and stratus materialize, afternoon high temperatures will almost certainly fail to reach the NBM's optimistically warm values for areas affected by fog/stratus.

By next weekend, ensemble clusters show some potential for a return to an active winter pattern, though with ensemble membership at roughly a 50/50 split between a troughing pattern and a ridging pattern, confidence in forecast details is low.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Fog and low stratus will result in conditions ranging from VLIFR to MVFR for all Columbia Basin/Gorge sites this morning. A cold frontal passage will (80 percent confidence) partially or fully scour fog and low stratus from the aforementioned sites by late morning and early afternoon. RDM/BDN are forecast to remain VFR through the period, albeit with breezy southwesterly winds (10-15 kts with gusts of 20-25 kts) late morning and afternoon. Elsewhere, anticipating lighter winds (5-12 kts) with periodic gusts of 15-20 kts during the afternoon hours. Have included some PROB30 and TEMPO groups for -RA with this morning and early afternoon at all sites.

Preliminary Point Temps/PoPs

PDT 34 53 34 50 / 0 0 0 0 ALW 38 54 38 51 / 10 10 0 0 PSC 34 51 34 50 / 0 10 0 0 YKM 31 49 33 49 / 0 10 0 0 HRI 33 52 33 50 / 0 0 0 0 ELN 31 43 33 45 / 10 30 0 0 RDM 26 54 28 56 / 0 0 0 0 LGD 31 50 33 54 / 0 0 0 0 GCD 30 51 33 55 / 0 0 0 0 DLS 37 51 36 51 / 10 20 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until 10 AM PST this morning for WAZ026. OR...Dense Fog Advisory until 10 AM PST this morning for ORZ507-508. Dense Fog Advisory until 10 AM PST this morning for ORZ510.