textproduct: Pendleton

KEY MESSAGES

- Overnight valley fog this morning. *Dense Fog Advisories Active*

- Periods of mountain precipitation into early next week.

- High pressure midweek promotes fog and stagnant air.

DISCUSSION

Current radar and infrared satellite imagery shows light returns moving across the Lower Columbia Basin under mostly cloudy skies. These light showers have resulted in a saturated lower level across the Kittitas and Yakima Valleys, resulting in dense fog developing as visibilities of one quarter of a mile are being observed. Thus, a Dense Fog Advisory has been issued for these areas until noon today. This advisory may need to expand to include the Tri-Cities area later in the morning, but confidence is low (20-30%) at this time. These conditions are in response to an approaching upper level shortwave that will pass along the Canadian border later in the day today, bringing light to moderate precipitation primarily across the Cascades. Lower chances (30-60%) reside along the east slopes of the Cascade and the Blue Mountains as slight chances (15-25%) reside across the Lower Columbia Basin and Blue Mountain foothills through the afternoon and early evening. This shortwave will briefly flatten the present upper level ridge that strengthened across the area on Thursday, leading to an overall lack of moisture associated with any of today's precipitation. A weak warm front will also clip the area associated with the passing shortwave, increasing snow levels to 6000-7000 feet this afternoon. These values will keep any mountain snow confined to higher elevations. Rain amounts of 0.10-0.25" are expected between 4000-6000 feet over the Washington Cascades, with up to 0.10" over the Oregon Cascades.

The upper level ridge rebuilds briefly on Saturday, bringing a return to dry conditions and partly cloudy skies. This ridge shifts east late Saturday as another shortwave approaches and passes through the Pacific Northwest Sunday into Monday to return mountain precipitation to the area. Snow levels will drop to between 4500-5500 feet on Sunday and linger at this level into Monday as the shortwave slowly passes. Again, due to the periodic ridging and lack of multi-leveled support in the form of a substantial upper level trough or surface frontal system, overall precipitation amounts will be minimal. Only up to an inch of snowfall across higher elevations are expected both days, with rain amounts of between 0.10-0.20" Sunday and 0.05-0.15" Monday primarily above 3500 feet.

Ensembles are in good agreement regarding an upper level ridge building late Monday into Tuesday and continuing through the midweek. These conditions will allow for the potential for overnight fog development across the Lower Columbia Basin, Columbia Gorge, Blue Mountain foothills, North-Central Oregon, and the Yakima/Kittitas Valleys as early as Tuesday morning. This aligns with WPC's surface analysis, which advertises surface high pressure building Tuesday through Thursday. Overall guidance struggles with fog development after day 3, but 58% of ensemble members do highlight a 40% chance or better of relative humidity values of 90% or greater for the Tri-Cities area Tuesday morning with all members suggesting this occurring across the Kittitas Valley (which should be the first location impacted by subsidence and cold pooling). 75

AVIATION...12Z TAFS

Variable conditions currently across all sites, with VFR at KPDT/KRDM/KBDN/KALW and IFR at KDLS/KYKM/KPSC due to reduced visibilities of 1-2.5SM and ceilings of 500-1000 feet observed at KDLS/KYKM. These conditions are expected to continue to degrade overnight, eventually dipping into LIFR as visibilities of 1/4-1/2SM are likely to persist through the morning. Chances of this occurring are lowest at KPSC (20-30%) and may be only for a couple hours mid-morning. Reduced ceilings of around 300 feet are also expected for these sites. Elsewhere, VFR conditions persist as winds stay light and below 10kts for all terminals. 75

Preliminary Point Temps/PoPs

PDT 37 52 33 51 / 20 0 0 10 ALW 40 51 37 51 / 20 10 0 10 PSC 34 49 34 52 / 20 0 0 10 YKM 34 48 34 49 / 10 10 0 10 HRI 35 49 34 51 / 20 0 0 10 ELN 33 43 33 43 / 20 10 0 30 RDM 33 56 32 52 / 10 0 0 10 LGD 35 50 38 51 / 10 0 0 10 GCD 36 54 34 52 / 0 0 0 10 DLS 40 53 39 51 / 30 10 0 40

PDT WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until noon PST today for WAZ026-027. OR...None.