textproduct: Pendleton

KEY MESSAGES

- Areas of dense fog redeveloping tonight and Thursday night in the Columbia Basin and adjacent valleys/foothills.

- Periods of breezy to gusty southerly winds along the Blue Mountain foothills, Grande Ronde Valley, and valley/canyon areas in Wallowa county through the weekend

- Active weather returns Friday through early next week with periods of moderate to locally heavy precipitation in the mountains and light rain in the lower elevations

DISCUSSION

Visible satellite imagery this afternoon shows the Lower Columbia Basin, eastern Columbia River Gorge, and the Yakima/Kittitas valleys under a stubborn stratus/fog deck, while patchy cirrus streams overhead. The stratus/fog deck has been slowly eroding way the last couple of hours, but has managed to hold on around the Walla Walla Valley and around Pendleton. IN other areas, the dense fog has lifted, but a status layer between 500ft to 2kft above ground level remains.

Today through Thursday Night: Mid to upper level high pressure aloft will continue to sit over the intermountain PacNW through Thursday night. This high pressure pattern has led to a strong layer of subsidence aloft (between 950-850mb) developing over the Lower Columbia Basin and adjacent foothills/valleys, which members of the REFS are in high confidence (80-90%) that this pattern will persist through Thursday night. As a result, fog/stratus is expected to persist through Friday morning across the lower Columbia Basin and adjacent areas, with the densest fog expected to redevelop in overnight and morning hours. While the fog/stratus layer will only strengthen over the next 24-36 hours, there is low confidence (20-35%) in the fog/stratus extending into central OR, as well as developing in the Grande Ronde valley and Wallowa valley.

Meanwhile, a persistent surface pressure gradient across the Blue Mountains will result in breezy southerly winds (15-25mph with gusts 35-45mph) developing along the base of the Blue Mountains, through the Grande Ronde valley, and through valley/canyon areas in Wallowa county. Thursday night, an approaching strong surface low will result in the pressure gradient tightening further and extending over central OR, resulting in winds across the aforementioned areas to further increase to near advisory criteria.

Friday through Tuesday: An active period of weather will be in store for the forecast area Friday into early next week. While mountain precipitation is expected, any snow accumulations will be limited to mainly above 5.5kft through the period, with a rain/snow mix to around 5kft at times (confidence 70-85%).

Starting Friday morning, an approaching vertically stacked low pressure system will result in a tightening of surface pressure gradients and a strong (40-60kts) 850-700mb jet developing over central OR and the eastern mountains. Winds will increase further through the morning and early afternoon, with moderate confidence (60-75%) that gusts between 45-55mph will mix down across portions of central OR, the eastern mountains, Grande Ronde Valley, and Wallowa county.

Other than winds, the approaching upper low will bring increasing chances of rain showers late Friday morning. Showers will push from west to east throughout Friday, with light showers in the lower elevations and moderate to locally heavy showers across most mountain areas. That said, 24-hr probabilities for 0.25 inches through Friday night are generally between 40-65% across the Blues and eastern mountains, with probabilities between 50-85% along the Cascade crest and east slopes. In the lower elevations, the probability of 0.1 inches is between 35-65%. Light Cascade mountain precip will continue into Saturday with dry conditions elsewhere while a transient ridge follows the exit of the upper low.

There's good agreement amongst ensemble cluster members that an upper level trough will extend equatorward in the northeast Pacific the first half of Sunday, then swing into western CONUS through Monday. Ahead of the upper trough approach, southerly breezy to locally gusty winds (wind gusts 35-50mph) will develop across central OR, the Blue mountain foothills, Grande Ronde valley, valley/canyon areas of Wallowa county Saturday night into Sunday. While the trough approaches the region Sunday, light to moderate rain shwoers will spread west to east across the region. By Monday, a colder airmass associated with the upper trough moving inland will lower snow levels enough for rain/snow showers developing across portions of the mountain zones and snow generally above 5-5.5kft. In most areas, there is moderate confidence (60-75%) in snow accumulations only amounting to an inch during this period, with higher accumulations along the Cascade and Wallowa peaks.

By Tuesday, there is moderate confidence (60-70%) in the upper trough weakening as it continues to swing across western CONUS, with light mountain rain and high mountain snow continuing.

AVIATION /06Z TAFS THROUGH 06Z FRIDAY/

Expect visibility to reach below airfield minimums at PDT and ALW, at least at times as visibility waivers between one quarter and one half SM overnight. Stratus in the gorge will lead to ceilings at DLS in the MVFR category and dropping to IFR/LIFR by 8-12z. BDN/RDM are the most probable to remain VFR although brief drops to 2SM occurred this evening at BDN. Least confidence in visibility trends is at PSC and YKM, where the probability of going below 1 mile is relatively small at 30% ans compared to 70% for PDT/ALW.

Preliminary Point Temps/PoPs

PDT 37 62 43 68 / 0 0 0 10 ALW 43 62 48 67 / 0 0 0 0 PSC 36 56 35 63 / 0 0 0 50 YKM 37 58 36 57 / 0 0 0 70 HRI 35 56 37 65 / 0 0 0 30 ELN 35 55 34 53 / 0 0 0 70 RDM 35 70 43 57 / 0 0 0 90 LGD 44 61 45 60 / 0 0 0 10 GCD 37 67 40 60 / 0 0 0 60 DLS 42 59 40 56 / 0 0 0 60

PDT WATCHES/WARNINGS/ADVISORIES

WA...Dense Fog Advisory until 11 AM PST Thursday for WAZ026-029. OR...Dense Fog Advisory until 11 AM PST Thursday for ORZ507.