textproduct: Pendleton

KEY MESSAGES

- Patchy dense fog and low stratus will continue to impact 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 continues to show the Lower Columbia Basin, Kittitas/Yakima valleys, and the Blue Mountain foothills under a stratus layer. Surface observations show that much of the dense fog that developed overnight has gradually lifted into the stratus layer, though some patchy dense fog remains along the Blue mountain foothills and higher terrain near the eastern Columbia River Gorge. Otherwise, decks of thin cirrus stream across the intermountain PacNW.

Today through Saturday: The PacNW will continue to feel the influence of a departing upper level ridge through tonight. This high pressure pattern will continue to support a strong layer of subsidence aloft (between 950-850mb) over the Lower Columbia Basin and adjacent foothills/valleys. As a result, fog/stratus is expected to persist through tomorrow morning across the Lower Columbia Basin and adjacent areas, with the densest fog expected to redevelop late this evening through the morning hours. There is increasing confidence (45-55%) that the fog/stratus will make a run for the Redmond/Bend area tonight, though impacts will be short lived through tomorrow morning.

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. Tonight through Friday morning, an approaching vertically stacked low pressure system will result in a tightening of surface pressure gradients and the development of 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- high confidence (70-85%) that gusts between 45-55mph will mix down across portions of central & north central OR, ridges of the eastern mountains, the base of the Blue Mountains, Grande Ronde Valley, and Wallowa county. Wind advisories are in effect from tomorrow morning until the afternoon for these areas.

Other than winds, the approaching upper low will bring increasing chances of rain showers through Friday. Showers will push from west to east throughout the day, 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 50-85% across the eastern mountains, Wallowa Mountains, and the Cascade crest/east slopes. In the lower elevations and northern Blues, 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.

Sunday through Wednesday: At the start of the day on Sunday, the LREF indicates that a closed upper low will be approaching the Northwest. Associated with this upper low, models indicate the formation of a pressure gradient across the Blue Mountains overnight Saturday into Sunday morning. As a result, models are suggesting breezy conditions will set up in the Blue Mountains and the foothills as well as the Grande Ronde Valley (80-90% confidence). Models suggest around a 30-50% chance of meeting wind advisory criteria (Gusts >45mph). In addition, the upper low will bring the chance of precip. Current model guidance indicates the rain will enter western portions of the area overnight Saturday into Sunday and overspread the region throughout the day on Sunday lingering through Monday (80% confidence). Sunday evening, snow levels are expected to fall to around 4500-5000 ft in the Cascades; Snow levels will drop to the same height in the Blue Mountains by Monday morning. Current model guidance supports a 10- 50% chance of receiving at least 0.25 in of liquid equivalent, a 50- 70% chance in the Blue Mountains, and a 80-100% chance in the Cascade Mountains.

After Monday, model spread starts to increase but models do suggest that there may be a lull in the precip as the trough moves overhead during the day on Tuesday (60-70% confidence) as the trough moves overhead. However, there are timing differences in when the trough moves overhead. After that trough moves through, models do support another trough approaching the region towards the end of the period. However, there are substantial differences regarding both timing and amplitude. The LREF and NBM currently both suggest lower probs of seeing 0.25 inches of rain which currently sit between 0-20% for lower elevations and 20-40% in the Cascades. Garty/26

AVIATION /00Z TAFS THROUGH 00Z SATURDAY/

Variable flight categories due to fog and low CIGs throughout the TAF period. KPDT: Wind shear beginning at 12z at 1,900ft at 31kt from 150 degrees. KALW: Wind shear beginning at 02z at 2,000ft at 40kt from 170 degrees. KBDN/KRDM/KYKM: Rain towards the end of the TAF period Fog should be similar to conditions we've experienced the last few days.

Preliminary Point Temps/PoPs

PDT 41 68 40 61 / 0 10 90 0 ALW 46 67 44 59 / 0 10 90 0 PSC 35 61 38 62 / 0 20 70 0 YKM 36 56 36 57 / 0 70 50 0 HRI 36 64 38 61 / 0 20 80 0 ELN 33 53 32 51 / 0 60 60 0 RDM 43 57 31 54 / 0 80 20 0 LGD 44 61 37 55 / 0 0 90 10 GCD 39 61 34 53 / 0 40 100 0 DLS 40 55 40 56 / 0 80 30 10

PDT WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory from 8 AM to 7 PM PST Friday for WAZ029. OR...Wind Advisory from 8 AM to 7 PM PST Friday for ORZ049-050-507- 508. Wind Advisory from 4 AM to 4 PM PST Friday for ORZ510-511.