textproduct: Pendleton

KEY MESSAGES

- Above-normal temperatures and dry conditions through tonight

- Return of showers and isolated thunderstorms to the eastern mountains tomorrow

- Breezy to gusty winds developing across the area Thursday night into Friday morning, with strongest winds through the Cascade gaps

- Moderate-high confidence (65-80%) in freezing morning temperatures across lower elevation area Saturday

DISCUSSION

Water vapor satellite imagery this afternoon shows an upper closed low off the coast of central CA, while a cold front boundary approaches Vancouver Island to the north. As for over the PacNW, little to talk about except for some partly cloudy conditions with a cumulus field developing over the Wallowas and Elkhorn Crest.

Today: While an upper low continues to sit off the central CA coast today, the forecast area will sit under a weak south to southwest flow aloft. Dry and seasonably warm conditions will continue to impact the area today. To the south, the upper low will begin to swing inland tonight, with precipitation ahead of the low reaching southern OR late this evening and overnight.

Tomorrow and Friday: The PacNW will see a brief return to unsettled weather as two separate systems arrive to the region. These two systems will come in the form of an upper closed low passing to the south and east and an upper trough with attendant cold front sliding across the PacNW.

The upper closed low passage will be the main precipitation maker through this period. Showers will begin to develop across the high terrain south and east of John Day late tonight, then spread to the remainder of the eastern mountains, central OR, and the eastern half of the Columbia Basin by tomorrow afternoon. Shower chances will dissipate from west to east tomorrow evening through Friday morning as the upper low continues east of the region. Between 5AM tomorrow and 5AM Friday, there will be a 70-95% chance of 0.25 inches of rain across the northern Blue Mountains, Elkhorn Crest, and Wallowa County; along the Blue Mountain foothills and the remainder of the eastern mountains, chances of 0.1 inches have come up 50-80%. In the remainder of the areas, there is a 30-60% chance of at least 0.01 inches.

The upper trough and cold front passage tomorrow night through Friday will bring wind impacts throughout the forecast area. Winds will begin to increase through the Cascade gaps tomorrow afternoon, and continue to strengthen overnight as the cold front drags across the intermountain PacNW. Winds are currently expected (confidence 70-85%) to peak overnight tomorrow into early Friday morning between 25-35 mph with gusts to around 45 mph through the eastern Gorge and Kittitas Valley, while winds will be slightly weaker (20-30mph with gusts up to 40 mph) in portions of the Columbia Basin, Central OR, Cascade east slopes, and eastern mountains (including Grande Ronde Valley). Breezy winds will carry on into the afternoon Friday, but become light by the nighttime hours.

Lastly, south to southwest flow aloft ahead of the low as well as the low becoming a negative tilt trough over the Great Basin will lead to increased instability over the eastern mountains. SBCAPE will increase to between 200-250 J/kg across the eastern mountains, leading to chances (15-25%) of isolated thunderstorms developing tomorrow afternoon.

Saturday through Tuesday: After Friday, ensemble members are still in some disagreement in the evolution of the synoptic pattern. About 60% of members from the ECMWF, GFS, and Canadian suites are in agreement of an upper low dropping down the BC coastline by Saturday afternoon, then slowly passing through the PacNW through early next week. That said, there is plenty of disagreement amongst these members in regards to timing, strength, and location of the low/upper trough moving over the region. For now, confidence is at least moderate (55-60%) in light mountain precipitation over the forecast area, but low confidence (15-20%) in timing and accumulation amounts through early next week.

Otherwise, there is good agreement that the earlier trough/cold front passage will allow for a colder/drier airmass to settle into the area Friday night through Saturday morning. The dry conditions and potentially clear skies will at least help with lowering temperatures to or below freezing in lower elevation locations Saturday morning (confidence 65-80%). Will continue to monitor for need of freeze watches/warnings. Lawhorn/82

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

VFR conditions are forecast through the period. Light, diurnally and terrain-driven winds of 10 kts or less are expected for all sites until sustained west to northwest winds ramp to 10-15 kts with higher gusts Thursday afternoon at DLS/RDM/BDN. Chances of precipitation increase Thursday morning and persist through Thursday afternoon at sites PDT/ALW/RDM/BDN/PSC; included PROB30 groups due to lingering uncertainty in how far west the band of precipitation will extend.

Preliminary Point Temps/PoPs

PDT 38 61 37 58 / 0 60 50 0 ALW 44 61 41 58 / 0 60 70 10 PSC 36 64 38 66 / 0 20 20 0 YKM 40 68 39 65 / 0 10 0 0 HRI 37 62 38 63 / 0 30 20 0 ELN 37 67 34 58 / 0 10 0 0 RDM 32 63 23 54 / 0 30 10 0 LGD 44 65 37 53 / 0 90 80 10 GCD 43 64 35 53 / 20 90 70 0 DLS 43 70 43 63 / 0 10 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Wind Advisory from 7 PM Thursday to 11 AM PDT Friday for WAZ024-026. OR...Wind Advisory from 7 PM Thursday to 11 AM PDT Friday for ORZ041.