textproduct: Pendleton

KEY MESSAGES

- Near-seasonal temperatures through the week.

- Breezy westerly winds through the Cascade gaps today.

- Transition to a convective weather pattern today with daily shower and thunderstorm chances into next week, mainly for the mountains.

DISCUSSION

Water vapor imagery reveals a pattern transition in progress with flow aloft turning southwesterly over the forecast area early this morning; a weak trough is noted over the Pacific Northwest, with an embedded shortwave diving southeast in the vicinity of Vancouver Island. At the surface, a modest cross- Cascade pressure gradient remains in place (4-6 mb difference between PDX and DLS as well as SEA and ELN) which is keeping breezy westerly winds in place through the Cascade gaps. As the day progresses, gap winds will strengthen, reaching widespread breezy to locally windy levels this afternoon.

NWP guidance is in excellent agreement that the shortwave mentioned above will continue to track southeast into Washington and northern Oregon today, serving as the synoptic- scale catalyst for a slight chance to chance (15-35 percent) of afternoon convection along the east slopes of the Washington Cascades. Lower chances (5-10 percent) of showers and thunderstorms extend across the Columbia Basin and portions of the Blue Mountains.

The wave will then exit to the northeast into the northern Rockies overnight into Thursday, albeit with a weak broad trough remaining over the Pacific Northwest on Thursday in its wake. Although PoPs are quite low, will mention that NWP guidance is advertising synoptic-scale forcing coupled with sufficient mid-level (700-500 hPa) moisture and negative mid- level (700-500 hPa) theta-e lapse rates to facilitate some weak nocturnal convection (predominantly showers with isolated embedded thunderstorms) overnight into Thursday morning across mainly the higher terrain of south-central Washington and eastern Oregon.

Looking ahead, ensemble NWP guidance suggests continued southwest flow aloft through the remainder of the forecast period. This will keep a convective pattern in place resulting in daily shower and thunderstorm chances, mainly over the mountains. That said, analysis of ensemble clusters reveals significant uncertainty in pattern details beyond Saturday. By Sunday and Monday, ensemble NWP indicates roughly equal chances (40-60 percent) of a deep offshore closed low or a weaker open wave trough. Either solution puts the forecast area under southerly to southwesterly flow aloft, favorable for convection.

Fire Weather Watches may be needed if confidence in widespread lightning increases this week or over the weekend.

Lastly, Air Quality Alerts/Advisories issued by air quality agencies remain in effect for the majority of our forecast area due to wildfire smoke from ongoing regional wildfires.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

VFR conditions currently across all sites, which will stay the course through the period. Elevated smoke will be present at KPDT/KALW/KPSC, but visibilities are expected to stay 6SM or above. Breezy winds at most sites, with sustained winds 10-15kts and gusts of 15-25kts. KALW/KPSC will experience lighter winds of 10kts or less through Thursday morning. 75

Preliminary Point Temps/PoPs

PDT 85 52 85 54 / 0 0 0 0 ALW 87 58 87 59 / 0 0 0 0 PSC 90 57 89 58 / 0 0 0 0 YKM 89 58 88 60 / 0 20 0 10 HRI 88 55 89 57 / 0 0 0 0 ELN 83 55 83 56 / 10 40 10 20 RDM 84 44 84 48 / 0 0 0 10 LGD 87 53 86 55 / 0 0 0 30 GCD 89 52 87 54 / 0 0 0 50 DLS 83 60 91 62 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until 11 AM PDT Thursday for WAZ026>030- 521>523. OR...Air Quality Alert until noon PDT today for ORZ041-049-050-502- 503-505-506-508>511.