textproduct: Pendleton

KEY MESSAGES

- Moderate HeatRisk through in the lower elevations through Wednesday

- Weak system passage Wednesday into Thursday will bring isolated thunderstorm chances to the Cascade east slopes, with breezy winds elsewhere

- Breezy winds, cooler temperatures, and mountain showers friday through the weekend.

DISCUSSION

Today through Tuesday: Despite a ridge of high pressure developing over the region today, satellite shows fields of cumulus forming over the WA Cascades and northern Blues this afternoon.

Warming conditions with a few afternoon breezes will be the main trend the rest of today and tomorrow as an upper level ridge slides across the PacNW. Winds will remain mostly light through tomorrow, while afternoon temperatures will warm another 2 to 4 degrees into the lower to mid 90s across the lower elevation areas.

Wednesday and Thursday: The upper ridge will push east Wednesday, allowing a mid-level shortwave to slide across the PacNW during the day. At the surface, a thermal trough will cross the Cascades into the intermountain PacNW, which will bring increasing unstable conditions and the warmest temperatures over the forecast area Wednesday afternoon. While conditions will remain mostly dry across the area, a weak moisture push with shortwave passage and increased instability across the Cascade east slopes will result in chances (15-30%) of isolated thunderstorms and showers developing in the afternoon and evening. Otherwise, the cross-Cascade pressure gradient will tighten by the late afternoon, resulting in breezy winds (10 developing through the Cascade gaps and portions of the Columbia Basin through the evening. Localized portions of the Columbia Basin/Gorge may briefly see low RHs (10-20%) combine with these breezy conditions late in the day, resulting in elevated fire weather conditions (confidence 60-70%)

Zonal flow will develop behind the shortwave exit early Thursday, keeping drier conditions across much of the forecast area. Breezy winds (15-25mph with gusts up to 40 mph) will redevelop late in the morning through the afternoon across the lower elevations as pressure gradients remain tightened ahead of a surface low swinging into central BC. The increased winds and low RHs (10-20%) across the Columbia Basin and portions of north central Oregon will result in elevated fire weather conditions in the afternoon, with increasing confidence (65-80%) that critical wind/RH thresholds will be met Thursday.

Friday through the Weekend: Cooler temperatures, breezy winds, and mountain showers will be the trend into the weekend, thanks to an upper low diving down the BC coastline into the PacNW. Ensemble guidance is in good agreement through the weekend that the upper low will approach the PacNW by Friday morning, with showers pushing east with a frontal boundary early Friday. By the afternoon, showers will retreat mainly to the mountains as the frontal boundary exits into the eastern mountains and the upper low enters into the PacNW (confidence 50-65%). Increasing instability combined with the frontal passage will also result in isolated thunderstorms developing across the eastern mountains Friday afternoon. Mountain showers will continue throughout the weekend as the low continues to swing through the area, with decent agreement amongst ensemble members that the upper low will be centered east of the PacNW by Sunday afternoon, with broad troughing remaining over the region. Other than precipitation, breezy winds (15-25mph with gusts up to 45 mph) will continue into the weekend across the lower elevations, with the strongest winds (20-35mph with gusts 40-50 mph) developing through the Cascade gaps Saturday afternoon (confidence 45-65%). Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

VFR conditions to prevail through the period. Winds will be light with diurnally/terrain driven wind shifts. Lawhorn/82

FIRE WEATHER

Dry and warm conditions will continue through Thursday. Breezy winds will develop through the Cascade Gaps Wednesday with localized critical fire weather conditions, winds and critical fire weather conditions becoming widespread Thursday. Isolated thunderstorms will also be possible along the Cascade crest and east slopes Wednesday afternoon. Breezy winds, cooler temperatures, and mountain showers are expected Friday through the weekend.

Preliminary Point Temps/PoPs

PDT 53 91 55 91 / 0 0 0 0 ALW 58 92 61 93 / 0 0 0 0 PSC 55 94 58 96 / 0 0 0 0 YKM 59 92 61 94 / 0 0 0 10 HRI 55 93 58 94 / 0 0 0 0 ELN 56 89 59 90 / 0 0 0 20 RDM 49 89 50 88 / 0 0 0 10 LGD 50 89 53 89 / 0 0 10 0 GCD 48 94 50 91 / 0 0 0 10 DLS 62 95 64 93 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Heat Advisory until 11 PM PDT Tuesday for WAZ024. OR...Heat Advisory until 11 PM PDT Tuesday for ORZ041.