textproduct: Pendleton

KEY MESSAGES

- Dry weather, seasonal temperatures, and breezy westerly winds through the Cascade gaps today.

- High confidence (70-95 percent) in a shift to a convective pattern Wednesday through the weekend.

DISCUSSION

Water vapor imagery shows zonal (Washington) to quasi-zonal (Oregon) flow over the forecast area early this morning. 00Z and 06Z soundings at UIL/SLE/MFR and OTX, respectively, corroborate satellite imagery with pure zonal flow aloft over UIL/OTX and west to west-southwest flow aloft over SLE/MFR. Of note, but perhaps of little consequence as far as today's weather is concern, a weak circulation is approaching the OR/CA coastal border.

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. Regarding fire weather conditions, 00Z HREF/REFS and latest NBM output suggest low chances (40% or less) of overlapping critical wind/RH thresholds today; highest probabilities are in the Kittitas Valley and lower Columbia Basin of Oregon west of Boardman.

Looking ahead, ensemble NWP guidance suggests southwest flow aloft amidst an amplifying 500-hPa height field Wednesday through the remainder of the forecast period. This will facilitate a change to a more convective pattern with daily shower and thunderstorm chances, though analysis of ensemble clusters reveals uncertainty in pattern details, especially beyond Thursday. 00Z/06Z CAMs and the NBM indicate the best shower and thunderstorm chances (5-20 percent) Wednesday and Thursday over the Washington Cascades and limited portions of the Blue Mountains. Shower and thunderstorm chances (10-30 percent) expand to encompass the Oregon Cascades and all of the Blue Mountains by Friday and Saturday. By Sunday and Monday, ensemble NWP indicates at least some potential (40-60 percent) of a deep offshore closed low, strengthening southerly to southwesterly upper-level winds, and ample moisture over the forecast area; this solution is quite favorable for robust convection. Fire Weather Watches may be needed if confidence in widespread lightning increases.

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 /00Z TAFS THROUGH 00Z THURSDAY/

VFR conditions currently across all sites, which will stay the course through the period for most sites. KRDM is the only terminal expected to drop to MVFR visibilities due to smoke from nearby wildfires. Breezy winds of 10-15kts with gusts of 20-25kts for KDLS/KRDM through the evening before subsiding later this tonight. 75

Preliminary Point Temps/PoPs

PDT 54 85 52 84 / 0 0 0 0 ALW 60 87 58 86 / 0 0 0 0 PSC 57 90 56 88 / 0 0 0 0 YKM 59 89 58 87 / 0 0 10 0 HRI 58 88 55 87 / 0 0 0 0 ELN 56 83 54 82 / 0 10 30 20 RDM 46 84 43 82 / 0 0 0 0 LGD 53 88 52 85 / 0 0 0 0 GCD 53 89 52 86 / 0 0 0 0 DLS 61 83 59 87 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Air Quality Alert until 11 AM PDT Thursday for WAZ024-026>030- 521>523. OR...Air Quality Alert until noon PDT Wednesday for ORZ041-049-050- 502-503-505-506-508>511.