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 /18Z TAFS THROUGH 18Z WEDNESDAY/

VFR conditions currently across most sites, with the exception of MVFR at KRDM due to reduced visibilities related to smoke from nearby wildfires. This is expected to persist through the day, improving slightly later this evening into the overnight period as winds subside. Elsewhere, VFR conditions will prevail, with breezy afternoon winds of 10-15kts and gusts of 20-25kts for KDLS/KYKM. All other sites will experience light winds of 10kts or less under mostly clear skies. 75

Preliminary Point Temps/PoPs

PDT 88 54 85 52 / 0 0 0 0 ALW 90 60 87 58 / 0 0 0 0 PSC 92 57 90 56 / 0 0 0 0 YKM 93 59 89 58 / 0 0 0 10 HRI 91 58 88 55 / 0 0 0 0 ELN 86 56 83 54 / 0 0 10 30 RDM 89 46 84 43 / 0 0 0 0 LGD 91 53 88 52 / 0 0 0 0 GCD 92 53 89 52 / 0 0 0 0 DLS 88 61 83 59 / 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.