textproduct: Pendleton

KEY MESSAGES

- Chances of an isolated to scattered thunderstorm across Central OR and the eastern mountains continue through this afternoon. Chances resume Wednesday and possibly Thursday.

- Models still advertise uncertainty with the evolution of the cut-off low pressure system offshore. Solutions trend toward the low pressure system moving into British Colombia.

- Warm/dry conditions with high temperatures in the low to upper 90s persist through the week.

DISCUSSION

Radar currently shows light returns across the Colombia Basin through the eastern mountains with isolated thunderstorms developing near Grant and southern Union county's. A cut-off low pressure system just off shore is allowing a lifting mechanism to initiate showers and thunderstorms through the middle of the week. A ridge extending over the Great Plains and into the Idaho/Eastern Oregon region is simultaneously allowing warm temperatures in the low to upper 90s to persist (50-70% confidence) and transporting mid-level monsoonal moisture. With the lift of the low pressure system, moisture transportation, and warmer temperatures, chances of isolated to scattered thunderstorms persist through Thursday. The RRFS model has been handling the convection the best through this afternoon. That same model takes convection up into the eastern mountains/northern Foothills of the Blues later this afternoon. Many short term models are in agreement that we'll see nocturnal light to moderate rain shower activity move-up Central OR through the Kittitas Valley by the early morning hours (40-50% confidence) with overnight thunderstorms a possibility.

Moving forward, the synoptic set-up is unclear with the cluster members struggling whether it wants to take the low pressure system on-shore or keep it off the coast. Clusters remain split over the next few days, with a somewhat even splitting for different scenarios whether it wants to keep the low offshore or onshore. Looking at clusters, 60% of members want to push the low just off-shore by late Wednesday afternoon, while the remaining 40% keep it well offshore and keep the weather drier. The split continues late afternoon on Thursday with 47% of members bringing the low into Washington and Oregon, with the remaining 53% keep it off the PacNW coast. Bottom line is that confidence is not great to either solution at this time, but there's been a slight trend towards the solution that favors the low pressure system moving onshore and then eventually into British Colombia.

The eventual location of the low-pressure system will effect our HeatRisk's over the next several days with a solution of the low staying offshore resulting in warmer conditions or cooler conditions if moved onshore. Current HeatRisk probabilistics advertise widespread 40-80% chances of a moderate or higher risk, with Wednesday looking to be the peak showing >90% virtually covering the entire lower elevations of south-central Washington and parts of northeastern Oregon. Chances for a major heat risk is around 20-40% Tuesday to Thursday. Heat Advisory's are possible across parts of Kittitas/Ellensburg area if solutions of the low pressure staying off shore verify, but with low confidence in determining the position of the low, a call to initiate a highlight hasn't been made yet.

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

VFR conditions are forecast through the period. There is a very low chance (5-10%) of isolated thunderstorms for BDN/RDM this afternoon and early evening, but confidence was too low to include mention in the 00Z TAFs. 12Z HREF and REFS guidance both advertise another round of high-based showers overnight into Wednesday morning, but confidence in measurable precipitation at any TAF site was too low (<30%) to mention. By Wednesday afternoon, isolated thunderstorms are anticipated to develop across the high terrain of central Oregon, spreading north in the vicinity of BDN/RDM by mid/late afternoon; for this convective potential, included a PROB30 group.

Winds will be diurnally and terrain-driven, mostly light with a couple exceptions. Periods of breezy winds are anticipated during the afternoon/evening hours for DLS/BDN/RDM, and any convection that develops will be capable of producing erratic, gusty outflow winds.

Preliminary Point Temps/PoPs

PDT 62 96 61 92 / 0 0 30 10 ALW 65 99 66 96 / 0 0 20 10 PSC 63 100 64 97 / 0 0 30 20 YKM 65 98 66 95 / 0 0 50 40 HRI 65 99 65 94 / 0 0 30 20 ELN 62 96 63 91 / 0 10 50 50 RDM 56 91 54 86 / 10 40 30 10 LGD 59 95 60 97 / 0 0 20 20 GCD 58 93 58 96 / 20 20 30 10 DLS 66 98 65 85 / 0 10 60 30

PDT WATCHES/WARNINGS/ADVISORIES

WA...None. OR...None.