textproduct: Pendleton

KEY MESSAGES

- Isolated to widely scattered thunderstorm chances this afternoon across central OR and the eastern mountains, with chances spreading to north central OR and south central WA Wednesday and Thursday

- Above normal temperatures with dry conditions will persist into early next week

- Uncertainty remains around synoptic pattern Thursday into early next week

DISCUSSION

Today through Wednesday: Satellite imagery tonight shows increasing mid to upper level cloud cover east of the Cascade crest, with radar imagery showing light to moderate returns pushing across east central OR. This activity is associated with a shortwave impulse currently over southeast OR, and is anticipated to push north along the OR/ID border then rotate northwest into ID/MT early in the afternoon. Showers associated with this impulse will be focused along the Southern Blues into Wallowa County throughout this morning, with shower activity waning mid to late afternoon. A secondary impulse riding up the south-southwest flow aloft will produce another round of shower activity across central OR mid afternoon into the overnight hours. In addition to orographic lift aiding shower development, increasing MUCAPE between 250-750 J/kg will allow for isolated to widely scattered thunderstorms to develop.

Wednesday, another shortwave impulse is anticipated to push north along the Cascade east slopes, producing showers from central OR to the central WA Cascades through Wednesday night, with isolated shower chances (20-35%) across western portions of the Columbia Basin. Much like this afternoon, marginal instability combining with orographic ascent will result in isolated to widely scattered thunderstorms across the Cascade east slopes from central OR to central WA.

Increasing near-surface monsoonal moisture will allow for precipitable water values to increase to between 0.75 to about 1.2 inches both Tuesday and Wednesday. This will allow for moderate to locally heavy rain cores to develop within showers/thunderstorms. That said, elevated fire weather conditions will remain present for the threat of lightning outside of the rain cores as well as for the potential of outflow boundaries.

Thursday through Monday: Thursday into early next week, uncertainty remains in the evolution of the synoptic pattern, especially in regards to an upper level low offshore the PacNW. Thursday into Friday, an ensemble cluster solution (~30-40% of members) driven mainly by members of the ECMWF ensemble favor an upper low lifting across the PacNW with cooler and wetter conditions as well as breezy to locally gusty Cascade gap and lower elevation winds. The remaining cluster solutions favor the low weakening and opening into a weak upper troughing offshore during this time, resulting in continued above normal temperatures with dry conditions and limited mountain shower activity.

Saturday into early next week, uncertainty remains with the overall pattern. The main clustering solutions are split between either a weakening upper trough extending from BC to offshore the PacNW (50-55% of members), or upper level ridging building into the region from the east(45-50% of members). The latter solution would result in a marked warming trend and increasing potential of heat impacts across forecast area while the warming of the former solution would bring a modest warming trend across the region. Other than warming temperatures, both solutions would result in a return to dry conditions with light winds into early next week (confidence 35-50%). Lawhorn/82

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/

VFR conditions are forecast through the period. Widespread mid/high cloud this morning is anticipated to thin or clear through afternoon, especially for western sites. There is a low chance (10-20%) of isolated thunderstorms for BDN/RDM this afternoon and early evening, but confidence was too low to include mention in the 18Z 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.

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 93 62 96 61 / 10 0 0 30 ALW 95 65 99 66 / 10 0 0 20 PSC 96 63 100 64 / 0 0 0 30 YKM 98 65 98 66 / 0 0 0 50 HRI 96 65 99 65 / 0 0 0 30 ELN 94 62 96 63 / 0 0 10 50 RDM 93 56 91 54 / 0 10 40 30 LGD 90 59 95 60 / 60 0 0 20 GCD 91 58 93 58 / 40 20 20 30 DLS 97 66 98 65 / 0 0 10 60

PDT WATCHES/WARNINGS/ADVISORIES

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