textproduct: Pendleton

KEY MESSAGES

- Above-normal temperatures through Monday.

- Low (5-15 percent) chance of thunderstorms Monday.

- Widespread breezy to windy conditions Monday night through Tuesday.

DISCUSSION

An upper-level ridge is in place over the Pacific Northwest early this morning, though broken high cloud is evident on satellite imagery as a weak shortwave approaches from the southwest. Ensemble NWP guidance is in excellent agreement that the ridge axis will continue to shift overhead today, resulting in very high confidence (95 percent) that temperatures will remain above normal. From a deterministic standpoint, widespread afternoon high temperatures in the 70s, and isolated lower 80s, are advertised by the NBM today and Monday. Glancing at probabilistic output, there is a high (70-95 percent) chance the eastern Columbia River Gorge, Yakima Valley, lower Columbia Basin, and lower elevations of north-central and central Oregon will reach 80 degrees Monday.

Dry conditions are expected (95 percent confidence) under the ridge today, but guidance is showing low (10-20 percent) chances of showers and very low (5-15 percent) chances of thunderstorms Monday as a weak shortwave wriggles into the Pacific Northwest from the Pacific and a closed low and attendant cold front slide southeast into British Columbia and northwest Washington. CAMs are showing a weakly unstable atmosphere (MUCAPE/SBCAPE 250-500 J/kg) over the mountains, primarily the Blues, but also portions of the Cascades of both Oregon and Washington. Thunderstorms, if any, will be of the air mass variety due to weak shear (15-25 kts effective) and slow storm motion of 5-10 kts to the east or east-southeast is anticipated. Gusty outflow appears to be the primary hazard with this convection, and CAMs are outputting up to 35 kts with the most robust cells. Forecast soundings suggest DCAPE of 500-800 J/kg which would support these sub-severe outflow winds.

By Tuesday, ensemble guidance generally depicts a closed low in the Pacific approaching the OR/CA coast while the aforementioned closed low treks across the Northern Tier. The trend in guidance has been farther north and east with the low from Canada, resulting in forecast temperatures that are near to above normal Tuesday and Wednesday.

Winds Monday night and Tuesday are still the best chance of a headline-worthy event for the next week. The NBM suggests a low- medium (30-60 percent) chance of reaching or exceeding advisory-level wind gusts (45 mph or greater) through the Cascade gaps and across wind-prone portions of the Columbia Basin and surrounding lowlands.

Looking ahead, there is significant uncertainty in the evolution of the 500-hPa pattern Wednesday through the remainder of the week as ensemble guidance struggles to resolve the track of the offshore cut-off low in the Pacific. Latest 00Z runs have trended less progressive and ensemble means keep the closed low in the vicinity of the OR/CA border or northern/central California Wednesday through Friday. The trend towards a split flow pattern has resulted in an upward trend in forecast temperatures and would keep above-normal temperatures in place for the duration of the week.

Delving into some analysis of uncertainty in the 500-hPa pattern, ensemble cluster analysis shows roughly 30-40 percent of members keep the closed low in a position that is favorable for shower and thunderstorm development for a small portion of our CWA across central and eastern Oregon Wednesday through Friday, but the bulk of the activity would remain in south- central Oregon and northern California. The remainder of the ensemble members are still advertising a more progressive pattern with the low diving southeast into California sooner and/or place the low farther south. 86

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

VFR conditions to prevail through the period. Winds 12kts or less will be diurnally driven. An occasional gust above 12kts will be possible this afternoon at sites BDN/RDM. Lawhorn/82

Preliminary Point Temps/PoPs

PDT 71 45 75 47 / 0 0 0 0 ALW 72 49 74 51 / 0 0 0 0 PSC 74 45 80 49 / 0 0 0 0 YKM 72 46 78 42 / 0 0 0 0 HRI 73 45 79 49 / 0 0 0 0 ELN 68 44 73 39 / 0 0 0 10 RDM 74 39 75 37 / 0 0 0 0 LGD 69 43 74 43 / 0 0 10 0 GCD 71 42 73 41 / 0 0 20 20 DLS 74 49 78 47 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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