textproduct: Pendleton

KEY MESSAGES

- Temperatures warming to above normal through early next week

- Uncertainty late Friday through early next week in potential for mountain shower and thunderstorm activity

DISCUSSION

Water vapor imagery shows a weak shortwave embedded within northwesterly flow aloft over western OR/WA. Some cirrus has overspread the forecast area in advance of the shortwave, and Day Cloud Phase RGB imagery reveals some cumulus development over the Blue and Cascade mountains. Shower activity within their respective cumulus fields has been isolated, with radar showing a few over the northern Blue, Wallowa, and Wenatchee mountains. Chances of lightning are low (10 percent or less) with the shower activity.

Wednesday and Thursday, an upper-level ridge in the northeast Pacific will move eastward over the region, resulting in rising heights and warming temperatures. While mostly dry conditions are forecast, yet another shortwave will dive south-southeast from Canada on Wednesday. Coupled with weak diurnally driven instability (CAPE of 500 J/kg or less), this will facilitate a slight chance (10-20 percent) of showers over the Blue Mountains.

By Friday, ensemble mean 500-hPa heights show a couple shortwaves moving eastward in the northeast Pacific towards the Pacific Northwest. Ensemble cluster analysis is in good agreement that the first shortwave will partially flatten the upper-level ridge, but its track well north of the region into British Columbia should keep the forecast area dry (90 percent confidence). Moreover, heights actually continue to rise over our area, so temperatures are forecast to continue to increase for most of our population centers relative to Thursday.

Uncertainty in the forecast grows considerably by later Friday through next week as ensemble cluster analysis suggests a high chance (80 percent or higher) of the latter shortwave trough deepening and cutting off into a closed low off the coast of Oregon and California. Based on the 00Z and 12Z ensemble runs, a little less than half of members keep the region warm to hot and dry under a ridge (Rex block pattern with a closed low offshore and ridging in the Pacific Northwest and an anomalous high offshore north of the low), while the remaining members place the low in a position near the OR/CA border that would provide precipitation, somewhat cooler temperatures (though still above normal), and potential for thunderstorms. While it is unlikely, analysis of QPF clusters suggests a roughly 25 percent chance of a widespread wetting rain for the Oregon Cascades and Blue Mountains as the aforementioned closed low eventually moves inland early next week (likely Monday through Tuesday).

While there is still uncertainty in how hot conditions will be under the ridge this weekend into early next week (largely due to the aforementioned uncertainty in shower and thunderstorm chances and resultant cloud cover), chances of afternoon highs exceeding 80 degrees across all major population centers are high (60-99 percent) per latest NBM calibrated probabilities. By Sunday through Tuesday, chances of reaching 90 degrees are low- medium (20-60 percent) for the Yakima Valley and lower Columbia Basin. Moreover, there is a low-medium (20-60 percent) chance of HeatRisk reaching Moderate (level 2 of 4) in the Yakima Valley and lower Columbia Basin on Monday and Tuesday. This level of heat affects those who are sensitive to heat, especially those without cooling/hydration. 86

AVIATION /00Z TAFS THROUGH 00Z THURSDAY/

VFR conditions currently at all sites, which will stay the course through the period. Breezy winds between 15-25kts are occurring at KDLS/KRDM/KBDN, and will slowly subside by early evening. 15-20kft ceilings continue overnight before breaking up and lifting early Wednesday morning. 75

Preliminary Point Temps/PoPs

PDT 66 40 69 43 / 0 0 0 0 ALW 66 44 69 47 / 0 0 0 0 PSC 71 43 74 44 / 0 0 0 0 YKM 69 40 73 45 / 0 0 0 0 HRI 68 41 72 42 / 0 0 0 0 ELN 62 38 69 42 / 0 0 0 0 RDM 64 29 66 34 / 0 0 0 0 LGD 63 40 66 38 / 10 0 10 0 GCD 63 38 66 38 / 0 0 20 0 DLS 66 41 74 47 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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