textproduct: Pendleton

KEY MESSAGES

- Warm and above normal temperatures persist through the week.

- Dry conditions through the workweek.

- Slight chance for mountain showers Friday.

DISCUSSION

Current radar and infrared satellite imagery shows dry conditions under mostly clear skies. This is in response to an upper level ridge encroaching in from the coast that will keep skies sunny and temperatures well above normal. High temperatures across the Lower Columbia Basin, Blue Mountain foothills, Central Oregon, and the Yakima/Kittitas Valleys should be slightly warmer (+1-2 degrees) from Sunday. Temperatures will cool 2 to 5 degrees on Tuesday due to northerly flow aloft becoming more enhanced and extending into the Yukon and Northwest Territory provinces. High temperatures rebound back into the upper 80s to low 90s across the Lower Columbia Basin, Gorge, Blue Mountain foothills, Central Oregon, and the Yakima/Kittitas Valleys on Wednesday. This is a result of a weak shortwave that suppresses the upper level ridge to bring more upper level flow from the west, as advertised by both the ECMWF and GFS AI ensembles. The best chance for any high temperature records being broken will be in Yakima today, with the current forecast being two degrees shy (88/90) set back in 1966. The NBM suggests there to be a 28% chance of tying the record and a 7% chance of breaking it.

The continued presence of the upper level ridge across the Pacific Northwest will keep conditions dry and humidities rather low. This is clearly visible across our Oregon zones when viewing the Hot-Dry-Windy Index (HDWI), which shows 75-95 median percentiles exceeded from climatology over the last 30 years. Afternoon humidities have dropped into the low teens across Central Oregon and the John Day Basin over the last 3 days, with areas of the Lower Columbia Basin, Blue Mountain foothills, Union county, and the Yakima Valley experiencing minimum humidities in the teens on Sunday. These conditions are expected to improve through Tuesday associated with northerly flow aloft, however the downward trend returns beginning Wednesday. Afternoon humidities are expected to bottom-out Thursday and drop into the low to mid-teens across Central Oregon, John Day Basin, Lower Columbia Basin, Gorge, Blue Mountain foothills, and the Yakima Valley. Humidities look to moderate on Friday as ensemble members suggest an incoming upper level trough clipping the region, but guidance hints at the ridge rebuilding and dropping humidities through the weekend.

Ensemble members are in fairly good agreement related to an incoming upper level shortwave Friday, that spins off its parent low pressure that is located off the Gulf of Alaska. The shortwave is expected to suppress the upper level ridge that set up earlier in the week, which effectively shears the shortwave north and into British Columbia. 86% of ensemble members keep precipitation confined to our Cascade and Blue Mountain zones, with 35% of members suggesting only precipitation over the Cascade crest. Guidance has trended toward a drier solution over the last 24 hours, which may continue. The more dry cluster ensembles (1 and 3) consist of 76% and 75% of ECMWF and CMC members, with 40% of GFS members advertising showers extending into lower elevations of the Basin and along the Blue Mountain foothills. Even with this wetter cluster scenario (14% chance), rain amounts are expected to stay below a wetting rain (0.10"). Depending on the strength of the incoming shortwave and how effective it is in breaking the ridge down, there is a slight chance (10%) of isolated afternoon/evening thunderstorms across Grant, Union, and Wallowa Counties on Friday. Not only will this potential be related to shortwave strength, but also the timing of the shortwave coinciding with peak afternoon heating. The ECWMF has slowed the shortwave passing to later in the evening over the past two model runs (18z and 0Z), which is also suggested via the LREF showing a passing over the Cascades in the early afternoon and between 7PM-9PM across Grant, Union, and Wallowa counties using a 567mb threshold in relation to the 500mb height timing product. Thus, showers outside the Cascades and isolated thunderstorms across our eastern zones on Friday is currently an unlikely scenario (10%). 75

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

VFR conditions currently across all sites, which will stay the course through the period. Breezy winds of 15-25kts are expected for KDLS/KRDM/KBDN/KYKM through the afternoon and evening, with winds of 10kts or less for all other sites. 25kft ceilings are also likely across KPSC/KRDM/KBDN toward the end of the period. 75

Preliminary Point Temps/PoPs

PDT 82 52 79 52 / 0 0 0 0 ALW 82 55 78 55 / 0 0 0 0 PSC 88 54 84 52 / 0 0 0 0 YKM 88 56 84 54 / 0 10 0 0 HRI 87 53 83 52 / 0 0 0 0 ELN 83 51 79 51 / 0 10 0 0 RDM 80 44 79 43 / 0 0 0 0 LGD 78 50 74 48 / 0 0 0 0 GCD 78 46 76 46 / 0 0 0 0 DLS 85 57 83 56 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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