textproduct: Pendleton

KEY MESSAGES

- Afternoon isolated thunderstorm chances today.

- Above normal highs through week, peak Monday and Wednesday.

- Mountain precipitation returns Friday.

DISCUSSION

Current radar and infrared satellite imagery shows dry conditions under mostly clear skies. This is in response to an upper level low pressure system located off the northern California coast as surface high pressure over southern British Columbia dives south through Washington later today. The upper level low will move slightly onshore later today and elongate east, which will spin off enough moisture and instability to promote isolated thunderstorm development over elevated terrain across Central Oregon, John Day Highlands, Blue Mountains, and the Elkhorns (15-25% chance). The HREF highlights timing between 2PM and 9PM with ensembles suggesting activity peaking around 4PM across Grant, Crook, Deschutes, Union, Wheeler, and Wallowa counties. There may also be weak cells that extend into southern Gilliam, Morrow, and Umatilla counties, but chances are rather low (<10%). The main concerns associated with any storm cells will be lightning and breezy winds.

The upper level system to our south will continue to bring a warm airmass into the region and keep temperatures above normal. Saturday's high temperature in Tri-Cities tied a record of 90 degrees set back in 1937 and 2014, which may occur today as the current record of 91 stands from 2016. Yesterday, the NBM advertised a 15% chance of the Tri-Cities reaching a high of 90 degrees or greater. Today's chance of reaching 90 degrees or greater is 22%, but tying the current record of 91 degrees is only 10%. However, the overall spread of ensembles is 2 degrees versus only 1 yesterday, which suggests less confidence in today's high temperature than yesterday's. Taking this into account, and that the NBM was two degrees off and outside its 25-75th percentile spread, it is expected that high temperatures across the Tri-Cities will tie and potentially exceed the current record of 91 later today. Elsewhere, high temperatures will break back into the mid-to upper 80s across the Lower Columbia Basin, Gorge, and the Yakima/Kittitas Valleys, and in the low to mid-80s through Central Oregon and the John Day Basin. High temperatures bump up another 1 to 3 degrees on Monday as highs reach into the low 90s through the Lower Columbia Basin.

The upper level low drops south into southern California as an upper level ridge slides into the Pacific Northwest on Tuesday, bringing northerly flow aloft and slightly cooler temperatures across the region. This is short-lived as the ridge axis moves overhead Wednesday, with flow aloft shifting more from the west. Highs increase back into the low 90s across the Lower Columbia Basin, Gorge, Blue Mountain foothills, and the Yakima Valley, with highs in the low to mid-80s through Central Oregon, John Day Basin, and the Kittitas Valley. Northwest flow aloft returns Thursday to cool temperatures about 5 degrees as additional cloud cover and rain chances return Friday to bring high temperatures further downward and closer to normal values early in the weekend.

ECMWF and GFS AI ensembles both showcase an incoming upper level trough Friday that may bring mountain showers to the Cascade and Blue Mountains, but differences are visible on how efficient the trough will be in breaking down the upper level ridge that develops earlier in the week. The ECMWF product suggests a stronger ridge that shears the incoming trough more north than its GFS counterpart. This is apparent within the LREF cluster ensembles as 35% of members (48% EMCWF ensemble members) keep the ridge stronger which confines any precipitation to along the Cascade crest on Friday. The remaining 65% of members do suggest showers extending into the Blue Mountains, Central Oregon, John Day Basin, and Wallowa county, but the progression from cluster 1 (35% of members) to cluster 4 (12% of members) is wetter which suggests more members hinting at a drier solution. This pattern may also lead to the development of isolated afternoon thunderstorms Friday as the trough breaks down the ridge, but this will rely heavily on these features' strength and progression late in the week. Stay tuned. 75

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

VFR conditions currently across all sites, which will stay the course through the period. Skies will clear through the morning as winds stay light and below 10 kts. The only exception will be KRDM/KBDN, which may experience breezy winds between 15-20kts and a slight chance (10-15%) of isolated thunderstorms developing through the afternoon and early evening. 75

Preliminary Point Temps/PoPs

PDT 83 52 83 51 / 0 0 0 0 ALW 83 56 83 53 / 0 0 0 0 PSC 88 52 89 53 / 0 0 0 0 YKM 86 55 88 55 / 0 0 0 0 HRI 86 51 87 53 / 0 0 0 0 ELN 82 51 83 51 / 0 0 0 10 RDM 81 43 80 43 / 0 0 0 0 LGD 79 48 78 49 / 0 0 0 0 GCD 80 45 78 47 / 20 20 0 0 DLS 88 57 84 56 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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