textproduct: Pendleton

KEY MESSAGES

- Isolated to scattered showers in the Cascade and Blue mountains this afternoon

- Isolated thunderstorms this afternoon in the Blue Mountains

- Temperatures gradually warming to above normal through the remainder of the week

DISCUSSION

Water vapor imagery shows broad troughing over the western CONUS this afternoon. A shortwave embedded within northwesterly flow aloft is evident just offshore of the Pacific Northwest, now approaching the OR/CA border. Similar to yesterday, predominantly clear to partly skies are present across the lower elevations, while the mountains have seen numerous cumulus develop. Scattered showers are evident within the cumulus field, mainly over the Blue Mountains. Anticipating convection will remain mostly shallow today given the lack of vertical extent of CAPE profiles in forecast soundings (EL around 12-18 kft) from HREF and REFS NWP guidance. However, modest CAPE (up to 500 J/kg per forecast soundings from the 12Z HREF/REFS) coupled with steep low-level lapse rates and the cool air mass aloft have enabled isolated lightning. For the remainder of the afternoon and evening the best chances for lightning (10-20 percent) will remain over the Blue Mountains, while other areas have very low chances (10 percent or less).

Looking ahead, northwesterly to northerly flow aloft will continue for Tuesday and Wednesday, albeit with rising 500-hPa heights. NWP guidance advertises yet more weak shortwaves and weak diurnally driven instability (CAPE of 500 J/kg or less per latest CAMs) which will facilitate low (5-15 percent) chances of showers, mainly over the Blue Mountains. Temperatures will warm several degrees each day through Wednesday as a result of the aforementioned rising heights.

Thursday through Friday, ensemble cluster analysis shows above- normal 500-hPa heights over the Pacific Northwest and vicinity. Afternoon high temperatures are forecast to rise to generally around 10 degrees above normal. It's still worthy noting all ensemble clusters show hints of a trough or closed low moving over southern Canada or the Northern Tier sometime between late Thursday and late Friday, but most (75 percent) members keep dry conditions region-wide through the period.

By next weekend, uncertainty in the 500-hPa height field burgeons as ensemble guidance struggles to resolve the evolution of an upper-level closed low and upper-level ridge in the vicinity of the Pacific Northwest. Based on the 00Z ensemble runs, over half of members keep the region warm 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, cooler temperatures, potential for thunderstorms.

By early next week, ensemble guidance shows very good agreement (about 80 percent of members) in a Rex block pattern, which would induce well-above-normal temperatures. Chances of afternoon highs exceeding 80 degrees across all major population centers are high (60-90 percent) per latest NBM calibrated probabilities, and chances of reaching 90 degrees are low-medium (10-40 percent) for the Yakima Valley and lower Columbia Basin. Moreover, there is a low-medium (20-50 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 WEDNESDAY/

VFR conditions are expected through the TAF period. Afternoon winds have become gusty in the 15 to 20 kt range at PDT, BDN and RDM and 20 to 25 kt range at DLS. Expect these winds to continue into the early evening before decreasing to 10 kts or less. Everywhere else winds should remain 10 kts or less through the night.

Winds should increase again on Tuesday and gust to around 20 kts at all sites except YKM and PSC, and gusts can't be ruled out at those locations either, just don't have enough confidence to include at this time.

Preliminary Point Temps/PoPs

PDT 41 66 40 69 / 10 0 0 0 ALW 45 67 44 70 / 10 0 0 0 PSC 43 72 43 74 / 0 0 0 0 YKM 40 70 41 73 / 10 0 0 0 HRI 43 69 41 73 / 0 0 0 0 ELN 40 62 38 69 / 10 0 0 0 RDM 29 64 30 67 / 0 0 0 0 LGD 35 64 40 67 / 10 10 0 10 GCD 32 64 38 68 / 10 10 0 0 DLS 44 66 42 74 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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