textproduct: Pendleton

KEY MESSAGES

- Cooler today.

- Above-normal temperatures this week, peaking Tuesday and Wednesday.

- Elevated fire weather concerns due to dry lightning through midweek.

DISCUSSION

Current radar and infrared satellite imagery shows light returns across Grant County extending northeast through the northern Blue Mountains as high level clouds pass south to north. This is in response to the strong upper level ridge over the Four Corners region moving northeast into Wyoming and the Dakotas, moving moisture north originating from a remnant trough over the Baja Peninsula. The clouds will shift east and break up through the day today as the upper level low near Vancouver Island advects a cooler airmass onshore strengthen by surface high pressure. Cooler temperatures are expected as a result, with afternoon temperatures around 5 degrees cooler than Saturday across the Columbia Basin, Blue Mountain foothills, and John Day Basin. Highs will hover in the low to mid-80s across these area, which is about 6 degrees below normal.

These cooler temperatures will be short-lived as the upper level ridge elongates off the southern California coast from the Dakotas and another upper level low pressure drops along the Washington/Oregon coasts. These synoptic features greatly increase the efficiency of southwest flow aloft and funnels sub-tropical heat into the Pacific Northwest. Highs will increase about 10 degrees on Monday, returning afternoon temperatures back into the low to mid-90s across the area. The offshore low continues to dig south along the Central California coast Tuesday and Wednesday to further increase high temperatures into the mid-to upper 90s. Guidance does suggest some areas may reach into the triple digits both days, as the NBM advertises a 20-45% chance on Tuesday and a 30-55% chance Wednesday for Walla Walla, Tri-Cities, Hermiston, Milton- Freewater and Yakima, with Walla Walla having the best chances (43% and 52%). Ensembles are hinting at higher temperatures for subsequent forecasts as 61% of members suggest higher temperatures than currently forecast on Tuesday, and 58% of members on Wednesday. This quick warmup, coupled with high temperatures around 100 degrees, will allow for widespread moderate HeatRisk (2 or 4) across the Columbia Gorge, Lower Columbia Basin, Blue Mountain foothills, North-Central Oregon and the Yakima/Kittitas valleys. Brief relief likely Thursday and Friday as ECMWF AI ensembles and GFS AI ensembles both show the CONUS high pressure shifting east as the offshore low weakens and moves inland, cutting off the enhanced southwest flow aloft.

The presence of the inland high and offshore low pressure systems sets up an extended 'ridge breakdown' weather pattern that will enhance dry thunderstorm potential during the afternoon hours. This potential peaks Tuesday and Wednesday as the offshore low digs south and taps into monsoonal moisture. ECMWF and GFS ensembles showcase precipitable water amounts of 0.85-1.1" which is 150-180% above normal. However, the NAM advertises marginal dewpoint depression at the 850mb level, and there is overall concensus between the NAM, GFS, and ECMWF with a vertical temperature difference between 850-500 mb of 30-35C. This large temperature difference above 30C will promote the dry lightning risk as cooler air aloft will product clouds and warmer/drier air closer to the surface will evaporate any moisture before reaching the ground surface. This will also lead to exceptional instability due to the less dense air mass at the surface being able to rise rapidly, leading to thunderstorm potential. 75

AVIATION /18Z TAFS THROUGH 18Z MONDAY/

VFR conditions are forecast at all sites for the next 24 hours, though there is low (30% or less) confidence in smoke from area wildfires affecting VSBYs at BDN/RDM.

Winds will be mostly 10 kts or less, except for periodic afternoon/evening gusts, and at DLS where breezy gap winds are forecast.

A band of mid/high cloud over PDT/ALW/PSC is anticipated to thin through the day, but some high cloud is expected (80% confidence) to persist through the period.

FIRE WEATHER

Dry conditions will still be present today with afternoon humidities of 20-25% after morning recoveries of 50-70%. These values should not waver too much through the week, as winds stay relatively light.

The primary fire weather concern this week involve the potential for dry lightning, primarily Tuesday and Wednesday across WA692, OR692-700, and OR705. As mentioned earlier with the presence of the high pressure to the southeast, lower levels of the atmosphere will be dry as cooler, more moist air slides over this layer. This will lead to moisture evaporating through the lower, dry layer as substantial instability is present. This instability will also lead to the potential for plume- dominated fires for any that are already on the landscape. There is also potential for erratic downdrafts and microbursts as the evaporative column of virga will still reach the ground surface as cooler air, accelerating toward the ground as it nears and extending outward when reached. 75

Preliminary Point Temps/PoPs

PDT 82 56 91 61 / 0 0 0 0 ALW 84 60 93 65 / 0 0 0 0 PSC 86 57 94 62 / 0 0 0 0 YKM 87 56 94 61 / 0 0 0 0 HRI 85 58 93 64 / 0 0 0 0 ELN 81 53 91 58 / 0 0 0 0 RDM 87 52 92 55 / 0 0 0 0 LGD 88 58 93 63 / 0 0 0 0 GCD 90 57 93 60 / 0 0 0 0 DLS 87 58 93 62 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

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