Walla Walla, WA Weather

Discussion

Water vapor imagery shows a large closed low over central California, and a weak mid-level circulation wrapping north- northwest around the parent low is evident over southeast Oregon. Meanwhile, a shortwave trough is approaching the Pacific Northwest from the northeast Pacific Ocean, and is forecast to move onshore late this afternoon.

Widespread showers have developed across central to eastern Oregon and south-central to southeast Washington, and isolated embedded lightning has been observed over the Blue Mountains and their adjacent foothills. Modest instability is present (100 J/kg or less MUCAPE per SPC's RAP-based mesoanalysis), and 12Z HREF mean forecast values remain below 500 J/kg through the day (highest for the northeast Blue Mountains). Aside from the isolated lightning within the ongoing band of showers, the best chance for notable convection will be this afternoon and evening across the Blue Mountains region of Grant, Union, and Wallowa counties; convection developing across southeast Oregon and southwest Idaho will track north to northeast and the portion in Oregon may (50% confidence) clip the aforementioned counties. Main hazards associated with this activity will be lightning and gusty outflow winds.

Looking ahead to late this afternoon into the evening hours, breezy to windy westerly to northwesterly winds are forecast to ramp through the Cascade gaps and across wind-prone portions of north-central Oregon as pressure gradients increase in strength (8-11 mb difference between PDX and GEG in latest NWP guidance) and a modest low-level jet (25-40 kts) develops. Confidence is medium-high (60-90%) in reaching advisory-level wind gusts (45 mph or greater) through the Kittitas Valley, eastern Columbia River Gorge, and wind-prone portions of north-central Oregon; Wind Advisories are in effect from 7 PM this evening until 11 AM Friday morning.

Precipitation is forecast to taper off through Friday morning from west to east as the shortwave trough moves overhead, and a very dry air mass (PWAT of 0.15-0.40") replaces the moist air mass currently in place. This will set the stage for near- to sub-freezing temperatures Friday night into Saturday morning. While most public forecast zones have observed a season-ending freeze, a widespread freeze has not yet occurred in the lowest elevations of our forecast area. Given the anticipated dry air mass, predominantly light winds, and mostly clear skies, confidence is medium-high (50-95%) that a season-ending freeze will be observed in the lower Columbia Basin, eastern Columbia River Gorge, Yakima Valley, and the foothills of the northern Blue Mountains, so have issued Freeze Watches.

Uncertainty in the 500-hPa height field burgeons Sunday through early next week due to continued ensemble spread in solutions regarding the development and evolution of a closed low in the Gulf of Alaska region. Roughly 50-60% of ensemble members depict a compact closed low moving inland Monday through mid- week, while another 20-30% of members show a weaker low, and the remaining members advertise an upper-level ridge over the Pacific Northwest. The scenario that shows a stronger closed low would result in shower chances and cooler temperatures, while the weaker low would warmer and drier, and the upper-level ridge would be warm and completely dry.

Aviation

Varied conditions across all sites this evening, with VFR at KDLS/KRDM/KBDN/KYKM and MVFR at KPDT/KALW/KPSC due to light rain reducing visibilities of 4 to 5 SM. Rain will continue to depart east overnight, improving visibilities to above VFR criteria. Breezy conditions with gusts of 20-25kts expected for most sites through the morning, with lower winds of around 10kts over KPDT/KALW. Clouds clear through the afternoon. 75

Pdt Watches

WA...Wind Advisory until 11 AM PDT Friday for WAZ024-026. Freeze Watch from Friday evening through Saturday morning for WAZ024-027>029. OR...Wind Advisory until 11 AM PDT Friday for ORZ041-510. Freeze Watch from Friday evening through Saturday morning for ORZ041-044-507.