Over the next couple of days, the Columbia Basin is forecast to see highs in the 100s, prompting the National Weather Service to issue a heat advisory for the region. It is in effect from 12:00 Monday to 23:00 Tuesday. This heatwave is much smaller and shorter in comparison to the one at the end of July, but temperatures will still be hot enough to warrant taking precautions when outside during the hottest time of the day on these days.
Tuesday evening and into the overnight hours, there is a chance of thunderstorms for the Columbia Basin. This comes as part of the weather pattern that will bring triple digit highs to an end for the week.
The primary driver for Tuesday’s thunderstorm activity is an upper level low that has been sitting off the coast of Northern California for the last week or so. It finally starts moving northward and closer to the coast pulling monsoon moisture from Southern California and Nevada up into the Inland Northwest, reaching the Lower Columbia Basin by the afternoon.
The ridge of high pressure to the east is likely to help limit thunderstorm activity in lowland areas by keeping temperatures aloft fairly warm. Because of this, most of the storms will be focused on mountainous regions of Oregon including around Bend and John Day as the terrain helps to lift moist air from the surface up to generate the storms.
Many of these are likely to weaken significantly as they move north out of the Blue Mountains into the flatter and lower elevation areas of the Columbia Basin. Under patterns like this, Pendleton, Walla Walla, and to a lesser extent Hermiston are more likely to benefit from rainfall but it is possible that a storm makes it further north to reach the Tri-Cities. The likelihood of thunderstorms for the Tri-Cities sits at 20% with a 30% chance for Walla Walla, Hermiston, and Pendleton.
Abundant moisture should allow for plenty of rainfall in and south of the Blue Mountains, limiting dry thunderstorm chances. Still, lightning caused fires are possible and any outflow boundaries generated may create chaotic behavior in existing fires. As the storms pass northward into the Columbia Basin, hotter temperatures at the surface may lead them to transition to become dry thunderstorms. Severe thunderstorms, which are defined as having 58+ mph winds and/or 1″+ diameter hail, are not expected.
As the low skirts the Washington coast on Wednesday, drier air closer to the center will be brought over the Columbia Basin so that thunderstorms are not likely near the Tri-Cities. The monsoon moisture will be located further east, more over Idaho and Utah, but a stray storm or two is possible close to the Blue Mountains.
Temperatures cool somewhat during this period, reaching into the mid 90s Thursday and Friday and possibly closer to average (low 90s) for the weekend. You can see current radar and lightning data by heading over to my Current Radar page.
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