The heat wave that is gripping the Columbia Basin will draw to a close over the next day or so. The upper-level ridge of high pressure influencing the pattern over the Inland Northwest will start moving to the east, allowing a trough located offshore to migrate closer to the Pacific Northwest. With the approach of the trough, marine air will start making its way through gaps in the Cascades and into the basin.
High temperatures will decline from around 100° on Tuesday afternoon to around 90° starting Thursday and lasting into the weekend. Overnight lows are also expected to decline, with morning temperatures expected to dip into the 60s again. These highs are much more typical for the Tri-Cities area for this time of year (average is 89°), but lows will remain above climatological norms (average is 55°).
The combination of the ridge moving east and the influence of cooler marine air from the west will also bring an end to isolated thunderstorms that have meandered around the region the past couple days. The moisture for these storms is monsoonal and was brought to the area within the clockwise circulation of a high pressure area under the ridge to our east, but storm development has been helped further by shortwave systems (small upper level troughs) that have been embedded in the mean upper level flow. The mid-level moisture can be seen in the water vapor image as the gray, white, and blue area stretching from the Bay Area north toward Central Oregon.
The movement of the ridge system away from the Tri-Cities will shift the monsoon moisture stream east. At the same time, the cooler marine air is much more stable, making it so there is less vertical motion in the atmosphere to move moist air from near the surface up to a point where the water will condense to make clouds and eventually thunderstorms. The general flow from west to east will bring intermittent rain to Western Washington and Western Oregon, but the Cascades will wring out that moisture before the air makes it to the Columbia Basin. Because of this, sunny to partly cloudy skies can be expected from Wednesday through the end of the week.
It looks as though the Columbia Basin will experience a dry cold front on Friday. This will help keep temperatures down near average and is likely to bring gusty winds to the area. Warm temperatures, low relative humidity, and the wind from this front will combine to raise fire danger. As always, areas of blowing dust and tumbleweeds can also be expected.
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