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Scattered showers and thunderstorms are expected through at least sunset across western and northern Wyoming. Heavy rainfall with localized flash flooding is possible in any thunderstorm, especially for west and southwest portions of the Bighorn Basin.

More shower and thunderstorm chances across western and northern through Saturday. The thunderstorm threat becomes more widespread across the state Sunday through the middle of next week, with a continued threat for flash flooding.

Hot temperatures are expected through Sunday with highs in the upper 80s to upper 90s. Some locations across the Bighorn Basin and Johnson and Natrona Counties could reach 100 degrees Friday through Sunday.

Update

As mentioned in the morning discussion, today will be a lot like yesterday with scattered showers and thunderstorms. However, unlike Wednesday/yesterday, most of the area has been sunny this morning. This morning sunshine has allowed the atmosphere to destabilize. This means that locations such as western Wyoming are expected to see more thunderstorm activity compared to the showery activity from yesterday. There is plenty of available moisture in the atmosphere, with the 18Z sounding from Riverton showing precipitable water amounts of 1.06", which is around 180% of normal. These high precipitable water amounts combined with slow storm motions of 10 to 15 knots will keep heavy rainfall with localized flash flooding as the primary thunderstorm threat today. The areas of greatest concern are the Absarokas, Cody Foothills, and Hot Springs County. These areas are particularly susceptible due to favorable easterly surface flow, thanks to a surface low over southern Wyoming, and 0.5" to 2.5" of rain in these areas Wednesday afternoon. Thus, the Flash Flood Watch remains in effect through midnight tonight for the aforementioned locations. Areas outside of the Flash Flood Watch still have the potential for heavy rainfall with localized flash flooding, so it is important to remain weather aware. A secondary threat from thunderstorms will be gusty 35 to 50 mph winds and small hail, with isolated gusts up to 60 mph not out of question due to DCAPE values around 1,200 J/kg. The threat for thunderstorms is greatest through sunset with the overall coverage decreasing through 3AM. Showers and thunderstorm chances exist across most of western and northern Wyoming, with Natrona County and eastern Fremont and Sweetwater Counties likely (90% or greater) remaining dry today.

Discussion

It is amazing how quickly things can change with the weather in Wyoming. Just a couple days ago we were talking about record heat and very dry conditions. Well, the weather has decided to do a Freaky Friday on us and flip into full monsoon mode. We had some reports of flooding today and the chance will be around through the next several days.

Water vapor imagery shows a large plume of monsoonal moisture continuing to stream northward from the Gulf of California and up through the Great Basin and into the northern Rockies. We continue to have some rain and some thunderstorms early this morning, especially in the south which just happened to light up when the previous shift left for the night. Luckily, the storms have some movement but there have been some good local rains, Big Piney got a quarter of an inch in only 20 minutes. The general trend is down through tonight and most places should be rain free by sunrise. However, things look similar to yesterday, with low level easterly upslope flow and abnormally high precipitable water levels, as high as 200 percent of normal. There is some steering flow but again, not a lot. So, it looks almost like a repeat performance of yesterday with slow moving showers and thunderstorms, although it could be a bit further east that yesterday. Instability parameters are decent as well with up to 1000 J/Kg of storm energy and lifted indices as low as minus 4. There is not a lot of upper level forcing though so most storms should largely be rain makers although you can't rule out small hail or a strong wind gust with any storm in Wyoming. With the local flooding we had yesterday and a similar setup to Wednesday, we have decided to issue a flood watch through midnight for the Absarokas, Cody Foothills as well as Hot Springs County given the favored low level easterly flow bringing upslope flow and slow movement of the storms. Like yesterday, the most numerous storms should end after sunset, but a few will linger after midnight. Temperatures should be a few degrees cooler than yesterday, but still above normal for this time of year.

I still feel there may be a bit less coverage on Friday and Saturday, especially in central Wyoming. It still looks like ridge may build a back to the west somewhat on these days. This should decrease available moisture and instability somewhat, especially in eastern portions of the area. However, it will be going back to 150 to 175 percent of normal versus 200 percent, so not a huge difference. The chance of rain reflect this fairly well again, with the highest in the western mountains on each day. And like the previous days, most should be daily but a few showers could linger after midnight and possibly until sunrise the next day. Temperatures will also warm a bit, especially East of the Divide, with some of the warmer spots getting close to the 100 degree mark once again.

Sunday still looks like the transition day when the monsoonal push will begin to move eastward, bring a better chance of showers and storms to areas East of the Divide and fewer storms across western Wyoming. Precipitable waters remain high all the way through much of next week as well, as high as 200 percent of climatological normals. There are still differences in the guidance in regards to which day will be the most active though, as is to be expected this far out. However, all areas should have a decent chance of getting some needed rain over the next several days. We just don't want too much at once. And unfortunately, that is a definite possibility as well.