Area Forecast Discussion · National Weather Service Riverton (RIW) · issued Sunday, August 23 at 12:00 PM MDT.
Revised in 4 passages since the 10:57 AM discussion. See every revision.
Messages
Monsoonal moisture continues this afternoon and evening but not as widely scattered as Saturday with localized flooding possible in the strongest storms.
Strong to severe storms possible (30-40%) looking more like the evening time frame after 6PM with a line of storms from Idaho capable of producing damaging winds up to 60 mph.
Monsoonal moisture will linger much of the week, with a chance of showers and thunderstorms each day. Temperatures will be closer to seasonable for this time of year nearing the end of August.
Update
Much more ample sunshine across the forecast area for the noon update as compared to yesterday. As such, temperatures will be a couple degree warmer this afternoon as Saturday afternoon as well. With better mixing, there will be a stronger breeze to go along with it as well gusting up to 25-30 mph at times outside of any storm activity and outflow winds. Nothing on radar currently but HiRes data is still pinpointing development by 2-3PM off leeside of the Absarokas and pushing in from the south off the Uintas for Sweetwater County. Much of this activity will be isolated in nature compared to yesterday's showers but expect lightning and a brief outflow over 40 mph with its eastward progression through the evening. With this, chance of rain have been adjusted and especially after the CRON wiped out the night shift's work to account for this heading into the evening hours.
This is where it will get a bit more interesting at least for Wyoming standards. A line of storm activity looks to form from southern Montana and across eastern Idaho by 6-7PM before pushing east into western Wyoming shortly there after along and ahead of the cold front. This will track through the forecast area by Monday morning. SPC continues with the Marginal Severe Risk, with an introduced Slight to the southwest portions of the state to include hatched wind. This looks to be overdone, even though damaging winds are possible up to 60 mph from the outflows ahead of the front. The instability with DCAPE values less than 500 J/kg just don't look to impressive as it nears radiational cooling towards sunset. There might be an hour or so before darkness, but the frontal passage will be later further south for the forecast area with its positive tilt and progressive eastward push. Stronger winds than that don't look great in the least but something to keep an eye on between 6-10PM west of the Divide. Beyond that as it makes its way to the east after midnight, the juice of the storms will be lost with mainly shower activity in its wake. The cold front will push the warmer moist air south and southeast.
Beyond that, status quo, as the monsoonal pattern continues but to a bit lesser extent. Temperatures behind this aforementioned cold front will bring afternoon highs closer to seasonable for this time of year ending the month of August with 80s east of the Divide even into next weekend.
Discussion
It was a typical monsoon day yesterday, with some areas, mainly in southwest and in Johnson and Natrona Counties getting a LOT of rain, causing some local flooding. Other areas saw nothing. And we continue to have some showers and embedded thunder around late tonight, although intensity is significantly less. And these will continue through the night, gradually becoming weaker and less numerous.
We will have another round of showers and storms today. There are some differences. For one, precipitable water values are less today by about 20 percent in most locations. This should bring less coverage and somewhat lesser rainfall rates. The other difference is that there will be more steering flow today, generally about 10 to 15 knots at 700 millibars and 25 to 35 knots at 500 millibars. So storms should have more movement. This should decrease the chance of flooding somewhat. We still have an excessive rainfall outlook across most ares West of the Divide though, so the threat is still there. There is also the risk of strong to severe thunderstorms. Most of the area is under a marginal risk of severe weather today. There could be some small hail, but this not the main risk. The main risk looks to be strong wind gusts. Roughly north and west of a Buffalo to Riverton to Lyman line has a Conditional 1 outlook for wind, or possible convective wind gusts to 75 mph. Soundings do show some inverted V profiles, especially across northwestern Wyoming where the surface will be drier, with low and mid level lapse rates close to 9 C/Km. This could lead to some strong downburst winds later today. Again, the best chance would be across the northwest but most areas will have a chance. And, like previous nights, most convection should end after sunset but a few showers and storms will likely linger through the night. As for temperatures, they should be a few degrees warmer than yesterday with more sunshine expected.
The rest of the week looks like more of the same as the annoying monsoonal moisture will hang around the area through at least Thursday and possibly into Friday as well. The models continue to have different ideas on which day will be more active with no real well defined shortwaves with changes from day to day. Precipitable water generally average about 125 to 150 percent of normal through the period though. So, at this point we went with a generic solution of mainly daily showers and storms each afternoon. Temperatures should remain fairly similar through the period, not very hot but averaging around 4 to 8 degrees above normal on average, with the possibility of a cooler day if there is more cloud cover than expected. Drier air may then work in for next weekend and bring a return to drier weather but uncertainty remains high on the exact timing. So, all in all, it continues to look like more of the same as the summer of the monsoon lives on in western and central Wyoming.
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