FXUS65 KRIW 210433

AFDRIW



Area Forecast Discussion

National Weather Service Riverton WY

1033 PM MDT Sat Jun 20 2026



.KEY MESSAGES...



- Scattered rain showers and isolated thunderstorms continue

  tonight. An isolated wind gust to 50 mph remains possible

  before midnight.



- Shower and thunderstorm chances continue for Monday and

  Tuesday, mainly across northern Wyoming.



- Elevated fire weather conditions remain into next week, given

  the low humidities, breezy winds, and lack of rain chances.



&&



.UPDATE...

Issued at 1037 AM MDT Sat Jun 20 2026



Not seeing any notable changes regarding the storm potential for 

today compared to the discussion below. The latest HRRR runs in 

particular continues to have a widespread push of 50 to 70 mph 

associated with storms this afternoon, from the Wind River Basin 

eastwards to Natrona and southern Johnson Counties. The HRRR can 

sometimes overdo these types of events, so caution is taken. The 

timing of this is between 2pm and 7pm (moving west to east in that 

time). Other storms will also have the capability to bring isolated 

gusts of similar magnitude. The HRRR also brings another push of 

storms and strong winds to Lincoln County and the Green River Basin 

between 6pm and 9pm, so another area to watch. This is all far from 

certain, but gives an idea of potential wind hazards today.



&&



.DISCUSSION...

Issued at 233 AM MDT Sat Jun 20 2026



It is always rather interesting when coming back from a vacation to 

see what you walk into. There are some things to talk about this 

morning, as we look to have an active few days across western and 

central Wyoming.



Even at the early morning hour this morning, a subtle shortwave is 

moving into western Wyoming and bringing a few showers and even an 

isolated lightning strike or two. There have been a few wind gusts 

with this, but the maximum has been around 40 mph or so. A few 

showers will continue this morning, but the main event still looks 

to be this afternoon. Most guidance continues to favor the areas 

East of the Divide as well as western Wyoming, where the deeper 

moisture will be found, with a maximum chance of around 3 out of 5 

across the northern border as well as Johnson and Natrona Counties, 

with tapering chances further south as you move southward into the 

drier air. The highest chance will be during peak heating, with a 

gradual decrease through the evening. However, a few showers will 

linger through the night. As for the chance of any stronger storms, 

most of the area is under a Marginal Risk of severe thunderstorms. 

There will be decent forcing and dynamics as a shortwave will 

approach the area and there will be a but of upper level support 

from the left front quadrant of a jet streak moving into southern 

Wyoming this afternoon. The limiting factor, as it usually is in 

this part of the country, is moisture. We have several threats, we 

will tackle each one of them individually. 



Severe wind gusts...Not surprisingly, this is the main threat from 

the storms, as it usually is in western and central Wyoming. Model 

soundings show well defined inverted V soundings across much of the 

the area, with an emphasis on the southwestern half of the area. 

There are steep lapse rates as well, approaching 9 degrees celsius 

per kilometer in some locations. In addition, shower and 

thunderstorm bases look to remain fairly high, generally more than a 

mile above the surface. Dew point depressions are not the highest I 

have seen, but are still decent. The largest are in southwestern 

Wyoming, where some approach 60 degrees. Depressions drop to around 

40 degrees in central Wyoming and to around 30 degrees in northern 

Wyoming where the deepest moisture will be found. However, any 

shower or thunderstorm could have a strong wind gust, even any 

cumulus cloud that could collapse. The dreaded "Little Green Blobs" 

are definitely possible this afternoon, especially in the southern 

half of our area.



Hail...There is a very small area of a marginal risk of large hail, 

largely along an east of a Glenrock to Midwest into eastern Johnson 

County. This does make sense since moisture will be deepest in this 

area, along with the lowest cloud bases. This is also the location 

of the best instability, with CAPE values approaching 1000 J/Kg and 

lifted indices as low as minus 4. Johnson County looks to have the 

best chance, as most guidance shows some weak upslope flow into the 

Bighorns this afternoon, which should aid in convective initiation. 

The best chance of large hail remains across eastern Wyoming, but 

there is around a 1 in 10 chance of some in our area, mainly along 

and east of Interstate 25.



Flooding...The chance of this is low but not zero, and mainly in the 

northern half of the state. As for climatology, the predicted 

precipitable water value of 0.63 at 00Z this afternoon puts moisture 

in the top 20 percent of historical values for this value and around 

125 percent of the daily mean (0.50 inches). The most likely 

locations will be, again across Johnson County, where some areas 

have over an inch or precipitable water, as well as the Big Horn 

Basin. However, there will be some steering flow today, so most of 

the storms will have some movement, mitigating the threat somewhat. 

In addition, it has been dry, so the soil does have a decent amount 

of capacity. The Weather Prediction Center does have eastern Johnson 

County under a marginal risk of excessive rainfall, and this looks 

reasonable given the pattern.



Dry Lightning and Fire Weather...This will largely be a threat 

across the southern half of the area, where cloud bases will be 

higher. This will obviously bring a threat of some fire starts, 

given the continuing drought conditions across the area. Elevated to 

near critical fire weather is likely across southern Wyoming where 

winds gusting over 30 mph and humidity falling to around 15 percent 

may bring erratic fire behavior if any fires can start.



And one bonus I will throw in, tornadoes. The threat is very low, 

less than 2 percent across the entire area as the best

instability and shear will be across eastern Wyoming. Like many

of the other threats, the main area of concern is Johnson 

County, where the most shear will be found and deeper moisture 

will allow for lower cloud bases and allow for the possibility 

of any rotation to reach the ground. This threat is by far the 

lowest of all the threats though.



Looking further out, things look to stay active over the next couple 

of days. Another shortwave will move across the area Sunday with 

another chance of convection. Instability values are around 25 

percent less with this wave and there is much less jet support. So, 

the threat of strong to severe thunderstorms will likely be less. 

And like today, the main threat will be East of the Continental 

Divide. 



Monday has the potential to be a bit more interesting. Another 

shortwave will be crossing the area. The main difference here is 

that there will be more jet support, with an 100 knot jet streak 

moving along the Wyoming / Montana border. The limiting factor here 

is moisture, with precipitable water values falling to as low as 

0.20 inches across western Wyoming. If the dry air wins out, the 

main threat would be fire weather with a gusty wind and lower 

humidity with the jet close by. However, if moisture could linger, 

we could have a situation with storms firing along a pseudo dry 

line. There is still a lot of uncertainty but we will have to watch 

this.



More settled conditions should move in for Tuesday as a building 

ridge brings upper level convergence. Uncertainty increases from 

midweek on as some guidance has ridging over the area and only 

isolated convection while others have the area near the top of the 

ridge with shortwaves topping the ridge and bringing a few chances 

of convection. Temperatures look to remain close to normal, with the 

biggest temperature anomalies across southern Wyoming where drier 

air and less cloud cover should allow for warmer temperatures.



&&



.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...

Issued at 1026 PM MDT Sat Jun 20 2026



Most areas continue to see decreasing activity as the main area

of thunderstorms moves east. Another smaller wave of light

showers is moving back into KJAC currently, which will bring

light impacts mainly through 09Z. Sites west of the Divide will

then see VFR conditions on Sunday with only varying high cirrus

expected, with no showers currently expected. East of the

Divide, another round of showers is possible late morning into

early afternoon. This will focus on KCOD/KWRL in the morning and

KLND during the afternoon. Main question is how much activity

will approach KCPR during the afternoon before the system shifts

east. Have kept only VC precip for KCPR for now, watching for

newer model output.



&&



.RIW WATCHES/WARNINGS/ADVISORIES...

None.



&&



$$



UPDATE...Wittmann

DISCUSSION...Hattings

AVIATION...Straub