textproduct: Western and Central Wyoming

KEY MESSAGES

- A weather system approaches today and brings another chance of thunderstorms, some could be strong to severe. The main threat from storms would be strong wind gusts and hail.

- Elevated fire weather remains likely east of a Rock Springs to Worland to Kaycee line through the weekend.

- Cooler weather moves in this weekend, especially West of the Divide. Snow is possible in the mountains tonight into Monday, with the potential for frost in the western valleys, especially on Monday night.

DISCUSSION

Issued at 227 AM MDT Sat Jun 27 2026

Right now, it relatively "Q-word" weather wise around western and central Wyoming and the humble Riverton abode. If you are wondering what the "Q-Word" is, just ask any people who are police officers, firefighters, paramedics or nurses. My mother, who is a retired night shift nurse, told me to never utter that word while at work, as if not to jinx yourself. But, as the pattern has been over the past several days, it will not last.

We continue to have multiple impacts to the weather today and tomorrow. And we will address them in this order: strong to severe thunderstorms, fire weather and much colder weather and the possibility of the "s" word, and by that I mean snow. The culprit of this will be another shortwave moving around a parent upper level low now moving onshore in the Pacific Northwest and progressing south and east toward the area before turning northeastward into Idaho and western Montana. This will bring somewhat cooler temperatures today, and the tightening pressure gradient and the nearby jet stream will bring breezy to windy conditions, especially across the southern two thirds of the area.

Strong to Severe Thunderstorms...For the seventh of the past eight days, we are under some kind of a convective outlook. In this case, a marginal risk that covers just about the entirety of western and central Wyoming. Instability is not as high as the best few days, with the highest in far western Wyoming where CAPE values rise as high as 1000 J/Kg, with lessening values further east as you head into drier air. One factor for severe thunderstorms will be a potent 100 knot jet that will be passing across southern Wyoming, bringing some upper level divergence and the approaching upper level low will bring some lift as well.

As for the threats, for tornadoes, we can't rule one out, but the chance is very small. The next threat, strong wind, looks to be most widespread threat. The main area of concern for this is East of the Divide and southern Wyoming, where model Skew T soundings have inverted V signatures and drier air at the surface should allow for somewhat higher cloud bases. Dew point depressions generally range from 35 to 45 degrees. not the biggest range. However, the strong jet energy should be able to mix winds down from aloft. In addition, wind the fairly strong mid level wind, steering flow will be fairly high and forward speed of the storms will aid in the wind. And there is still the risk of dry microbursts. We do have a conditional intensity group 1 for thunderstorm wind (gusts to 75 mph) from the eastern Green River Basin through most of the Big Horn Basin and into Johnson County. As for large hail, the threat is somewhat less. Much of the area, except for far southeastern parts of the area, have a marginal risk of this though. The atmosphere East of the Divide remains drier than earlier in the week, so the risk is lower. The highest risk of hail would be across the western mountains where deeper moisture would be found. Shear is somewhat limited though. However, with low wet bulb zero levels, small hail is a greater possibility here. Any storm could have some hail, but this is the lesser threat. As for heavy rain. precipitable water levels are not that high for most locations. The best chance would be in the western mountains and in northern Wyoming where precipitable water levels would be higher but nothing extreme. Most convection should end this evening as the shortwave lifts northward, but a few will linger through the night, especially in western Wyoming.

Fire Weather...This will mainly be a concern for southern and eastern Wyoming. Relative humidity will be a but higher and temperatures a bit cooler than yesterday. However, elevated fire weather is still possible in the afternoon across portions of Sweetwater, Fremont, Hot Springs, Washakie, Natrona and southern Johnson Counties where humidity will fall under 25 percent and wind gusts over 25 mph will be common this afternoon. The area of most concern is Natrona County, where humidity will fall into the teens. It doesn't quite look like Red Flag Conditions but an hour or two of critical fire weather has about a 1 in 2 chance this afternoon. Continued elevated fire weather is possible again on Sunday afternoon although cooler temperatures may mitigate the threat somewhat.

Colder Temperatures and Snow...Temperatures will be cooler across the entire area, with an emphasis on western Wyoming where Jackson will likely be close to 20 degrees colder than Friday afternoon. The coolest day across the area looks to be Sunday, as more cloud cover lingers across the area and cooler air moves in behind a cold front. Now, the main thing people are wondering about, the chance of snow. Chances of this really begin tonight as cooler air moves in tonight and drops 700 millibar temperatures down to minus 3, which could lower snow levels to 7000 to 8000 feet. Any snow at this time would be light though. Occasional light snow continues in the mountains through Sunday and possibly into Monday. The heaviest may be Monday as the low lifts north and a trough axis swings across the area. Through Monday, the NBM ensemble gives a greater than 2 out of 3 chance of an inch or more of snow above 9000 feet. A few locations, like southwestern Yellowstone Park, the highest elevations of the Tetons and Wind Rivers above 10000 feet have a greater than 1 in 3 chance of 4 inches or more. It shouldn't be a problem for travel but people venturing into the mountains should prepare for cool and wet weather, this includes Yellowstone Park. If it gets a bit colder, I can't rule out some snowflakes into even into the Jackson Valley, although no accumulation is expected. And there is one more thing, the potential for frost. It will turn much cooler tonight in the west, but cloud cover should keep most temperatures above freezing. Things get a little cooler again Sunday, but again cloud cover may keep things warmer. The real day on concern may be Monday night, when the low lifts out and the sky clears. Ensemble guidance gives a greater than 1 in 2 chance of low temperatures 32 or lower in places like the Green River Basin, southern Lincoln County and even portions of the Jackson and Star Valleys. We could see some record lows on these days. It is too early for freeze highlights but we will have to watch this.

Otherwise, cool weather will last through Monday and possibly Tuesday. With decreasing instability on Sunday, the chance of thunderstorms will be much less. Cyclonic curvature remains over the area on Monday as well, keeping temperatures cool with most precipitation chances across the north and west with drier conditions further south. Going into next week as we head into July, we will see ridging slowly build into the area with temperatures gradually warming to near to above normal temperatures, with the warmest temperatures in eastern portions of the forecast area. Wyoming will be stuck between a trough over the western United States and strong ridging over the East. That means unsettled weather is likely to continue with shortwaves topping the ridge from time to time, chances of convection and breezy to occasionally windy conditions. There is a lot of spread in guidance in timing of the waves, so confidence in timing the periods of showers and storms remains low at this time.

AVIATION /18Z TAFS THROUGH 18Z SUNDAY/

Issued at 1049 AM MDT Sat Jun 27 2026

An approaching disturbance will bring chances for showers and thunderstorms through much of the day. Showers and thunderstorms have already begun developing nearby western terminals such as KJAC, KPNA, KBPI, and KRKS. Other terminals such as KLND, KRIW, KWRL, and KCOD will see showers and thunderstorms shortly after the start of the period around 20Z Saturday. KCPR has a less than 20% chance of seeing a shower or storms so no precipitation groups have been added. The main concern with any nearby showers or storms will be strong gusty outflow winds in excess of 40 knots, frequent lightning, small hail, and heavy rain. Brief periods of MVFR to IFR conditions cannot be ruled out if any storms move overhead. Aside from convection winds will become breezy around the start of the period. Winds look to prevail from the west/southwest at 15 to 20 knots with occasional gusts of 25 to 30 knots. Winds should decrease shortly after 00Z with a light breeze expected through the overnight into Sunday. Showers and thunderstorms look to start dissipating after 00Z Sunday with a few lingering east of the Divide until around 03Z Sunday. A few nearby showers cannot be ruled out around KRKS Sunday morning. Winds increase during the late morning hours Sunday with most terminals seeing west/southwest winds around 10 to 20 knots and gusts of 25 to 30 knots by the end of the period. Mountains will likely be obscured throughout much of the TAF period with some improvement possible by the morning hours Sunday.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts.

FIRE WEATHER

Issued at 227 AM MDT Sat Jun 27 2026

Elevated fire weather remains likely roughly along and east of a Rock Springs to Riverton to Worland to Kaycee line. Relative humidity will fall below 25 percent with the lowest humidity in Natrona County where it will fall into the teens. A gusty wind will develop this afternoon with wind gusts to 30 and in some locations over 40 mph this afternoon. Lightning strikes from thunderstorms are also possible. Elevated fire weather may continue tomorrow as gusty wind and low humidity continues although cooler temperatures may mitigate conditions somewhat.

RIW WATCHES/WARNINGS/ADVISORIES

None.