textproduct: Grand Junction

KEY MESSAGES

- Widespread rain continues through early Friday.

- Strong to severe thunderstorms will be possible favoring southeastern Utah and far Western Colorado generally from I-70 south through tonight. - The potential for flooding increases today and Friday, especially for portions of southeast Utah and southwest Colorado.

- Rainfall amounts in the southern mountains range from 1-3 inches with higher amounts of 2-4" or more possible in the San Juans and southern foothills.

DISCUSSION

Issued at 1038 PM MDT Wed Oct 21 2026

A deep trough off the West Coast and High pressure to the southeast near Big Bend is keeping the Desert Southwest under a strong south to southwesterly flow aloft advecting deep subtropical moisture north through Arizona into eastern Utah and Western Colorado. Models are in good agreement with satellite imagery showing the low just turning the corner in the base of the trough off the SoCal Coast before ejecting to the northeast into the Great Basin Thursday evening and continuing into Montana Friday evening taking most of the moisture with it. Meanwhile, another shortwave system descending into the trough Friday cuts off over SoCal Friday night to eject farther east over the region Saturday night into Sunday to keep unsettled weather in the forecast through the weekend. Models stay in fairly good agreement through the weekend, but are diverging on the timing and track of the second system by Sunday with some solutions tracking it to the west over northern Utah. The models do generally agree on setting up a dry zonal flow across the region next week that could extend into the next weekend, but that a long way out and confidence is lot on the extended period.

Today we saw quieter showers with rain rates remaining on the low side only around a half inch per hour in the heavier showers. Even though we had strong vertical sheer and plenty moisture on hand, with all the heavy cloud cover and the warm air aloft, we just didn't have any CAPE to feed stronger storms. Both the 18Z and 00Z soundings showed a saturated atmosphere nearly up to 200mb for efficient rain making, but with zero CAPE there wasn't the strong convection. That being said, much of the southern half of the region still received an inch or more rain today saturating the soil with water and priming the area for flash flooding should heavier showers come overnight into tomorrow.

These conditions look to change overnight into tomorrow as we come more under the influence of the trough to the west cooling the air aloft to provide some CAPE to support convection. Additionally, upper- level dynamics increase as a 70-90 kt jet moves in over the region. The primary concern with convection will be strong winds over 60 mph possible as the winds aloft mix down. The atmosphere remains saturated through the day, making localized heavy rain in showers and thunderstorms much more likely leading to flash flooding, especially near recent fire scars and areas prone to flooding like slot canyons and steep terrain. Stay tuned to the forecast for updates as weather develops through the day. Drier air starts moving into the region late tomorrow which looks to scatter out the showers and thunderstorms, but also looks to increase the potential for stronger thunderstorms overnight into Friday. Most of the moisture is swept out of the region Friday limiting convective activity through the afternoon, but has the second low starts moving in from the west, showers and thunderstorms will pick up again Friday night into Saturday feeding on the residual moisture and continue through Saturday night and Sunday as the low tracks over the region. The storms move on off to the east as a zonal pattern sets up across the region Sunday night through mid next week.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 525 AM MDT Thu Oct 22 2026

Widespread rain and rainshowers continue today with some thunderstorms possible from KMTJ southward this afternoon. Low ceilings will be common as will visibilities under the stronger cells. Conditions will fluctuate between VFR down to IFR through the day with the worst conditions expected south of the I-70 corridor. ILS breakpoints will be met for most TAF sites thanks to the same low ceilings and visibilities. Generally light winds are expected though gusty outflows can be expected near any showers or storms. Flight conditions will slowly improve after 12Z tomorrow, if not a bit before.

HYDROLOGY

Issued at 257 PM MDT Wed Oct 21 2026

Hydrologic concerns remain elevated through Friday due to the influx of highly anomalous tropical moisture across the 4 Corners area into the Central Rockies. Potential hazards include localized flash flooding from thunderstorms producing heavy rainfall rates and river flooding due to a prolonged period of widespread moderate to heavy rainfall.

Southeast Utah and the lower valleys along the CO/NM border will be under the highest threat of strong to severe thunderstorms capable of producing flash flooding across desert floors and dry washes the through tomorrow evening.

There is a moderate risk of excessive rainfall leading to flooding across portions of the San Juans and adjacent southerly valleys where precipitation totals of 2 to 4 inches will be possible. The bulk of this precipitation will fall overnight through early Friday morning. Higher precipitation totals over portions of the southern San Juans of 4 to 5 inches are definitely possible. An additional concern of rain on snow leading to additional runoff making it into the waterways. Snow levels are forecast above 12-13 kft throughout the day Thursday, which will likely result in melting of some of the existing early-season snow pack. The combined runoff from rain and rain- on-snow will result in a rapid rise in many of our creeks, streams and rivers. Modeled responses suggest several rivers reaching bankfull or higher at river forecast points across southwest Colorado and southeast Utah. The highest confidence in minor flooding and related impacts along the San Juan River, Piedra, Animas, La Plata Rivers and above Vallecito Reservoir. However, other creeks and tributaries within the Four Corners region remain susceptible. All river responses are heavily dependent on where the heaviest precipitation falls, so please continue to monitor the forecast as the week unfolds.

GJT WATCHES/WARNINGS/ADVISORIES

CO...Flood Watch through Friday morning for COZ019-021>023. UT...Flood Watch through Friday morning for UTZ022-027>029.