textproduct: Grand Junction

KEY MESSAGES

- Widespread rain is expected tomorrow through early Friday.

- Strong to severe storms will be possible near the Four Corners...favoring southeast Utah...on Wednesday and Thursday.

- The potential for flooding increases Thursday 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 1019 PM MDT Tue Oct 20 2026

HAZARDOUS WEATHER POSSIBLE THROUGH FRIDAY:

Remnant tropical moisture will once again take aim at the Intermountain West and Rockies. The first wave of this deeper moisture arrives to the Four Corners region by sunrise and track northeastward across the remainder of eastern Utah and western Colorado through the day before being swept away on Friday. A full array of hazards are possible with this prolonged event including river flooding, flash flooding, strong to severe thunderstorms, and snow above 12,000-13,000 feet. A Flood watch has been issued for parts of southeast Utah and southwest Colorado beginning 12 PM tomorrow and persisting through 12 PM Friday.

Tomorrow through Thursday:

The 21/00Z KGJT RAOB is sitting at 2/3 of an inch of PWAT which is already anomalous and in the 90th percentile. Impressive for mid October but just the tip of the iceberg. GOES All Layer PWAT imagery is showing the nose of true tropical mositure moving into south-central AZ this evening and should arrive to the 4 Corners in the pre-sunrise hours. PWAT is projected to push above an inch by sunset at GJT which would be a new daily record and some 300+ percent of normal. So the stage will once again be set for excessive rainfall rates and impressive total precipitation amounts through the event. This moisture will also effectively lighten and destabilize the atmosphere and allow even minimal lift to produce ascent and efficiently squeeze out this anomalous PWAT. Ascent over the next few days will aided by a strong SubTrop jet aloft tomorrow followed by the Polar jet on Thursday which is associated with the main upstream trough over the West. CAPE will be modest at best over the 4 Corners region the next few afternoons. The wind profile will however be impressive...with low level east/southeast flow over-topped by the strong southwest flow aloft. This shear will be favorable for producing periods of organized convection even though plenty of cloud cover will inhibit full surface heating. Strong outflow winds will remain the main threat but some storms will also be capable of producing larger hail. The veering profile and local terrain effects could also lead to some brief spin ups...not probable but not zero either. Lastly the CAPE profile will be long and skinny...an indication of deep warm cloud processes which favor high precipitation rates as well as high snow levels. It should be no surprise that the fertile crescent of the San Juans will also be effective at producing the ascent in this southerly flow pattern. With the fire hose pointed directly at them precipitation amounts in the 2-4 inch range are still forecast with higher amounts possible due to convection. Rain or a nighttime mix at pass level could translate to 6 to 12 inches or more of snow over the highest peaks.

Friday:

Unlike the last event the upstream system will be lifting northward through the Great Basin instead of sending the main circulation overhead. Drier air will be pulled in as this system lifts into the northern Rockies through the day on Friday. A decrease in precipitation coverage and intensity is expected through the day as PWAT is forecast to crash below a half of an inch by midday. Even after it stops raining, the risk for flooding will remain elevated on Friday as water continues to work through the system. Some main stem rivers are forecast to crest on Friday morning. We'll have a better idea of how long flooding may continue to cause issues after we get a better sense of how much rainfall is received on Thursday.

LOOKING AHEAD:

Chances for light precipitation (valley rain, mountain snow above 10,000 feet) continue through the weekend as a low pressure system, currently spinning off the California coast, traverses the Intermountain West. Plenty of uncertainty remains in how this low will evolve as it moves inland, but ensemble guidance tends to show a weakening system by the time it reaches our forecast area. Regardless, this system will provide enough forcing for scattered to widespread precipitation as it moves through, alongside a brief reduction in temperatures.

AVIATION /18Z TAFS THROUGH 18Z THURSDAY/

Issued at 1139 AM MDT Wed Oct 21 2026

A weather system has made its way into the region from the southwest. Showers are already being reported at TAF sites along and south of I-70. Precipitation coverage will become more widespread by the afternoon hours and will persist through the overnight hours and into Thursday for nearly all TAF sites. Potential impacts include periods of lower ceilings and visibility in and around shower activity. Additionally, periodic gusty and variable winds should not be ruled out as this system propagates the area. MVFR conditions are a possibility, along with ceilings falling below ILS breakpoints.

HYDROLOGY

Issued at 1019 PM MDT Tue Oct 20 2026

Hydrologic concerns ramp up tomorrow (Wednesday), due to the potential for thunderstorm development. Atmospheric moisture increases under strong south-southwesterly flow, lofting an abundance of warm tropical moisture across the forecast area. While bands of showers expand as early as Wednesday morning, thunderstorms are expected to move into east/southeast Utah tomorrow afternoon. Southeast Utah and lower valleys along the CO/NM border show the highest pockets of instability, which focuses the better confidence for flash flooding across desert floors and dry washes. Coverage of rainfall rates exceeding 0.5 inches in an hour remains low even 24 hours out. However, whatever embedded thunderstorms that develop tomorrow will only help saturate soils leading into the main surge of moisture expected Wednesday night through Thursday.

The bulk of precipitation remains on track for Thursday, with generally 1 to 3 inches of rain falling across the southern half the CWA within a 24 to 36 hour window. Bullseye QPFs are highest over the San Juan's, where locally higher amounts of 4 to 5 inches are possible. This system continues to track anomalously warm, as the main moisture plume remains well ahead of a cold, closed low deepening along the Pacific Coast. Thus, rain on snow is a viable concern. Snow levels are forecast above 12-13 kft throughout the day Thursday, which will likely result in melting much 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 also nudged upward across many river forecast points across southwest Colorado. We have the highest confidence in minor flooding and 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.