textproduct: Grand Junction

KEY MESSAGES

- Periods of moderate to heavy rain will impact southern Colorado the next few days with drier conditions across the north and far West.

- Widespread rainfall amounts in excess of an inch to over 2 inches in some areas could lead to flooding across the southern San Juans and adjacent valleys through Thursday. A Flood Watch is in effect for those areas.

- Warmer and drier conditions arrive for the weekend with another push of moisture arriving by early next week.

DISCUSSION

Issued at 108 PM MDT Wed Sep 23 2026

A splitting trough to our west and a ridge to the east is keeping moist southerly to southwesterly flow aloft over much of western Colorado. Over eastern Utah, a push of drier air is working in from the west as the trough continues to split into a northern stream trough and southern stream closed low. Along the edge of the boundary between the much moisture airmass and the drier one to our west, a few orographically driven showers and thunderstorms have developed and will continue through this afternoon and evening. The main concerns with these storms will be gusty outflow winds in excess of 50 mph and frequent lightning, along with heavy rain. The best chances for heavy rain and flash flooding will be with storms closest to the moisture boundary. Meanwhile, across southwest Colorado, a Flood Watch is in effect for the southwest valleys and southern San Juans. This area has already seen half an inch to an inch of precipitation in the last 24 hours, and stands to see another half an inch to an inch by tomorrow evening. As a result, there's concerns for flash flooding in steeper, small basins, as well as potential for rises in mainstem rivers in this region.

The best of the moisture plume gets shunted to the east by Friday. However enough above normal moisture will stick around to interact with a passing shortwave Friday afternoon to bring one last round of showers, storms, and heavy rain to the southern and central mountains. Through the end of the work week, temperatures remain near to a few degrees below normal, as we remain moist and cloudy. Transient ridging and drier westerly to southwesterly flow moves in for the weekend, with temperatures jumping to near or a few degrees above normal.

A return of deep tropical to subtropical moisture is expected early next as troughing once again slides into the West Coast and ridging dominates over the middle of the country. Something to watch with this pattern shift is the interaction between the West Coast trough and Hurricane Polo in the tropical eastern Pacific. Should the remnant moisture from this hurricane get swept up in the southwesterly flow aloft, the Four Corners region could see potentially intense rainfall. The southwest Colorado valleys and the southern San Juans could be particularly sensitive, thanks to this week's multiple rounds of heavy precipitation. There's a lot that can change about the forecast between now and early next week, but this is a situation to keep an eye on over the coming days. Regardless of whether the moisture is tapped from Hurricane Polo, or just from more typical subtropical Pacific sources, the return of showers and cloud cover will see temperatures drop below normal once more.

AVIATION /00Z TAFS THROUGH 00Z FRIDAY/

Issued at 535 PM MDT Wed Sep 23 2026

Above normal moisture flowing in on southerly flow will continue to bring rain to the southern mountains with widespread mountain obscuration through tomorrow. On the periphery of the thicker cloud cover thunderstorms will be possible with impacts to mainly near the I-70 airfields. VFR/ILS conditions can be expected with a low probability of MVFR occurring when showers intensity increases over the southern TAF sites. The far northern and western TAF sites should keep solid VFR conditions in place over the next 24 hours.

HYDROLOGY

Issued at 108 PM MDT Wed Sep 23 2026

Widespread moderate to heavy rainfall is expected today and Thursday, focused across the southern San Juans and adjacent valleys. A Flood Watch has been issued through Thursday night for this area, which remains somewhat saturated from the heavier precipitation last week. The forecast continues to indicate a total of 1 to 2 inches of precipitation through Thursday night. This could result not only in excess runoff over the smaller basins but lead to rises in the main stem rivers of Southwest Colorado. If you are in these areas, you should monitor forecast updates and be alert for possible flood warnings and advisories. Those living in areas prone to flooding should be prepared to take action should flooding develop.

GJT WATCHES/WARNINGS/ADVISORIES

CO...Flood Watch through late Thursday night for COZ019-022-023. UT...None.