textproduct: Grand Junction

KEY MESSAGES

- Periods of moderate to heavy rain will impact southern Colorado through Friday 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 1026 PM MDT Wed Sep 23 2026

A shortwave trough is getting pinched by high pressure ridge trying to build back over the west, forming a Rex Block with a low over the Baja. This is introducing some drier air in from the northwest and across eastern Utah, resulting in clearing skies across the northwest portions of the CWA and shunting the deeper moisture plume further east. This moisture is currently being squeezed between the shortwave trough to our west and high pressure to our east, with showers currently concentrated across southwest Colorado and up the central divide and areas east of the divide. These showers are fairly shallow with low tops but still producing light to moderate stratiform rainfall underneath across our southern mountains and valleys. Over the last 36 hours, precipitation totals across these areas range from 0.5 to 1.5 inches, with the higher amounts concentrated the southern and eastern San Juans as well as San Juan River Basin and eastern portions of the Animas River Basin. This is providing a good soaking rain, which this area has been getting plenty of over the past couple weeks. Little instability and lack of forcing isn't quite resulting in the heavier rates or convection needed to cause flash flooding and the rivers have seen a little response, but nothing concerning at the moment. Winds have also been on the lighter side due to the cloud cover and more stratiform nature. However, as anticipated, thunderstorm activity this afternoon was confined to the periphery of this cloud shield along the areas that received more sunshine. Overnight tonight, there is still some indications of a weak circulation moving up the divide from the south which could result in continued stratiform rainfall. However, this moisture gets shunted further to the east by Thursday with a Rex Block forming to our west and the high pressure on top of the Rex Block introducing drier air across the west, keeping this moisture to our east. Storms will fire Thursday afternoon across the high terrain along this periphery, so there is a chance that we see some heavier rates with thunderstorm activity. However, the instability and forcing is lacking so do not expect storms to last very long and be more of the pop and drop variety while the southern divide might see continued stratiform rainfall. QPF numbers have come down even further with the latest model runs, probably due to the eastward shift in the better moisture, less instability and lack of forcing. Decided to keep the Flood Watch going through Thursday night given the amount of rainfall received thus far across southwest San Juans and southern valleys, in case any area rivers see some rises in response and any convection could lead to heavier rates on already saturated soils.

By Friday, the deepest moisture gets shunted eastward as the Rex Block breaks up and the low over the Baja weakens and lifts across the Desert Southwest and Four Corners as an open shortwave Friday afternoon. More sunshine Friday morning will result in more instability to fire scattered showers and storms. The focus does seem to be across southwest Colorado where abundant rainfall has already fallen. This shortwave moves east by Friday evening, with drier air behind it and a high pressure ridge building in for the weekend. So we will see another break in the active weather this weekend.

Looking ahead to early next week, the models are struggling with whether or not we will see remnants of Hurricane Polo get pulled into the region. While the weather does get more active with some moisture returning and scattered showers and storms, the latest model runs of the GFS and ECMWF are also bringing in a rather progressive low pressure trough into the Great Basin and across our region around the same time frame that Hurricane Polo moves inland across the Baja. So, some tropical moisture may get pulled northward but the deeper trough may keep the deeper moisture south of the Four Corners as another high pressure ridge tries to build across the west. GFS is trending drier while the EC is trending wetter. 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 /06Z TAFS THROUGH 06Z FRIDAY/

Issued at 1125 PM MDT Wed Sep 23 2026

The precipitation and cloud cover isn't over yet as we can look for another day of midlevel broken skies especially after 18Z. Showers and storms remain a possibility for several TAF sites except the far north and west. VFR conditions will remain predominant but brief MVFR can't be ruled out either if a stronger cell/shower moves over a TAF site.

HYDROLOGY

Issued at 1026 PM MDT Wed Sep 23 2026

Widespread moderate to heavy rainfall is expected through Thursday, focused across the southern San Juans and adjacent valleys. A Flood Watch remains in effect through Thursday night for this area, which remains somewhat saturated from the heavier precipitation last week and the last 24-36 hours. 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.