textproduct: Grand Junction

KEY MESSAGES

- Period 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 these areas.

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

DISCUSSION

Issued at 1039 PM MDT Tue Sep 22 2026

The H500 hand analysis map at 23/00Z has a complex pattern downstream with an compact Omega block over the Great Lakes region on top of the larger SubTrop High still centered over the western Gulf states. Downstream of us there is a splitting trough extending from the N.Rockies to SoCal/Baja. We are squeezed between the trough and high which has allowing for a deep fetch of subtropical moisture to invade our CWA over the past 24 hours with the KGJT RAOB PWAT gaining a third of an inch over the past 24 hours. This of course has fed the uptick in convective coverage today with a mix of moderate/heavy rainfall storms and gusty outflow winds over 50 mph. The interaction between the upstream trough and large high will play a role in the heavy rain threat over a portion of our CWA over the next few days. The northern split in the upstream trough looks to have enough influence to keep a fetch of drier air flowing into our far western and northern CWA through tomorrow evening. While to the south moisture will be streaming in unabated East of the 4 Corners into our southern and central mountains tonight through tomorrow morning. PWATs in this area look to be 150-200 percent of normal equating to the 99the percentile with a 10 year return interval for early Fall. So the juice is there and will be in the area through Thursday before flushing to the East on Friday. Southerly flow into the terrain will create on avenue of lift for squeezing this moisture out. A large anticyclonic jet in the faster flow aloft will be another which will be joined by another speed max arriving Wednesday night into Thursday. This secondary jet will be forced by a stronger wave/closed mid level system allowed to lift northward around the SubTrop High by a weakening downstream trough. This forcing should help provide stronger lift to help release instability and produce efficient rain processes early Thursday through the afternoon. This system will also help pull moisture back westward and spread precipitation/cloud cover farther north and west on Thursday. With moisture and lift in place there are high probable chances of rainfall(and snow on some peaks?) across SW Colorado tonight thru much of Thursday. The missing ingredient is instability to help drive heavier rainfall rates and thunderstorms. This should happen on the periphery of the cloud cover and deeper moisture today but under the rain shield to the South this isn't as clear. Sounding show moderate lapse rates and any CAPE in a long skinny profile. So some enhancement from little pieces energy and maybe orographics will help but the stronger storms remain on the edge where more sunshine is likely to occur. As the system lift through on Thursday there could be enough mixing to bring some peaks of sunshine and help produce some stronger storms down South behind the lifting rain shield. Friday with the better moisture moving east and more possibility of sunshine should lead to more scattered thunderstorms. When all is said and done the most probable outcome for the heaviest precipitation remains over the southern San Juans and adjacent valleys and where PoPs will be the highest for this event. QPF amounts have come down slightly from previous forecast but still widespread 1-2 inches in this area have at least a 70 percent chance of probability. Therefore the Flood Watch in place looks on track. So we will be watching for the threat of flash flooding in smaller basins and as in the last event some of the main stem rivers in southern Colorado are likely to see a jump in stages as this precipitation runs off. We should see a drying trend for the weekend with another push of moisture coming back late Sunday to start the new week on the wetter side. Temperatures will be seasonable to cool depending on cloud cover and precipitation overhead the next few days with a warmer weekend on tap.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 1125 PM MDT Tue Sep 22 2026

Showers and a few storms will continue overnight and do expect several TAF sites to see some precip, albeit fairly light. Moisture continues to stream into the area with another round of showers and storms anticipated with the best chances for the southern half of the CWA. KTEX and KDRO continue to have the best chance to see MVFR due to low clouds and vis thanks to the convection. That being said, KASE, KMTJ, KGUC may also see VC showers or storms in the afternoon.

HYDROLOGY

Issued at 1039 PM MDT Tue Sep 22 2026

Widespread moderate to heavy rainfall is expected Wednesday and Thursday, focused across the southern San Juans and adjacent valleys. A Flood Watch has been issued for Wednesday morning through Thursday night for this area which remains somewhat saturated from the heavier precipitation last week. The forecast continues to indicate 1 to 2 inches of additional precipitation during the Wednesday and Thursday timeframe. 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 from Wednesday afternoon through late Thursday night for COZ019-022-023. UT...None.