textproduct: Grand Junction

KEY MESSAGES

- Flash flooding will be possible as potentially record moisture passes through western Colorado and eastern Utah.

- Flood Watch in effect for SE Utah and SW Colorado Wednesday through Thursday.

- While vulnerable locations like slot canyons and burn scars will be at the highest risk of flash flooding and debris flows, impacts could be observed at multiple locations across the region.

DISCUSSION

Issued at 1004 PM MDT Tue Sep 15 2026

With moisture still plentiful and an afternoon sounding bolstering the most CAPE we've seen in quite some time (nearly 2000 J/kg), today saw, and continues to see, heightened storm activity. High rainfall rates have prompted numerous flash floods and powerful storms have dropped severe winds and small hail. As we progress through the overnight hours, a mixture of stratiform rain showers and clusters of stronger convection are expected to continue. While the wind and hail threat should diminish with lessened instability, rain rates could continue to be a concern and a few additional flash floods or debris flows cannot be ruled out.

EXTREMELY MOIST WEDNESDAY AND THURSDAY:

Believe it or not, we will transition into an even wetter environment over the next 48 hours as yet another surge of sub- tropical moisture pushes into the Intermountain West through the persistent southwesterly flow. Ensembles are keying in on PWAT's nearing 300% of normal, particularly focusing on the southwestern corner of Colorado. At this magnitude of an anomaly, PWAT's are not just forecasted to break records, but to crush them.

With that much moisture, it's almost a certainty that cloud cover will be widespread for the vast majority of the next 48 hours. This is the one factor dampening storm potential, and CAM's are responding by depicting a stratiform storm mode dominating the region. While we often talk about stratiform rainfall being less threatening to flash flooding concerns due to weaker rainfall rates, the Wednesday/Thursday event isn't quite in the same boat. For one, the low-level environment and soils will have both sufficiently saturated by this point thanks to many portions of the region seeing 1+ inches of rainfall over the past couple days. This will lead to efficient rainfall and minimal absorption by said soils. We've already seen some vulnerable locations produce flash flooding and debris flows with weaker rainfall rates, an indication that there is little room left for the land to take on more water. Secondly, just because stratiform storms will dominate does not mean embedded stronger cells cannot develop. Between upper-level jet support, a pronounced shortwave passing through (roughly between 6 PM local time Wednesday through 6 PM local time Thursday), and enhanced PVA with the southward digging of the west coast low, the dynamics are in place for a handful of strong updrafts and especially wet rainfall rates. Even so, it is likely that we will see less of the hail and wind threat over the next couple days, and more of the hydrologic threats. Refer to the hydro section below for more detailed information on what to expect.

FRIDAY AND BEYOND:

On Friday, the moisture pool driving this wet weather rapidly diminishes. Though, given just how wet we are expected to peak at, even with this decrease in moisture we are confident that the majority of the region will still see above normal PWAT's through the weekend. With the low beginning its eastwards movement though, and with little in the way of forcing, storm activity should become less pronounced. On Friday, there looks to still be enough moisture to keep storm coverage scattered. By Sunday though, look for most shower and thunderstorm development to be confined to the high terrain as convection will be driven by orographics.

AVIATION /06Z TAFS THROUGH 06Z THURSDAY/

Issued at 1137 PM MDT Tue Sep 15 2026

Unsettled conditions will persist over the next 24 hours with periods of showers and thunderstorms impacting all the TAF sites. Through sunrise much of this activity will be over the northern TAF and KTEX is in line for another batch of storms moving in from the Southwest. Gusty outflow winds and heavy rainfall rates have a low probability of dropping flight conditions to MVFR overnight. Look for more active weather with thunderstorms and heavier showers to pick up after 18Z. Heavy rain and wet downburst with 50KT gusts are possible with these storms. The mountain TAF sites can expect periods below ILS breakpoints for low ceilings and reduced visibility with these storms along with areas of terrain obscuration.

HYDROLOGY

Issued at 1004 PM MDT Tue Sep 15 2026

It's a difficult forecast when the dominant storm type will be stratiform rain, but soils have been so saturated over multiple days of moderate to heavy rainfall. What will be more pertinent to the event: lighter rain rates or the lack of absorption from soils? In the end, the answer will likely vary across the region. In this, as we approach this Wednesday through Thursday period of record moisture (for this time of year), it's safest to assume that flash flooding will be a possibility across most to all portions of the CWA. This is, as usual, especially important for vulnerable areas like slots canyons and burn scars. But the sheer persistence of precipitation will mean even less vulnerable areas are at risk of hydrologic complications. Additionally, QPF for multiple forecast cycles has highlighted an especially wet bullseye on the southern slopes of the San Juans and into the Animas and San Juan River Basins where 48-hour totals are exceeding 2 inches and nearing 3 inches in some locations. As a result, southern portions of the CWA are in a Flood Watch for portions of the coming 48 hours. Stay aware of road conditions, avoid locations known to be prone to flash flooding, and report flooding or other weather impacts to local law enforcement.

GJT WATCHES/WARNINGS/ADVISORIES

CO...Flash Flood Watch until 3 AM MDT Wednesday for COZ002-003. Flood Watch from Wednesday afternoon through Thursday evening for COZ018-019-021>023. UT...Flood Watch through late Wednesday night for UTZ022-028-029.