FLAGSTAFF, ARIZONA — The National Weather Service’s Weather Prediction Center issued Mesoscale Discussion 1274covering a broad corridor from southern Utahthrough central Arizona, warning that slow moving storms carrying high rainfall ratescould produce isolated instances of flash flooding through 2300 UTC, or 23Z. The discussion, tagged with the #utwx and #azwxhashtags, was tied to a GOES-18 visible satellite loop timestamped 1720 UTC on September 13, which showed active convection developing across the outlined region.
Weather Prediction Center Flags Slow Moving Storms From Southern Utah Into Central Arizona
Forecasters with the Weather Prediction Center outlined a long, narrow corridor stretching roughly north to south from the Utah-Arizona border down through the heart of central Arizona, encompassing a wide stretch of high terrain along the way. Within that corridor, the primary concern was storms moving slowly enough to allow rainfall to pile up over the same locations repeatedly, rather than storms racing through and limiting total rainfall at any one spot. That slow storm motion, combined with tropical moisture typical of the late-summer monsoon patternacross the Southwest, was the key ingredient forecasters cited for the flash flooding threat.
Mesoscale Discussion 1274 Warns of Flash Flooding Through 2300 UTC
The bulletin specifically warned that slow moving storms with high rainfall rates may result in some instances of flash floodingthrough 23Z, or roughly early evening across the Mountain time zone. Mesoscale Discussions, commonly abbreviated MPDs, are short-fused products the Weather Prediction Center issues when a specific, localized rainfall threat needs closer attention than a standard forecast provides, typically valid for a window of just a few hours. MPD 1274 carried the discussion forward from the early afternoon issuance time through the remainder of the afternoon and into the early evening hours.
Satellite Imagery Captures Heaviest Rainfall Bands Across Central and Southern Arizona
The accompanying GOES-18 Channel 2 visible satellite image, timestamped 1720 UTC on September 13, showed the outlined threat corridor overlaid on real-time cloud cover across the Four Corners region. Forecasters highlighted several specific pockets within the corridor that had already recorded the heaviest rainfall totals over the prior 24 hours, including clusters over the higher terrain of central Arizona and a larger, more expansive area of heavy rain across south-central Arizona, where one zone was specifically flagged as carrying the highest rainfall totals within the entire outlined region. Those areas were considered most susceptible to additional flash flooding if slow moving storms redeveloped over the same saturated ground.
Broader Monsoon Pattern Keeps Flash Flood Risk Elevated Across the Southwest
The corridor highlighted in MPD 1274 sits within a broader stretch of the Southwest that has seen repeated rounds of monsoon moisture streaming north out of Mexico and the Gulf of California in recent weeks, keeping soils saturated and drainage systems strained across much of Arizona and Utah. Forecasters noted that even modest additional rainfall on top of already-saturated ground can be enough to trigger flash flooding in narrow canyons and desert washes that drain quickly during storms. With the pattern showing no clear signs of breaking down immediately, additional Mesoscale Discussions and localized flash flood products remain possible in the days ahead across the same general corridor.
How a Mesoscale Precipitation Discussion Differs From a Flash Flood Warning
Unlike a Flash Flood Warning, which is issued by local National Weather Service forecast offices once flooding is imminent or occurring, a Mesoscale Precipitation Discussion is a broader, earlier-stage product meant to draw attention to an evolving rainfall threat before it necessarily reaches warning criteria. MPDs are often used by local forecast offices as supporting justification when they do issue Flash Flood Watches or Warnings, and they give the public and emergency managers additional lead time to prepare. MPD 1274 did not by itself constitute a warning, but it flagged the southern Utah into central Arizonacorridor as an area where local offices would likely need to consider flash flood products if storms behaved as forecast.
What Residents in the Corridor Should Watch For as Slow Storms Continue
With the monsoon-driven pattern expected to keep producing slow moving, high-rainfall-rate storms across the region, forecasters urged residents across the outlined corridor, spanning southern Utah into central Arizona, to stay alert for rapidly rising water in washes, canyons and low-lying areas, particularly near the zones already flagged for the heaviest rainfall over the past 24 hours. Because the threat was tied to storm training over the same locations rather than a single fast-moving system, forecasters noted the risk could persist in pulses through the remainder of the valid period, with additional flash flooding possible anywhere storms redeveloped over already-saturated terrain.
