Forecast Discussion
520 FXUS65 KTWC 202012 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 112 PM MST Mon Jul 20 2026 .SYNOPSIS... The combination of ample moisture across Southeast Arizona and an upper-level disturbance moving into the area from New Mexico today will result in widespread showers and thunderstorms, especially to the east and south of Tucson. These storms will be capable of producing heavy rainfall. A Flash Flood Watch is in effect for Graham, Greenlee, Cochise and Santa Cruz counties this through Tuesday evening. Otherwise, daytime temperatures will remain below average by several degrees into midweek increasing to above normal late this week into the weekend. && .DISCUSSION... Biggest change from this morning is extending the Flash Flood Watch through tomorrow evening for Graham, Greenlee, Cochise, and Santa Cruz Counties. The main threat will be very heavy rainfall and flash flooding, with gusty winds a secondary threat. There could be a localized gust to 50 kts as convection moves off the terrain, but 30-40kts are more likely. A deeper dive into the flash flood setup shows abundant moisture and a susceptible area. Widespread 0.5"+ with localized amounts of 2"+ fell across a good chunk of far SEAZ, especially Cochise County yesterday. This area will be the most vulnerable. In addition, CAMs show weakly organized clusters/single cell with the potential for cell mergers and at least some training/backbuilding as flow remains relatively weak. The best chance for training or backbuilding will be across Cochise and Santa Cruz Counties this evening. As the mid/upper level low noses into SEAZ from NM/MX, convection could go well into the evening and perhaps into the overnight. Confidence is low at this time regarding the overnight, but there is at least some signal in the HREF of higher rainfall totals across central Pima County. In addition, at least some higher res solutions now show additional convective development early morning across areas from Tucson south to Nogales and west toward Sells. There still is not enough confidence to expand the Flash Flood Watch west into eastern/central Pima County, but its is not out of the question we have a flash flood warning or two this evening or even tomorrow morning. As for the Tucson Metro, confidence is again low on the evolution and propagation of storms off the rim this evening/overnight. We will likely have storms in the metro, but once again favoring the Catalina Foothills, Oracle/Catalina, the far east side, and south into the Corona De Tucson. We cannot rule out localized flash flooding, but probabilities are rather low. The more likely scenario is seeing light to moderate rain showers with embedded storms late evening into overnight/early morning hours as the mid/upper level low begins to provide more lift and elevated instability to the area. Moving into Tuesday still remains a little tricky given how active today will be and how convection lingers into tomorrow morning, but enough CAMS, HREF probs of 3"+ amounts, and the mid/upper level low moving across the CWA is more than enough to bump up confidence. So the watch for tomorrow looks good, with uncertainty remaining for areas along the watch in Pinal/Pima County. As we move into Wed/Thu/Fri, there should be a noticeable downtrend in daily showers/storms. While it will trend drier, there will still be daily convection, most likely favoring the typical areas south and east of Tucson. The flash flood threat will also lessen and allow for soils to recover as we heat up. While the flash flood threat trends down, the threat for gusty winds will increase with better mid and upper level flow/drier air. Monsoon activity looks to ramp back up for the weekend as the high moves into a more favorable region and multiple waves look to track across the area. Daytime temps below average early this week, transitioning to above average by midweek. Spotty major heat risk is possible for portions of Pima County, generally around the metro and across the western third of the county. In addition, the upper Gila River Valley has some spotty major heat risk. As of now, confidence is not high enough for an Extreme Heat Watch, due to the limited coverage and possibility of monsoon activity/clouds lowering aftn temps. We will monitor for now and message an overall upward trend in temps and heat risk by mid week. && .AVIATION /18Z MONDAY THROUGH 18Z TUESDAY/... FEW/SCT cloud decks AOA 8-11k ft agl this afternoon, becoming BKN/OVC as coverage of SHRA/TSRA expands through the evening. KOLS/KDUG has the best chance to see TS with brief MVFR possible under heavy +TSRA. Sfc winds generally less than 12kts, except for around storms where VRB gusts to 40kts will be possible. -TSRA/-SHRA continues into the evening and perhaps overnight with BKN/OVC decks lowering, but VFR will prevail. && .FIRE WEATHER... Daily chances for thunderstorms will continue each afternoon and evening through the next 7 days with day to day variability. Widespread showers and thunderstorms are expected east and south of Tucson today, and again Tuesday. These storms will be capable of producing heavy rainfall with the potential for flash flooding. A Flash Flood Watch is in effect today/tomorrow for the eastern portions of Southeast Arizona. Min RH 25-30% in the lower desert locations and above 40% in the mountains through mid-week. By Thursday into the weekend, RH could dip into the 17-22% range for the lower deserts west of Tucson and the upper Gila River Valley as temperatures warm to above average. Winds this week will remain mainly light and under 15 mph, though strong and erratic winds will be possible with any thunderstorm. && .TWC WATCHES/WARNINGS/ADVISORIES... Flood Watch through Tuesday evening for AZZ503-507>513. && $$ Visit us on Facebook, X, YouTube, and at weather.gov/Tucson
NWS TWC Office Area Forecast Discussion