Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
935 FXUS65 KTWC 181630 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 930 AM MST Sat Jul 18 2026 .SYNOPSIS... Widely scattered light showers with a few embedded thunderstorms will develop this morning to the north and northeast of Tucson (mainly across Pinal and Graham counties). Scattered showers and thunderstorms will then develop this afternoon across Santa Cruz and Cochise counties. These storms will have the potential for heavy rainfall. Otherwise, chances for showers and thunderstorms will continue into Sunday with a possible uptick in thunderstorm activity next Monday and especially Tuesday. && .UPDATE... Mostly cloudy skies this morning across a large majority of southern Arizona with cloud cover beginning to clear. Lingering showers from overnight continue in Pinal and eastern Pima counties with showers starting to pop up in Graham and Cochise counties. Light showers will continue for the next few hours producing minimal rain up to 0.25 inches in heavier showers otherwise less than a tenth of an inch is expected. Substantial moisture remains in place today with surface dewpoints in the mid 50s to upper 60s and satellite derived PWATs between 1.5 to 1.8 inches. CAMs are having a hard time capturing what is going to happen today, showing varying outcomes with each run of the models. In general, chances for thunderstorms seem to be focused to areas east and south of Tucson, initiating off the terrain and moving to the south at 5 to 10 mph. The main concern associated with storms today is heavy downpours leading to flash flooding and strong to gusty outflow winds. && .DISCUSSION... The upper pattern this morning is defined by an upper-low positioned over WRN Texas and SE New Mexico, and an elongated upper ridge of high pressure extending from WRN Arizona into NE Utah/central Wyoming. Weakness trapped in the mid-to-upper levels across central Arizona will drift SE today into Southeast Arizona. There should be enough mid-level moisture around to support some light showers and embedded thunderstorms to develop this morning across ERN Pinal/WRN Graham counties. I would not anticipate these storms to be too impactful though. This feature will continue to get sheared from aloft as it continues to drift SE today with the flow at 300 mb shifting from the north to the northeast. This should create an environment for some weak lift across Cochise and Santa Cruz counties this afternoon. Although I would expect to see widely scattered to low-end scattered convection across those counties this afternoon, they will not be as capable of producing heavy rain as we have seen the past few days. PWATs will continue to get eroded today as the dry slug seen on the GOES Precipitable Water imagery early this morning to the south of Pima county in NW Sonora Mexico advects into the area from the west to southwest. This is supported by the 18/00Z HREF neighborhood probabilities of 3-hr rainfall within 25 miles of a given point generally decreasing across the forecast area with the highest probabilities at 18/21Z this afternoon between 30-50 percent limited to Santa Cruz and Cochise counties. The upper-low to the east remains quasi-stationary today with a more favorable easterly flow developing Sunday into early next week as the upper-ridge consolidated across the Rockies generally along an E/W orientation. The upper-low eventually becomes an inverted trough and nudges into the eastern portions of Southeast Arizona Monday. This feature then rotates northward through our neck of the woods Tuesday into NRN Arizona by 22/06Z Tuesday evening as the upper ridge positions itself near the Oklahoma Panhandle. This should result in our next up- day across Southeast Arizona. Given the increase in PWATs and slower mid-level southerly flow, it should provide a favorable environment for heavy rain/flash flooding Tuesday. Per coordination with the Weather Prediction Center (WPC), they have updated their Day 4 Excessive Rainfall Outlook (ERO) to include SLGT for Southeast Arizona. This category represents at least 15% probability of rainfall exceeding flash flood guidance within 25 miles of a point. In any event, the Monsoon season rolls-on. && .AVIATION /15Z SATURDAY THROUGH 15Z SUNDAY/... Valid through 19/12Z. SCT-BKN 8-12k ft AGL BKN-OVC 12-15k ft AGL. Widely scattered to scattered light rain showers and thunderstorms will develop this morning through 18/18Z across portions of eastern Pinal and western Graham counties. Scattered showers and thunderstorms will then develop after 18/20Z across Santa Cruz and Cochise counties with brief heavy rain and erratic strong wind gusts of 30-40 kts possible with any storm. Outside of thunderstorms, surface winds generally under 10 kts. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER... Daily chances for thunderstorms will continue each afternoon and evening through the next 7 days with day to day variability. An increase in thunderstorm activity is expected Monday and especially Tuesday of next week. Minimum relative humidity 25-30 percent in the lower desert locations and above 40 percent in the mountains this week. 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... None. && $$ UPDATE...Malarkey Visit us on Facebook, X, YouTube, and at weather.gov/Tucson