Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
385 FXUS65 KTWC 181646 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 946 AM MST Mon Aug 18 2025 .SYNOPSIS....Isolated to scattered thunderstorms this week across Southeast Arizona, with day to day variability in intensity and chances. The big story for Southeast Arizona will be the extreme heat returning the second half of the week. An Extreme Heat Watch is in effect Wednesday morning through Friday evening. && .UPDATE... No major changes in thinking for today`s outlook from the previous discussion. 12Z forecast soundings continue to show weaker mid- level lapse rates, thanks to warming aloft, inhibiting the development of strong instability today. There will be adequate moisture in place to develop isolated to scattered convection over elevated locations of Santa Cruz through Cochise and northward into Graham and Greenlee counties today, with far lower chances towards Pima/Pinal including the Tucson metro. 12Z HREF probabilities for flash flood inducing rainfall rates are low, but non-zero across the far south and far east. && .PREV DISCUSSION... /issued 102 AM MST Mon Aug 18 2025/ The upper pattern this morning is defined by a ridge of high pressure extending from southern New Mexico and northern Chihuahua Mexico toward the northeast into the Midwestern states. At 1 am MST, radar indicated scattered moderate-intensity showers with embedded thunderstorms persisting along the NW periphery of the 300 mb high center extending from northeast Sonora Mexico into Southwest New Mexico. We have actually seen a few light showers develop early this morning across portions of Graham and Greenlee counties in the mildly diffluent southerly flow aloft. The upper high is expected to slowly move into southwest/west-central New Mexico by this afternoon. As this happens, we will see a slight warming aloft across the eastern portions of the CWA today which should inhibit deep convection. Granted, we will still have enough moisture and instability around from strong surface heating to result in isolated to scattered thunderstorms across Southeast Arizona, there is no large scale forcing mechanism at play. The upper high is still progged to shift into the Four Corners region Tuesday afternoon, where it will reside and strengthen the second half of next week. Although this location is usually active for our neck of the woods, it all depends on the moisture that is available. The 18/00Z deterministic GFS still indicated a plume of higher precipitable water values (1.50+ inches) moving up the Gulf of California into the lower deserts of SW Arizona this week. Unfortunately, we also see a general drying across the eastern half of our forecast area, with PW values below an inch for the majority of the week. As mentioned in previous forecast discussions, this will still likely setup a pattern where we have isolated storms east of Tucson capable of producing strong and gusty thunderstorm outflows but little rainfall, then moving west to southwest across the CWA, increasing in coverage and becoming capable of producing heavy rainfall the farther west the storms make it. In any event, the bigger story may end up being the increasing afternoon high temperatures warming back up to 4-6 degrees above normal Wednesday through Friday, increasing HeatRisk to Major by Wednesday and locally Extreme Thursday. There is an Extreme Heat Watch in effect Wednesday through Friday for the majority of the lower elevations of Southeast Arizona. && .AVIATION...Valid through 19/12Z. SCT-BKN clouds 10k to 14k ft AGL through 18/17Z with a few light -SHRA possible thru daybreak near KSAD and KDUG. SCT-BKN low clouds 7k to 10k ft AGL will develop around 18/19Z and persist thru 19/05Z. ISOLD-SCT -SHRA/-TSRA will be possible mainly east and southeast of KTUS in Cochise, Graham and Greenlee counties. SFC winds will be light an variable this morning becoming north-northwest less than 12 kts after 18/20Z. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Expect daily chances for thunderstorms and wetting rain showers into next week. Min RHs will generally be 15-20 percent in the lower elevations and 20-30 in the mountains. Winds will generally be less than 15 mph with typical afternoon gusts up to 20 mph into next week. Temperatures will be trending above normal the second half of this week, with extreme heat expected in the lower elevations of Southeast Arizona. && && .TWC WATCHES/WARNINGS/ADVISORIES... Extreme Heat Watch from Wednesday morning through Friday evening for AZZ501>507-509. && $$ Update...Edwards Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson