Area Forecast Discussion

SYNOPSIS

The combination of ample moisture across Southeast Arizona and an upper low moving across portions of the area from the east 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 remains in effect for Graham, Greenlee, Cochise and Santa Cruz counties through 10 PM MST this evening. Otherwise, daytime temperatures will remain below average by several degrees into midweek increasing to above normal late this week into this weekend.

DISCUSSION

Abundant moisture remains in place with areas in Graham, Greenlee, Cochise, and Santa Cruz saturated and seeing above average stream/wash/river flows. These areas will be the main focus with the Flash Flood watch remaining in effect through late evening.

Current thinking remains the same as development and expansion of convection around the periphery of the upper low continues. With already susceptible conditions, it will not take much to get flash flooding. We have already issued one FFW, where nearly 2" fell in less than 1hr. We are seeing similar rates from additional development across SEAZ. HREF once again paints a large area of 10% probs of greater than 3" in much of the watch area. The entire watch has a risk, but there is one area of more concern this afternoon into the evening. WoFS runs are consistently highlighting QPF MAX values nearing 4" in localized spots, with reasonable worst case (90th) percentile showing 2-3" from western Cochise, eastern Santa Cruz into extreme eastern Pima/Catalina and Rincon Mtns as well as extreme eastern Pinal and western Graham.

As for the Tucson Metro, similar flavor as yesterday. High likelihood of convection south and east of Tucson, including the Santa Catalina/Rincon Mtns. The best chance for storms in the metro will be the Catalina Foothills, Tanque Verde/east side, and north into Catalina/Oracle. Anything that makes it into the city center should be more of moderate to locally heavy rain with embedded storms situation. Nonetheless, with abundant moisture and above average soil moisture, we cannot rule out an isolated instance of flash flooding. Latest WoFS runs continue to support this scenario, but does provide a little more confidence of storms migrating off the higher terrain into the immediate communities from the Catalinas/Rincons. Given the rates seen and the WoFS output, a localized flash flood threat remains possible for these areas.

Upper level low meanders NW of the area by Wednesday with a downward trend in convection. There still should be enough moisture/instability that we could see some isolated to perhaps scattered storms, but confidence is low on the area.

Despite the fact that we will still have daily afternoon and evening showers and thunderstorms the second half of the work week, there will be a distinct downturn in activity. The upper- high center will move from Oklahoma today to southern New Mexico 24/00Z Thursday afternoon then up to the Four Corner by 26/00Z Saturday afternoon. The approaching upper-high will increase afternoon temperatures once again to around 5 to 8 degrees above normal. The combination of drier air and modest mid-level flow will increase the potential for showers and thunderstorms next weekend to be more of the severe variety. In other words, the Monsoon rolls-on.

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 /19Z TUESDAY THROUGH 19Z WEDNESDAY/

Valid through 22/18Z. SCT-BKN 6-8k ft AGL with FEW down to 4k ft AGL around KOLS. SHRA/TSRA, some +TSRA developing and expanding in coverage this afternoon. VFR, with the exception of brief MVFR due to +TSRA/SHRA. Best chance is for KDUG/KOLS seeing these impacts. VCHS/VCTS for KTUS, as activity should stay mainly south and east. Sfc winds generally less than 12kts, except for around storms where VRB gusts to 35kts will be possible. 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. Widespread showers and thunderstorms are expected east and south of Tucson again today. These storms will be capable of producing heavy rainfall with the potential for flash flooding. A Flash Flood Watch remains in effect until 10 PM MST this evening for the eastern portions of Southeast Arizona. Min RHs 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 until 10 PM MST this evening for AZZ503-507>513.