Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
206 FXUS65 KTWC 310900 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 200 AM MST Sat Oct 31 2026 .SYNOPSIS...Much of the day today will be dry with only very limited shower activity, however precipitation will increase after sunset especially from Tucson westward as the next weather system starts to impact the area. This weather system will bring widespread showers, heavy at times, with scattered thunderstorms Sunday into Monday morning. A Flood Watch is in effect for all of southeast Arizona from 11 PM Saturday through 5 AM Monday. && .DISCUSSION...While the area will mostly be between weather systems today, there will be enough lingering moisture for limited/isolated shower activity (10 to 20 percent chances). Most locales will stay dry but can`t rule out a passing shower anytime from early this morning through the day with light rainfall amounts. As the next weather system approaches tonight, precipitation chances start to pick up after 5 PM and continue through the evening hours, especially from the Tucson area westward as showers become more widespread with a slight chance of thunderstorms. Overnight tonight through Sunday and Sunday night, a potent weather system riding the subtropical jet will move through the region as a west coast upper level trof moves inland. A strong atmospheric river (AR), level 3 on a AR 1-5 scale, will bring near record to record levels of PWAT of (1.25" to 1.50") for November. As this AR moves over the area, expect widespread moderate to localized heavy rain and isolated embedded thunderstorms on Sunday. The timing for the most widespread precipitation will initially be over the western deserts during the morning, shifting east into the Tucson area late morning through the afternoon, and then points east of Tucson during the afternoon and continuing into the evening. Most locales will see a 4-6 hour period of moderate to at times heavy rainfall with a few embedded thunderstorms. Upslope flow into the mountains will also enhance rainfall rates/totals for these locales. Additionally, given the 30-40 kt 850-700mb southwest flow, expect breezy conditions with gusty winds over 40 mph especially within any convective lines capable of bringing some of those stronger winds aloft to the surface. While the most widespread heavier precipitation will end Sunday night into Monday morning as the AR moves east of the region, PoPs still remain elevated as the upper trough moves across the northern half of Arizona. Most of the area is under slight risk from WPC for excessive rainfall Sunday with a very small snippet of far northern Graham, Pinal and Greenlee counties under moderate risk. We are looking at rainfall records for November 1st falling across much of the area. A Flood Watch is in effect for all of SE AZ from 11 pm tonight to 5 am Monday. While confidence is increasing in rainfall totals, there is still some spread in the guidance. Below is a look at the expected totals, high and low end totals. -Expected: Valleys 0.50"-1.75", Mountains 1"-3+" -Low end (9 in 10 chance): Valleys 0.20"-1.25", Mountains 0.50"-1.5" -High end (1 in 10 chance): Valleys 1.50"-3.00", Mountains 2"-4" locally higher With the expected rain on Sunday into early Monday we will see a response in the area rivers, creeks and washes across SE AZ. See hydrology section below for further details. Thereafter, we`ll get into a quieter weather regime early next week. Any lingering precipitation should come to an end by Monday afternoon for most locales with shortwave ridging and dry conditions Tuesday and Wednesday. Thursday still looks dry as well as the flow becomes southwest ahead of the next weather system. Ensembles are showing more troughing Friday and Saturday but still some uncertainty on the specifics. Nonetheless, it`s not looking too impactful but 10 to 30 percent PoPs return Friday and Saturday. Daytime temperatures will mostly be below normal through the forecast period, with the coolest highs Sunday and Monday. && .AVIATION...Valid through 01/12Z. Mostly BKN clouds 7-10k ft AGL thru valid pd through some brief periods of SCT clouds may occur from time-to-time. Only slight chances for -SHRA/-TSRA through 00Z, then more numerous -SHRA with slight chc -TSRA mainly from KTUS/KOLS westward through end of valid period. SFC winds mainly SELY/SLY 5-10 kts thru the forecast period. Outlook btwn 01/12z and 02/00z, widespread MVFR conds, isold IFR conds, due to CIGS, SHRA and VSBY. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Isolated showers with a slight chance of thunderstorms today as the area will mainly be between weather systems. Precipitation chances increase tonight with widespread showers and isolated thunderstorms Sunday into Monday. Moderate intensity rainfall with pockets of heavy rain is expected Sunday into Sunday night, with locally heavy storm total rainfall amounts possible, especially in the higher terrain. Afternoon high temperatures will lower below normal through the forecast period. Breezy to locally windy southwest winds expected Sunday, turning northwest Monday. Minimum RH levels values into early next week will be 40+ percent. && .HYDROLOGY...As mentioned in the discussion section above, with the expected moderate to heavy rain on Sunday into early Monday we will see a response in the area rivers, creeks and washes across SE AZ. There is the potential for significant rises on the San Francisco, Blue, Gila and Sabino Creek with Minor to Major flood stages possible. This will continue to be evaluated over the next day and through the event. && .CLIMATE...Below is a listing of probability of exceedance of 36-hour rainfall totals from 5 pm Saturday Oct 31 to 5 am Monday Nov 2. Chance of seeing more than 1" 2" 3" Tucson 84% 35% 1% Marana 94% 37% 3% Mount Lemmon 97% 81% 23% Ajo 64% 18% 3% Picacho Peak 88% 34% 3% Oracle 95% 56% 5% Hannagan Meadow 100% 93% 39% Safford 69% 17% 0% Clifton 93% 50% 4% Willcox 46% 4% 0% Douglas 23% 0% 0% Sierra Vista 35% 3% 0% Nogales 71% 31% 0% Tucson rainfall records October 31st: 0.44" in 1926 November 1st: 0.42" in 1987 November (one-day): 2.09" on November 22, 1931 && .TWC WATCHES/WARNINGS/ADVISORIES... Flood Watch from Saturday evening through late Sunday night for AZZ501>515. && $$ Visit us on Facebook, X, YouTube, and at weather.gov/Tucson