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Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
396 FXUS65 KTWC 201554 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 854 AM MST Sat Jul 20 2024 .SYNOPSIS...Temperatures will remain 4 to 7 degrees above normal through the weekend, with increasing moisture levels across the region into early next week. This will result in increasing daily chances for showers and thunderstorms. The main threats will continue to be strong gusty outflow winds and heavy rainfall leading to localized flash flooding. && .UPDATE...Lingering showers and cloud cover remain from last night`s activity from Tucson westward. East of Tucson a few clouds remain but are quickly dissipating. The upper level high is centered over southern Nevada putting Arizona under northeast flow aloft. This is evident in the 20/12Z KTWC sounding which shows mid level northeast flow around 10 kts. PWAT was measured at 1.57", which is about 0.2" above the climatological normal. The atmospheric profile is moist above about 600 mb with a pronounced inverted V profile below that. Forecasted CAPE values will be around 1500 J/kg. All the ingredients are in place for storms, but the main uncertainty today will be how long the cloud cover sticks around and will it affect afternoon convection. Hi-res models show initiation between noon and 1 PM along the higher terrain with storms then slowly expanding west from outflows. The flow isn`t quite strong enough for a rim shot, but models do show fairly strong outflows coming off the rim. Tucson could see storms by late afternoon initiated off outflows moving in from the east and south. Wet microbursts will be possible with storms capable of strong to severe winds. The HREF has most of Arizona within the 70 percent chance contour for winds in excess of 30kts this afternoon and evening. A couple HREF members show a fairly strong outflow moving through Pinal County, which could also lead to blowing dust issues. Heavy rainfall and flash flooding will also be another threat. Locally heavy rainfall of 1-2 inches will be possible with the HREF highlighting the sky islands, Santa Cruz County, and also parts of the Tohono O`odham later this evening. The flash flooding threat will also be aided by slower moving storms. Please see the previous discussion for longer term details. && .PREV DISCUSSION... /issued 450 AM MST Sat Jul 20 2024/ PWATs across southeast Arizona have been increasing notably through the week with just over 1.5 inches recorded overhead per yesterday afternoon`s rawinsonde compared to 1.15 inches reported in Wednesday afternoon`s sounding. WPC has included south central CO, southeast AZ, and most of NM in a slight risk for excessive rainfall today, i.e., at least a 15 percent chance of exceeding flash flood guidance within 25 miles of a point. The 20/00Z run of the HREF depicts a widespread swath of 30 percent of exceeding an inch of rainfall in a given 3-hour period this afternoon through midnight. Tomorrow, the HREF shows even better chances (50-70 percent) for exceeding an inch of rain in a given 3-hour period and even a slight chance (10 percent) of exceeding 3 inches of rain in a 3-hour period over portions of Graham/Greenlee counties. Analyzing the wind potential, the HREF shows a widespread 70 percent chance of outflow winds in excess of 30 kts beginning over northern Graham/Greenlee counties associated with convection beginning as early as 2 pm MST. There is even a 90 percent along the AZ/NM border between 4-6 pm. There is also an area of higher probabilities to the south of Tucson over Santa Cruz county. The wind potential appears to spread west/southwest through midnight following the propagation of storms off of the White Mountains/higher terrain. There will also be decent outflows to consider propagating north/northwest from convection in northern Sonora and Santa Cruz county. HREF shows a widespread 70 percent chance of outflow winds in excess of 30 kts in this area as well. Outflow mergers from these two complexes will need to be considered over the Tucson Metro which may locally increase the thunderstorm potential between 4-7 pm. Based on the above analysis, today`s forecast focus is geared toward the potential impacts from scattered storms across southeast Arizona this afternoon and evening. The primary hazards will be isolated strong to severe storms capable of producing gusty outflow winds greater than 35 mph and locally heavy rainfall. Impacts resulting from the locally stronger cells may include isolated severe wind gusts, localized reduced visibility in areas of blowing dust, and isolated to scattered instances of flash flooding. Despite increased confidence in seeing heavy rain out of individual thunderstorms today, there is simply not enough confidence in the spatial coverage of thunderstorms to warrant a flash flood watch. That being said, if today ends up being a washout, PWATs increase again, and HREF provides increasing confidence in widespread flash flood potential, then antecedent conditions may warrant a headline for tomorrow afternoon and evening. Similar story on the possibility for strong/severe outflow winds resulting in areas of blowing dust. The 20/12Z run of the HREF should add confidence in specific areas to include in a Blowing Dust Advisory if necessary, but for now the confidence isn`t quite there for widespread areas of blowing dust as HREF didn`t highlight any areas with potential for 50 kt winds. Looking ahead, the upper air pattern remains favorable for daily chances for Monsoonal thunderstorms through the middle of next week with up and down days to be highlighted in future forecast packages. && .AVIATION...Valid through 21/12Z. FEW-SCT clouds at 8k-12k ft AGL with gradually diminishing coverage through mid morning. SCT-BKN clouds at 8k-12k ft AGL developing after 20/18Z. SCT TSRA/SHRA between 20/19Z and 21/07Z with decreasing coverage thereafter. MVFR conditions in TSRA, with mountain obscurations, vsby reductions and wind gusts to 45+ knots. Outside of TSRA, SFC wind at most locations 12 knots or less favoring NWLY direction during the afternoon/early evening hours and variable in direction at other times. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Deeper moisture remains across southeast Arizona, with typical July moisture levels expected into at least early next week. This will result in the daily cycle of mainly afternoon and evening showers and thunderstorms. The main threats will be strong, gusty and erratic outflow winds and heavy rainfall leading to localized flash flooding. Minimum relative humidity values will be 20-30 percent in most lower elevation locations and 25-40 percent in mountain areas. Away from thunderstorms, 20-foot winds will generally be 15 mph or less, with afternoon/early evening gusts to 20-25 mph and light winds overnight. Wind direction during the afternoon hours will primarily be west to northwest through the weekend, especially from Tucson westward and in the upper Gila River Valley. && .TWC WATCHES/WARNINGS/ADVISORIES... None. && $$ Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson