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Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
916 FXUS65 KTWC 171628 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 928 AM MST Wed Jul 17 2024 .SYNOPSIS...Temperatures 4 to 7 degrees above normal through the rest of this week, with typical mid July moisture levels across the region. This will result in the 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...One of the old-school proxies that would hint at an active afternoon/evening here in the Desert Southwest was the presence of ACCAS in mid-morning hours. We see that across the south central portions of Arizona this morning, with the latest satellite visible image indicating it extends from Santa Cruz northward across the Tucson Metro Area and into southeast Pinal. It usually indicates that there is quite a bit of instability in the mid-levels. However, sometimes the old rule-of-thumb techniques can be misleading. The upper high is centered over east central New Mexico this morning and will drift west into west central Arizona this afternoon. As this occurs, we will see a warming in the mid-levels. This won`t completely erode away all the CAPE seen on the 17/12Z KTWC upper air sounding, but the warm air AOA 500 mb will choke off deep sustained updrafts and thus limit the overall coverage of thunderstorm to isolated from Tucson east today. So, the ACCAS we see this morning won`t come to fruition with regards to an active day in Tucson. However, this isn`t true for the entire forecast area. Today the main focus for thunderstorm activity will be a bit farther west today compared to yesterday because of the westward drift of the upper high, so primarily across western Santa Cruz and south central Pima county. A slug of dry air noted in the lower and mid-level water vapor imagery across Chihuahua and NE Sonora Mexico will rotate NW into south central Arizona this afternoon. This dry air will be available to entrain into the mid-levels of thunderstorms, thus increasing the potential for strong downdrafts/outflows. The 17/12Z HREF is picking up on this, and has a 50-70% chance bullseye of wind gusts in excess of 30kts within 25 miles of a given point over W Santa Cruz and the southern Tohono Oodham Nation. The drier air aloft will also reduce the threat of widespread heavy rain. The HREF isn`t too excited for heavy rain, with only a 10% chance of a 3-hour rainfall in excess of 1 inch within 25 miles of a given point in the same location discussed above. So, today will be more of a low-grade Monsoon day with the best chance for strong winds/heavy rain south of Tucson along the International Border. && .PREV DISCUSSION... /issued 250 AM MST Wed Jul 17 2024/ Residual showers and embedded thunderstorms continue early this morning to the west of Tucson. Much of this activity has formed on the outflow from a weakening MCS in far northern Sonora. It`s a bit difficult to discern at this juncture but it appears there is an MCV to the south of Nogales. CAMs have been struggling to pick up on this activity but regardless, expect it to trend downward through the early morning hours. The overall larger scale pattern remains quite persistent for our region as the mid/upper level high remains to our northeast, to the south of the Four Corners along the AZ/NM border. That continues to result in mid level easterly flow. Moisture levels are certainly adequate ranging from around 1 inch PWAT near the NM border to 1.5 inch over western Pima County. These levels are close to normal for this time of year. HREF/CAMs show scattered showers and thunderstorms developing mainly to the south and east of Tucson this afternoon, attempting to push west courtesy of outflows into the lower elevations/deserts this evening but confidence is low on how much storm activity there will be. There isn`t very much discernible in terms of disturbances/perturbations in the flow to hang our hat on today. Regardless, as we`ve seen over the past couple of days, there will continue to be a threat for gusty winds and localized flash flooding. Over the next few days and into the weekend there will be some minor changes to the flow pattern. The mid/upper level high to our north will begin to shift westward and slightly strengthen as it does so. Not much change to our easterly steering Thursday and perhaps Friday but by this weekend and into next week, northeasterly flow will dominate and that is a favorable direction for bringing storms off the Mogollon Rim/White Mtns into the lower elevations. Meanwhile, temperatures will nudge upward slightly, peaking Saturday about 5 to 8 degrees above normal. A few locales mainly to the north and west of Tucson will be teetering close to major Heat Risk levels so this is something we`ll be keeping an eye over the next couple of days. && .AVIATION...Valid through 18/12Z. BKN-OVC clouds 8-12k ft AGL south and west of KTUS will tend to diminish aft 17/12Z. Elsewhere, SKC-SCT clouds this morning. SCT-BKN clouds redevelop again aft 17/18Z, with diminishing cloud cover aft 18/05Z. SCT TSRA/SHRA thru 17/12Z west of KTUS. Otherwise, SCT TSRA/SHRA developing again for much of the area aft 17/19Z. MVFR conditions in TSRA, with mountain obscurations, vsby reductions and wind gusts to 40+ knots. Outside of TSRA, SFC wind at most locations 12 knots or less favoring a SWLY thru 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 mid July moisture levels expected through at least the upcoming weekend. 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 around 20 mph and light winds overnight. Wind direction during the afternoon will primarily be west to northwest into the weekend. && .TWC WATCHES/WARNINGS/ADVISORIES...None. && $$ Visit us on Facebook...Twitter...YouTube...and at weather.gov/Tucson