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Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
570 FXUS65 KTWC 192126 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 226 PM MST Fri Jul 19 2024 .SYNOPSIS...Temperatures will remain 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. && .DISCUSSION...The big forecast question for today is to gage how much activity will we see across Southeast Arizona today. At 2 PM MST this afternoon, we are seeing two distinct areas of thunderstorm activity. The first is up along the Mogollon Rim and the higher elevations in northern Graham and Greenlee counties. The second is down along Santa Cruz and southern Cochise counties. The upper pattern is defined by the 500 mb centered over central Arizona this morning. This feature will slowly drift WNW toward west central Arizona by this afternoon, then up toward southern Nevada by Saturday afternoon. This will serve to setup a more favored NE flow off the White Mountains/Mogollon Rim (rimshot pattern) that generally results in stronger thunderstorms impacting Southeast Arizona. Today is kind of that transition day where all the ingredients are not setup yet like it will be for this weekend. Expect storms to fire up in the higher terrain and push SW into the lower elevations by late this afternoon into this weekend. The 19/12 HREF reflects this thinking by showing a 70% chance of outflow winds in excess of 30 kts begining across northern Greenlee at 23Z, then expanding SW into the rest of Graham and SE Pinal counties by 02Z. The latest GOES derived precipitable water imagery shows a general increase in PWAT values east and NE of Tucson (increasing from near 1 inch up to 1.25 inches) over this time yesterday morning. Dry enough to aid strong thunderstorm downdrafts/outflows, but also enough to help increase the potential for heavy rainfall. The 19/12Z HREF depicts a 50% probability of 3-hour rainfall exceeding an inch with the storms that move off the higher terrain in central Graham and Greenlee counties. Not great, but not bad. It just means that we will begin to transition from just wind was the primary thunderstorm threat to a more dual threat with wind and heavy rainfall as we transition into the weekend. A weak mid-level disturbance moving into the far SE corner of the state this afternoon should help initiate convection across Cochise and Santa Cruz county during the mid-afternoon hours today. We have moistened up a bit in these areas as well over the past 24 hours, so the treat of a strong thunderstorm outflow is present...but lower than yesterday. The 19/12Z HREF has this area painted with a 50% chance of outflow winds 30+ kts starting at 22Z. As mentioned yesterday, as we move into this weekend and next week, the upper high positions itself near southern NV which would suggest the stronger NE flow pattern will continue into next week for our neck of the woods. The strongest days are still looking like Saturday and Sunday as the mid-level flow is the strongest for Southeast Arizona. && .AVIATION...Valid through 21/00Z. SCT-BKN clouds at 8k-12k ft AGL with SCT TSRA/SHRA through midnight, with similar conditions expected Saturday between 20/18Z and 20/00Z. 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