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Area Forecast Discussion
Issued by NWS Tucson, AZ
Issued by NWS Tucson, AZ
964 FXUS65 KTWC 202056 AFDTWC Area Forecast Discussion National Weather Service Tucson AZ 156 PM MST Sat Jul 20 2024 .SYNOPSIS...Temperatures will remain 4 to 7 degrees above normal through the weekend, with abundant moisture in place through Monday. This will result in daily chances for showers and thunderstorms with the main threats being strong gusty outflow winds and heavy rainfall leading to flash flooding. After Monday expect normal afternoon and evening Monsoon thunderstorm activity. && .DISCUSSION...Radar and satellite shows storms already forming over the higher terrain. Expect this activity to continue through the afternoon with hi-res guidance showing storms moving into the lower elevations along outflows. Coverage of storms will increase as the day goes with storms impacting Tucson as early as 4 PM. Aloft winds are northeasterly around 10 kts with GOES 16 precipitable water values ranging from 1.2" along the AZ/NM border and 1.75" in western Pima County. Mesoanalysis shows 1000-1500 J/kg of MLCAPE, which will be available into the evening hours. Wet microbursts will be possible thanks to DCAPE values around 1000 J/kg, leading to strong to severe winds. The HREF has most of southeast 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. 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. WPC has areas east of Tucson in a slight risk for excessive rainfall. Sunday looks to have a similar set up, although mid-level winds look to be more northerly and slightly stronger around 10-15 kts. Moisture amounts look to be roughly the same as today with perhaps a small increase along the AZ/NM border. Hi-res guidance is hinting at a rim shot type of day with storms forming along the rim early afternoon then pushing southward into Pinal and Pima Counties, then down to the international border. The main caveat to this scenario will be if mid-level winds will be strong enough to sustain storms traveling from the rim down into the lower deserts. However, any storms that do form will again be capable of strong to severe wind gusts as well as heavy rainfall. HREF probabilities for one inch or greater range from 20-50 percent across much of the CWA. Also, areas that receive rainfall today will be at greater risk of flash flooding Sunday, which could expand the threat. It may be necessary to consider putting out a Flash Flood Watch based on what happens today. WPC also has most of the CWA in a slight risk for excessive rainfall Sunday, with the area expanded further west compared to today. The upper level ridge stays centered over southern Nevada until at least the middle of the week. However starting Monday areas east of Tucson begin to dry out a little bit. PWAT values drop to around normal or a little below normal. This will diminish the larger areal threat for flash flooding, but daily isolated to scattered showers and thunderstorms are still expected. Threats will be more localized compared to this weekend. Late next week ensembles show the high moving south thanks to a shortwave trough moving through northern California. This doesn`t seem to affect moisture in southeast Arizona, but with the high being closer overhead this could lead to some down days for the monsoon by next weekend. && .AVIATION...Valid through 22/00Z. SCT-BKN clouds at 8k-12k ft AGL this afternoon and evening. SCT TSRA/SHRA through 21/05Z with decreasing coverage thereafter. MVFR conditions in TSRA with wind gusts in excess of 40 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 is in place this weekend leading to a threat of heavy rainfall and flash flooding and strong winds. After the weekend moisture decreases a little, although daily thunderstorms are still expected with heavy rainfall and strong winds being more localized. 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