794 FXUS65 KPSR 181808 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1108 AM MST Thu Sep 18 2025 .UPDATE...Updated Aviation Discussion. && .KEY MESSAGES... - A Flood Watch has been issued from early this morning through this evening for southeastern California and southwest Arizona, due to the potential for locally heavy rainfall leading to flash flooding. - Lingering shower and thunderstorm chances will be possible Friday, primarily focused across the Arizona high terrain. - Near normal temperatures over the next couple of days should warm to above normal this weekend into early next week with Moderate Heat Risk returning across the lower deserts. && .SHORT TERM /TODAY THROUGH FRIDAY/... Deep tropical moisture has advected northward into southeast California and southwest Arizona over the past 12-18 hours with PWATs at around 2" in Yuma and 1.6-1.9" for the rest of the aforementioned areas. Good moisture also extends eastward through much of south-central Arizona with PWATs between 1.3-1.6". The remnants of TC Mario have also merged with a weak upper level low off the coast of southern California providing ample dynamics for showers and a few embedded thunderstorms across southern California early this morning. The forecast for the rest of today into tonight remains challenging due to a number of factors. The moisture has basically reached its peak across the region and drier air above 15-20K feet is already starting to push north northeastward across northern Baja. The drier air should reduce the cloud cover to a degree later this morning allowing for better destabilization across southeast California and southwest Arizona. Assuming this occurs, it should give rise to scattered thunderstorm development as early as mid to late morning, but more so during the afternoon with PWATs still hovering somewhere between 1.7-1.9" across the aforementioned areas. Hi-res guidance is still quite unsure on the areal coverage for thunderstorms today, but it has at least been consistent in showing the potential for heavy rainfall. It seems quite likely a portion of this area will see anywhere from 1-2" of rainfall today with some low-end potential for very localized amounts exceeding 2". The best potential for heavy rainfall and any flooding impacts are likely to occur across southeast California into La Paz County. We have expanded the areal coverage of the Flood Watch to include all of La Paz County and much of Yuma County, although Hi-res CAMs have not specifically shown much activity for Yuma County. Looking farther to the east across south-central and eastern Arizona, guidance continues to struggle with the rainfall potential for today. Some model runs have indicated heavy rainfall potential into western and southern Maricopa County, but they have been quite inconsistent and have somewhat backed off on this threat over the past 24 hours. Given moisture will still be quite high into south-central Arizona, we can't rule out some thunderstorms producing heavy rainfall but the chances are much lower compared to out west. It would not be surprising to see at least one or two good thunderstorms later today that drop 1" rainfall amounts outside of the Flood Watch area, but given the low probability we do not see a need to expand the Watch into Maricopa County. Rainfall amounts across south-central Arizona may only average around 0.1" with a good portion of the area barely seeing any rainfall. The drier air aloft will spread across the rest of the region by tonight with any shower and thunderstorm activity diminishing considerably by around midnight. Forecast PWATs are expected to drop to between 1.2-1.5" already by Friday afternoon with the best rain chances shifting over the Arizona high terrain. Some locations to the north and east of Phoenix may see an additional 0.25" of rain on Friday, but most locations will barely see any measurable rainfall. The 06Z HRRR is quite optimistic for showers and thunderstorms between Phoenix and Flagstaff Friday afternoon/early evening, but that seems to be an outlier at this point. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Model uncertainty will continue over the weekend into early next week as models are having a difficult time resolving what will happen with a cut-off low that is likely to form off the California coast Friday into Saturday. Yesterday's model runs had mostly dismissed this cut-off low as it called for it stay well to our west, but the latest trends indicate it may get quite close to southern California Sunday into Monday. We are still expecting our region to largely be under the influence of the subtropical ridge this weekend with overall dry conditions, but if the low does creep close enough to our area we may see some moisture return Sunday into Monday. The NBM shows 10-20% rain chances for later Sunday into Monday, but this seems overly optimistic given the amount of moisture that is likely to be available. We tend to believe the ridge will win out keeping the cut-off low far enough to the west resulting in the bulk of our area staying dry through at least early next week. Temperatures will be trending upward this weekend into early next week as the ridge builds in from the south, but with the model trends of keeping the cut-off low closer forecast highs are noticeably lower than what we saw 24 hours ago. Highs are currently expected top out at around 100 degrees this weekend before pushing closer to 105 degrees by around next Tuesday. The upper end of guidance does still show the potential for record highs for Phoenix next Tuesday and Wednesday, but that will only happen if the cut-off low stays far enough to our west. && .AVIATION...Updated at 1808Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary weather concern during the TAF period will be the potential for thunderstorms across the Phoenix area, though forecast confidence is low. Morning shower activity will continue to lift north of the terminals over the next hour or so with clearing conditions. Confidence detioriates later this afternoon through this evening on how thunderstorm activity will evolve. Latest hi-res guidance continue to show large discrepancies in the overall coverage of thunderstorms this evening, yielding low forecast confidence. Have maintained the PROB30 in the TAFs for now when thunderstorm chances will be the highest. Any thunderstorms that develop will be capable of producing gusty erratic outflows, localized blowing dust, and locally heavy rainfall. Guidance then suggest the potential for additional shower/storm activity overnight tonight. SCT-BKN cloud bases will mostly remain aoa 10 kft throughout the period, though could briefly drop lower in thunderstorms. Outside of any thunderstorm impacts, winds will generally follow diurnal trends with speeds mostly below 10 kts. Southeast California/Southwest Arizona including KIPL and KBLH: The primary weather concern during the TAF period will be increasing chances for thunderstorms going into this afternoon. Hi-res guidance show thunderstorms developing across SE CA this afternoon that will be capable of producing gusty erratic outflow winds, localized blowing dust, and locally heavy rainfall. Chances for outflow wind gusts in excess of 30 kts climb to 50% during this time. The primary window for these thunderstorms looks to be between 21Z-02Z. SCT-BKN clouds will mostly remain aoa 8-10 kft through this evening, briefly dropping lower in thunderstorm activity, before clearing this evening. Outside of thunderstorm impacts, winds will favor the W-NW at KIPL and fluctuating between the S to NNW at KBLH with speeds mostly below 10 kts. && .FIRE WEATHER... Abundant moisture with a weather system residing across western portions of the region today will help to bring fairly widespread showers and thunderstorms. Best chances will be across southeast California and southwest Arizona where a Flood Watch will be in effect for much of today. Rain chances will continue into Friday, but mainly across the Arizona higher terrain. Humidities will be elevated over the next couple of days with MinRHs mostly between 30-40% before gradually lowering closer to 20% by Sunday. Outside of any storms, winds will remain light over the next few days with directions mostly following diurnal trends. High pressure should settle back into the region over the weekend and into early next week resulting in a warming trend and overall dry conditions. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Flood Watch through late tonight for AZZ530>533-536. CA...Flood Watch through late tonight for CAZ560>570. && $$ SHORT TERM...Kuhlman LONG TERM...Kuhlman AVIATION...Smith FIRE WEATHER...Kuhlman