642 FXUS65 KPSR 272322 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 422 PM MST Tue May 27 2025 .UPDATE...Updated 00z Aviation Discussion... && .KEY MESSAGES... -Seasonably hot and dry through Thursday, with highs in the upper 90s to lower 100s. -A pattern shift late this week will result in a slight boost in temperatures, with highs peaking around 99-107 across the lower deserts on Friday leading to areas of Moderate HeatRisk. -An influx of tropical moisture over the weekend may cool temperatures to near normal by Sunday and lead to slight chances for showers (20-40%) and isolated thunderstorms for portions of Southwest and South Central Arizona. && .DISCUSSION... The latest objective analysis data shows weak troughing meandering over the Desert Southwest, while upper level ridging continues to build over much of the western CONUS with positive height anomalies extending into western Canada. Temperatures continue to run close to seasonal normals as a result with lower desert highs forecast to top out in the 97-103 degree range for most lower desert locales through Thursday. Dry conditions will continue across our area over through the end of the workweek with light winds and some afternoon breezes expected each day. Getting into the end of the workweek, a shortwave that is currently about 800 miles off the California coast over the eastern Pacific will continue to propagate eastward before cutting off near the northern Baja Peninsula. As this occurs, upper level ridging will again build across much of the western CONUS leading to increasing positive height anomalies over our area. Operational and ensemble guidance show 500 mb heights rising to around 586-589 dm over the forecast area, which is about the 90th climatological percentile. Forecast highs top out Friday in the 99-107 degree range for most lower elevation areas and 97-102 degrees for some of our higher elevation communities such as Globe and San Carlos. As a result, we will see more widespread coverage of Moderate HeatRisk Friday, so folks and those particularly sensitive to the heat should exercise the necessary heat safety precautions to avoid any heat related illnesses. In addition to the aforementioned cutoff low, a tropical system currently south of the southern Mexico coast will continue to advance northward off the coast of western Mexico. The latest NHC update now shows a 100% chance of tropical depression or tropical storm developing from this area of interest tonight or tomorrow. While we aren't expecting this tropical system to move into our area, ensemble and deterministic guidance continue to show tropical moisture getting transported northward into the southwest CONUS thanks to the cutoff low. Global ensembles have trended up on moisture and now have PWATs closer to 1.2-1.3" with the latest EPS now showing most of its members showing values around 1.3-1.4". Given that this time of year is typically very dry, PWAT anomalies as result climb in excess of 200-250% of normal. This moisture combined with forcing from the low will support increasing rain chances for the upcoming weekend. Guidance continue to come into better agreement in the evolution of this cutoff low, though there are still discrepancies in the exact timing and track of the low as it moves inland. As of now, ensemble and deterministic guidance support the low moving inland Sunday into early Monday. Forecast soundings show increasing instability Sunday afternoon and will support increasing thunderstorm chances across the region. Gusty winds and locally heavier rainfall totals may accompany any thunderstorm development that we see. Exact QPF remains uncertain, but for now the latest WPC amounts support Arizona lower desert areas seeing generally less than 0.10" with higher amounts across the high terrain north and east of Phoenix. Stay tuned as we continue to fine-tune the forecast. && .AVIATION...Updated at 2320Z. South Central Arizona including KPHX, KIWA, KSDL, KDVT: No aviation weather concerns will exist during the TAF period. The overall wind pattern will continue to exhibit the typical diurnal tendencies with speeds aob 10 kts, although some occasional gusts into the mid to upper teens will be possible through early this evening and once again for Wednesday afternoon/early evening. There will also likely be some periods of light and variable winds, especially during diurnal transitions. The SCT to BKN high clouds currently overhead will gradually clear out heading into the evening and overnight hours. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns are expected through Wednesday afternoon under generally mostly clear skies with some passing high clouds. At KIPL, current SE winds will shift out of the SW for a period this evening before shifting back out of the SE during the overnight hours. At KBLH, winds will generally fluctuate between the S-SSW. The overall wind speeds at both terminals will remain aob 10 kts. && .FIRE WEATHER... Very dry conditions will continue through the next couple of days as Min RHs range between 5-15%. Overnight Max RHs will generally range between 20-40% with locally higher values across Imperial County. Winds will favor typical diurnal tendencies with afternoon gusts to around 15-20 mph. Temperatures will be near to slightly above normal over the next couple of days before peaking on Friday. An unsettled weather pattern then takes shape for this upcoming weekend and will lead to cooler temperatures and increasing chances for showers and thunderstorms. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Smith AVIATION...Lojero FIRE WEATHER...Smith