Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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869 FXUS65 KPSR 242028 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 125 PM MST Tue Sep 24 2024 .SYNOPSIS... An unseasonably strong high pressure system building over the region will strengthen through the weekend bringing a return of excessive heat to much of the region. In fact, temperatures resembling the middle of June will be more common through early next week. Dry air will also remain entrenched over the area well into next week resulting in mostly clear skies and a lack of rainfall. && .DISCUSSION... An unseasonably strong high pressure ridge will build and continue to strengthen into the SW Conus and Rockies over the next several days as the entire flow pattern across the country becomes more blocked. An essentially stagnant synoptic pattern will take hold across the country later this week with zonal jet energy retreating into southern Canada yielding a near persistence forecast across the CWA into at least the weekend. As a result of the evolution, H5 heights currently analyzed around 586dm this morning will rise closer to a 588-591dm range during the latter half of the week with excellent agreement among ensemble members that H7-H8 temperatures feeding the boundary layer will reach the 99th or greater percentile of climatology for this time of year. Correspondingly, guidance spread remains extremely narrow yielding excellent forecast confidence that surface readings will warm into a 6F-12F above normal range. Areas of Major HeatRisk will become fairly widespread Wednesday, initially focused more over south-central Arizona due to the ridge center setting up over New Mexico, however then expanding westward as the breadth of high pressure expands. With numerical guidance continuing to push temperatures higher with each model run over the last several iterations due to blended models finally catching up to the majority of operational and ensemble members, the hottest days and most expansive major HeatRisk appears to be on tap Friday and Saturday. Forecast confidence is excellent that multiple days of temperatures nearing and exceeding record territory will be common through the latter half of the week (see Climate section). In fact, its becoming a near certainty that Phoenix Sky Harbor will experience its latest 110F day on record (since 1896) as the previous latest 110F was September 19, 2010. Given the increased magnitude and expansion of major HeatRisk, have expanded the warning across much of the CWA with the Friday-Saturday time frame highlighted as the most impactful. Despite the mandated, blended NBM suggesting cooler temperatures arriving early next week, the majority of ensemble members suggest this blocking pattern persisting with anomalously high H5 heights remaining stationed over the SW Conus into the middle of next week. Thus, there is now higher confidence this early autumn heat episode will continue through the weekend and perhaps well into next week with some expectation that the ongoing warning may very well be extended. With a large subset of models indicating H5 heights being maintained above 590dm into the middle of next week, many in the region may be dealing with record or near record temperatures well into the middle of next week. && .AVIATION...Updated at 1700Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns are expected through the TAF period. Winds will overall favor light, diurnal tendencies with extended periods of variability. There is uncertainty in regards to whether or not winds this afternoon will make a definitive westerly switch. As of now, expect light and variable winds to prevail while favoring a light westerly component at times this afternoon. Clear skies will persist. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are expected through the next 24 hours. Winds will generally favor the west at KIPL and NNE at KBLH with extended periods of light and variable winds throughout the period. Clear skies will continue. && .FIRE WEATHER... Strong high pressure will settle over the region today and remain in place at least through the weekend. This will result in well above normal temperatures and continued dry conditions. Lower desert highs temperatures are likely to reach 9-13 degrees above normal by Wednesday and stay there through the weekend. MinRHs will continue to dip down to around 10% over the lower deserts to 15-20% over higher terrain areas through the rest of the week, while winds remain light generally following diurnal wind patterns. && .CLIMATE... Record highs this week: Date Phoenix Yuma El Centro ---- ------- ---- --------- Sep 25 108 in 1989 112 in 1963 110 in 2015 Sep 26 108 in 1989 111 in 2010 110 in 2010 Sep 27 107 in 2009 111 in 1963 111 in 2009 Sep 28 108 in 1992 108 in 1994 109 in 2009 Sep 29 107 in 2003 110 in 1980 110 in 1969 Sep 30 107 in 2010 109 in 2012 108 in 2020 && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning from 10 AM Thursday to 8 PM MST Saturday for AZZ530-533-534. Excessive Heat Warning from 10 AM Wednesday to 8 PM MST Sunday for AZZ537>544-546-548>551-553>555-559. Excessive Heat Warning from 10 AM Friday to 8 PM MST Saturday for AZZ531-532-535-536. CA...Excessive Heat Warning from 10 AM Thursday to 8 PM PDT Saturday for CAZ569. Excessive Heat Warning from 10 AM Friday to 8 PM PDT Saturday for CAZ566-567-570. && $$ DISCUSSION...18/Kuhlman AVIATION...RW FIRE WEATHER...Kuhlman CLIMATE...18