Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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693 FXUS65 KPSR 232010 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 110 PM MST Mon Sep 23 2024 .SYNOPSIS... Strong high pressure will build into the region over the next couple of days resulting in rapid warming and temperatures reaching well above normal by Wednesday. An Excessive Heat Watch has been issued for the Phoenix area for Wednesday through Saturday. Dry air will also remain entrenched over the area this week with mostly clear skies persisting. && .DISCUSSION... Residual positively tilted troughing will finally exit the region over the next 24 hour replaced by anomalous high pressure ridging folding equatorward from the Pacific NW as the entire Conus pattern transitions towards a higher amplitude blocking regime. By the middle of the week, pronounced ridging approaching 2 normalized standard deviations above normal will extend from Arizona through the northern plains yielding an expansive area of much above normal temperatures. With H5 heights over the forecast area hovering very near 588dm, high temperatures 5F-10F above normal should be common across the CWA during the middle and end of the week. Numerical guidance seems to have consolidated into a narrow range yielding an increasingly large area of major HeatRisk across south-central Arizona and high probabilities of multiple record high days in Phoenix. In fact, it would not be out of the question for KPHX to reach 110F during the latter half of the week which would easily be the latest recorded in the period of record (since 1896 the latest 110F day in Phoenix is Sep 19 2010). While much above normal temperatures will also occur in SW Arizona and SE California, readings should not be quite as excessive and remain a few degrees short of records. This early autumn heat episode will likely be prolonged through the weekend as the weather pattern remains blocked with a deep cutoff low meandering around the middle Mississippi River valley forming the base of an eastern Conus Rex block. Although the SW Conus ridge center could oscillate a bit into the weekend, ensemble guidance shows a high correlation of H5 heights remaining around 588dm with some guidance indicating a potential strengthening above 590dm. Not surprisingly, trends in NBM forecast temperatures have continued to increase during the weekend pointing towards a more persistence type forecast instead of any substantial cooling. There remains a minority of ensemble members suggesting a deeper low pressure system entering central/southern California early next week, however the preponderance of modeling shows a general maintenance of anomalous ridging over the Southwest. As has been the case the past several days, it would be fully expected that the mandated official NBM forecast will warm in future iterations such that any notable cooling would not enter the region until the middle and end of next week. Regardless, there is still essentially no evidence of any measure of moisture return such that rainfall chances the remainder of September are almost nil. && South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation weather concerns are expected during the TAF period. Winds will be light out of the E before becoming more variable during the afternoon hours. Low confidence exists regarding a W`rly shift this afternoon/evening, but if this were to occur, it would likely only last a few hours before winds switch back to the E again. Clear skies will prevail through Tuesday afternoon. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns expected during the TAF period over the next 24 hours. Variable winds will eventually become established out of the W at IPL, while NE winds will shift out of the NW at BLH this evening. Clear skies will prevail through Tuesday afternoon. && .FIRE WEATHER... High pressure will build into the region over the next couple of days resulting in warmer temperatures and continued dry conditions. Above normal temperatures with lower desert highs topping 100 degrees are expected today with many locations topping 105 degrees by Wednesday, or near 10 degrees above normal. MinRHs will fall to around 10% over the lower deserts to 15-20% over higher terrain areas. Winds are expected to remain light through at least mid week, generally following diurnal wind patterns. Much of the same is expected through the rest of the week as the high pressure system keeps above normal temperatures and dry conditions over the region. && .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 && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Watch from Wednesday morning through Saturday evening for AZZ537-540>544-546-548>551. CA...None. && $$ DISCUSSION...18/Kuhlman AVIATION...RW FIRE WEATHER...Kuhlman CLIMATE...18