Area Forecast Discussion
Issued by NWS Phoenix, AZ

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189 FXUS65 KPSR 220010 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 510 PM MST Fri Jun 21 2024 .UPDATE...
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Updated Aviation section.
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&& .SYNOPSIS... Elevated moisture content will continue to provide daily thunderstorm chances during the next several days, with the best chances for activity across the higher terrain areas of eastern AZ. Excessive heat conditions will continue today across south- central AZ before subsiding this weekend. As high pressure strengthens over the region through the middle of next week, temperatures are expected to heat up once again. && .DISCUSSION... The overall synoptic pattern continues to feature a weak trough over the Great Basin and a retrograding ridge of high pressure centered over the Tennessee Valley. This overall combination is continuing to enhance southerly flow, which is advecting moisture from Mexico, with the majority of it coming from the remnants of Tropical Cyclone Alberto. Latest SPC mesoanalysis shows PWATs ranging between 1.2- 1.4", mainly across central and southeast AZ along with MLCAPE between 500-1000 J/KG across eastern AZ, which will be sufficient to generate thunderstorms this afternoon across the higher terrain areas to the east of Phoenix. With DCAPE approaching 2000 J/KG, the environment once again will be favorable for the generation of strong downburst winds. Given that the steering flow will remain from the south to south-southwest, any thunderstorm activity that develops over the higher terrain areas is not expected to impact the lower deserts. However, gusty outflow winds emanating from the thunderstorms could reach the Phoenix metro area with the HREF indicating a 30-50% chance of 35+ mph winds. These outflow winds could also generate areas of blowing dust as well. Heading into the weekend, southeasterly flow will continue to bring surges of moisture into central and southern AZ and as far west as southeast CA, with ensembles showing PWATs incrementing to between 1.6-1.9", which for this time of the year is close to 250% of normal. As a result, more thunderstorm activity is expected, especially across the higher terrain areas of northern and eastern AZ. Even though the large-scale forcing for ascent will be lacking, given that the subtropical ridge will be building over the region, there are a couple of disturbances that will traverse northern Mexico that could increase the convective coverage, even across the lower deserts. For Saturday, there is subtle inverted trough in the 700 mb layer on the northern periphery of the large moisture plume from the remnants of Alberto that could help enhance the convective activity with several Hi-Res models indicating some morning showers across southern and even into southwest AZ followed by afternoon convection developing across the Mogollon Rim. With the steering flow expected to shift more out of the east, the activity across the Mogollon Rim could make its way down into the lower deserts, including the Phoenix area, as some of the hi-res guidance is showing for late Saturday afternoon/early evening. Latest NBM PoPs range between 40-60% across the higher terrain areas and between 20-40% across the south-central AZ lower deserts. On Sunday, another inverted trough associated with a tropical disturbance located over the western Gulf of Mexico is projected to move into northern Sonora, which could help increase the convective coverage once again. However, the convective coverage on Sunday will be dependent on how Saturday`s activity pans out given that the atmosphere could be overworked and usually it is not that common to get widespread activity on back to back days as the atmosphere needs a day of recovery. With the thunderstorm activity this weekend, strong gusty winds and locally heavy rainfall due to the higher moisture content will be the main hazards. Heading into next week, the subtropical ridge is forecast to strengthen with 500 mb height fields expected to increment to between 594-597dm through the middle of the week. The strengthening ridge will increase the subsidence aloft and thus moisture content will gradually decrease. Thus, a decrease in storm activity is likely heading into the middle of next week with activity confined mainly to the higher terrain areas of eastern AZ. Ensembles indicate that the ridge will likely weaken heading into the latter half of next week as a large-scale trough moves across the Pacific Northwest, however, it appears that abundant monsoonal moisture will still remain in place to continue to result in daily chances for afternoon and early evening convection, especially across the higher terrain areas of eastern AZ. Excessive Heat will continue to remain an issue today, particularly across south-central AZ, where high temperatures today are expected to top out between 110-114 degrees. This will result in areas of Major HeatRisk materializing and thus an Excessive Heat Warning remains in effect through this evening. With more cloud cover and higher moisture content this weekend, temperatures are expected to slightly cool off with a moderate HeatRisk in place. Heading into the early to middle portion of next week, as the aforementioned ridge strengthens overhead, temperatures are expected to heat up once again with most of the lower desert areas likely to exceed 110 degrees. && .AVIATION...
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Updated at 0009Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: A surface wind convergence zone is in place over the Greater Phoenix area with southeasterly winds meeting southwesterly winds. For most of the TAF sites, this has resulted in southerly or southwesterly with KIWA being an exception (southeasterly). As mixing weakens this evening, anticipate an overall trend to an easterly component due the orientation of the pressure gradient. However, a major consideration is thunderstorm activity. So far, activity over the higher terrain to the east of the metro area has been quite isolated. But there is more significant activity over eastern Pinal County. Outflow winds from there are expected to reach the metro area this evening. This will reinforce southeasterly winds at KIWA and cause a more distinct shift at the other sites including KPHX. At this time, best estimate of arrival at the TAF sites is in the 01Z-03Z time frame from southeast to northwest. In addition, there may be visibilities reduced below 10 miles (mainly at KIWA). This scenario assumes that there will be not be new storm development over the Valley floor which is not a certainty. At this time, probability of TS or VCTS is too low (20-30% to reflect in the TAFs). As for sky cover, anticipate ceilings remaining AOA FL150. Remnants from storm activity over Mexico could conceivably produce some pockets of virga late tonight and Saturday morning but expect that to remain near and south of I-8. During the day Saturday, anticipate a change to the wind regime with significantly lighter winds until perhaps late afternoon/early evening when storms over Gila and far northeast Maricopa Counties may spread outflow winds across the Valley floor. Southeast California/Southwest Arizona including KIPL and KBLH: Skies at the TAF sites will be mostly clear except for some minor mid and high clouds Saturday. Winds at KIPL will have a lot of variability this evening waffling between westerly and southeasterly. Expect predominantly southeasterly to prevail late tonight before more distinct S/SEly winds develop by late morning. At KBLH, expect a familiar change from southerly early in the TAF period to southwesterly before nocturnal drainage winds (NWly) develop late tonight. Expect an uptrend in southerly breeziness (gusts to 25kts) starting by late morning.
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&& .FIRE WEATHER... Moisture will continue to increase across the region into the upcoming weekend bringing daily chances for afternoon thunderstorms, with the best chances expected across the higher terrain areas of south-central AZ. MinRH values this afternoon will range between 30-40% across the far eastern districts to between 10-15% across the western districts. MinRH values will then increase for Saturday, ranging from 40% across the eastern districts to 20-25% across the western districts, with similar levels through Sunday as well. Winds will follow their typical daily tendencies, with gusty outflows from any thunderstorm activity anticipated over the next several days. High pressure is expected to strengthen over the region by early to middle of next week leading to even hotter temperatures along with slightly drier conditions. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ534- 537>555-559. CA...None. && $$ DISCUSSION...Lojero AVIATION...AJ FIRE WEATHER...Young/Lojero