Area Forecast Discussion
Issued by NWS Phoenix, AZ

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203 FXUS65 KPSR 211750 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1050 AM MST Fri Jun 21 2024 .UPDATE...
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Updated 18Z Aviation Discussion.
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&& .SYNOPSIS... Monsoonal moisture has made its appearance into the Desert Southwest, with well above moisture levels continuing over the next several days. As a result, daily thunderstorm chances will continue, peaking over the weekend. Excessive heat conditions will continue today before subsiding this weekend. Temperatures are expected to warm once again early next week as thunderstorm chances diminish. && .DISCUSSION... Satellite imagery early this morning depicts a plume of moisture, courtesy of former tropical cyclone Alberto, streaming into southeastern Arizona. While the cyclone center will remain well south of the region, elevated moisture levels arrived yesterday, marking the first signs of monsoonal thunderstorm activity. As a mid-level high continues to retrograde westward across the country from the Ohio Valley region over the next several days, elevated moisture levels will continue through at least this weekend as the general flow pattern will remain southerly to southeasterly. Thus, the general outlook over the next several days is an uptick in thunderstorm activity through this weekend, and cooler temperatures (although apparent temperatures will remain in the upper 100s to lower 110s) across the region. An Excessive Heat Warning continues today for the lower desert areas of southcentral AZ, as areas of Moderate HeatRisk continue as lows this morning are now in the low to mid 80s (upper 80s to low 90s in the Phoenix metro) due to the increased moisture levels and highs today ranging from 110-115 degrees. While mid-level heights will continue to build due to the aforementioned high center moving westward, the increasing convective activity is expected to keep somewhat cloudier conditions, thus moderating the high temperatures across the region through this weekend. Looking closer at the moisture levels, PWAT`s generally around 1.25"-1.50" have entered southcentral Arizona (dewpoints in the upper 50s to low 60s) this morning. Ensembles depict PWAT`s continuing to rise upwards of 1.75" (generally the consensus ensemble means) going through this weekend. Thus, today continues this transition period to better thunderstorm chances, as the main activity is expected to remain primarily in the higher terrain areas of southcentral AZ once again this afternoon and through the evening hours. However, strong winds and areas of blowing dust in association with any outflows remains a decent threat today. The latest HREF guidance depicts another corridor of enhanced outflow wind potential across the higher terrain and edging into the Phoenix metro for this afternoon/evening, with upwards of 50-70% (20-40%) chance of 35+ mph wind gusts across the higher terrain (Phoenix metro) and a sizable 10% contour of 55+ mph wind gust potential across the higher terrain. Thus, very isolated severe thunderstorm potential continues today for strong/damaging winds, as was the case yesterday where there were a few severe wind reports across eastern Arizona. Given that there are pockets of 1000 J/kg surface based CAPE and DCAPE in excess of 1000-1500 J/kg available as antecedent ingredients for today, this severe wind threat should come as no surprise. The general steering flow continues to be southerly across the region, so storm progression should preclude most, if not all, activity from the lower deserts, as depicted by most hi-res guidance at this time. Thus, general thunderstorm chances are 15% or less across the Phoenix metro (up to 15% from the east valley to near 0% for the west valley) and 30-40% across the higher terrain areas. .PREVIOUS DISCUSSION... Heading into the weekend, the model guidance is showing additional surges of moisture moving into central and southern AZ and as far west as the southeast California lower deserts with PWATs increasing to between 1.6-1.8". However, at the same time that the moisture is on the increase, the upper-level ridge is expected to move directly overhead, which will increase the subsidence aloft. Therefore, the overall convective coverage will likely be limited to the higher terrain areas of AZ. The one caveat is that guidance is indicating that an inverted trough moving westward through northern Mexico will reach southern AZ on Sunday, which could potentially provide greater convective coverage. This is reflected in the latest NBM PoPs, which shows the greatest areal coverage for Sunday. High temperatures over the weekend, with the increasing moisture and potential cloud cover will be slightly cooler with overnight lows expected to more elevated. In fact, overnight and early low temperatures are forecast to remain near to just above 90 degrees across central Phoenix. Thus, even though the overall HeatRisk is expected to drop into the moderate category, the necessary heat precautions should still be performed, especially that there will not be as much relief during the overnight hours. Ensembles are strongly favoring the subtropical ridge to strengthen even further heading into the first half of next week with 500 mb height fields climbing to between 594-597dm. The high will likely be centered between eastern AZ and western NM with subsidence aloft increasing even further. This will cause some moisture erosion and thus convective potential will decrease as a result starting on Tuesday. With the strong ridge positioned directly overhead, extreme heat will be a concern with temperatures heading into the middle of next week rising to near 115 degrees with the areal coverage of major HeatRisk becoming more widespread. By the latter half of the week, there are indications from the ensembles that the ridge will gradually weaken as a trough moves through the Pacific Northwest and thus a slight cooling trend would ensue. && .AVIATION...
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Updated at 1750Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Winds over the next several hours will follow diurnal trends, gusting 20-25 kts at times. SHRA and TSRA are expected once again to the east of the Phoenix metro, but should remain outside the vicinity of terminal sites. However, it is possible (30-50% chance) that outflow winds from this activity advances toward the metro this afternoon/evening, generating strong gusts out of the E/SE upwards of 30 kts. Any potential outflows should keep winds out of the E through the remainder of the nighttime hours, with speeds calming during this timeframe. FEW-SCT mid and high level clouds will prevail over the region during the TAF period, with occasional periods of BKN skies. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation concerns are expected during the TAF period. Winds at IPL will be generally out of the E, through tomorrow morning, though directions may briefly shift to the W this evening. At BLH, diurnal tendencies will prevail, with a few afternoon gusts in the mid-teens. Clear can be expected this afternoon before FEW high clouds begin to move this evening.
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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 for today will range around 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 next week leading to even hotter temperatures along with slightly drier conditions. && .PSR WATCHES/WARNINGS/ADVISORIES...
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AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ534- 537>555-559. CA...None.
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&& $$ DISCUSSION...Young PREVIOUS DISCUSSION...Lojero AVIATION...RW FIRE WEATHER...Young/Lojero