Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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119 FXUS65 KPSR 290548 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 1048 PM MST Fri Jun 28 2024 .UPDATE... Updated Aviation section. && .SYNOPSIS... Above normal temperatures will persist well into next week, with many lower desert locations seeing highs near or exceed 110 degrees each afternoon. Sunday looks to be the hottest over the next week, resulting in an Excessive Heat Watch for portions of Southeast California starting Sunday. Minimal monsoonal rainfall is expected through the start of the weekend, with only slight chances for an isolated shower or thunderstorm for areas east and southeast of Phoenix. Moisture and subsequent rain chances will increase starting Sunday, likely lasting through the middle of next week. && .DISCUSSION... Current atmospheric analysis continues to show the sub-tropical ridge stretched out over parts of Southern Arizona and Northern Mexico, while a compact area of low pressure moves over the Northern Plains and Southern Canada. In between these two systems, westerly flow aloft will prevail through today before becoming more southerly by Saturday as the center of ridge becomes established over the Southern Plains. This high pressure is also expected to strengthen over the weekend, resulting in increasing heights aloft and a concurrent rise in temperatures. Day-to-day highs will increase about a degree a day, maxing out between 110-114 degrees by Sunday across the lower deserts. The westerly flow will continue to funnel dry air towards the Desert Southwest through the remainder of today, especially aloft, where a noticeable dry pocket was observed between 500-300mb in this mornings Phoenix sounding. However, enhanced boundary layer moisture is expected to stick around for longer than originally forecasted, which could play into shower and thunderstorm potential this afternoon and evening for south-central Arizona. Subsidence aloft provided by the aforementioned ridge has been one of the main limiting factors for monsoon activity over the past few days but with the high starting to retreat to the east today, this subsidence may not be as much of a factor. As a result, hi-res guidance is now hinting at the potential for some isolated shower and thunderstorm activity across parts of south-central Arizona this afternoon and this evening. With forecast soundings projecting DCAPE values this afternoon exceeding 1600 J/kg, enhanced wind gusts cannot be completely ruled out with stronger cells. By Saturday, as the sub-tropical ridge becomes established over the Southern Plains, flow across Arizona will become more southerly to southeasterly, allowing for continued moisture flux over the state. At the same time, an easterly wave will traverse northern Mexico, eventually making its way toward southeast Arizona by late Saturday/early Sunday. The dynamics associated with this wave, combined with PWAT values between 1.5-1.75", should create a favorable environment for potentially widespread shower and thunderstorm activity across south-central Arizona. NBM PoPs have trended downward, mostly for lower elevation area, since the previous forecast update. However, given the favorable setup, it appears that rain chances are being underdone at this time. One potential factor that may inhibit convection during this timeframe will be the presence of cloud cover, with greater coverage likely resulting in less thunderstorm activity. The main uncertainty appears to be how long these clouds may stick around for, but we should have a better idea of how things will evolve during the next 12-24 hours. The overall pattern will remain favorable for monsoon activity through at least the middle of next week as the ridge over the Southern Plains remains mostly stationary. At the same time, an anomalously strong ridge will develop over the eastern Pacific, eventually moving onshore by late next next week. Based on where the models position this feature, dry northerly flow would likely take over, limiting further rain chances, while temperatures potentially climb toward the 115 degree mark for areas across the lower deserts. && .AVIATION...Updated at 0548Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: A broad swath of southerly outflow winds originating from earlier storms that were over Pinal and Pima Counties has overspread the Valley floor. Expect southerly breeziness (gusts to 20kts) to slowly weaken and become light and variable favoring southeasterly before sunrise. Expect SW/Wly winds to develop by mid afternoon. Anticipate an increase in high clouds late tonight and Saturday morning. As for storm development Saturday afternoon/evening, confidence in impacts at the TAF sites too low to reflect in the forecasts at this time. Southeast California/Southwest Arizona including KIPL and KBLH: Overall, expect familiar warm season wind patterns at the TAF sites through Saturday. At KIPL, westerly winds will shift to southeasterly by sunrise. At KBLH, SWly winds early in the TAF period will trend to southerly by 09Z or so. Southerly breeziness (gusts to 25kts) resumes around midday Saturday. Otherwise, expect clear skies. && .FIRE WEATHER... Unfavorable monsoon conditions today into Saturday will continue to lead to clear to mostly clear skies with only minimal (10%) chances for isolated showers and maybe a thunderstorm across Pinal and Gila Counties. Above normal temperatures will persist with readings running 3-5 degrees above normal. Humidities will continue to drop across the western districts today and likely into south-central Arizona on Saturday with lower desert MinRHs between 15-20%. Humidities over the Arizona higher terrain should stay above 25%. Afternoon breeziness will be common today with gusts reaching to or just over 20 mph in many locations before winds settle down a bit on Saturday. Starting Saturday night, moisture will increase across much of southern Arizona leading to a return of shower and thunderstorm chances on Sunday. Monsoon storm chances should persist through at least south-central Arizona for much of the first half of next week, while humidities rebound to 20-25% across the lower deserts to upwards of 35-40% over the eastern Arizona high terrain. Temperatures next week will remain several degrees above normal each day. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Watch from Sunday morning through Friday evening for AZZ530. CA...Excessive Heat Watch from Sunday morning through Friday evening for CAZ562-566-567-569-570. && $$ DISCUSSION...Kuhlman/RW AVIATION...AJ FIRE WEATHER...Kuhlman