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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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822 FXUS65 KPSR 150825 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 125 AM MST Thu Aug 15 2024 .SYNOPSIS... Rather quiescent weather conditions will prevail through the beginning of next week with thunderstorm activity starting to return to the region over the weekend, albeit mainly confined to higher terrain areas of Arizona. Temperatures will remain in a slightly above normal range with afternoon highs reaching near 110 degrees over the warmest lower desert communities. && .DISCUSSION... With pronounced negative height anomalies sunken across the Pacific NW, objective analysis indicates a strong zonal jet continuing to progress through central California and the Great Basin acting to dampen subtropical ridging over the SW Conus. South of this jet core, an impressive midtropospheric subsidence inversion has expanded with a large pool of H5 temperatures -3C or warmer covering the entire Southwest. With westerly flow also scouring moisture from the top of the boundary layer, convective inhibition will be quite overwhelming precluding much, if any deep convection through Friday across the CWA. Despite the thermal anomalies aloft, H5 heights are not particularly unusual (~595dm) resulting in a continuation of slightly above normal temperatures and widespread moderate HeatRisk the remainder of the week. By Friday night, the center of the H5 anti-cyclone will have shifted across the forecast area into the southern high plains undergoing a strengthening and amplification phase. This transition will be in response to an entire Conus pattern amplification with deep negative height anomalies becoming re-established along the Pacific NW coast as well as the Great Lakes. This pattern re-alignment will place the region back into a more traditional monsoon south/southeast flow capable of importing better quality moisture northward. However, despite models suggesting low level mixing ratios rebounding into a more supportive 10-12 g/kg range over the weekend, a measure drier air residing at the top of the boundary layer and strengthening of the subtropical high may relegate thunderstorm activity to mountainous regions of Arizona. Should better deep moisture advect north from larger Sonoran storm complexes and multiple outflow boundaries collide over lower elevation population centers, rainfall prospects could be somewhat better than the paltry 10-15% POPs advertised by the official NBM forecast. Through the first half of next week, the upper level flow pattern will not change tremendously other than the expansion and further strengthening of subtropical high pressure over New Mexico. Ensemble mean H5 heights rise closer to a 595-597dm range though some individual members suggest the center of the anti-cyclone closer to eastern Arizona with heights peaking closer to 597-599dm. Should these latter solutions come to fruition, temperatures would naturally be warmer than forecast with the 75th percentile of the model spread indicating readings potentially flirting with excessive heat criteria towards the middle of next week. As the high pressure system expands, subsidence over much of Arizona should increase with better moisture and forcing mechanisms displaced into northern Mexico. This pattern would still support higher terrain storms and conceivably some activity into lower desert locations, albeit not widespread by any stretch, though mandated NBM POPs seem a bit low given the prevailing synoptic pattern. && .AVIATION...Updated at 0500Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No aviation concerns are expected during the TAF period. Winds will follow typical diurnal trends, with occasional gusts 15-20 kts during the afternoon and early evening hours. Other than a FEW high clouds, skies will be mostly clear through Thursday evening. Southeast California/Southwest Arizona including KIPL and KBLH: No aviation weather concerns are anticipated through the next 24 hours. Winds will follow familiar diurnal trends, with some periods of variability during directional shifts, especially at BLH. Other than a FEW high clouds filtering in Thursday afternoon, skies will be mostly clear. && .FIRE WEATHER... Slightly above normal temperatures will prevail through the beginning of next week as conditions remain drier than normal by mid monsoon standards. Higher terrain areas of eastern districts will stand the best chance for thunderstorms over the weekend, however wetting rainfall should be very isolated. With drier air already in place, minimum afternoon humidity levels will fall to around 10-15% in western districts and between 15-25% for the Arizona districts. Overnight recovery should be fair to good between 30-60%. Wind patterns should follow a more typical upslope/drainage pattern without much chance of gusty outflows the next few days. However, local afternoon upslope gusts of 20-25 mph will be common. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...18 AVIATION...RW FIRE WEATHER...18