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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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943 FXUS65 KPSR 110010 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 510 PM MST Sat Aug 10 2024 .UPDATE...Updated 00z Aviation Discussion. && .SYNOPSIS... Daily shower and thunderstorm chances will continue through early next week, favoring the higher terrain areas of northern, eastern, and southeastern AZ, with the best chances for activity across the lower deserts this afternoon. By the middle of next week, drier air will filter into the region resulting in a significant downtrend in thunderstorm activity across the entire region. The hottest temperatures during the next week will be this afternoon as highs across most of the lower desert communities approach 110 degrees. Starting on Sunday and continuing though the first half of next week, temperatures will cool down slightly with readings closer to normal levels. && .DISCUSSION... Convective onset has been much earlier today than yesterday. Additionally, instability is higher and the cap is weaker. Noontime mesoanalysis has MLCAPE values between 500-1000 J/kg with MLCIN greater than -50 J/kg across most of southern AZ and southeastern CA. Highest instability is being analyzed in western Pima and southern Maricopa counties, where a weak upper level vorticity wave is also swinging through. Robust thunderstorms developed quickly in this area shortly after afternoon, realizing the instability. The other area to see early storm development was near the Kofas in La Paz County. Latest HREF keeps the best coverage of this afternoon`s thunderstorm activity generally between the Colorado River and just west and southwest of Phoenix, with isolated activity elsewhere. There is still uncertainty in the chances for the Phoenix area this afternoon. Recent hi-res model runs keep thunderstorm activity just outside of the Phoenix area. However, a strong outflow and colliding outflows will remain capable of generating storms in the Valley. DCAPE values are in the 1000-1500 J/kg range, so strong to locally severe downburst winds will be possible with any storm this afternoon and evening. The strong downburst winds will be capable of generating localized dense blowing dust. Moisture levels are also high enough, with mixing ratios near 11 g/kg, to support heavy rainfall that could lead to localized flash flooding concerns with any storms that anchor to a location or if multiple storms hit the same area. While most of the thunderstorm activity should dissipate after sundown, there is support for some overnight and morning elevated convection development in parts of south-central and southwest AZ. The best chances for elevated convection may coincide with a low and mid level moisture push from northern Mexico after midnight. Overnight and morning activity typically is not too strong, but still capable of producing wetting rains, lightning, and some gusty winds. The synoptic setup will remain nearly the same tomorrow through Monday with a low circulation over the Baja Peninsula and light southeasterly flow into the southern deserts. Following the moisture push tomorrow morning and with this synoptic setup, there will continue to be chances for lower desert thunderstorms and one of the two days may be an active one for the Valley. It is difficult to say which day is more favorable to see storms in the Phoenix metro area. Latest NBM PoPs would suggest tomorrow, with 20-30% chances, while global models support a vorticity wave from the south moving through south-central AZ Monday afternoon and evening. Regardless, both days will see MLCAPE values up to 1000-1500 J/kg and mixing ratios closer to 12 g/kg. This will support potential for strong storms, that could briefly become severe, and storms capable of producing heavy rainfall with the potential for localized flash flooding. Blowing dust will remain a risk as well with any thunderstorm outflows. .PREVIOUS DISCUSSION... Heading towards the middle to latter of the week, a significant pattern change is expected to occur across the western CONUS as a polar trough is expected to move from the Pacific Northwest through the Intermountain West Region. This will result in southwesterly flow across the region, which will significantly scour out the monsoonal moisture with the latest GEFS and EPS indicating mean PWATs between the Wednesday through Friday time frame decreasing to between 1-1.2" across south-central AZ the lower deserts to less than one inch across the western deserts. Therefore, storm activity is expected to be mostly absent and this is reflected in the latest NBM guidance which shows PoPs across the lower deserts at less than 10% with slight chances remaining across the higher terrain areas of eastern AZ. Heading into next weekend, the ensembles indicate that as the aforementioned polar trough moves into the eastern half of CONUS and the subtropical high rebuilds near Four corners region, a return to easterly flow and thus increasing monsoonal moisture is likely, with storm chances returning. High temperatures today are expected to be the hottest of the next 7 days as highs across most of the lower desert communities approach 110 degrees. Then, starting on Sunday and continuing through early next week, temperatures are expected to slightly cool off with readings returning back closer to normal. Temperatures during the second half of the week may trend slightly warmer but still remain below 110 degrees across the majority of the region. && .AVIATION...Updated at 0010Z. South Central Arizona including KPHX, KIWA, KSDL, and KDVT: VCSH/VCTS conditions have developed at KIWA, with VCTS conditions developing going through the evening hours across all TAF sites. Outflow winds have shifted winds southwestward at KPHX, but a southerly to southeasterly outflow is expected to shift the winds shortly after 00z. Best TS chances at KIWA at this time until around 01z. VCTS conditions possible across the metro from 00-03z. Lower confidence in wind trends should TS conditions develop across portions of the metro, but generally brief periods of southeasterly winds with initial outflows should turn back southerly to southwesterly into the overnight hours. Winds will then resume typical diurnal tendencies through tomorrow morning and into the afternoon hours. Sky conditions will remain SCT-BKN through the evening to overnight hours before becoming FEW-SCT by tomorrow morning. Cloud bases should remain aoa 9 kft, except for any TS development, where cloud bases could dip to around 6-8 kft. Southeast California/Southwest Arizona including KIPL and KBLH: VCTS/TS conditions at KBLH over the next several hours will be the most noteworthy aviation impacts through the forecast period. An outflow boundary emanating from convection moving westward across Yuma and La Paz Counties will reach both terminals this evening. For KIPL, winds should remain southeasterly, with some gustiness with 20-25 kts with the outflow passage. Can`t rule out some haziness or reduced visibilities due to lofted dust as the outflow winds move through KIPL, but there are no VCSH/VCTS expected to develop. For KBLH, VCTS conditions, with TS conditions possible, over the next several hours are possible as thunderstorms approach from the east. Initially, blowing dust reducing visibilities may become an issue, followed by TS developing in the vicinity or at the airfield, causing further reductions to visibility between 00-02z. Winds could also gust 30-40 kts from the east to southeast direction during this period, as seen in the TEMPO group. After TS conditions subside, winds should remain out of the southeasterly direction, before veering back to the southerly direction through the overnight hours. Gusty winds are expected to develop during the late morning hours tomorrow at KBLH, gusting 20-30 kts going through the afternoon hours. && .FIRE WEATHER... Shower and thunderstorm chances will persist into early next week, mainly favoring the higher terrain areas of AZ, with the best chances and coverage across the lower desert areas this afternoon and early evening. Any thunderstorm will remain capable of producing locally wetting rains and strong winds, with potential for outflow winds to travel long distances and cause sudden wind shifts. Storm activity is expected to trend downward significantly by the middle to latter half of next week as drier air filters into the region. Outside of the thunderstorm activity, the general winds will continue to exhibit the typical diurnal tendencies with the typical afternoon upslope breeziness. MinRHs will remain somewhat elevated through early next week with values of between 20-30%, before trending lower into the 15-25% range heading towards the middle of next week. The hottest temperatures over the next 7 days are expected today as highs across most of the lower desert communities approach 110 degrees. Slightly cooler temperatures are expected starting on Sunday and will persist through the first half of next week as readings trend closer to normal. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Benedict PREVIOUS DISCUSSION...Lojero AVIATION...Young FIRE WEATHER...Lojero/Benedict