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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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416 FXUS65 KPSR 141158 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 458 AM MST Sun Jul 14 2024 .UPDATE...Updated 12Z Aviation Discussion... && .SYNOPSIS... Thunderstorm chances will increase today with the potential for some activity to get to the Valley floor. Strong winds and localized dense blowing dust will be the primary concerns with these thunderstorms. Thunderstorm chances will then become mostly confined to the higher terrain through at least the first half of the week before chances increase across the lower deserts for the second half. Temperatures through the week will remain above normal with widespread Moderate to locally Major Heat Risk. && .DISCUSSION... Early morning satellite and radar showed a decaying thunderstorm complex over northern Sonora with isolated to scattered new thunderstorm development occurring along the international border in southern Arizona. Elsewhere across the forecast area, quiet conditions are in place with a gulf surge helping to boost dew points into the 60-70 degree range across much of central Arizona to southeast California. The aforementioned decaying northern Sonora thunderstorm complex will also act to increase moisture into southern Arizona. Meanwhile, objective analysis showed an upper level ridge centered near the Four Corners region with 500 mb heights around 594 dm. Temperatures today are forecast to be around 2-5 degrees cooler than yesterday due to the increase in moisture into the region along with the gradual weakening of the ridge. Lower desert highs today will top out in the 106-111 degree range for most places. However, the increased moisture will help push heat indices to upwards of 113-115 degrees across southeast California and the Lower Colorado River Valley. Thus, an Excessive Heat Warning remains in effect through this evening for the areas mentioned. The increase in better boundary layer moisture and higher instability will provide better thunderstorm chances to south- central Arizona heading into this afternoon and evening. Ensemble guidance show PWATs continuing to rise to around 1.3-1.5" today and forecast soundings and HREF data showing CAPE values increasing to around 1000-1500 J/kg. DCAPE values will also be high at around 1500- 2000 J/kg, which will support strong to potentially severe winds and long traveling outflows. HREF paintball plots show convection firing up this afternoon in southeast Arizona near the international border and along the Rim. These thunderstorms are then expected to send outflows traveling toward central Arizona, though there is uncertainty in exactly how strong these outflows will be and whether or not new convection will fire along them. Some of the hi-res guidance has backed off on convection getting into the lower elevations of south-central Arizona with modest CIN in place. However, strong, colliding outflow boundaries could certainly invigorate shower and thunderstorm activity across parts of the lower deserts this evening. As mentioned before, thunderstorms will be capable of producing strong winds with 00Z HREF probabilities for winds in excess of 35 mph climbing upwards of 50-70%, particularly across southern Gila County. Strong outflows will be capable of producing areas of dense blowing dust in dust prone regions, which can quickly reduce visibilities and make travel dangerous. Locally heavy rainfall will be possible for areas that end up under thunderstorms and could lead to an isolated flooding concern. Going into the first part of the new workweek, moisture in the region will slightly decrease while subsidence over the area increases as the ridge center migrates over Arizona. The latest global ensemble runs have backed off on the amount of drying that will take place in the atmospheric column early to midweek with ensemble mean PWATs sticking around 1.3-1.5". Daily thunderstorm chances will continue across the Arizona high terrain through the upcoming week with chances increasing across the lower elevations during the latter half of the week as moisture increases. Aside from thunderstorm chances, temperatures will once again approach or exceed 110 degrees for many areas across the lower deserts by the middle part of the week. With these temperatures, some areas could flirt with excessive heat conditions, particularly across southeast California where there are pockets of Major HeatRisk. Elsewhere, most places will be under Moderate HeatRisk. Folks should continue to exercise the proper heat safety precautions this week, including staying cool, limiting time outdoors, and staying hydrated. && .AVIATION...Updated at 1155Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: The potential for thunderstorm activity as well as outflow winds will be the primary weather issues heading into this evening. Westerly winds will continue through most of the day. There continues to be some lingering gusts near 20 kts, however, that is expected to subside during the next couple of hours with speeds generally aob 12 kts through this afternoon. Heading into this evening, thunderstorms are likely to develop across the higher terrain areas to the north and east of the metro. Confidence continues to remain low on the overall convective coverage across the area and as a result there is no mention of VCTS/TSRA for this TAF package but may need to be included in subsequent packages if confidence for greater coverage increases. What is a bit more certain is the potential for outflow winds to impact the terminals, most likely from the north to northeast between the 03-05z timeframe. There is a 30-50% chance of gusts exceeding 30 kts with the outflow. Sky coverage through this afternoon will generally remain FEW, increasing to SCT-BKN this evening depending on the overall convective coverage. Southeast California/Southwest Arizona including KIPL and KBLH: Southeast winds will be favored at KIPL while south winds will be favored at KBLH through the period. Thunderstorms are likely to develop over the higher terrain areas, especially over the San Diego Mountains later this afternoon. This could result in some outflow winds, especially at KIPL where a TEMPO for westerly wind gusts is in place. Otherwise, periodic gusts into the 20-25 kt range are likely at both sites, especially this afternoon and evening. FEW-SCT passing mid to high level clouds will prevail through the period. && .FIRE WEATHER... Chances for thunderstorms and wetting rains will increase this afternoon and evening, particularly over the higher terrain east of Phoenix with lesser chances in the lower elevations. Thunderstorms will be capable of producing strong erratic outflow winds with up to a 50-70% chance of outflow winds in excess of 35 mph across southern Gila County this afternoon/evening. Through the first half of the week, thunderstorms and wetting rainfall chances will be primarily concentrated over the higher terrain of eastern districts before chances across the lower deserts begin to increase during the latter half of the week. Gusty outflow winds will be possible on many days this week exacerbating issues on any ongoing wildfires and new starts. Otherwise, afternoon upslope gustiness of 20-25 mph will be periodically common. Min RHs will be in the 15-25% before decreasing to around 10-20% by the middle part of the week. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530. CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ560>570. && $$ DISCUSSION...Smith AVIATION...Lojero FIRE WEATHER...Smith/18