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Area Forecast Discussion
Issued by NWS Phoenix, AZ
Issued by NWS Phoenix, AZ
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330 FXUS65 KPSR 171023 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 323 AM MST Wed Jul 17 2024 .SYNOPSIS... Daily thunderstorm chances will continue through the next several days, with most of the activity expected over the higher terrain areas. The chances for thunderstorms over the lower deserts will increase heading into this weekend and through early next week as moisture increases. Temperatures are expected to remain above normal through the next several days, with excessive heat conditions likely across the lower deserts starting Friday and lasting through the weekend. && .DISCUSSION... The latest objective analysis continues to show the subtropical high centered across central New Mexico. This high through the end of the week will slowly drift westward through the northern half of Arizona and then be situated across southern Nevada by the weekend. As it migrates westward it will also strengthen from 593dm it is currently to 596-597dm by the end of the week. As the high moves into Arizona, there will be increased subsidence aloft leading to drying in the low to mid level of the atmospheric column. Thus, low-level mixing ratios through the end of the week will generally remain under 9 g/kg across the lower elevations. However, enough moisture will still be present to result in daily chances for thunderstorm activity over the AZ high terrain. With the drying in the atmospheric column from the increased subsidence, there will be marginal instability as well as decent convective inhibition across the lower deserts through the end of the week as well. Thus, thunderstorm activity across the lower deserts will continue to be hard to come by and the overall coverage will be isolated at best. Even though storm activity will remain sparse across the lower deserts, outflow boundaries emanating from the higher terrain activity could still impact the region with gusty winds. Latest HREF shows around a 10-30% chance of gusts exceeding 35 mph across the south-central AZ lower deserts later today and on Thursday. A more active monsoon pattern looks to set up heading into the weekend through the first half of next week. As the subtropical high positions itself over Nevada and a trough dives through the Plains, a diffluent flow aloft pattern looks to be setting up across the region, increasing the upper-level forcing. In addition, moisture will be on the increase as well with the global ensembles showing PWATs rising to 1.5"+. This will lead to the low-level mixing ratios to rise above 10 g/kg. Therefore, the combination of the increased upper-level forcing and moisture will result in a more conducive environment for more widespread thunderstorm activity to materialize, including the lower deserts. At this time, it appears that the time period between Sunday through early next week will have the best shot of seeing at least a day of widespread strong thunderstorm activity. This far out, it is difficult to pinpoint exactly what day is likely to see the most widespread convection as it be dependent on the overall mesoscale details. The overall steering flow will be from the north/northeast and thus the overall flow pattern will be ideal for storms that develop over the Rim to move into the lower deserts. As mentioned in previous discussions, historically, the northeasterly flow events have led to severe weather materializing across the Phoenix metro area. Thus, the potential will be there for one day between the Sunday and early next week time frame to have an impactful convective event across the Phoenix area. This is something that bears watching as we get closer. In addition to the storm perspectives during the next several days, the other main weather headline heading towards the end of the week and into the upcoming weekend will once again be the potential for excessive heat conditions. As the aforementioned subtropical ridge moves westward and strengthens, surface temperatures will on a gradual warming trend through the end of the week. Highs today and Thursday will generally range between 108-113 degrees across the lower deserts with widespread moderate HeatRisk expected. However, by Friday and Saturday, as the 500 mb height fields from the subtropical high strengthen to around 596-597dm, surface temperatures are expected to increase even further with highs ranging between 110-116 degrees across the south-central AZ lower deserts to between 112-117 degrees across the western deserts. Temperatures of these magnitudes will result in a widespread coverage of major HeatRisk and thus an Excessive Heat Watch has been hoisted for the lower deserts. The Excessive Heat Watch for south-central and southwest AZ is in effect for Friday and Saturday but from the Lower Colorado River Valley westward through southeast CA, it is in effect from Friday through Sunday. Temperatures on Sunday across the AZ lower deserts are expected to cool down by about 3-5 degrees due to increased low-level moisture and thus the overall HeatRisk drops into the moderate category whereas across the Lower Colorado River Valley through southeast CA, temperatures are not expected to cool off as much and thus areas of major HeatRisk will still persist. Therefore, given the very hot conditions that will be materializing through the weekend, it will be crucial for anyone partaking in outdoor activities to take all the necessary heat precautions to prevent any heat-related illnesses. && .AVIATION...Updated at 0530Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Outflows emanating from recent shower/storm activity remains the biggest uncertainty at this point, at least for KPHX and KIWA, due to the rapid decay of the gust front as it moves southward. At this point, expecting both terminals to remain out of the southwest, with a chance of variable winds if the gust front reaches either terminal. Otherwise, light and variable winds expected to settle into the metro through the overnight hours, with KIWA the highest chances of having a discernible southeasterly wind develop going into tomorrow morning. Winds will then go westerly with afternoon gusts 15-20 kts. Outflows will again be a possibility tomorrow evening with convective activity in the higher terrain. Best chances will be out of the easterly or northeasterly direction, but probabilities are too low to include in this TAF package. SCT-BKN into the overnight hours will clear to FEW-SCT by tomorrow morning and remain this way until the evening hours where skies will become more cloudy due to convection nearing the metro. Southeast California/Southwest Arizona including KIPL and KBLH: Typical diurnal winds are expected to continue, with SE at KIPL and S-SW at KBLH. Limited convective activity across the region will only provide a very low (<10%) chance of an outflow reaching either terminal through the forecast period. Skies will remain FEW-SCT through the period. && .FIRE WEATHER... Sufficient moisture will remain in place to result in daily thunderstorm activity through the next several days, with activity mainly confined to the AZ high terrain through the end of the week. Thunderstorm chances start expanding into the lower elevations, especially by the upcoming weekend. Gusty, erratic winds will be the biggest hazard with these thunderstorms, exacerbating issues on any ongoing wildfires and new starts. Aside from the thunderstorm winds, the typical afternoon upslope gustiness upwards of 20-25 mph will be common. MinRHs during the next several days will range between 15-25% with fair to good overnight recoveries between 30-60%. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Watch from Friday morning through Sunday evening for AZZ530. Excessive Heat Watch from Friday morning through Saturday evening for AZZ531>546-548>555-559. CA...Excessive Heat Watch from Friday morning through Sunday evening for CAZ560>570. && $$ DISCUSSION...Lojero AVIATION...Young FIRE WEATHER...Lojero/Berislavich