Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/16/21

Area Forecast Discussion
National Weather Service Phoenix AZ
459 PM MST Sun Aug 15 2021 .UPDATE... Updated 00Z aviation discussion. && .SYNOPSIS... Abundant monsoon moisture will remain entrenched across the Desert Southwest into the middle part of the week maintaining daily chances for showers and thunderstorms. The primary threats with any storms will be localized heavy rainfall that may cause flash flooding. Localized strong winds and some blowing dust will also be possible for desert locations. A gradual decrease in moisture during the latter half of the week should reduce rain chances across the lower deserts as early as Thursday night. Temperatures will remain below normal for most of Arizona, while near normal temperatures are mostly expected for southeast California. && An anomalously moist airmass continues to dominate the region today. This morning`s PSR 12Z sounding registered an anomalously high PW of 2.19" and mean W of 16.5 gm/kg. The objective analysis shows the UL high centered over the NE CA/NW NV border with pronounced mid level ENE flow along its S flank which is introducing some moderate mid level drying, with an area of mid-level elevated mean RH sliding into S AZ/N MX. Vort max`s and inverted troughs are over this same borderland NW MX/N Baja area with an additional connected trough over NW Chihuahua/SSW NM. The disturbances were supporting a small area of rain/showers over Imperial Cty and modest isolated thunderstorms over S-SE AZ. On the NE flank of the high was another shortwave trough pushing SSEwrd into N CO/N UT. An area of isolated thunderstorms was developing ahead of this trough in N AZ and CO. The midday ACARs mean sounding confirmed some pronounced mid level drying centered at H5 with PW down to 1.11", although elevated lower mid level and BL moisture were still supporting SB-MU CAPE in the low 1000s with modest CIN. The slight-moderate instability was held in check with a strong CAP just below H7. Early afternoon METSAT showed decreasing cloudiness with partly- mostly sunny conditions across much of the deserts with mostly fair weather CU. The HREF members are in much better agreement for this afternoon and evening`s storm prospects. They are picking up on the short term drying and decreasing instability trend by agreeing on keeping storms rather isolated this afternoon and evening and confined mainly to the high terrain of N-E AZ, due to the approaching shortwave to the N, and the deserts of S-SE AZ due to the inverted troughs. The NBM has also locked onto the trend as well with only 10-20% POPs depicted for this afternoon and into Monday. Some isolated storms to the north may try to impact the far N portions of La Paz Cty from near Parker to Alamo Lake/State Park. Any isolated thunderstorms that manage to form later today will still be capable of producing localized flash flooding across the deserts or near Phoenix due to the wet soils. Beginning later tomorrow and through the middle of the week the shortwave trough and associated vort max`s up north will push Swrd into NM and E-AZ as PW once again recovers and inversion CAPS are weakened through the column. The HREF members are now fairly bullish on either a scattered, or scattered to numerous, line of organized thunderstorms forming along the Rim on Monday afternoon and descending S into Phoenix and the lower deserts Monday evening. These storms will be capable of producing strong winds, heavy rainfall, and localized flooding as potential threats. HREF also calls for a >70% chance of 35 mph wind gusts Monday evening in and around Phoenix. In preparation for this system the Flash Flood Watch for S-Cent AZ has been extended through 2 AM MST on Monday night. The active weather pattern should continue into at least Wednesday as the inverted trough that will exit to our southwest today, likely comes back around into our region on Wednesday. This weak cut off low (what was the inverted trough) looks to get picked up in the flow as an anomalously strong trough dives through the Pacific Northwest into the Great Basin. There is good model evidence of a fairly strong upper level jet entering Arizona from the south Tuesday night into early Wednesday while there may also be increasing jet forced ascent from the trough diving into the Great Basin. With plenty of moisture likely still in place across our region, Tuesday night into Wednesday could end up being another fairly widespread (modest) rain event across south-central and eastern Arizona. For the late week and weekend model uncertainty increases, but the majority of the ensembles show the deep trough to our north and then to our northeast becoming the dominant feature. With broad troughing likely taking over our region, we should be under a drying westerly flow by Thursday, likely lasting through next weekend. Models are in better agreement showing this drier air working into our region Thursday into Friday, likely ending or significantly reducing our rain chances over the lower deserts on Thursday with only very limited rain chances over the Arizona high terrain. Temperatures this week are likely to be fairly stable with the western deserts mostly around seasonal normals each day. For the south-central Arizona deserts, including the Phoenix area, below normal temperatures are likely to continue with highs mostly in the mid 90s to around 100 degrees. && .AVIATION...Updated at 2359Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Aside from a 20% chance of convective outflows this evening, minimal aviation impacts are expected until early Monday evening when another round of strong to severe thunderstorms are possible. Light and variable or south-southwest winds below 10 knots will persist at the Phoenix TAF sites until easterly flow returns around 05-06Z. There is only about a 10% chance of storms at any terminal tonight if any weak northeasterly outflows push into the lower deserts due to a low-level stable layer. Westerly flow should return by late Monday morning through the afternoon hours, with a 40% chance of strong to severe thunderstorms during the evening. The best timing of outflows and storms is between 01-04Z Tuesday, with a 70% chance of winds exceeding 35 knots and at least a 10% chance of winds exceeding 50 knots. BKN-OVC ceilings in these storms should generally remain above 6-8 kft. Southeast California/Southwest Arizona including KIPL and KBLH: Minimal aviation concerns are expected through Monday morning, with gusty south-southeast winds expected on Monday afternoon. Winds will generally favor a southeast component at KIPL with speeds generally remaining below 10 knots aside from gusts up to 20 knots on Monday afternoon. At KBLH, winds will generally favor a southeast to southerly component with speeds below 10 knots aside from gusts up to 25 knots on Monday afternoon. FEW-SCT ceilings should remain above 10 kft. && .FIRE WEATHER... Tuesday through Saturday: Elevated moisture levels and a threat of showers and thunderstorms will continue through at least Wednesday. A drying trend should then set in late in the week with dwindling chances for wetting rains on Thursday and likely little if any rain chances by Friday. The strongest storms early in the week will be capable of producing localized flash flooding, particularly in the higher terrain north and east of Phoenix. Minimum humidities mostly between 30-45% early in the period will gradually lower to 20-30% late in the period. Temperatures will remain below normal across much of Arizona through the period, while the western deserts see near normal readings. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Flash Flood Watch through late Monday night for AZZ534-537>563. CA...None. && $$ DISCUSSION...Sawtelle/Kuhlman AVIATION...Hopper FIRE WEATHER...Kuhlman