Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/02/19

Area Forecast Discussion
National Weather Service Phoenix AZ
455 PM MST Sun Sep 1 2019 .UPDATE... Updated 00Z aviation discussion. && .SYNOPSIS... Large areas of strong winds and blowing dust are expected over south-central and southwest Arizona late this afternoon and evening. Isolated to scattered thunderstorms are expected as well, mainly over south-central Arizona. Any given storm will be capable of producing extremely strong winds and brief heavy rain. Storm chances will trend down Monday and remain marginal for much of the workweek before an uptrend later next weekend. Expect above normal high temperatures for much of the workweek as well. && .DISCUSSION... Rest of Today and Tonight... Convection over the Rim and White Mountains is underway but even the lower elevations have started (a bit early). The basic ingredients in play for this afternoon include 500 to 1500 J/kg of MLCAPE, 1300-1500 J/kg of DCAPE, and moderate to strong (for our area this time of year) easterly steering flow and associated effective shear. This scenario is supportive of storms capable of producing strong downdrafts and possibly organizing into linear structures. However, there still remains CIN to overcome over much of our forecast area. There is still time for additional surface heating so expect some more erosion of the CIN but likely some areas will remain. To varying degrees, the CAMS have been depicting a scenario of storms over east-central Arizona and southeast AZ (especially central Pima County) tracking west and northwestward and producing strong outflows. In turn, colliding outflows lead to new storm development. Of note, there is generally good agreement that storms will get to the northern and eastern edges of metro Phoenix but otherwise not become widespread on the Valley floor. This is likely due to CIN but also there isn`t a bunch of activity over Graham and eastern Pinal Counties track west/northwestward due to drier air there. At a minimum though, anticipate broad swaths of outflow causing varying degrees of blowing dust over portions of Pinal, Maricopa, and Yuma Counties (less so over metro Phoenix). In anticipation of this, a Blowing Dust Advisory was issued earlier to provide a heads up. Where storms do occur, they will be capable of producing localized winds of 50+ mph and possibly even 60+. Later this evening, storms are anticipated to develop over northern La Paz County as outflows emanate from southwest Yavapai County (and possibly with help from outflows coming in from the southeast). As for southeast California, there is good agreement on the lack of storm development (too much CIN). Of note, the HREF depicts potential for outflows to move all the way through Imperial County. Eastern Riverside County could get some outflow from storms emanating from over northern La Paz and southern Mohave Counties. Monday through Sunday... Moisture values trend down during the first half of the workweek as drier air from the east moves in. Thus, storm potential trends down but not to zero. High temps nudge upward with portions of south-central AZ flirting with high Heat Risk values. Will need to keep an eye on this for possible need for a Watch. The GEFS plumes depict an increase in precipitable water during the latter half of the week. Possibly a reflection of the tangential impacts of Tropical Storm Juliette as well as the subtropical high being centered further east for a time. There looks to be a bigger increase for Sunday possibly with the aid of an inverted trough. That is when the next noticeable uptrend in PoPs takes place. && .AVIATION...Updated at 2354 UTC. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: An outflow boundary from the southeast has already pushed through KIWA with another outflow boundary from the northeast expected to push through KSDL by 00Z. This pair of outflow boundaries should merge together and continue to support thunderstorm development as it pushes to the west-southwest, moving through KPHX and KDVT between 0015-0030Z. Winds at KPHX may initially be out of the E-SE if that outflow boundary arrives first, but the faster-moving NE boundary should quickly overtake it. Midlevel stability evident on aircraft soundings into KPHX that may weaken convection that develops to some extent despite its current organization driven by 25-30 knot shear. Nevertheless, wind gusts of 30-35 knots are expected with the boundary with gusts in excess of 40-45 knots in thunderstorms , some which may be severe with winds above 50 knots as all of Phoenix is currently under a Severe Thunderstorm Watch. Once the initial outflow boundary and line of storms pass, shower activity with vicinity thunderstorms should continue through 02-03Z. Beyond this, the forecast is more uncertain as a strong line should stabilize the atmosphere enough to prevent additional thunderstorms from developing, but continued showers are possible. For now, we are not showing any shower activity with easterly winds generally persisting into the overnight hours even though there may be a brief period of W-SW winds associated with outflows on the backside of more organized convection. Assuming today`s storms hold together, we expect much less convective activity tomorrow under weak subsidence with diurnal winds below 15 knots. Southeast California/Southwest Arizona including KIPL and KBLH: Outflow boundaries and wind shifts later tonight resulting from storms in Arizona will be the greatest aviation impact, especially at KBLH. Through the daylight hours, sustained S/SE winds upwards of 15 kt will be common with a few gusts into the upper teens/lower 20s. Confidence is moderate to high that strong easterly outflow winds will sweep into SE CA (particularly KBLH) late this evening. Although timing is relatively uncertain and tied to the strength of storms and outflows currently pushing into the Phoenix metro, we currently expect the boundary around 05Z with a two-hour period of convection in the vicinity. Blowing dust and reduced visibilities may be possible as well. Storms are currently not expected to affect KIPL, but the TAFs for both sites will be amended as confidence on the timing of the easterly outflow and potential for convection increases. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Tuesday through Saturday: There is no significant change to the outlook forecast. Thunderstorm chances will continue across most of southwest and south-central Arizona, and southeast California, on Tuesday. Locally heavy rain and gusty outflow winds will be possible. Despite the increase in moisture and slight decrease in temperatures, daytime minimum humidity values will still fall into the 15-20% range with overnight recoveries into the 30-60% range. Outside of any outflows, the winds will favor diurnal trends with typical afternoon breezes. A gradual drying trend will occur Wednesday into Friday, keeping the highest thunderstorm chances across the higher terrain of south- central Arizona. Temperatures will remain a few degrees above normal each day. && .SPOTTER INFORMATION STATEMENT... Spotter reports may be needed this afternoon and evening. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Blowing Dust Advisory until 8 PM MST this evening for AZZ531-532- 535-536-538>540-550-551-553-554-559. CA...None. && $$ DISCUSSION...AJ AVIATION...Hopper/Hernandez FIRE WEATHER...Rogers