Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/10/19

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
510 PM MST Fri Aug 9 2019 .UPDATE...Updated aviation discussion. && .SYNOPSIS... Increasing moisture and disturbances across eastern Arizona and the southern Gulf of California will cause periods of scattered showers and thunderstorms through Sunday morning over south central Arizona. Temperatures will remain slightly below normal for most areas except southeast California where temperatures will be near normal with drier air preventing any rainfall chances to the west. Dry air will begin to sweep eastward late this weekend through most of next week, pushing thunderstorm chances east into New Mexico and allowing above normal temperatures to return. && .DISCUSSION... Very isolated showers with broken to overcast midlevel clouds continue to persist across south central Arizona this afternoon. Although aircraft soundings currently suggest that there may be a small amount of surface-based instability, there is a stable layer evident at 500 mb just above the melting layer where drier air is present, but is slowly moistening. In general, this should keep any convective activity that forms mostly in the form of shower activity with just a few thunderstorms. Thunderstorms this afternoon and early evening will be most likely at the fringes of the cloud shield out over the higher terrain of La Paz County where daytime heating is considerably higher, likely causing enough instability to overcome the midlevel stable layer. Showers and very isolated thunderstorms will be possible over the higher terrain north and east of Phoenix and the Phoenix metro as well, but in general the cloud cover should prevent instability from increasing enough to allow more than a rogue thunderstorm or two through early evening. This could change over the Phoenix metro if we warm up a few more degrees more towards our convective temperatures in the 103-104 degree range, but this is not currently expected. Storm coverage over the higher terrain should remain very isolated, with weak steering flow this afternoon keeping outflow boundaries out of Phoenix. Confidence is increasing in strengthening southerly steering flow later tonight with an increased coverage of showers and thunderstorms over southern Arizona. A weak inverted trough currently in far southeast Arizona and southwest New Mexico into Mexico near the Sonora-Chihuahua border is currently associated with convection over Cochise County and northern Sonora. Models generally do not appear to be handling the position of this feature very well, with perhaps up to a 100 mile bias south and a little east of where convection is developing now. In addition, it may be combining it with the weak inverted trough that is located further south over the Gulf of California and Sinaloa that is expected to move through the region later tomorrow. Given that confidence in any particular model solution is low, we are favoring ongoing observational trends which models are slowly latching on to that suggest the inverted trough over southeast Arizona combined with one further south over the southern Gulf of California may help trigger convection over southern Arizona tonight. If this occurs, some model solutions show scattered to widespread showers and a few thunderstorms moving into Pinal, Gila, and perhaps southern Maricopa Counties overnight which would have relatively efficient rain rates capable of producing flash flooding. Thus, we have issued a Flash Flood Watch for these areas beginning at 1 AM MST tonight to account for this potential, though the best chances for heavy rainfall associated with flash flooding are likely not until later on Saturday. At some point tomorrow, the much advertised pronounced inverted trough that is currently triggering convection over the Gulf of California, Baja California, and southern Sonora will begin to provide larger-scale lift to help get convection going across south central Arizona. Although what happens tomorrow afternoon and evening will be heavily modulated by whatever evolves tonight, the broader model ensemble continues to show a swath of heavier rainfall from Pinal County north-northeast into Gila and far eastern Maricopa Counties. Specifically, the operational GFS shows a large area of 1-1.5" rainfall totals over three hours in southern Gila County late tomorrow afternoon and early evening. Although the general depiction of this seems reasonable given PWAT values in the 1.6-1.8 range with favorable upslope development, the axis of deeper moisture could end up further west or east and the timing could be earlier or later. The Flash Flood Watch is primarily for this feature and is lowest in confidence for areas at the far western and northern fringes of the watch including the East Valley of the Phoenix metro. There is higher confidence for the watch the further south and east you go, particularly for Gila and Pinal Counties. Models continue to trend faster, so it seems reasonable that Sunday may end up being drier and warmer than previously thought with PoPs generally in the 10 percent range for Phoenix by Sunday afternoon, with isolated to scattered storms confined to the higher terrain of Gila County. Through much of next week, a series of progressive northern stream waves will create unusually strong troughing over the western Conus and further dampen and force subtropical ridging southward. By Sunday evening, most of southwest and south central Arizona will be out of its monsoonal pattern with the entire atmospheric column comprised of westerly flow. This pattern will result in PWATs plummeting down towards 1 inch or below with boundary layer mixing ratios falling to 5-7 g/kg which generally does not support convection. Even storms over favored higher terrain locations will be tough to come by, with most deep convection relegated to areas along the New Mexico border for multiple convective days. As this drier pattern evolves, H5 heights will still hover in a 592-595dm range yielding another round of potentially excessive heat as afternoon highs approach or exceed 110 degrees over the lower deserts by Wednesday. && .AVIATION...Updated at 1755 UTC. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Very interesting weather pattern with potential aviation concerns through the TAF period. Initially, there is the possibility of a first weak wave moving north out of southeast AZ, bringing a few showers or a stray storm into the greater Phoenix area after midnight and into early morning tomorrow. Currently, we are rather stable over the central deserts due to cloud cover/cool high temps, and storms not that widespread from Tucson south and east. So, confidence is low in this initial wave so decided to remove the VCSH from the TAFs later tonight. Decided to just go with variable mid/high level debris decks with FEW-SCT down to around 7k feet. If showers develop will amend the TAFs later tonight or in the 06z package. Much stronger and more vigorous wave will move north and through eastern AZ later Saturday and through Saturday night, bringing scattered to numerous showers and embedded storms...some of which may affect the greater Phoenix area. Before this big wave arrives, may see some clearing during late morning and early afternoon before convection really spins up. This will be an issue for the subsequent 06z and 12z TAF packages to add this increasing/lowering cloud and weather. Actually, blowing dust Saturday night may be less of an issue if widespread rains or heavy rain develop and really wet the ground. One of the problems in the TAFs lately have been winds. Although winds have not been very strong (mostly less than 12kt), the directions have been tricky and poorly forecast by much of our guidance. Currently it appears that the preponderance of our wind guidance suggests winds turning to the southwest or west early this evening and continuing much of tonight. Decided to make the shift to light southwest after 02z...but turn winds back to the south or southeast after about 06-07z. Most of the showers/storms that develop tonight will be over southeast AZ and we may eventually see a broad, weaker SELY outflow wind move up into the Phoenix area and winds will probably stay out of the south or southeast rest of tonight and into the day Saturday. After 19z opted to turn winds back to the southwest less than 10kt due to model consensus and the fact that that conforms to typical diurnal tendencies. Again, with the weather/disturbances moving through over the next 24+ hours, winds will be very trick and often will not conform to guidance. Thus there will be numerous updates likely needed to the TAFs as winds make sudden and unexpected changes. Stay tuned with regards to the winds tonight through Saturday. Southeast California/Southwest Arizona including KIPL and KBLH: Dry southwest flow aloft will keep skies mostly clear next 24 hours. Winds to favor the south at KBLH through the period, with gusts OVER 20KT likely during the afternoon. Winds to favor the south to southwest at KIPL with elevated wind speeds in the afternoon between 10-15 kts. Some tendency for winds to turn to the southeast by mid morning Saturday with speeds below 10kt. Aviation Discussion not updated for amended TAFs. && .FIRE WEATHER... Monday through Friday: A much drier pattern will take hold beginning Monday with thunderstorms very limited even over higher terrain area. Temperatures will warm above normal amidst the drier air mass, with winds following typical diurnal trends. Humidity values will generally drop into the upper single digits to mid-teens across the lower elevations while remaining slightly higher above 20 percent over the higher terrain. Overnight recovery will be poor to fair except at higher elevations where it will be better. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Flash Flood Watch from 1 AM MST Saturday through late Saturday night for AZZ547>563. CA...None. && $$ DISCUSSION...Hopper/MO AVIATION...CB FIRE WEATHER...Hopper/Rogers/MO