Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/30/21

Area Forecast Discussion
National Weather Service Phoenix AZ
505 PM MST Sun Aug 29 2021 .UPDATE... Updated 00Z aviation discussion. .SYNOPSIS... The best storm chances will continue to favor higher terrain areas through Monday, but some isolated showers or storms and gusty winds could impact the lower deserts. Very warm conditions will remain across the region through Monday, but with highs lowering to near normal by Monday. Attention then turns toward an influx of tropical moisture into the region starting Tuesday with shower and storm chances increasing starting Tuesday afternoon. A threat for heavy rainfall and localized flooding should then affect portions of southern and central Arizona for Wednesday, possibly lasting into Thursday. Below normal temperatures are also expected for the latter half of the coming week. && .DISCUSSION... A persistent band of showers and storms, with moderate to heavy rainfall, developed this morning near Yuma and lasted into the early afternoon. The rain-cooled air and cloud cover through these areas will likely keep temperatures from reaching excessive levels this afternoon. Whereas, areas in southeast CA and southwest AZ with more sunshine are likely to still reach highs up to 110-113F. Temperatures through south-central AZ are expected to be a few degrees warmer than yesterday with highs in the mid to upper 100s. In regards to storm chances through the rest of this afternoon, objective analysis shows the greatest moisture and instability being in the Lower Colorado River Valley, with MLCAPE up to 1000-1500 J/kg, and marginal instability across the rest of the CWA. There is still a considerable amount of CIN analyzed across the lower deserts, with ACARS soundings in Phoenix showing roughly -150 to -200 J/kg. This should greatly limit convection over the lower deserts as long as it hold. With the cap over the lower deserts, most convection is expected to develop over higher terrain of northern and eastern AZ, as well JTNP in southeast CA and portions of La Paz county. Mid level lapse rates are steep, increasing to the north, with 8 C/km and above in northern AZ. So, there is potential for robust updrafts with strong storms and perhaps a few severe storms this afternoon. With high DCAPE, up to 1500 J/kg, damaging outflow winds will be the primary threat. As outflows translate to lower elevations blowing dust will be a threat (especially in areas that have not seen recent extensive flora growth). New storm development from outflows will be possible through the evening, dependent on the strength of the cap. Forecast soundings in Phoenix show the cap holding through the rest of this afternoon, however lingering elevated convection, above the boundary layer, late this evening cannot be ruled out. Nora has been downgraded to a tropical storm this afternoon with a center on the Mexico coastline near the mouth of the Gulf of California. The latest track from the NHC keeps the central circulation along the Mexico coastline with a gradual weakening. The circulation from Nora is still not expected to be a threat to our area, with it more likely becoming an open wave within the steering flow before reaching the US border. Tropical moisture associated with Nora advecting into the Desert Southwest and leading to scattered to widespread rain remains the primary threat. Steering flow begins shifting more southerly to southeasterly in our region Monday as the anticyclone that has been positioned over northern AZ slides east. Moisture increase into the region will initially be gradual Monday. 12Z HREF is suggesting Monday afternoon will be slightly more active than today with a subtle increase in instability, but still with most showers and storms favoring higher terrain. NBM PoPs climb to 20-30% across the lower deserts Monday afternoon as there will be a slightly better chance for storms to survive to lower elevations over today. .PREVIOUS DISCUSSION... Tropical moisture will continue to stream northward into our region on Tuesday, likely creating a more unstable environment for further chances for some stronger storms. The steering flow will turn southerly on Tuesday and vertical ascent fields should still be weak with an upper level jet max to our southwest. Given that MUCAPE is likely to increase to above 1000 J/kg and there is no obvious strong capping inversion, we should have some scattered afternoon storms Tuesday with gusty winds, heavy rainfall, and flooding all potential threats. Models are still showing late Tuesday night through Wednesday evening as being the prime time for an area of moderate to heavy rainfall to impact southern and central Arizona, possibly centered initially over the Phoenix area. The ingredients for heavy rainfall should be in place as PWATs are forecast to increase to around 2" across much of south-central Arizona and vertical ascent may become quite strong over central Arizona. A band (not sure how wide) of moderate to heavy rain is likely to develop by Wednesday morning within the increasing southerly upslope low level flow and within the right rear quadrant of the 60-70kt upper level jet positioned across southern California into the Great Basin. Warming aloft will eventually weaken instability on Wednesday, so the likely threats should completely shift over to heavy rain and flooding across south-central Arizona and then into eastern Arizona later Wednesday into early Thursday as the band is likely to slowly drift eastward. A Flash Flood Watch will likely be needed for portions of the area during the middle part of this week. WPC forecast QPF amounts show the heaviest rainfall focused across Pima and Maricopa Counties, and areas to the north of Phoenix with a large swatch of 1.25-1.75" of potential rainfall. Some areas are likely to exceed those amounts, especially in the more favored upslope foothills areas just north of Phoenix. Although models have been consistent with showing the heaviest rainfall falling over south-central Arizona, some east or westward shift is still possible. Considerable forecast uncertainty is then seen for Thursday into Friday with some ensemble members ending rain chances fairly quickly across the western deserts and maybe as far east as the Phoenix area. However, other members show a continued chance of showers and storms through next weekend with moisture remaining in place over much of the region. For now, the NBM favors the drier solution. Temperatures will for sure take a big hit into the middle part of this week with highs likely falling to 10-15 degrees below normal in some areas through Thursday. Temperatures are then likely to gradually warm up going into next weekend, but below normal readings are still favored. && .AVIATION...Updated at 0005Z South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Convective outflows this evening continue to be the main aviation impact through the TAF period, with a low chance (10-15%) of isolated thunderstorms developing along outflows, especially across the northwestern and eastern parts of the Phoenix area. Current thinking is that the main outflow will be from storms that are now located over parts of Pima and SE Pinal Counties. Current timing of this outflow into the Phoenix area is in the 02-03Z timeframe. There is about a 40% chance of outflow winds in excess of 30 knots, so for now we have included 20-25 knot gusts in the TAFs. There is also the possibility of a 2nd, weaker outflow moving into the Phoenix area from storms that are now located over the Rim, but confidence is too low to include any sort of 2nd outflow in the TAF`s at this point. Once the storms dissipate, expecting lighter southeasterly winds to prevail the the rest of the night and into Monday morning. latest high-res model output is now indicating a weak weather disturbance will move northwestward across the region early Monday morning. This disturbance could set off some showers, and even a TS or two, but confidence is too low to include either VCSH or VCTS in the TAF`s at this point. A more typical switch to westerly winds is anticipated on Monday afternoon. Monday evening appears it will be more activity convectivewise, with at least a good chance of significant outflows from the east of northeast moving to the Phoenix area along for a better chance for TS at one or more of the TAF sites (20-30%). Cloud bases should remain elevated above 10-12 kft aside from lower ceilings down to 8 kft in storms. Southeast California/Southwest Arizona including KIPL and KBLH: Winds at KIPL to remain from a south-southeasterly direction thru the TAF period, aside from a brief period of downsloping westerlies this evening. At KBLH, isolated showers and thunderstorms may spread from the northeast later this evening towards KBLH. Still low confidence (<20% chance) that the terminal will be directly impacted, but outflow winds from a generally easterly direction may be possible. Otherwise, a quieter period of mostly southerly winds is anticipated overnight into Monday morning. && .FIRE WEATHER... Tuesday through Saturday: Elevated moisture levels from an influx of tropical moisture will bring very good rain chances across the region through at least Thursday. Heavy rainfall and flooding may impact area burn scars, particularly Wednesday into Thursday. Much cooler conditions will also move into the region with highs dropping well below normal starting Wednesday. The increased moisture and cooler temperatures will keep minimum RHs mainly above 40% for much of the period before some gradual drying occurs Friday into Saturday. Winds should be relatively light with an afternoon upslope component and overnight drainage expected for a good portion of next week, but southerly winds are also likely to be seen a bit more than typical. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530>533-535-536. CA...Excessive Heat Warning until 8 PM PDT this evening for CAZ560>570. && $$ DISCUSSION...Benedict PREVIOUS DISCUSSION...Kuhlman AVIATION...Percha/Rogers FIRE WEATHER...Kuhlman