Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/18/21

Area Forecast Discussion
National Weather Service Phoenix AZ
522 PM MST Tue Aug 17 2021 .UPDATE... Updated Aviation Discussion. && .SYNOPSIS... Thunderstorm chances will increase by late evening and overnight before falling off Wednesday evening and night, leaving very little storm chances remaining by Thursday. Until that time, a very moist environment will support scattered to isolated showers and storms through south-central Arizona at times. Flooding rainfall remains the greatest threat followed by gusty winds and lightning. With much drier conditions arriving by the end of the week, temperatures slightly below average will slowly warm back to near normal levels early next week. && .DISCUSSION... Convection commenced by late morning again today as a quasi- stationary upper low/inverted trough and vort max disturbances remained positioned across S-SE-Cent to E to NE AZ. Midday radar and METSAT imagery showed areas of developing thunderstorms mostly over SE AZ, the White Mountains and along the Rim NE of Phoenix. The anomalously moist airmass remains in place across the region today and was assisting the aforementioned triggers. The UL ridge axis was weakened and repositioned from S CA to the 4-Corners in advance of a low pressure system moving through the PAC-NW with drier, weak NE flow aloft. Late morning ACARs mean soundings maintained elevated mean PW of 1.78" due to a moist column from the BL through mid levels/H5. Winds from just above H7 to H5 were still NEerly. With an overturned column the very skinny MU CAPE was below 800 j/kg. High to decent chances for Monsoon storm activity will persist through Wednesday for Phoenix and the deserts. However the very best chance of thunderstorms, including a WPC Day 1 slight risk of excessive rainfall, through Wed 12Z, could occur on and off from early Tue evening to early Wed morning when NBM PPI/POPs spike up to 70% between 06-12Z. HREF members vary on the timing of on and off storms through tonight as the upper level disturbances essentially remain in place. The NW-PAC low will slowly advance inland into the Intermountain West through Wed night. On Wed morning SW flow develops aloft in advance of the trough and begins to eject the local disturbances NE into E AZ and beyond. During this time the HREF shows a second round storm chances associated with an area of pre-trough instability. Again the HREF members vary on their timing and coverage of thunderstorms tracking NE through Phoenix and the deserts into the E AZ high country. Some members favor storms developing in the mid- late morning while others favor early-mid afternoon. The forecast coverage also varies from very isolated to a bit scattered. The uncertainty is likely due to the beginning of an accelerated drying trend starting Wed although during a time when PW are still a bit elevated near 1.4- 1.5". The south-central AZ deserts are in a WPC Slight Risk of flash flooding through tonight while S. Gila County remain in a Slight Risk through Wed night. The latest HREF probability of 3-hr QPF >1" for very early Wed morning for Phoenix and the deserts is ~10-20%, and 30% for S Gila Cty and far E Maricopa Cty and far NE Pinal county by Wed afternoon. However given the wet soils and limited amount of rainfall needed to create impacts over burned areas, some flood threat will persist into Wed evening. The threat for strong thunderstorm wind gusts remains in the forecast as well. The HREF chances of strong thunderstorm winds >=35 mph is 10-20% late tonight and 10% again on Wed afternoon. Virtually all ensemble output indicates a rapid drying trend Thursday and beyond with forecast soundings showing 850-700mb mixing ratios dropping below 8 g/kg Wednesday night, then eroding the lower level moisture during diurnal mixing Thursday. Remaining south of the trough base, anti-cyclonic midlevel warming will also contribute to the lack of convection. With ensemble data converging on a common solution, NBM POPs have fallen solidly below 10% for the entire latter half of the week. Into the first half of next week, some ensemble members indicate subtropical ridging over the SE Conus retrograding back into the Southwest and starting the cycle of increasing storm coverage again while other members reload the Great Basin trough and maintain convectively unfavorable westerly flow. So far, nothing suggests a return to the current very active type pattern for at least the next 7-10 days. && .AVIATION...Updated at 0022Z... South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Thunderstorms over Gila and eastern Yavapai Counties are expected to struggle to survive on the Valley floor this evening. Thus, not anticipating anything like last night. However, they could get close enough for VCTS (especially at KIWA and KSDL) and cause a wind shift. Will need to continue monitoring this closely. Otherwise, anticipate south-southwest winds early this evening before transitioning back to southeast (light and variable) by 03Z. Later tonight, a disturbance currently centered over the AZ/Mexico border is expected to track northward and generate shower and thunderstorm activity over and near the Greater Phoenix area. Once again, not anticipating a severe squall line situation but instead scattered activity that has a good chance of occurring within 5SM of each of the TAF sites roughly between 06Z-14Z with most active period 08Z-12Z. This would likely create erratic winds in addition to the reduced ceilings and visibility with the precipitation (remaining AOA FL050). As an aside, higher terrain to the east of Greater Phoenix will have a better chance of storms. Anticipate thinning of cloudiness after 14Z as southwesterly flow aloft develops. But, there will be one more window of opportunity for storm development Wednesday afternoon. There is more uncertainty about this period than tonight`s. Wouldn`t be surprised to see development over the Estrella`s begin by midday and then move east-northwestward. Activity Wednesday afternoon, if any, would likely be finished by sunset if not sooner. Southeast California/Southwest Arizona including KIPL and KBLH: Winds will favor southerly directions at KBLH through Wednesday with varying speeds (gusts 25-30kts Wednesday afternoon). Imperial will have southeasterly winds early this evening before transitioning to southerly then westerly late this evening. Anticipate westerly winds to strengthen at KIPL during the afternoon Wednesday with gusts to 25kts. Otherwise, no significant cloudiness. && .FIRE WEATHER... Thursday through Monday: Substantially drier conditions will overspread all districts during the latter half of the week with almost no chance of thunderstorms or wetting rains. These chances only increase marginally over eastern districts early next week. With temperatures mostly hovering slightly below normal, minimum afternoon humidity levels will generally range between 20-40% through Friday, then potentially falling closer to 15-25% over the weekend and early next week. Overnight recovery will be good to excellent in a 40-80% range. Winds should be relatively light with an afternoon upslope component and overnight drainage common. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Sawtelle/18/AJ AVIATION...AJ FIRE WEATHER...18