Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/21/21

Area Forecast Discussion
National Weather Service Phoenix AZ
435 PM MST Tue Jul 20 2021 .UPDATE...Updated Aviation Discussion... && .SYNOPSIS... Abundant monsoon moisture will continue to bring daily chances for at least isolated to scattered showers and thunderstorms across much of Arizona with a tendency to mostly favor the higher terrain of eastern Arizona, especially for the first half of the week. Near normal temperatures are favored through midweek, followed by below normal by late in the week. More scattered to possibly widespread storms are expected for the second half of the week and into the weekend as a more organized weather system is expected to affect the region. Storms will be capable of producing both locally heavy rain, strong winds, and blowing dust. && .DISCUSSION... Showers and thunderstorms scooted across the Phoenix metro and southwest AZ/southeast CA this morning, dropping 0.10 to 0.25" of rain over a few fortunate spots, in response to a passing inverted trough and nearly 1500 j/kg of CAPE. Often, morning convection will spoil storm chances for the afternoon and the morning HREF showed a relatively quiet day for our forecast area. However, latest radar trends indicate that may not be the case with multiple thunderstorms now developing southwest, east, and north of the Phoenix metro. Storms forming over the higher terrain is not surprising but does cause some pause as to whether or not we can get more storms into the lower desert today than previously thought. The latest aircraft soundings show less inhibition with a weaker midlevel cap and much more instability than guidance suggested...but those soundings may not be adequately sampling the storm environment either. Lower desert storm chances for this afternoon are ~10-25%. Overall, water vapor analysis shows a ridge of high pressure centered over the Colorado Rockies with a fairly moist air mass stretching from tropical Mexico through the Intermountain West. The H5 ridge center will continue to weaken while settling close to the Four Corners region over the next day or two, providing stronger southeasterly flow through southern Arizona. This position will keep the monsoon in gear with chances for thunderstorms seemingly every day. PV anomalies associated with a modest midlevel jet streak over western TX and New Mexico are expected to move through the region later this week, potentially increasing chances for thunderstorms perhaps as early as Wednesday evening, but more likely Thursday and/or Friday. Aside from the aforementioned 1500 j/kg CAPE, the 12z KPSR sounding also showed a whopping 2.13" in precipitable water, exceeding the daily maximum for date according to SPC Sounding Climatology. Daytime mixing should lower PWs somewhat, but not by all that much, as the latest aircraft soundings out of Sky Harbor also measured 1.98" PW. These moisture levels are much more typical of a late August tropical airmass rather than July 20th. This ultimately means that any thunderstorms that do develop will have the potential for producing heavy rain that could cause flash flooding. To illustrate, a thunderstorm that developed rapidly over the Bradshaw Mountains around noon dumped nearly 3.00" of rain over one gauge within one hour, and exceeded rainfall rates of 1.00"/hr over a few other rain gauges! That does not mean every storm will dump that much rain but it does not take nearly that much rain to cause flooding in our arid environment. The threat for flash flooding is anticipated to increase daily through the end of this week as storm chances and coverage are also forecast to increase. Temperatures through Wednesday are expected to be near seasonal norms to slightly above normal across western deserts before dropping several degrees as the storm chances increase and the upper low moves into the area late week. Latest NBM has Wednesday as the warmest day and Friday and Saturday as the coolest. NBM is even supporting a 30% chance for sub-90 degree highs Friday and Saturday in Phoenix. Temperatures should warm back up during early next week as the pattern continues to shift with ensemble guidance supporting a transition to increased south to southwest flow and a gradual drying trend. && .AVIATION...Updated at 2335Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: As expected, storm activity today has been fairly limited with the current activity well west of Phoenix. For the rest of the afternoon/evening, expect little if any aviation impacts with prevailing light westerly winds. Can`t completely rule out a minor outflow during the evening hours, potentially shifting winds, but chances are pretty low (less than 20%). Tonight, expect a more typical diurnal shift to easterly winds. As far as Wednesday is concerned, expecting easterly winds to persist into the early afternoon hours with a typical diurnal switch to westerly during the mid/late afternoon hours. Prospects for convective activity impacting the terminals look better for Wed evening (30-40%) with an even better chance for easterly outflows (60-70%). Current timing of a potential wind switch to easterly Wednesday evening is somewhere in a 01-03Z time period. Southeast California/Southwest Arizona including KIPL and KBLH: Generally quiet conditions will prevail with winds favoring the south at both KIPL and KBLH through Wednesday afternoon. Can`t completely rule out a few isolated showers or storms Wednesday afternoon, mainly across southwest Arizona, but it is unlikely to impact the terminals. && .FIRE WEATHER... Friday through Tuesday: No major changes to the forecast...Lower than normal afternoon temperatures and ample monsoonal moisture will persist. Storm activity is anticipated to spread as far west as southeast California with most storms having good potential for wetting rain (0.10" or more). The best potential for more widespread storm activity will likely be Thursday through Friday and into Saturday as an upper level disturbance moves through the region. Threats from storms will continue to involve locally heavy rainfall and flash flooding along with strong outflow winds and localized downbursts within storms. Significant flooding may be experienced on previously burned areas of Gila County, particularly during the second half of the week. Min RH levels are expected to be in the 25-40% range across south-central Arizona (30-50% higher terrain of south- central AZ) and 20-30% across southeast California and southwest Arizona. Max RH levels, outside of storms, will be 45-70% most places (60-90% higher terrain of south-central AZ). Apart from thunderstorms, winds will favor familiar diurnal patterns. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...AD/Benedict AVIATION...Kuhlman FIRE WEATHER...Smith/Kuhlman