textproduct: NWS Phoenix

UPDATE

Updated 18Z Aviation Discussion.

KEY MESSAGES

- Hot and muggy conditions will continue today across the western deserts, however temperatures will begin to cool across much of Arizona dropping to below normal Friday through the weekend.

- Showers and thunderstorms are expected to become more widespread across eastern and central Arizona by this evening and last into early Friday with the potential for heavy rainfall and localized flash flooding.

- A decrease in monsoon activity is expected starting Saturday with chances favoring the Arizona high terrain.

PREVIOUS DISCUSSION

The synoptic pattern will become even more conducive for robust thunderstorms starting this afternoon as at least one shortwave shed from the aforementioned Texas cutoff progresses towards south-central Arizona during peak heating. With convective debris and outflows pumping additional moisture into the middle and lower troposphere, total column PWATs 1.75-2.00" should be common while boundary layer mixing ratios settle around 12-13 g/kg by the afternoon. This moisture profile should result in a near equal MLCape/DCape balance in a "sweet spot" of 1000-1500 J/kg historically present during big monsoon thunderstorm events. Convective inhibition should be nearly absent late afternoon/early evening, and with underlying deep layer ascent, it should only take weak outflows to initiate additional storms. Intersecting outflow boundaries are also likely with consolidating storms over parts of the CWA resulting in heavy rainfall and flash flooding. Rainfall rates at least 2-3"/hr appear likely given the moisture profiles, and a Flash Flood Watch has been issued for south- central Arizona counties. HREF and global model output suggests northern Pinal and SW Maricopa counties as the focus for the heaviest rainfall.

Considerable forecast uncertainty still exists Friday contingent upon the strength and expanse of thunderstorms this evening and tonight. A large subset of modeling indicates the atmosphere becoming completely overturned by storms and outflows Friday morning with forecast soundings depicting a classic, post-event moist adiabatic profile. Should this come to fruition (better than a 50% chance), convective potential the remainder of Friday would become minimal, at best, given the combined extensive cloud cover and poor lapse rates resulting in essentially no instability. A slight alternative outcome is hinted at by recent operational ECMWF and NAMNest output which suggests a well defined MCV juxtaposed with a pool of higher theta-e advecting north through the Phoenix metro Friday morning. While these type model forecasts with morning storms forced by an MCV and copious deep moisture are common during the monsoon, they only come to fruition occasionally every summer, but when this scenario unfolds, very efficient rain rates and flooding ensues. Regardless, the strongest model evidence indicates only small pockets of instability remaining Friday afternoon with reduced chances for showers and thunderstorms through the remainder of the day.

LONG TERM /Saturday through Wednesday/

Although some uncertainty remains, guidance continues to trend drier for this weekend, particularly the GEFS. This drier air is expected to advect in from the south southwest starting Friday morning and continue through most of Saturday, lowering PWATs to as low as 1.4-1.5", but the Euro maintains a more optimistic 1.7-1.9". The easterly wave is also forecast to weaken while it briefly shifts back toward central Texas. The combination of the expected drying, very little instability, and weaker upper level support is likely to limit shower and thunderstorm potential on Saturday. As of now, much of the lower deserts should stay quiet on Saturday, while scattered showers and storms are much more likely over the Arizona high terrain. Temperatures will begin to recover starting Saturday, but daytime highs are still likely to fall short of 100 degrees in the Phoenix area.

Starting Sunday into early next week, the subtropical ridge is likely to become more elongated from the Great Basin to the Lower MS River Valley. The remnants of the easterly wave or even a newly developed disturbance may also become more of a player for monsoon activity by around Monday or Tuesday as moisture remains plenty adequate for convection. An increase in easterly upper level winds across Texas may eventually stretch into the Desert Southwest by late Monday into Tuesday which could help drive monsoon activity throughout portions of the lower deserts again. NBM PoPs still favor the Arizona high terrain through the period, but it seems likely the lower deserts will see another day or two of active weather.

Forecast temperatures for next week show a gradual upward trend, despite fairly high model spread. Lower desert daytime highs may again reach 100 degrees in the Phoenix area by Monday and eventually back to near normal during the latter half of next week. Locations across southeast California and southwest Arizona are likely to maintain near normal temperatures throughout next week.

AVIATION

Updated at 1746Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concern will be an extended period of erratic winds and VCTS/TSRA impacts this evening through tonight. There is already some development of light showers west of the terminals, however moving to the SW away from the terminals and further development over the next several hours should stay outside of the terminals range. Storms later this afternoon are forecast to focus initially to the north and then to the south and southeast, which may lead to prominent outflows from both directions, with a northerly outflow favored first in the late afternoon. There are good chances (40-60%) for VCTS/TSRA & VCSH/SHRA at terminals heading into the late evening and overnight hours tonight. Total accumulation over the TAF period is expected to be mostly between 0.25"-0.75" however some storms may produce localized very heavy, blinding, rainfall, whether the terminals will be impacted or not will depend on the location of these isolated heavier storms. Cloud bases will mostly stay at or above 8-10K ft AGL, but there is potential for lower cloud bases, down to 6-7K ft AGL. The dust potential is low.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours under increasing midlevel clouds. SE winds will be common at KIPL, with speeds aoa 10kts through the first portion of the TAF then weakening later this evening/tonight. Southerly winds will prevail at KBLH. A period of gusts around 20-25 kt may develop at KBLH late this morning through the afternoon/early evening. There is low potential (10-20%) for an isolated shower or storm this afternoon and evening, conditional on the evolution of disturbances to the east of the area.

FIRE WEATHER

Active monsoon weather with elevated moisture levels will continue through the rest of the week. Shower and thunderstorm chances will increase over the eastern districts today with only isolated chances over the western districts. Localized heavy rainfall is expected today into early Friday across the eastern districts with some potential for gusty outflow winds late this afternoon and early evening. MinRHs will be 35-55% for the eastern districts through Saturday to 20-35% across the western districts. Outside of potential thunderstorm outflows, winds for the eastern districts will tend to be light and somewhat favor diurnal trends. Winds will predominately favor the south and be breezy at times across the western districts.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST this evening for AZZ530-532.

Flood Watch from 2 PM MST this afternoon through late tonight for AZZ534-537>563.

CA...Extreme Heat Warning until 8 PM PDT this evening for CAZ562-563- 565>567-569-570.