textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Saturday across much of the area.

- Lower desert highs in excess of 110 degrees and overnight lows in the eighties to lower nineties will be common this week, resulting in Major to Extreme Heat Risk.

- Monsoon storm chances will be very limited before a pattern change occurs this weekend, which will result in an increase in moisture and promote temperatures dropping closer to late August normals by Sunday or early next week.

SHORT TERM /TODAY THROUGH THURSDAY/

The 12Z PSR sounding provides further evidence that lower desert storm chances are essentially out of the picture for today, with mean low level mixing ratios below 9 g/kg and PWAT around 1.25". For perspective, values around 10+ g/kg and 1.3-1.5+", respectively, would be marginally supportive for formation and/or maintenance of lower desert storms this time of year. However, enough moisture combined with terrain-driven convergence will support chances for isolated thunderstorms over prominent features like the Kofa Mtns, and maybe some weak storms or just isolated convective showers over portions of Southern Gila County. Also evident in the 12Z PSR sounding is a more prominent midlevel inversion and warmer H5 temperatures compared to the same time yesterday, indicative of increasing subsidence over the forecast area. This comes as the subtropical high over Southern NM continues to expand westward, increasing its influence over the Desert Southwest. RAP analyzed H5 heights around or just above 596 dam have now expanded to encompass almost all of AZ. Ensembles remain in excellent agreement that H5 heights will peak between 595-598 dam over the forecast area and remain fairly constant through Thursday as the subtropical high centers itself over Southeast AZ/Southwest NM. NAEFS mean 850 mb temperatures will increase to 29C-32C Wednesday and Thursday, above the 99th percentile of climatology and near climatological maximum values, supporting lower desert highs near 115F for the typically hotter spots, challenging records in several spots (see Climate section). As a result of the well above normal temperatures, and overnight lows in the 80s to lower 90s providing little relief, HeatRisk will solidly fall in the Major to Extreme category heading through the rest of the week. Therefore, Extreme Heat Warnings will remain in effect through the end of the week and even into the weekend.

Moisture will remain meager through Thursday, with 1000-700 mb mixing ratios between 6-8 g/kg and ensemble mean PWATs around 80% of normal. More isolated, terrain-anchored thunderstorms are likely to develop each afternoon for the next few days before a pattern change occurs heading into this weekend, drawing in better moisture from over Sonora/the Gulf of California.

LONG TERM /FRIDAY THROUGH MONDAY/

Guidance continues to point towards the subtropical high shifting east of the region starting Friday as a troughing pattern develops over the Northwest US/along the West Coast. As the trough pushes eastward, PWATs will begin to improve, reaching near 1.5"-1.7" by Saturday and lasting through at least the weekend. 1000-700MB mixing ratios are forecast to increase to 10-12 g/kg, enough to support rain chances even over the lower deserts, with NBM/WPC PoPs currently reflecting 20-30% chances across South-Central AZ. This is a decent increase from previous runs but still several days out, so models are still subject to change. Temperatures are forecast to drop closer to late August normals by Sunday or early next week as falling midlevel heights and an increase in moisture/cloud cover over the eastern CWA support a cooling trend (more pronounced across South-Central AZ).

AVIATION

Updated at 1720Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No aviation weather concerns are anticipated under periods of FEW to SCT mid/high cloud decks. The overall wind pattern will follow the typical diurnal trends with speeds generally aob 8 kts. Periods of light and VRB can also be expected, especially during diurnal transitions. Winds may be slower to transition out of the west on Wednesday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation weather concerns are expected under periods of FEW to SCT mid/high cloud decks. Winds will generally fluctuate between the SE to W at KIPL and S to SW at KBLH with speeds remaining aob 10 kts at both terminals. Extended periods of light and VRB winds will be common during the early morning hours.

FIRE WEATHER

Hot and seasonally dry conditions will prevail for much of the rest of the week. Strong high pressure will settle over the region midweek with gradual drying conditions. Afternoon minimum humidities will mostly fall into the teens through the rest of the week before improving this weekend. Temperatures will be around 5-10 degrees above normal, with lower desert highs eclipsing 110 degrees nearly every afternoon through at least Friday. Outside of distant thunderstorm outflows, expect overall light winds (below 15 mph) following typical diurnal tendencies with some modest afternoon upslope gustiness.

CLIMATE

Record highs:

Date Phoenix Yuma El Centro ---- ------- ---- --------- Aug 25 113 in 2011 116 in 1962 117 in 1962 Aug 26 117 in 2011 115 in 2020 115 in 1981 Aug 27 114 in 2023 120 in 1981 117 in 1981 Aug 28 117 in 2023 118 in 2023 117 in 2009 Aug 29 116 in 2023 117 in 1954 117 in 2005

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Saturday for AZZ530>555-559- 560.

Extreme Heat Warning until 8 PM MST Friday for AZZ556-561-562.

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

Extreme Heat Warning until 8 PM PDT Friday for CAZ561-563-564- 568.