textproduct: NWS Phoenix

UPDATE...Updated 18z Aviation Discussion

KEY MESSAGES

- Extreme heat will persist through the start of the week with high temperatures topping out in the 110 to 118 degree range over much of the lower deserts, hovering right around record levels.

- Extreme Heat Warnings are in effect regionwide through Tuesday.

- High pressure and drier air over the region will confine showers and thunderstorms primarily to far eastern and southeastern Arizona areas through at least Monday.

- Shower and thunderstorm chances increase across south-central Arizona by the second half of the week.

SHORT TERM /TODAY THROUGH TUESDAY/

Near record strength high pressure remains in place over AZ this morning and the 00Z sounding from Flagstaff once again measured a 600dam 500mb height. This strong high has led to extreme temperatures across the region. Heights at 500mb are expected to gradually lower a few decameters through Monday, down to 594-595dam, which is still right at the 90th percentile of climatology. So, there will at least be a slow decline in ambient temperatures through the first half of this week. For today though, high temperatures will be nearly identical to yesterday, with most lower deserts topping out around 112-118F after only cooling off into the middle 80s to lower 90s. These temperatures will once again lead to widespread Major to Extreme HeatRisk, and both high and low temperatures will be near daily records. Highs for Monday will only drop 1-3 degrees from today and most major populated areas will remain under Major to Extreme HeatRisk. Another 2-4 degrees of cooling is expected heading into Tuesday, bringing the lower desert highs down to 107-113 degrees. However, conditions will become much more humid Tuesday, as most guidance show a strong push of boundary layer moisture from the south Monday night through Tuesday morning. Surface dew points are forecast to reach the middle 60s to lower 70s across the lower deserts. This added humidity will make temperatures very uncomfortable and lead to heat indices up to 111- 117 degrees. An Extreme Heat Warning remains in effect through Monday for all areas, but has now been extended through Tuesday for all lower desert locations, largely due to the high humidity. The warning for higher terrain areas will be allowed to expire Monday evening.

The atmospheric setup will likely remain unfavorable for storms in south-central AZ through at least the start of the week, due to the very warm temperatures aloft and subsidence with the high pressure overhead. Storms the next few afternoons will continue to favor eastern and southeastern AZ, with a northeasterly steering wind. Storms will be strong to severe across these areas. Storms moving out of the White Mtns this afternoon may clip into portions of far southeast Gila County, including the San Carlos area, and will be capable of producing locally strong to severe wind gusts. Monday looks to be a particularly active day for southeast AZ, with favorable shear and instability for some organized storm clusters. Given high DCAPE magnitudes (>1500 J/kg), the storms in southeast AZ Monday will be capable of sending outflows, potentially strong and with dust, westward and northward through Pinal County. These outflows and a gulf surge Monday night will be the driver of the increase in moisture heading into Tuesday. While this sounds promising for storm chances across the lower deserts heading into Tuesday, with much better instability, subsidence, persistent very warm temperatures aloft, and fairly high inhibition will continue to counter/hinder convective potential. Latest NBM PoPs show slight chances (15-20%) for storms in southern Gila County Tuesday and 10% PoPs for the Table Top area of western Pinal and southern Maricopa counties. Any storm that is able to develop over lower desert areas will be capable of producing very strong to locally severe wind gusts.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

After weakening slightly to start the weak, global ensembles support the subtropical high restrengthening during the second half of the week, back up to a 600dam high, but also drifting a little further north toward/into UT. H5 heights over the local area are forecast to climb back up to a peak of around 596-597dam by Friday. Coolest temperatures over the next week are forecast for Wednesday, but temperatures will still be above normal with lower desert highs in the 106-112 degree range. Despite the restrengthening of the high, high temperatures are not forecast to see a significant increase again, roughly 1 degree each day through Saturday, likely due to the the increased moisture. However, the forecast calls for just enough warming to once again increase the coverage of Major HeatRisk. Morning low temperatures will likely remain quite warm - barring any cooling from storms - mostly in the 80s to right around 90 degrees.

With the small movements of the subtropical high midweek and shift north late week, there will be a better fetch of easterly winds across southern New Mexico, southern Arizona, and northern Mexico. Above normal moisture is also shown to stick around through the end of the week and next weekend, with 60s dew points. This pattern will at least support daily robust thunderstorms developing over the AZ high terrain, but there is still uncertainty whether storms can fire off or survive into lower desert areas like Phoenix and points further west. Still, this looks like the best chance of the summer so fat for thunderstorms in portions of southwest AZ and southeast CA, who have been lacking for rain so far this summer. Latest NBM/WPC forecast continues to show 10-30% PoPs across much of south-central Arizona by Thursday and continuing through the end of the week. A period of stronger mid-level easterlies and perhaps and easterly wave/IT will certainly improve the odds of storm chances across the area Strong to severe wind gusts will be possible with most storms and an increase in mixing ratios up to 11-12 g/kg late week will support very heavy rainfall.

AVIATION

Updated at 1715Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns under FEW to occasionally SCT mid to high-level clouds can be expected throughout the TAF period. The overall wind pattern will generally exhibit the typical diurnal tendencies with occasional afternoon/early evening gustiness near 20 kts.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under mostly clear skies can be expected throughout the TAF period. At KIPL, southeast winds will prevail through the afternoon before another temporary shift out of the west occurs during the evening hours. At KBLH, winds will generally fluctuate out of the south to southwest. Overall sustained wind speeds will generally range between 8-12 kts, with some occasional gusts between 20-25 kts at KBLH during the afternoon.

FIRE WEATHER

Strong high pressure and very hot temperatures, with dry conditions, will persist through this weekend. Afternoon high temperatures will be up to 112-118 degrees across lower elevations and minimum RH values will be around 10-15%. Temperatures then decline a few degrees each day through the first half of the week and moisture levels will slowly rise. Minimum RHs will still be around 15-20% Monday, but will increase up to 20-30% starting Tuesday. Overnight RH recoveries will be fair to good this weekend at 30-45% and then increase up to 45-70%. The increasing moisture will result in an increase in shower and storm chances. Initially, storm chances this weekend through the start of the week will focus in far eastern and southeastern AZ, with potential for storm outflow winds to progress into south-central AZ. Slight chances for storms (15-30%) then spread to south-central AZ by the middle and second half of the week. Outside of storm outflows, winds will mostly follow diurnal patterns, but west southwesterly winds will be dominant much of the time with afternoon gusts of 15-25 mph at times.

CLIMATE

Record high temperatures:

Date Phoenix Yuma El Centro ---- ------- ---- --------- Aug 2 114 in 2025 117 in 1918 117 in 1969 Aug 3 116 in 2024 116 in 1998 117 in 2021

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Tuesday for AZZ530>544-546- 548>551-553>555-559.

Extreme Heat Warning until 8 PM MST Monday for AZZ545-547-552- 556>558-560>563.

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

Extreme Heat Warning until 8 PM PDT Monday for CAZ560-561-564- 568.