textproduct: NWS Phoenix

UPDATE...Updated 00z Aviation Discussion

KEY MESSAGES

- High temperatures reach or exceed 110 degrees nearly every day through the next week and will be near daily records, while morning lows remain unusually warm.

- Extreme Heat Warnings are in effect through next Saturday evening with daily Major to Extreme HeatRisk.

- Afternoon and evening strong to locally severe thunderstorms will primarily favor higher terrain areas through the start of next week, with drier and less favorable conditions next week.

SHORT TERM /TODAY THROUGH MONDAY/

Extreme heat will be the primary weather hazard through the short and long term forecast with very strong high pressure persisting over the region. Latest objective analysis still shows a 599dam anticyclone centered over northern NM, with H5 heights around 595-597dam over the local area. These magnitudes are at or above the 99th percentile of climatology. This will continue leading to well above normal high and low temperatures through this weekend. A drying and warming of the PBL was observed over the last 24 hours, as seen in observed soundings. This should result in high temperatures this afternoon 1-3 degrees warmer than yesterday, with lower deserts topping out around 112-116 degrees. A slight weakening of the subtropical high, and repositioning to eastern NM, Sunday into Monday will support a slight cooling of temperatures through Monday. Potentially more active showers and storms Sunday evening/night in south-central AZ may add a bit to the cooling into Monday. Latest NBM forecast highs for Sunday are around 108-114 degrees and then 106-112 degrees for Monday. Morning low temperatures will remain very warm, up to 5-10 degrees above normal, in the mid 80s to around 90 degrees across the lower deserts. The unusually hot temperatures will lead to widespread Major HeatRisk today and Sunday, with a decrease in coverage of Major HeatRisk for Monday. An Extreme Heat Warning remains in effect through this weekend.

Afternoon thunderstorms today should mostly favor the same areas as yesterday, but with the drying of the PBL, surface-based instability will be lower today. Mean mixing ratios are down 8 g/kg or lower and surface dew points at midday were mostly around 50-55 degrees. Lower instability means overall less intense updraft strength. This does not mean storms that develop cannot still become strong to severe. With the high T-Td spreads and high LCLs, DCAPE remains quite high (>1500 J/kg), which will promote strong to locally severe downbursts. Latest hi-res models favor isolated storms developing mostly along the mountains east of Phoenix and over terrain features south of Phoenix, with a focus mostly in central Pima County and portions of southern Maricopa and western Pinal counties. The storms to the south will be capable of sending dusty outflows northward heading into the evening, along with debris clouds and perhaps some weak showers or virga. Just like yesterday, the odds of an outflow popping off storms closer to or in Phoenix are low (5-10%), but it is still not impossible for 1-2 strong to severe storms to pop up with the right amount of forcing. Some ACCAS showers also cannot be ruled out across south-central AZ heading into Sunday morning within some of the debris clouds moving across the area.

For Sunday, models continue to support a strong moisture return, albeit brief, in the evening and overnight hours. There are indications that instability will increase again south and southwest of Phoenix with storms again developing in similar locations with similar potential for strong to locally severe wind gusts. Then heading into the evening and overnight hours it looks like there will be a strong gulf surge as well as indications of perhaps a subtle shortwave/vort max moving up out of Sonora. This may increase storm survivability into the Phoenix area. NBM PoPs remain quite low despite this. The 12Z HREF shows a 30% neighborhood probability for dBZ>40 in south-central AZ, including Phoenix, Sunday evening/night. So, PoPs were increased slightly to at least show a slight chance (10-20%) across south-central AZ. Isolated storms are again forecast over higher terrain features for Monday afternoon and evening, but there may be stronger storms and greater coverage over some of the features west of Phoenix, like the Kofas to Harcuvar Mtns and portions of eastern Riverside County following the moisture increase the night before.

LONG TERM /TUESDAY THROUGH NEXT FRIDAY/

Ensemble members are in good agreement that by Tuesday drier air will start to filter back into the region as the subtropical high restrengthens and starts to retrograde westward back to over our region. By the middle of the week the center of the high will be straddling the AZ/NM border and by the end of the week the center of the high is forecast to be over AZ. As the high moves overhead, PWATs are shown to drop into a 0.9-1.2" range across our region and with warmer temperatures aloft and subsidence chances for thunderstorms will be poor across the region. The NBM shows that Tuesday and Wednesday there could be enough moisture around to support isolated thunderstorm chances for the higher terrain areas. But by the end of next week thunderstorm chances for our area shut off and will mainly be relegated to the Mogollon Rim.

Unusually hot temperatures will be the main story next week. As mentioned above the subtropical high will slowly retrograde back over our region beginning on Tuesday, which will result in a quick warming trend for our area. The high is also expected to strengthen to around 596-598dam during this time. The combination of the center settling over our region and strengthening will result in temperatures heating up to around 10 degrees above normal for this time of year. Afternoon highs (morning lows) are forecasted to be in the 112-118 (85-93) degree range across the lower deserts and in the 102-112 (75-85) degree range across the higher terrain areas. There is a good chance daily record highs and warm lows will be tied or broken and temperatures may even near monthly records. The increase in temperatures will also result in an increase in coverage of Major HeatRisk, back to widespread beginning Tuesday and then widespread Extreme HeatRisk by Thursday. Confidence in the heat forecast for next week remains high. So, the Extreme Heat Warning will continue through the workweek and has now been extended through Saturday, in collaboration with the Tucson office. The warning will likely be expanded to our entire CWA by the middle of the week with the development of widespread Extreme HeatRisk.

AVIATION

Updated at 2325Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The potential for a gusty outflow this evening continues to be the main aviation weather challenge through Sunday afternoon. Light westerly winds will persist through the early evening hours before the potential for an outflow to impact the terminals after 02-03z. If an outflow were to impact the area, it will most likely come from the south. Along with some gusty winds, there will also be the potential for some BLDU issues as well, especially at KIWA and KPHX. Otherwise, conditions should become more tranquil after 04-05z, with easterly winds likely during the overnight and early morning before the westerly shift late morning/early afternoon. FEW-SCT mid/high cloud decks will be common throughout the period, although a period of BKN coverage will be possible, especially this evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues under FEW-SCT mid/high cloud decks can be expected throughout the TAF period. Trends in the overall wind direction should be similar to the last 24 hours, with westerly winds this evening and overnight turning out of the southeast early Sunday morning at KIPL and winds fluctuating between the south-southeast and southwest at KBLH. Overall speeds will generally remain under 12 kts with some modest afternoon/early evening gusts.

FIRE WEATHER

Hot and seasonally dry conditions are expected nearly every day, through next week. Aside from a brief increase in moisture Sunday- Monday, minimum RH values each afternoon will be around 12-18% (15- 25% Sun-Mon). Temperatures will be around 5-10 degrees above normal through the next week, with lower desert highs eclipsing 110 degrees nearly every afternoon. There will be slight chances for storms each afternoon through Monday, but they will mostly favor the higher terrain areas. Drier conditions, with lower storm chances are expected by the middle of next week. Outside of thunderstorm outflow winds, winds will follow the typical diurnal tendencies with some modest afternoon upslope gustiness.

CLIMATE

Record highs:

Date Phoenix Yuma El Centro ---- ------- ---- --------- Aug 22 113 in 2011 115 in 1969 117 in 1969 Aug 23 114 in 2011 115 in 2011 116 in 1969 Aug 24 115 in 2020 119 in 1985 117 in 1985 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

PSR WATCHES/WARNINGS/ADVISORIES

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

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