textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Saturday with daily Major to Extreme Heat Risk.

- Lower desert highs are likely to reach or exceed 110 degrees through the rest of the work week and even into the weekend, peaking around daily records during the middle of the week.

- Monsoon storm chances will be very limited this week and essentially come to an end by Tuesday before slowly returning this coming weekend.

SHORT TERM /TODAY THROUGH THURSDAY/

An expansive subtropical anticyclone can be seen in midlevel water vapor imagery centered over Southern NM/far Western TX, resulting in southerly to southeasterly flow aloft. A pocket of considerably drier air is evident nudging into Southeastern AZ and will continue to filter into the area as we head into the middle of the work week. Shower/thunderstorm activity from yesterday evening has effectively overturned the atmosphere over South- Central AZ, evident in the 12Z PSR sounding which shows a nearly moist adiabatic temperature profile above the boundary layer and only skinny elevated CAPE between approximately 600-400 mb. Inhibition is very unlikely to be overcome across the eastern CWA this afternoon/evening, except maybe for a couple thunderstorms over the most prominent terrain of Southern Gila County. However, the same cannot be said about portions of the western CWA. Marginally supportive low level moisture remains in place across Southwest AZ and Southeast CA with near surface mixing ratios between 9-11 g/kg and RAP analyzed PWATs between 1.4-1.7". While CIN is still high (-50 to -125 J/kg), there may be a window late this afternoon where outflows from the Baja Spine could spark some strong to severe storms over Imperial County. This is a very low probability, but the potential is worth mentioning. Otherwise, anticipate 10-20% chances for isolated thunderstorms over prominent terrain features near JTNP, the Kofa Mountains in Southwest AZ, and perhaps the Harcuvar Mountains.

Ensembles remain in excellent agreement that the subtropical anticyclone will continue shifting westward and strengthen gradually heading into the middle of the work week. With the center of the anticyclone settling along the AZ/NM border, ensemble mean H5 heights by Wednesday will reach a 595-597 dam range, around the 98th-99th percentile of climatology. NAEFs mean 850 mb temperatures respond quickly, with values peaking around 29C-32C, near climatological maximum for late Summer. This will support lower desert afternoon highs around 10F above daily normals, near record highs through the middle of the week. This will follow overnight lows in the 80s to around 90F, offering little overnight relief from the heat. Extreme Heat Warnings remain in effect through Saturday across much of the area and will likely need to be expanded to cover communities such as Globe, Tonto Basin, and rural communities outside the typically hotter lower desert locales during the latter half of the week. The other result of the high settling closer to the forecast area will be increasing midlevel subsidence and weak, if any, midlevel flow. Better quality moisture will be confined to our south, mostly over Sonora, with 1000-700 mb mixing ratios remaining low, 6-8 g/kg over the forecast area. The result of this increased subsidence and meager moisture will be a near complete suppression of thunderstorm activity by Tuesday-Wednesday.

LONG TERM /FRIDAY THROUGH SUNDAY/

Ensemble guidance favors some weakening of the subtropical high starting Friday as it starts to shift back to the east. This shift assumes a Pacific trough moving into the Northwestern U.S. late week. Models also favor some gradual moisture advection by Friday and lasting through the upcoming weekend. Given the expected increase in moisture and weakening subsidence, rain chances should return as early as Friday but more likely over the weekend. NBM/WPC PoPs show 10-15% chances over high terrain areas Friday and Saturday before increasing and expanding into the lower deserts by Sunday. These PoPs may be underdone somewhat considering what the latest models runs show, so we anticipate higher PoPs soon from the NBM. Temperatures should also begin to improve late week and through the weekend as moisture increases and heights gradually lower. The latest forecast highs still maintain or even top 110 degrees through Saturday before dropping to below 110 degrees by Sunday or Monday of next week.

AVIATION

Updated at 1755Z.

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

No major aviation concerns are expected through this TAF period. As morning storms continue to move out of the region expect periods of southerly winds gusting up to 20kts. Diurnal shift out of the west will occur around the 20z timeframe. Winds will generally be aob 10kts or less. However, some elevated wind speeds with gusts in the mid-upper teens will be possible later this afternoon/evening. SCT-BKN mid/high cloud decks will be common throughout this afternoon into this evening, clearing out tomorrow morning. The diurnal shift out of the east this evening is forecast to occur around 11z with wind speeds expected to be aob 5-8 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns under FEW-SCT mid/high cloud decks can be expected throughout the TAF period. Winds will generally be out of the south aob 8-10 kts becoming light and variable later this evening with mostly clear skies.

FIRE WEATHER

Hot and seasonally dry conditions will prevail for much if not all of the coming week. Strong high pressure will settle over the region by midweek with gradual drying conditions. Minimum RHs today will mostly fall between 20-30%, before dropping back into the teens starting Tuesday. Temperatures will be around 5-10 degrees above normal, with lower desert highs eclipsing 110 degrees nearly every afternoon. Outside of distant thunderstorm outflows, expect overall light winds following typical diurnal tendencies with some modest afternoon upslope gustiness.

CLIMATE

Record highs:

Date Phoenix Yuma El Centro ---- ------- ---- --------- 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 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.

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