textproduct: NWS Phoenix

KEY MESSAGES

- Much above normal temperatures will continue through the weekend with readings approaching near record levels and Extreme Heat Warnings in effect.

- The high pressure system responsible for the extreme heat will also restrict thunderstorm development with minimal chances today.

- Temperatures will cool slightly early next week as increased thunderstorm coverage returns to the region.

SHORT TERM /Today through Sunday/

Upper level water vapor satellite imagery early this afternoon shows a broad area of high pressure sprawled across southern CONUS with the ridge axis across western CONUS. The center of the high pressure system is currently centered over the the northern AZ-NM boarder. The main story through the weekend will be the extreme heat that this strong high pressure system is bringing to the area. This high will remain around 596-598 dm through tomorrow before beginning to weaken slightly and shift north/northeastward slightly on Sunday. Model guidance continues to show temperatures peaking today and tomorrow with afternoon highs around 10 degrees above normal (in the 112-120 degree range across the lower deserts and in the 100-110 degree range across the higher terrain areas) and morning lows of 8- 12 degrees above normal (in the 90s across the lower deserts and in the mid to upper 80s across the higher terrain). Record morning warm lows were either tied or set this morning at all three climate sites (Phoenix Sky Harbor, Imperial, and Yuma) , and in fact Phoenix Sky Harbor tied their all time record warm low (97 degrees originally set on July 19, 2023). Similar morning lows, if not a few degrees warmer, are expected again tomorrow morning. Phoenix Sky Harbor could also tie or break today's record high temperature this afternoon, which is 116 degrees set in 2014, 2018, and 2023. However, the other two climate sites are expected to fall a few degrees shy of their record highs for today. An increase in moisture on Sunday paired with the high weakening slightly and moving further north/northeast will lead to temperatures cooling around 3-5 degrees on Sunday, but remain around 5 degrees above normal. An Extreme Heat Warning remains in effect for most of the area through the weekend. The Extreme Heat Warning for the locations in southern Gila County is set to expire tomorrow evening.

Some shower and thunderstorm activity has already popped up along the higher terrain features across eastern AZ. Much like yesterday, activity will be isolated and short-lived in nature. But, shower and thunderstorm activity is expected to gradually increase this weekend as a well defined easterly wave/inverted trough comprised of tropical remnants cross through northern Mexico. The northern extent of the inverted trough and jet streak is expected to cross into southern AZ tomorrow afternoon providing ample lift for storms to develop in SE AZ tomorrow. While mixing ratios languishing under 10 g/kg and a warm midlevel thermal layer should preclude storm development over most lower elevation locations, DCape values 1500- 2000 J/kg will promote strong, long lived outflow boundaries racing into lower deserts creating elevated chances of dense blowing dust (primarily over Pinal County). This outflow will also enhance and deepen the moisture pool bringing boundary layer mixing ratios back into a 11- 12 g/kg range and spiking total column PWATs near 1.75". Depending on how Saturday's activity pans out will influence activity on Sunday. The environment on Sunday remains fairly favorable for thunderstorm development with MLCAPE exceeding 1000 J/kg, and DCAPE around 1500 J/kg. Forecast BUFR soundings suggest low elevation CinH in excess of 25 J/kg. However, if outflow boundaries collide they should be able to break the cap.

LONG TERM /Monday through Thursday/

Strong, expansive ridging will remain nearly in place next week across the southwest and central United States with the anti- cyclone center rotating back from central Colorado into New Mexico and Arizona. Early in the week, sufficient easterly flow will prevail over the region juxtaposed with marginally supportive mixing ratios of 9-11 g/kg for scattered storms. There remains considerable uncertainty regarding a more preferred time frame for higher POPS in any given location with mesoscale details likely becoming more important towards convective evolution. Unfortunately, the mandated NBM/WPC forecast during this time only further complicates and diminishes the forecast confidence with numerous instances of spurious QPF forcing procedural, un- scientific higher POPs spread over uncommon time periods based on climatology and the preponderance of trusted model output. In reality, this appears to be a very typical monsoon pattern with alternating convectively active and inactive days. By the latter half of the week, there are a handful of NAEFS members indicating notable strengthening of the high pressure with 600dm H5 heights materializing over northern AZ/southern UT. It appears the NBM/WPC forecasts are latching onto this scenario as POPs dwindle to much lower than expected values given the ensemble mean synoptic pattern. The larger impact of this outcome would be a return to major HeatRisk and extreme heat thresholds towards the end of the week.

AVIATION

Updated at 1730Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No significant aviation weather impacts are expected through early Saturday afternoon, with FEW to SCT mid and high clouds. The main forecast uncertainty lies with wind directions, however, they will generally maintain a westerly component through the TAF period with perhaps a briefer than usual east switch or period of VRB around sunrise Saturday morning. A distant, decaying outflow boundary may reach the Phoenix area this evening, but confidence is too low to include in the TAFs. Occasional gusts to between 15-20 kts look likely this afternoon, and a boundary may increase SW'rly winds once again overnight into early Saturday morning.

Saturday late afternoon/evening outlook: Chances are better for at least one outflow boundary to affect all the terminals, with some Hi-Res guidance suggesting 2 distinct boundaries: an initial boundary out of the NE in the late afternoon/early evening and a second out of the south during the mid-late evening. These would cause abrupt wind shifts, temporary stronger gusts (upwards of 25-35 kts), and possible surface visibility restrictions in lofted dust/haze (especially if out of the south). VCSH/VCTS conditions cannot be ruled out Saturday evening, more likely with a second boundary in the mid-late evening if it comes to fruition.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather impacts expected through Saturday morning under FEW mid and high level cloud decks. Wind directions will generally fluctuate between the southeast to southwest. Some elevated gusts between 20-25 kts are likely this afternoon at KBLH.

FIRE WEATHER

Thunderstorm chances will remain low today with only isolated showers and storms expected over the high terrain areas in eastern Arizona. These storms are expected to be short-lived. Better chances for more expansive storm coverage and wetting rainfall will arrive later in the weekend and early next week with activity focused over higher terrain locations. With temperatures hovering much above normal and near record levels the next several days, minimum afternoon humidity in a 10-25% range will improving closer to a 20-30% range next week. Correspondingly, overnight recovery will mostly be in a fair to good range of 30-60% with somewhat higher values next week. Temperatures are expected to cool back to near to slightly above normal early next week. Outside of thunderstorm outflows, wind speeds should be fairly light through the period with only local afternoon gusts 20-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

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

Extreme Heat Warning until 8 PM MST Saturday for AZZ556-560>562.

CA...Extreme Heat Warning until 8 PM PDT Monday for CAZ560>570.