textproduct: NWS Phoenix

KEY MESSAGES

- Temperatures much above normal will persist through the weekend with readings approaching near record levels resulting in Extreme Heat Warnings.

- The high pressure system responsible for the extreme heat will also restrict thunderstorm development with chances retreating into the mountains through the end of the week.

- Above normal temperatures should cool slightly early next week as increased thunderstorm coverage returns to the region.

SHORT TERM /Today through Saturday/

Early morning WV imagery and objective analysis depicts a retrogressive anti-cyclone over west Texas while the easterly cyclonic wave that passed through the region earlier in the week has lifted into northern Colorado. This places the region under a more subsident regime with some modest drying aloft as total column PWATs slowly deteriorate. While current boundary layer mixing ratios sit around 10-12 g/kg, even these levels will be pushed closer to 8-10 g/kg the next few days. As warmer temperatures aloft advect into the area with subtropical ridging building into Arizona, instability will be reduced while adding substantial convective inhibition. HREF members suggest a few pop-up pulse showers/storms over the most favored locations today and Friday, though chances are essentially under 10%. As the center of the anti-cyclone migrates north into the four corners over the weekend, this will allow easterly waves and better convective potential to return to east/southeast Arizona. While distinct differences in thunderstorm coverage among models exists, there is sufficient evidence that more aggressive outflows and isolated storms descend into lower elevations.

Models remain in good agreement showing a 598dm H5 anti-cyclone center becoming situated over western New Mexico on Friday with 596- 598dm readings across the forecast area. This will easily result in widespread major HeatRisk with afternoon highs 5F-10F above the daily normals Friday and Saturday. Numerical guidance spread still exhibits a wider than expected range likely due to uncertainty in surface dewpoints. With the expectation of boundary layer moisture lingering through the weekend, official NBM 50th percentile forecasts are likely sufficient, though some areas that may scour away moisture could achieve levels closer to the 75th percentile of the full ensemble envelop (i.e. some lower deserts locations 115F to 120F). Regardless, the hottest temperatures will settle over the region Friday and Saturday with excellent forecast confidence of most lower desert communities reaching or exceeding 115F. Even some higher elevation valley population centers in Gila County will attain major HeatRisk Saturday, and the Extreme Heat Warning has been expanded in these locations prior to better thunderstorm potential Saturday evening.

LONG TERM /Sunday through Wednesday/

During the first half of next week, the anti-cyclone center will propagate across the four corners into central Colorado allowing enhanced easterly flow and better chances of inverted shortwaves entering the SW Conus. However, there is considerable uncertainty regarding the timing and location of any particular feature which would enhance thunderstorm chances, not to mention mesoscale details from decayed convective complexes which further complicates the forecast. However, as early as Saturday night, long tracking, strong outflows may be sufficient to incur isolated storm development over lower elevations, while deepening moisture profiles, increasing total column PWATs, and setting the stage for enhanced convective coverage on following days. Various modeling suites highlight different days with a more preferred chance of storms sweeping into lower elevations early next week, but all models indicate at least one easterly wave influencing enhanced activity.

This easterly flow will also help erode higher midlevel heights and temperatures; and combined with the convective influences, may significantly reduce and complicate heat related impacts. A growing trend in models shows outflows and convection rapidly cooling temperatures across Arizona early in the week such that by Monday, readings return to normal levels. GFS BUFR soundings are even more aggressive towards outflows affecting temperatures suggesting extensive cloud cover much of the day Sunday, and readings falling well short of 110F. Not putting too much stock in these types of details 3-4 days in advance, but certainly the pattern supports this evolution where somewhat lower temperatures and a return to only moderate HeatRisk arrives quicker than previously forecast.

AVIATION

Updated at 0500Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns are expected throughout the TAF period. The overall wind pattern will continue to favor light and diurnal tendencies along with extended periods of variable conditions. FEW to occasionally SCT mid to high level cloud decks with bases as low as 10 kft will be common throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under a FEW mid to high level cloud decks are expected throughout the TAF period. The overall wind pattern will favor light and diurnal tendencies along with extended periods of variable conditions.

FIRE WEATHER

Thunderstorm chances will remain quite low the rest of the week with only slight chances remaining over high terrain areas of far eastern districts. Better chances for more expansive storm coverage and wetting rainfall will arrive next week. Temperatures will warm significantly with much above normal readings resulting in Extreme Heat Warnings for lower elevations and foothill locations. Despite the warmer and drier conditions, humidity levels will not fall substantially with minimum afternoon readings in a 10-25% range improving closer to 15-30% next week. Correspondingly, overnight recovery will mostly be in a fair to good range of 30-60% with somewhat higher values next week. Winds 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 AZZ530-532.

Extreme Heat Warning from 10 AM this morning to 8 PM MST Monday for AZZ531-533>555-559.

Extreme Heat Warning from 10 AM Friday to 8 PM MST Saturday for AZZ556-560>562.

CA...Extreme Heat Warning until 8 PM PDT Monday for CAZ562>570.

Extreme Heat Warning from 10 AM this morning to 8 PM PDT Monday for CAZ560-561.