textproduct: NWS Phoenix

KEY MESSAGES

- Temperatures will increase significantly the remainder of the week with Extreme Heat Warnings in effect as readings approach near record levels.

- High pressure responsible for the extreme heat will also restrict thunderstorm development with chances retreating well into the mountains through the weekend.

- Above normal temperatures may begin to relax early next week as better thunderstorm coverage returns to the region.

SHORT TERM /Today through Friday/

A well defined easterly wave, now in the form of a closed circulation was slowly migrating north along the Arizona/New Mexico border early this morning. With this evolution, favorable divergence aloft and downstream vorticity advection on the northern and western periphery of the gyre were exiting the forecast area. In its place as the vorticity center ejects into the central Rockies, deeper NW flow and subsidence will become more pronounced over the next 36 hours as subtropical ridging retrogrades from the southern plains. While this pattern will not completely scour away moisture, the current 10-14 g/kg boundary layer mixing ratios will be replaced by 9-10 g/kg levels. Coincident with notable warming aloft, this will heighten CinH and limit available MLCape such that even deep convection over higher terrain locations may struggle. HREF output reflects this with only isolated pulse driven deep convection this afternoon, followed by essentially a completely dry forecast Thursday and Friday.

The larger weather hazard over the next several days will be building heat as strong subtropical ridging retrogrades back into the SW Conus. Ensemble members are in excellent agreement depicting the central anti-cyclone becoming established over western New Mexico by Friday with H5 heights settling near 598dm. Surprisingly given the agreement among the entire ensemble suite, numerical spread is fairly wide likely owing to differences in boundary layer moisture content across the CWA. Given the current low level dewpoint status in south-central Arizona and lack of strong flow throughout the troposphere, it's more likely modest moisture lingers and some of the more extreme forecasts above the 75th percentile will not come to fruition. However across the western CWA where moisture is not as plentiful, upper end readings above 115F will be more likely. Regardless, widespread major HeatRisk (and local extreme HeatRisk) with afternoon highs 5F-10F above normal will become common, and an Extreme Heat Warning continues.

LONG TERM /Saturday through Tuesday/

Strong high pressure will continue to be the dominant synoptic feature through early next week with H5 heights 596-598dm sprawling across the forecast area. Unsurprisingly given this pattern, widespread major HeatRisk will continue with clear cut warning criteria persisting through the weekend as temperatures hover 5F-10F above normal. The preponderance of ensemble output suggests the center of the subtropical anti-cylcone slowly lifting through the four corners and into Colorado during the first part of next week allowing easterly flow, subtle shortwaves, and stronger, moist outflows to return to Arizona. Easterly wave energy will also help erode some of the higher midlevel heights and temperatures while also aiding the displacement of the anti-cyclone center. These combined efforts should allow local H5 heights to fall below 594dm though the timing at which this evolution occurs is somewhat uncertain. Among GEFS members, there is fairly convincing evidence of enhanced convection Sunday allowing deeper outflow intrusions while H5 heights fall below 594dm reducing or eliminating the extreme heat threat Monday. EPS members are more suggestive of better convective potential delayed until Monday evening, but regardless temperatures should return to near normal levels by the middle of next week with the diurnal thunderstorm risks also returning to the region.

AVIATION

Updated at 0515Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns are expected throughout the TAF period. VCSH will prevail through the first part of the overnight, especially at KDVT and KSDL. Otherwise, fairly tranquil conditions will prevail through Tuesday with no outflows/thunderstorms expected at this time. Winds through Tuesday morning will prevail out of the east at less than 10 kts with light west to northwest expected Tuesday afternoon, although extended periods of variability will be common. SCT-BKN clouds aoa 10 kft will be common through the overnight with coverage decreasing to FEW throughout the day Tuesday.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather issues under few passing mid/high cloud decks can be expected throughout the TAF period. Winds will generally be out of the west at KIPL and out of the south to southeast at KBLH through the overnight period at less than 10 kts. Mostly light and variable winds can be expected throughout the day Tuesday, with KIPL shifting out of the west during the evening hours.

FIRE WEATHER

Thunderstorm chances will dwindle the remainder of the week with only slight chances remaining over high terrain areas of far eastern districts. Instead, temperatures will warm significantly with much above normal readings and Extreme Heat Warnings. Despite the warmer, drier conditions, humidity levels will not fall substantially with minimum afternoon readings 15-30% the next couple days falling closer to a 10-25% range over the weekend. Correspondingly, overnight recovery will mostly stay in a fair to good range of 30- 60%. Winds should be fairly light through the period with only local afternoon gusts 20-25 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning from 10 AM today to 8 PM MST Monday for AZZ530-532.

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

CA...Extreme Heat Warning from 10 AM today to 8 PM PDT Monday for CAZ562>570.

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