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/

A sprawling subtropical high pressure system was covering the southern Conus early this morning with one 597dm H5 center over southern New Mexico. The overall trend in the anti-cyclone center is towards a retrogressive phase, however this will change over the next 48 hours with the core lifting towards central Colorado. Despite this movement, the forecast area will maintain H5 heights in a 596-598dm range with H8 and H7 temperatures well above +30C and +15C respectively which are general proxies for lower elevation temperatures reaching/exceeding 110F. Numerical guidance spread has narrowed recently providing increased confidence of many lower elevation communities touching 115F today and Saturday yielding widespread major and areas of extreme HeatRisk. Confidence in forecast temperatures deteriorates significantly Sunday as strong, moist thunderstorm outflows Saturday night may substantially alter boundary layer moisture and mixing depths, as well as introducing pesky midlevel cloud decks detrimental to daytime insolation. This outcome is reflected in numerical guidance where the full range in ensemble spread widens, but also abruptly drops from the Friday/Saturday levels.

As the center of the subtropical ridge shifts into Colorado, this will allow better easterly flow to develop on the southern periphery of the anti-cyclone over northern Mexico/southern Arizona. Subsidence and somewhat drier air will be maintained today over the CWA in proximity to the high pressure center. However much like yesterday, this will not completely preclude deep convective development over the most favored mountain locations, though this activity should be very isolated and short-lived in nature. This will gradually change over the weekend as a well defined easterly wave/inverted trough comprised of tropical remnants crosses into northern Mexico.

Confidence is growing that the northern extent of the inverted trough and jet streak will cross into southern Arizona Saturday afternoon providing a very convectively favorable difluent flow aloft. 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". Surprisingly, models are not overly enthusiastic regarding more expansive thunderstorm development Sunday with a continuation of a favorable synoptic pattern, MLCape exceeding 1000 J/kg, and DCape still near 1500 J/kg. Forecast BUFR soundings suggest lower elevation CinH in excess of 25 J/kg, though multiple colliding outflows should conceivably breach this inhibition suggesting there may be other factors contributing to models not producing better storm chances.

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 0450Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: No major aviation weather concerns under FEW mid to high level cloud decks are expected throughout the TAF period. The overall wind pattern will continue to exhibit the typical diurnal tendencies with speeds generally aob 10 kts, although some occasional afternoon gusts in the mid to upper teens will be possible Friday.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation weather concerns under FEW mid to high level cloud decks are expected throughout the TAF period. Wind directions will generally fluctuate between the southeast to southwest. Some elevated gusts upwards of 20-25 kts are likely Friday afternoon at KBLH.

FIRE WEATHER

Thunderstorm chances will remain low today with only a limited opportunity remaining over high terrain areas in Arizona. 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. 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 from 10 AM this morning to 8 PM MST Saturday for AZZ556-560>562.

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