textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

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

- Strong outflow boundary winds will move through south-central Arizona late this afternoon and evening, with a slight chance to generate showers and and storms over the lower deserts. - Temperatures will cool slightly early next week as high pressure weakens, but will rise again heading into the latter half of the week with widespread Major and areas of Extreme heat risk returning.

- Typical monsoon conditions are expected next week with daily thunderstorm chances favoring northern Arizona high terrain and southeast Arizona and low chances in the south-central Arizona lower deserts.

SHORT TERM /Today through Monday/

The temperature at midnight in Phoenix was 107 degrees, which is quite frankly disgusting and very unpleasant. This is not the first time though that temperatures were this hot in the midnight to 1 AM hour. The temperature was 107 at midnight back on July 30th, 2023 and was nearly as hot during the middle of the same month, when the 97 degree record warm low was first set. There is a decent chance to break the all time record warm low this morning, especially considering the record was tied yesterday morning and the temperature shortly after midnight this morning was 3-4 degrees warmer than the same time yesterday morning. Whether the record is broken is largely dependent on the winds. If a westerly breeze can be maintained through sunrise, then the record will probably be broken and it is not out of the realm of possibility to see the first 100 degree low.

After another hot morning temperatures this afternoon will again heat up to similar levels to yesterday, if not 1-3 degrees hotter, with lower desert highs forecast to reach 114-119 degrees. This combo of forecast record warm lows and near record highs is leading to widespread Major HeatRisk and areas of Extreme HeatRisk in the most populated areas. Combining this with the weekend only increases the already heightened risk for heat-related health impacts. Even the morning hours are not recommended for outdoor recreation given the very hot start and rapid warming through the morning - 110 degrees by 11 AM. The mugginess, dew point temperatures in the upper 50s to 60s will only add to the discomfort. While not much, there will be a small degree of relief to the temperatures, at least with the highs, heading into Sunday and Monday as H5 heights come down a few decameters and 850mb temps cool a few degrees C. The mornings will remain uncomfortable with lows in the upper 80s to low 90s. High temperatures Monday are forecast to drop to within a few degrees above normal, but the added humidity may add a few degrees onto the early afternoon apparent temperatures. So, for now the Extreme Heat Warning will be maintained through Monday.

The prospect of showers and thunderstorms in the lower deserts of South-Central AZ are slightly better today-tonight than they were the past few days, for a few factors coming together. Broad anticyclonic flow remains in place across the Desert Southwest, with an IT (Bertha remnants) seen moving across Sonora, MX this morning and there is a small mesoscale low pressure over central NM. Latest guidance shows the mesoscale low will progress into east-central AZ this afternoon and evening, assisting Rim convection and inducing a light east-northeast steering wind off the Rim. DCAPE will be fairly high today across South-Central AZ (1200-1800 J/kg), given the very hot temperatures, and latest HREF favors moderate to strong outflows (30-50 mph wind gusts) moving off the Rim into the lower deserts in the late afternoon and evening. It remains to be seen if the outflows will generate new storms over the lower deserts or if the cap/CIN will will be too much to overcome. Even if it is overcome, showers and storms over the lower deserts are not expected to be too intense, due to sub-1000 J/kg CAPE, owing to the very warm air aloft and deep boundary layer. The strongest storms today will be around Tucson and points south, where moisture and instability will be greatest, with modest shear (20-25 kt) as well. These storms will likely push outflows north-northwestward toward South-Central AZ later in the evening, which may collide with the southwestward-moving outflows to spark activity. Mid and upper level ascent will increase heading into the overnight hours tonight, to perhaps support lingering showers during the night, as an upper level jet streak nudges in from the east.

Heading into Sunday and Monday, the synoptic setup will take on more of a traditional monsoon regime with high pressure centered to the northeast and increasing southeast flow. Storms will primarily focus over N AZ and SE AZ, with the strongest storms still near the international border. Despite an increase in boundary layer moisture (11-12 g/kg mixing ratios) and instability model soundings show a moderate-strong capping inversion that may be difficult to overcome for convection to ensue over the lower deserts, away from terrain features. Latest NBM has 10-20% chances in Maricopa and northern Pinal counties Sunday and Monday.

LONG TERM /Tuesday through Friday/

Strong, expansive ridging will remain nearly in place through 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. Overall, this appears to be a very typical monsoon pattern with alternating convectively active and inactive days. However, 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 and the center of the anticyclone shifting to 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, after the very short-lived reprieve at the beginning of the week. As a result, the current Extreme Heat Warning may actually be extended as one prolonged warning event.

AVIATION

Updated at 1842Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty winds with the potential for multiple wind shifts this evening/overnight will be the main weather issue through the period with occasional passing mid/high level cloud decks potentially lowering and becoming thicker this evening. Confidence is high that at least one outflow boundary will move through the metro this evening with gusty winds (25-30 kt, 50% chance of some gusts in excess of 35 kt). Confidence is lower on how many outflows will move through the metro, from which directions, and if they will be carrying any BLDU leading to reduced VIS. Models continue to show a NE'rly outflow arriving between 00-01Z this evening. Models also hint at an additional SE'rly outflow will try to make it into the metro a couple hours later, however they show the NE'rly outflow dominating over the SE'rly outflow. If the NE'rly outflow dominates this evening, as currently expected, then little to no reductions in VIS are expected from BLDU. However, if a SE'rly outflow can make it into the metro BLDU and reduced VIS will become more of a concern, depending on the strength of the outflow. The possibility of SHRA/TSRA behind the wind shift remains around 20% and therefore has not been included in the TAF. If there are multiple colliding outflows over the metro, confidence will increase. Confidence is low on the wind direction after the outflow moves through the metro. Currently looks like winds will southerly during the overnight hours before going SE'rly early tomorrow morning.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues will exist through Sunday morning under occasional passing mid/high cloud decks. Wind patterns will be nearly identical to the past 24 hours, generally oscillating between SE and SW. Gusts may be more prevalent at KBLH this afternoon. May have to watch for an additional period of gusty winds at KBLH during the overnight hours. It will depend on if an outflow can make its way into the area.

FIRE WEATHER

There will be better chances for showers and storms this weekend into early next week, than the previous few days, but with activity still mostly favoring the high terrain of northern AZ and southeastern AZ and slight chances (10-20%) in the lower deserts of south-central AZ. There is high confidence that strong gusty thunderstorm outflow winds will progress into the lower deserts late this afternoon and this evening, which may offer slightly better odds for rain over the lower deserts today than in the following few days. Extreme, well above normal, temperatures will contribute to seasonally dry afternoon conditions. Hottest temperatures will be today, with minimum afternoon RH values in the 10-20%. Temperatures then fall several degrees heading into the beginning of next week and moisture increase slightly, leading to minimum RH values improving slightly to 20-30%. Temperatures will warm back up again during the latter half of next week and minimum RHs will again fall to the 10-25% range. Overnight recoveries will be fair most nights with RH values mostly in the 35-50% across lower deserts to 45-65% across southern Gila County. Daily afternoon thunderstorms will continue through next week, mostly across higher terrain and 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 this evening for AZZ556- 560>562.

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