textproduct: NWS Phoenix

KEY MESSAGES

- Extreme Heat Warnings remain in effect through the weekend as afternoon highs will hover several degrees above normal while humidity values remain higher than normal, resulting in areas of Major HeatRisk.

- Shower and thunderstorm activity throughout the weekend will remain confined to the higher terrain areas with gusty outflow winds and areas of blowing dust the most likely impacts for the lower deserts.

- Deeper moisture will increase the chances for more widespread shower and thunderstorm activity heading into early next week along with cooling temperatures.

SHORT TERM /Today through Sunday/

Latest objective analysis early this morning indicates the subtropical high centered over Utah with 500 mb heights ranging between 596-598dm across the region. These upper-level heights are above the 99th percentile for this time of the year, indicating a very warm airmass is in place throughout most of the atmospheric column. As a result, afternoon high temperatures today will generally range between 110-115 degrees across most of the lower desert communities, which is a good 4-8 degrees above normal. The subtropical high will weaken slightly on Sunday, resulting in slightly cooler afternoon highs but still exceeding 110 degrees across most of the lower desert communities. Given the afternoon highs exceeding 110 degrees along with early morning lows bottoming out in the middle 80s to lower 90s, high-end moderate to areas of Major HeatRisk can be expected throughout the weekend and thus the Extreme Heat Warnings remain in effect across all of the lower deserts through Sunday evening.

The strong subtropical high sitting just to the north will continue to result in strong subsidence and thus a good deal of convective inhibition across the lower deserts today as well as into Sunday. As a result, similar to the last several days, thunderstorm activity is expected to develop across the higher terrain areas to the north and east of Phoenix as well as across southeast AZ during the afternoon hours, and migrate westward but likely weakening as the activity approaches the lower deserts during the early evening hours. This scenario is indicated by most of the hi-res guidance. However, outflow winds from these storm complexes that do develop will more likely affect the lower deserts with some gusty winds and perhaps areas of blowing dust if the outflows that materialize move through the dust prone areas. It will have to take a strong outflow or collision of multiple outflows to overcome the convective inhibition and result in the development of activity in the lower deserts. Although this scenario is of low probability, it cannot be totally ruled out.

LONG TERM /Monday through Friday/

The overall pattern heading into the first half of next week is likely to become more favorable for more widespread convective activity across the region, including for the lower deserts. The subtropical high is projected to weaken even further and migrate more towards the Four Corners Region, resulting in the overall flow to shift more out of the southeast. This will cause moisture values to increase with the latest EPS and GEFS indicating PWATs settling around 1.5-1.8" and low-level mixing ratios exceeding 11-12 g/kg. This increase in moisture will result in better instability and weaker convective inhibition, making the environment more favorable for storms that do develop across the typical higher terrain areas to survive into the lower deserts. This more favorable environmental setup is depicted in the latest forecast from the NBM, which shows PoPs above 40% across the south-central AZ lower deserts starting on Monday with elevated chances continuing through the middle of the week. Which day early next week could be the most active is difficult to pinpoint at this time as it will be dictated by mesoscale processes that are difficult to predict this far out.

Heading towards the latter half of the week, latest guidance indicates the subtropical high migrating further eastward towards Texas while an upper-level low sets up off the California coast. This overall setup is likely to shift the flow more out of the southwest, bringing in some drier air as PWATs drop under 1.5". As a result, an overall downtrend in storm activity is likely by the end of the week.

With the weakening of the subtropical high and in increase in convective activity and cloud cover, temperatures next week are expected to gradually cool with afternoon highs expected to fall to near seasonal levels by Tuesday with afternoon highs of 102-108 degrees across the lower deserts. The NBM is even showing temperatures potentially cooling even further through the end of the week as it shows afternoon highs of 100-106 degrees, which if it materializes, would be slightly below normal for the middle of August.

AVIATION

Updated at 0900Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concern will continue to be gusty outflow winds during the afternoon and evening hours. Current TAFs reflect a "best guess" given current guidance, but a number of adjustments are certainly likely in subsequent forecasts given the very low confidence of timing, strength, and orientation of potential boundaries. It is not out of the question that no outflow moves through metro terminals at all given the expectation that convective activity across Arizona will be more muted compared to days prior. In the meantime however, winds are expected to follow diurnal trends into the afternoon hours, though the switch to the west may take longer than usual as several hours of southerly flow is being suggested. Outside of brief window of mostly clear skies late this morning, SCT to BKN cloud decks will be common over the region during the forecast window.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Familiar diurnal trends will prevail at each terminal, with SE'rly winds switch out of the W at KIPL, and S/SW'rly flow being common at KBLH. There is a low end chance (~10%) of an outflow from distant convection moving through KBLH later in the period, but confidence is too low to warrant mention in the TAF at this time. FEW high clouds will be present through much of the forecast window before mid-level decks move in early tonight.

FIRE WEATHER

Above normal temperatures will prevail throughout the weekend before gradually cooling to more seasonal levels by early next week. Afternoon MinRHs will bottom out between 15-25% today and increase slightly on Sunday closer to 20-25% areawide. Increasing moisture next week will lead to MinRH values bottoming out above 20% along with good overnight recoveries. Shower and thunderstorm activity will be mainly confined to the higher terrain areas to the north and east of Phoenix as well as southeastern Arizona throughout the weekend with more widespread activity, including the lower deserts, more likely by early next week. Outside of thunderstorm outflows, winds will follow the familiar diurnal trends, with typical afternoon upslope breeziness around 20 mph.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Sunday for AZZ530>544-546- 548>551-553>555-559.

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