textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Sunday evening with highs and lows several degrees above normal and higher than normal humidity, resulting in areas of Major HeatRisk.

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

- Deeper moisture will provide a better opportunity for more widespread shower and thunderstorm activity by early next week, including in the lower deserts, along with cooler temperatures.

SHORT TERM /Today through Sunday/

Current GOES mid-lvl wv imagery and 500 mb streamline analysis reveals an unseasonably strong subtropical high centered near the 4-Corners region with height fields approaching 596-598 dam. Heights of this magnitude are above the 98th percentile of climatology for early August. Due to the overall strength of this ridge, and responding low-lvl thermal provide, temperatures will once again top out well above normal (5-8 degrees) this afternoon. Most lower desert communities will see highs in the 110-115 degree range, resulting in a widespread Major HeatRisk. The Extreme Heat Warning remains in effect today across the lower deserts of AZ and southeast CA, thus it is important to continue practicing smart heat safety. The center of the subtropical high will begin to slowly shift ewd on Sunday which will allow 500 mb height fields to lower ever so slightly across the Desert Southwest. This will cause temperatures to tick down a couple of degrees, but still remaining in a well above normal range Sunday afternoon with lower desert highs reaching 108-112 degrees. Given the afternoon highs exceeding 110 degrees along with early morning lows bottoming out in the middle 80s to lower 90s, the Extreme Heat Warnings will remain in effect across all of the lower deserts through Sunday evening.

Similar to the last several days, thunderstorm activity is expected to develop across the higher terrain areas to the north and east of Phoenix this afternoon and migrate westward but likely weakening as the activity approaches the lower deserts during the early evening hours. Forecast soundings for the lower deserts indicate a strong inversion in the 700-600 mb layer. Latest RAP analysis also shows MLCIN values approaching -90 to -110 J/kg across Maricopa County which will likely limit storm activity across the lower deserts. What is more likely will be an outflow boundary associated with a decaying complex of storms over the foothills NE of Phoenix which could bring strong, gusty winds into portions of Maricopa County. Winds could gust between 25-35 mph for a couple hours as the outflow sweeps through. Patchy blowing dust will also be possible, especially for areas south and west of the Phoenix Metro. Very similar conditions are anticipated on Sunday in terms of areal coverage of thunderstorms with NBM PoPs ranging from 10-25% across the lower deserts of southcentral AZ. Again the main impacts from any storms will be gusty outflow winds in excess of 30 mph.

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 0021Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty thunderstorm outflow winds will be the primary forecast challenge and potential impact the next couple evenings. There is moderate confidence that an outflow will arrive from the ENE to NNE in the 02-04Z timeframe this evening with potential for wind gusts up to 20-30 kts and around 30-40% chance for peak gusts >30 kts. Odds of VCSH/VCTS are low (10%) this evening. Preceding outflow wind shifts, winds will likely continue to favor light variability with a weak S-SW component. There is low confidence on wind directions tonight and Sunday morning, with potential for more variability through the night. There will be potential for outflow winds from the south and east Sunday evening, which with any outflow collisions may be able to spark VCSH/VCTS (20% chance). FEW to SCT clouds will be common through the period, with lowest bases staying at or above 10K ft AGL.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Both terminals will mostly favor a SW-SE wind direction with light winds in the morning and overnight hours and 8-12 kts with gusts to 20 kts in the afternoon. There is a low chance (~10%) of an outflow from distant storms moving through KBLH tonight. FEW to SCT mid and high clouds will be present through much of tonight and Sunday morning.

FIRE WEATHER

Above normal temperatures will prevail through this 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. Low-level moisture will continue to rise through early next which will promote much better overnight recovery. Shower and thunderstorms 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 arriving by early next week. Chances of wetting rainfall will also increase significantly early next week. Outside of gusty and erratic winds due to thunderstorms, 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.