textproduct: NWS Phoenix

UPDATE..Updated 06z Aviation Discussion

KEY MESSAGES

- Extreme heat will continue through the weekend, with afternoon highs hovering near to several degrees above normal while humidity values remain higher than normal.

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

- Monsoon rainfall chances become more favorable and will begin to expand into the lower deserts by late this weekend and into the start of next week.

SHORT TERM /Today through Saturday/

Latest visible satellite imagery indicates a growing cu field over the Mogollon Rim and White Mountains as well as across portions of southeast AZ. Mesoanalysis indicates MLCAPE values increasing to around 1200-1500 J/kg over the higher terrain of Gila County and far E Maricopa Counties where skies are clear and sfc dew points are in the mid 50s to lower 60s. High-res guidance suggests storms continuing to develop over the rim and pushing into S Gila County by early this evening. We will continue to monitor for a potential outflow boundary that could surge into E Maricopa County, however subsidence aloft from a strong 596-598 dam high and a strong capping inversion (MLCIN -80 to -100 J/kg) will likely limit storm activity across the lower deserts tonight. Therefore, PoPs remain on the order of 10-25% across Maricopa and Pinal Counties. If any storm does develop along an outflow boundary, strong to marginally severe wind gusts will be possible which could generate areas of blowing dust. Storm chances will diminish after 06Z, leading to a quiet rest of the night. With the additional cloud cover and higher dew points, overnight lows will remain above normal in the mid 80s to around 90 degrees.

Even though the high will remain in a favorable position over the 4- Corners region for continued monsoonal activity, high pressure aloft will strengthen further on Saturday. This will again provide a strong subsidence inversion over the lower deserts. Slightly drier air is also anticipated with sfc dew points falling into the low to mid 50s. These conditions will likely limit storm activity across the lower deserts aside from another potential outflow emanating from convection over the high terrain. NBM PoPs do appear to reflect this scenario as rain chances are very similar to today (10-20% for the lower deserts and around 30% over the foothills and mountains NE of Phoenix).

The unseasonably strong sub-tropical high will continue to remain entrenched over the 4-Corners region through this weekend with 500 mb hghts remaining around 595-597 dam through Sunday. Hghts of this magnitude are in the 90th-95th percentile of climo for this time of year. Although record breaking temperatures are not anticipated, afternoon highs will run a good 4-8 degrees above normal for early August, translating to readings ranging near 107-115 degrees. These temperatures will continue to promote a Moderate to Major HeatRisk and thus an Extreme Heat Warning will remain in effect through the remainder of this weekend.

LONG TERM /Sunday through Thursday/

The relative pause in convective activity should be fairly short lived as the high will eventually become parked over the Four Corners, inducing southeasterly flow over the region which should provide renewed moisture flux. PoPs have fallen slightly compared to 24 hours ago, mainly for lower elevation areas, but that is not much of a surprise as it make take a little bit more time to get moisture profiles back to more favorable levels. The more favorable spots to see convection will once again be the High County and Southeast Arizona, but downstream effects from any activity that does form could have impacts on our lower elevation zones. Once we get closer, and get inside the hi-res window, we will have a better idea on what impacts to expect.

Daily chances for shower and thunderstorms will reach into the front half of the next workweek as sufficient moisture stays in place. The lower deserts, if not by Sunday, may finally get in on some of the action as low-level moisture looks to improve with model soundings indicating mixing ratios reaching 11-12 g/kg. That will aid in increasing instability and limiting the cap that has been so prevalent the past few days. As typical with the monsoon, how one day plays out will heavily influence what happens the subsequent day, so day-to-day forecast changes should be expected, but with moisture levels not likely to fall off through at least the middle part of the week, we may be looking at one of the more active periods we have seen so far this summer.

Temperatures will remain several degrees above seasonal levels through Sunday, but there appears to be at least some relief on the horizon. With increased moisture, storm coverage, and cloud cover, afternoon highs over South-Central Arizona are expected to fall much closer to normal for this time of year with projected lower-desert highs between 101-106 degrees by Tuesday. Areas further displaced from the storm activity, namely Southwest Arizona and Southeast California, will see temperatures gradually decrease too thanks to a weakening trend with the prevailing high, but readings should remain slightly higher relative to locations further east. Highs for those western desert locations will be closer to 105-110 degrees through at least Thursday.

AVIATION

Updated at 0520Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Winds overnight will follow typical diurnal trends, transitioning to light east-southeast at KPHX and KIWA, and becoming light and variable at KSDL and KDVT. Expect a westerly shift by late Saturday morning to early afternoon, with speeds generally around 8-12kts and occasional afternoon gusts near 15-20kts. Mid to high level cloud cover SCT-BKN will thin overnight into Saturday morning, before increasing again by late Saturday afternoon. Looking ahead to Saturday evening, convective outflow uncertainty returns to the forecast. Model guidance remains highly divergent regarding potential storm evolution and boundary propagation, though HiRes solutions suggest a potential outflow pushing through the Phoenix Metro around the 02-03Z timeframe. Confidence in timing, wind direction, and exact strength remains low at this time, but solutions are pointing to winds emanating from the north- northeast.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns expected through the TAF period. At KIPL, southeasterly winds will shift westerly overnight before returning to a southeasterly direction by late Saturday morning. At KBLH, winds will maintain a S to WSW direction throughout the period. Speeds at both terminals will remain light, generally under 10kts. Sky conditions will feature mostly clear skies overnight with FEW high clouds returning during the day on Saturday.

FIRE WEATHER

Hot temperatures with elevated humidity levels will continue through the weekend. Although humidity levels will trend down, regional readings should remain at 15% or above through Saturday, and increase back to around 20% or greater on Sunday. Overnight recovery will offer modest to decent relief as MaxRHs range generally between 30-55% through Sunday, with even better readings expected by early next week. Rain chances through Saturday will be mostly confined to high terrain areas and southeastern Arizona, with lower desert chances remaining 15% or below. By Sunday, increasing moisture will result in an uptrend in lower elevation shower and thunderstorm chances, which appears likely to persist into the following week. Outside of thunderstorm outflows, winds will follow 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.