textproduct: NWS Phoenix

UPDATE..Updated 00z 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 2340Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation concern this evening and tonight will revolve around convective outflow boundaries and the potential for blowing dust (BLDU) with reduced visibilities. Confidence remains low regarding a distinct outflow, and with it dust impacts, specifically around the 05Z timeframe. However, there is still a possibility for outflows, likely originating from thunderstorms to the south and east, to push through the Phoenix Metro anytime between the 04Z and 12Z window. If an outflow does arrive, a sudden wind shift with gusts up to 20-25kts and temporary restrictions in visibility due to dust can be expected. This could lead to slantwise visibility issues Saturday morning and afternoon. Otherwise, expect typical westerly winds this evening to eventually transition to light east-southeast winds overnight. Winds will resume their typical westerly diurnal direction by late Saturday morning. Cloud cover will feature SCT-BKN mid to high- level decks overnight, gradually scattering out to FEW-SCT on Saturday.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns expected through the TAF period. At KIPL, southeasterly winds will shift to the west this evening before returning to the southeast early Saturday morning. Ar KBLH, winds will generally favor southerly direction with a westerly shift for a few hours late this evening then returning southerly. Wind speeds at both terminals will remain light, generally aob 10kts. However, there will be some periods of gusts at KBLH this evening up to 20 kts. Skies will remain mostly clear with just FEW-SCT high clouds passing through.

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.