textproduct: NWS Phoenix

UPDATE..Updated 12z Aviation Discussion

KEY MESSAGES

- Extreme heat conditions 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 confined to higher terrain areas and Southeast Arizona through at least the start of the weekend with gusty outflow winds and blowing dust the most likely impacts for lower elevations of South-Central Arizona.

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

SHORT TERM /Today through Saturday/

Current objective analysis reveals broad anti-cyclonic flow over the Great Basin and Four Corners regions, helping to keep parts of Arizona active with convective activity. After yesterday, in which Southeastern and Southern Arizona saw widespread severe weather, the atmosphere down there was incredibly worked over, limiting thunderstorm coverage and allowing northern portions of the state become the main focus for monsoon storms. Meanwhile, the lower deserts of South-Central Arizona remain quiet, not because the ingredients are not there, but rather a strong cap has been in place, and there has not been any significant triggering mechanisms to tap into available elevated instability. Model soundings point towards a weaker cap across the eastern half of our coverage area, so perhaps we see a bit more convective coverage for the population centers of Maricopa and Pinal Counties. However, PoPs do not project much optimism as rain chances remain at 15% or less. It would likely require a strong enough outflow to set off any convection over these areas, and as of now, guidance does not favor that outcome. However, this does not completely rule out lower elevation activity, as there are some hints amongst the hi-res data that favor a more active result. Regardless of all this, typical orographically induced shower and thunderstorms can still be expected for areas to the north and east of the Phoenix metro.

Even though the high will remain in a favorable position for continued monsoonal activity, Saturday may turn out out to be a bit of down day compared to what we have seen across Arizona this week. The reason for this downtrend will be the introduction of drier air originating from the Rockies where PWATs are running well below normal for this time of year (about 30-60%) of normal. The easterly flow that usually provides the region the moisture needed to get our seasonal rains will do the opposite for a time and erode at the moisture that is currently in place. PWATs for Eastern and Central Arizona will fall to around 70-90% of normal, which should greatly inhibit storm activity. Convection should not completely cease, but coverage should be focused primarily around the most enhanced terrain features. NBM PoPs do appear to reflect this scenario as rain chances are on par with chances for this afternoon, even though moisture profiles will be much better today. Therefore, PoPs have been lowered slightly for Saturday afternoon and evening.

The sub-tropical high will continue its trek eastward through the near term, peaking at 594-597dm as it migrates. These heights aloft are above the 90th-95th percentile for this time of year, indicating a very warm airmass throughout the column for this time of year, even for our standard. Now, this does not mean we are expecting record breaking temperatures, but afternoon highs will run a good 4- 8 degrees above normal for early August, translating to readings ranging near 107-115 degrees. As a result, high-end Moderate to Major HeatRisk remains in the forecast and the Extreme Heat Warning will remain in effect during this timeframe.

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 1120Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Outflow winds and slight potential for thunderstorm activity this evening will be the main aviation weather challenges during the upcoming TAF period. In the meantime, conditions will remain relatively tranquil through this afternoon with light easterly winds to variable conditions prevailing through the early morning hours before the typical westerly shift materializes mid to late morning. Some occasional afternoon gusts in the mid to upper teens will be possible. Heading into this evening, thunderstorm activity across the higher terrain to the north and east may approach the Phoenix metro, mostly likely after 02-03z, however, confidence of any direct impacts is low at this time with the more likely outcome that the activity dissipates before reaching the terminals. There will be higher potential for an outflow boundary to move through, shifting the winds out of the east to northeast. SCT-BKN mid to high clouds will prevail through early this morning before coverage decreases to FEW late morning through the afternoon. Cloud coverage will increase to SCT-BKN once again this evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns under FEW to occasionally SCT mid to high-level cloud decks can be expected throughout the TAF period. At KIPL, southeasterly winds will prevail through this afternoon with a westerly shift likely this evening. At KBLH, winds will generally fluctuate out of the south to southwest. Overall speeds will remain aob 10 kts.

FIRE WEATHER

Hot temperatures with relatively elevated humidity levels will continue through the weekend. Though afternoon RHs do trend downward through Saturday, regional readings should remain at 15% or above into next week. Overnight recovery will offer modest to decent relief as MaxRHs range generally between 30-55% through Sunday, with even better readings expected by 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.