textproduct: NWS Phoenix

UPDATE...Updated 12z Aviation Discussion

KEY MESSAGES

- Extreme heat conditions will persist through the weekend with temperatures hovering near to several degrees above normal while humidity values remain higher than normal.

- Shower and thunderstorm chances will favor high terrain areas and Southeast Arizona the remainder of the week with gusty outflow winds and blowing dust the most likely impacts for south-central Arizona.

- Shower and thunderstorm chances start to increase across the lower deserts late this weekend and into next week.

SHORT TERM /Today through Friday/

Current 500mb analysis reveals an elongated area of sub-tropical high pressure, stretching from California to Atlantic Coast while broad troughing advances through the Great Lakes Region. Closer to home, another round of showers and severe storms were observed across portions of Arizona yesterday into very early this morning, especially across Southeastern and far Southern Arizona, aided by enhanced moisture profiles provided by regional southeasterly flow. As for our forecast area, the moisture is there, but the atmosphere is heavily capped so we were unable to tap into the fair amount of elevated instability. Unfortunately, this scenario looks likely to play out again this afternoon and perhaps again on Friday as the afternoon cap looks to remain in place, heavily limiting convective activity. The one exception to this is, of course, where orographically induced showers and storms are favored over eastern portions of Gila County, where PoP chances currently stand around 20- 40% in the near-term. As for the lower deserts, any rainfall potential will be heavily conditional on if a strong enough outflow, or colliding outflows, can move through, but based on current hi-res data, that outcome appears to be in the minority. This doesn't mean that outflows for areas such as the Phoenix metro and Pinal County are being completely ruled out of the forecast, but any that were to move through these areas are not expected, at least at this time, to be strong enough to spark convection.

Much like the past few days, temperatures this afternoon will be conditional on how much cloud cover sticks around from a fairly expansive MCS down in the Tucson CWA. Current NBM forecast has widespread highs for the lower deserts ranging between 105-111 degrees, but these may need to be adjusted during the next forecast update if there is high enough confidence that lingering clouds will limit surface heating. With how worked over the atmosphere is likely to be down in Southeast Arizona, convective activity should be more suppressed, so there is reason to believe that cloud cover will not be as widespread Thursday night into Friday, therefore leading to higher confidence that current projected afternoon highs should be reached as we wrap up the workweek. With more limited cloud cover, and the sub-tropical high expected to strengthen slightly, widespread readings 110-115 degrees and above will make their return. Regardless of all this, the enhanced moisture in the air will make apparent temperatures feel even hotter, with "feels like" temperatures ranging from 110 to even around 120 degrees for some spots out in Southeast California.

The impact of the MCS down in Southern Arizona on our area may not be limited to temperatures as there are signs that an associated strong outflow may push through the Lower Colorado River Valley into the morning hours. The HREF indicates the potential to see gusts pushing toward 30-40 mph, which would likely be accompanied by blowing dust that could result in hazardous travel conditions. Depending how winds evolve, it is not out of the question that hazy skies could linger through much of the day, but if winds remain elevated through the morning, then that should be enough to clear most of the dust out of the area. A similar scenario may play out for areas of Pinal and Maricopa Counties as well, similar to what occurred just a few days ago when dust hazy conditions lingered for more than 12 hours. It is tough to say exactly how long any dust may stick around, as many factors need to be taken into consideration, but more than likely it would clear out by the afternoon hours at the latest.

LONG TERM /Saturday through Wednesday/

Heading into the weekend, the center of the anti-cyclone will shift eastward towards the Four Corners region. As it does this, it will actually pull in some slightly drier air off the Rockies, helping to add to what is already a relatively unfavorable environment for lower desert convection. It should not inhibit convection completely in the typically active spots, but coverage may be a bit more limited. This "drier' period should be short lived however as the center of the high will settle off to our north and east, inducing southeasterly flow once again and providing renewed moisture flux. Rain chances for Sunday bump back up, and the lower elevations may get in on some rain well as PoPs increase to around 20-30%, some of the better chances seen in a while. There still might be some recovery time from the drier air needed, so it would not be surprising that activity Sunday evening is not very robust for areas such as Maricopa and Pinal Counties, but it is a positive sign to see an uptrend in rain potential nonetheless.

For next week, the forecast appears that it will not change much in terms of convective activity as ensembles have the high parked over the Four Corners. Less subsidence aloft, and continued moisture flux bodes well for continued rainfall chances and that is reflected in current PoPs sit between 20-40% basically from Monday through Thursday. As typical with the monsoon, what days are active will have heavy influence on what occurs the day after, so day-to-day forecast changes should be expected once we get into this more active period.

Temperatures over the next seven days will be at their hottest on Saturday as current projections have widespread readings between 110- 116 degrees for the lower deserts. Add the humidity to that and it will not feel particularly great outside. Some more good news is that as that temperatures will begin to "cool" as the high shifts east and weakens slightly, we may be able to finally escape extreme heat criteria by the start of next week, at least for now.

AVIATION

Updated at 1135Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Slantwise visibility issues this morning due to lingering lofted dust and gusty outflow winds this evening are the main aviation weather challenges during the upcoming TAF period. Light easterly winds to variable conditions will be common through early this evening before a shift out of the west materializes mid to late morning. Lofted dust is likely to linger throughout the area through most of the morning, creating slantwise visibility issues with conditions improving late morning into the afternoon. Thunderstorm activity is likely across the high terrain areas to the north and east of the terminals this afternoon into early this evening with the potential for a northerly to northeasterly outflow boundary to progress through the terminals with some gusty winds this evening, mostly likely after 02-03z. However, there is a chance that the outflow does not materialize if the storm coverage across the higher terrain is more isolated and thus wind directions will remain out of the west with an easterly shift expected during the overnight period. SCT-BKN mid to high clouds will be common through this morning, with coverage decreasing to FEW-SCT this afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues under periods of mid/high cloud decks are expected throughout the TAF period. Winds will generally be out of the southeast at KIPL and out of the south at KBLH, with some modest afternoon/early evening gusts. Some brief gusty winds between 20-25 kts is likely early this morning due to an outflow passage.

FIRE WEATHER

Hot temperatures with relatively elevated humidity levels will continue through the next several days. Though afternoon RHs do trend downward for Friday, 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%. Rain chances through this weekend will be confined mostly confined to high terrain areas and southeastern Arizona, with lower desert chances remaining 10% or below through most of the weekend. 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.