textproduct: NWS Phoenix

UPDATE

12Z Aviation Discussion

KEY MESSAGES

- Extreme Heat Warnings remain in effect through Saturday with areas of Major to Extreme Heat Risk, despite temperatures slowly decreasing.

- Today and Friday will bring low end chances for showers and a few thunderstorms, focused more across southern and central Arizona.

- A low pressure system nearing the region over the weekend will bring much better chances for rain, including a chance of strong to severe storms across southern Arizona from mainly on Sunday or Monday.

SHORT TERM /TODAY THROUGH FRIDAY/

An unseasonably strong subtropical ridge remains positioned directly over the Desert Southwest, but it is getting infiltrated by a good amount of high level moisture peeled away from the remnants of TC Iselle. Water vapor imagery shows a decent stream of upper level moisture just west and southwest of northern Baja into Arizona. We anticipate partly to mostly cloudy skies across southeast California and partly cloudy skies over Arizona today providing a slight reprieve from the scorching temperatures. Despite the higher clouds, highs are still expected to easily top 110 degrees across the lower deserts today, but they should be a few degrees cooler than yesterday's.

In the lower levels, a modest Gulf surge of moisture is currently working its way into the area with 65-70 degree dew points already into Yuma and the Imperial Valley and eventually to around 60 degrees in the Phoenix area later this morning. However, even with some improvement in moisture for today forecast soundings show little support for convection. Low level mixing ratios will at most reach 8 g/kg this afternoon which is barely adequate for even high terrain storms for an ideal monsoon season day. The continued strong subsidence from the ridge overhead should be enough to keep any potential convection confined to higher terrain areas this afternoon/early evening with at most isolated coverage. Slightly more favorable conditions should at least allow for some more scattered convection over far southeast Arizona which eventually may be enough to kick off a northward progressing outflow toward Pinal and Maricopa Counties late in the evening. The HRRR suggests a combination of this potential outflow with some mid-level moisture advection and/or gravity waves from more organized convection over Sonora may be enough to spark off some elevated late evening showers and weak storms. Have adjusted PoPs upward to 10-20% over south-central Arizona from 10 PM to 3 AM tonight to account for this potential.

Friday is shaping up to be relatively quiet through the majority of the day with no additional moisture advection tonight and through the first half of Friday. The higher level moisture and clouds for today are also expected to push out of the area leaving more sunshine for Friday. CAMs show less convective activity than today, but that is only through Friday afternoon. The subtropical high is also forecast to weaken and push into northern New Mexico. Given the lowering heights, but more sunshine forecast highs on Friday are very similar to today's, keeping widespread Major to Extreme HeatRisk over the area.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

A significant pattern change is set to begin this weekend with a large Pacific trough slowly moving southeastward across the Western U.S., eventually pushing the subtropical high into the Southern Plains by Sunday. Our main forecast concern with this change will be the increasingly favorable conditions for thunderstorms, especially by Sunday and Monday. The influence of the trough may occur as early as late Friday night and early Saturday as a relatively strong jet streak moves through California into the Great Basin. This should result in increasing upper level support for showers and thunderstorms, but we will also have to have enough moisture. For now, guidance is unclear on the rain chances for early Saturday, but the NBM is trying to put a 30-40% chance in after midnight. The daytime hours Saturday are just as unclear as moisture is likely to continue to improve, but will it be enough to overcome a modest capping inversion. Guidance does show PoPs of 30-40% during the evening hours Saturday from Phoenix and areas to the north and east of Phoenix, but little chance to the west. Temperatures Saturday will cool a bit from Friday, but there should still be enough residual heat to push daytime highs to around 110 degrees across the western lower deserts to 107-109 degrees in the Phoenix area.

Sunday into possibly Monday very well could be quite active as guidance is pointing at very good moisture advection into Arizona, somewhat directed by a passing tropical system to the southwest of southern Baja. Atmospheric parameters improve on Sunday with MUCAPEs anywhere from 1000-2000 J/kg and little if any CIN during the afternoon hours. The presence of the trough over or just to the northwest of our area should also provide some modest shear and upper level support for storm potential on Sunday and/or Monday. Depending on the evolution of the trough, heating, and how much moisture gets pulled into our area, we may need to worry about a strong to severe storm potential as well as a heavy rainfall threat as PWATs may reach near 1.75". Drier air will also be trying to work into western portions of our area by Sunday, leaving little chance for rain over southeast California and potentially southwest Arizona. NBM PoPs reflect this with at most 10-20% chances over the western deserts, but PoPs rise to 40-60% from Phoenix and areas to the north and east of Phoenix on Sunday. There is much higher model uncertainty going into the mid to latter half of next week as GEFS favors moisture staying mostly in place while the EPS favors some more significant drying. Part of this uncertainty is due to a highly active tropic region to our southwest which very well could influence our region at times.

Temperatures are at least slated to drop a good amount over the weekend into early next week with forecast highs falling back to near normal on Sunday and then at or just below the normal range early next week. We can't even rule out having highs in the 90s for a day across south-central Arizona if the rain activity is widespread enough on Sunday and/or Monday.

AVIATION

Updated at 1112Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:

No major aviation concerns will exist through the TAF period. Winds are expected to mostly follow light and diurnal trends through the period. Some VCSH cannot be ruled out during the mid to late morning timeframe, but forecast uncertainty remains too high to warrant inclusion of any rainfall mention in the TAFs. If any SHRA were to occur, operational impacts should be minimal. Otherwise, a southerly outflow with gusts between 20-25kts is likely late tonight to near midnight before switch back easterly for the remainder of the the overnight period tomorrow. SCT-BKN high clouds will persist through the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected through the TAF period. Winds will follow familiar diurnal trends, and some VCSH cannot be ruled out during the mid to late morning timeframe, mainly for KBLH, but forecast uncertainty remains too high to warrant inclusion of any rainfall mention on the TAF. KBLH is likely to see gusty conditions again near midnight tonight from the south with speeds between 20-25kts. SCT-BKN high clouds will persist through the period.

FIRE WEATHER

Hot and relatively dry conditions will continue over the next couple of days with only widely isolated showers and thunderstorms possible. Afternoon minimum humidities will improve slightly to 15-20% through Friday before improving further over the weekend as rain chances increase over Arizona. Winds through Friday will overall be light and follow diurnal tendencies, with modest afternoon upslope/upvalley breeziness.

CLIMATE

Record highs:

Date Phoenix Yuma El Centro ---- ------- ---- --------- Aug 27 114 in 2023 120 in 1981 117 in 1981 Aug 28 117 in 2023 118 in 2023 117 in 2009 Aug 29 116 in 2023 117 in 1954 117 in 2005

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Saturday for AZZ530>555-559- 560.

Extreme Heat Warning until 8 PM MST Friday for AZZ556-561-562.

CA...Extreme Heat Warning until 8 PM PDT Saturday for CAZ562-565>567- 569-570.

Extreme Heat Warning until 8 PM PDT Friday for CAZ561-563-564- 568.