textproduct: NWS Phoenix

KEY MESSAGES

- The Extreme Heat Warning has been extended through Sunday.

- 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 east and southeast Arizona the remainder of the week with gusty outflow winds and blowing dust the most likely impacts for south-central Arizona.

SHORT TERM /Today through Thursday/

Hot and somewhat humid conditions will prevail over the next couple of days as the heat dome remains in place over the Southwestern U.S. The center of the subtropical high is currently located across central Arizona, likely staying there through Thursday, while enough moisture remains trapped under the high to create daily chances of high terrain shower and thunderstorms.

Although temperatures have dropped off from the 115+ degree readings from this past weekend, the heat dome will continue to be enough to bring uncomfortably hot conditions, especially when considering the increased humidities. The increased low level moisture, patches of clouds, and even suspended dust held back temperatures yesterday. As of 2 AM, most locales are still holding in the low to mid 90s with forecast lows mostly in the mid 80s to upper 80s. Morning clouds are again expected today which will initially hold back temperatures, but decent clearing in the afternoon should still allow temperatures to reach near 110 degrees for highs. High surface dew points holding above 60 degrees all day today will also result in heat indices breaching 110 degrees for the bulk of the lower deserts.

Convective potential for this afternoon and evening remains quite uncertain, at least for the south-central Arizona lower deserts. Guidance continues to point toward a more active storm day today than yesterday, as long as enough insolation is realized this afternoon. The 00Z HREF shows strong instability developing this afternoon over the region, with MUCAPE as high as 1500-2000 J/kg over Gila County through southeast Arizona and CIN eroding sufficiently. CAMs show storms firing mid afternoon along the Mogollon Rim southeastward into southwest New Mexico and then across southeast Arizona late afternoon. A few strong to possibly severe storms will be possible over the higher terrain with winds being the main threat. A more definitive 5-10 kt easterly steering flow today should aid in progressing the storm activity a bit more quickly with some storms likely approaching the lower deserts between 01-03Z. The storms will however encounter lower instability and higher CIN over the lower deserts, likely causing most of the activity to dissipate. Strong outflow winds with areas of dust seems like at least a 50/50 chance of reaching Pinal and Maricopa County this evening. The latest 06Z HRRR has also introduced redeveloping storms along an advancing outflow boundary, potentially affecting the Phoenix metro and other parts of Maricopa County through late evening. As with the past couple of days, there will be plenty of elevated instability with HREF forecast MUCAPE of 1000-1500 J/kg, but the convective inhibition will be the main issue. Have increased PoPs to 10-20% over the south-central Arizona lower deserts to account for the HRRR's optimism.

The subtropical high is forecast to start strengthening again by Thursday with the center drifting to over northern Arizona and southern Utah. Slightly less convective potential is expected for Thursday across the east-central Arizona high terrain as some drier air is likely to move in from the east. This should also lead to less potential over the lower deserts.

LONG TERM /Friday through Tuesday/

Strengthening of the ridge is forecast through Friday before gradually weakening this weekend into Monday. Guidance agrees H5 heights will rise to 596-598dm on Friday before dropping back to 594-596dm by Sunday. The strength of this ridge will easily be around the 99th percentile of climatology across the region on Friday, dropping back to 90-97th percentile on Sunday. Due to the strength of the ridge and some slight drying, temperatures are forecast to peak on Friday and/or Saturday with highs 110-115 degrees. Surface dew points should fall back into the upper 50s to lower 60s during this time, but it will still feel somewhat humid as heat indices could reach as high as 115-120 degrees across the lower deserts of southeast California. There was enough confidence in guidance to also extend the Extreme Heat Warning through Sunday as the convective potential should be fairly limited. The strength of the high and slight dip in moisture is expected keep CIN values quite high across the area, especially over the lower deserts. Higher terrain areas of eastern Arizona will keep daily 15-30% chance PoPs, while the Phoenix area mostly stay between 10-15%.

By early next week, models show the ridge weakening again, while most likely staying centered to our north. This should allow for better monsoon storm chances with models leaning toward some improving moisture and better upper level support. NBM/WPC PoPs seem too high for Sunday at 30-40% over central and eastern Arizona, but those chances may be more reasonable for Monday and/or Tuesday of next week. Temperatures will also drift lower early next week due to the weakening of the ridge and with more convective activity in the area. The latest NBM shows temperatures back into the normal range by around Monday or Tuesday.

AVIATION

Updated at 0900Z.

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

The main aviation concern will be the potential for strong outflow winds Wednesday afternoon/evening. Details about this boundary remain uncertain at this time, but an initial best guess has it originating out of the east with gusts around 25-30 kt, though stronger gusts are not out of the question. If a due east origin is favored, dense blowing dust will likely be less of a concern, but a brief window of sub-VFR conditions in haze cannot be ruled out. There are signs that point toward VCSH/VCTS popping along this outflow, but chances of that remain at 15% or less, so any mention of rain or thunderstorms has been omitted from the TAFs at this time. Bottom line is, forecast confidence for this window of time is low and multiple adjustments and amendments are likely to be needed.

In the mean time, winds appear likely to favor a westerly component through the remainder of the morning, though some brief periods of variability would not be surprising. Westerly flow will continue into the afternoon and will be accompanied by gusts around 20 kt. SCT to sometimes BKN mid-level clouds will be common through most of the forecast.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns are expected during the next 24 hours. KIPL, will continue to experience southeasterly flow with occasional gusts and a brief period of VRB conditions late this morning. KBLH, will see generally southerly winds with gusts close to 25 kt developing by late morning. FEW to sometimes SCT mid and high level clouds will be present over the region through the TAF period.

FIRE WEATHER

Hot temperatures will continue all week with fairly elevated humidities, but showers and thunderstorms will mostly remain over far eastern and southeastern Arizona. Gusty outflow winds are likely to impact the south-central Arizona lower deserts again today with a 10% chance of storms. MinRHs will remain elevated at 20-30% through Thursday before dropping back into the teens over the lower deserts Friday into Saturday. Overnight recoveries will remain fair to good at 40-60%. Outside of storm outflows, winds will mostly follow diurnal patterns, with upslope afternoon gusts of 15-20 mph at times.

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.