textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Isolated showers and thunderstorms will remain possible mainly during the afternoon hours today and Friday with the best chances across the Lower Colorado River Valley.

- Near to below normal temperatures are forecast through Friday before heating back up over the weekend into early next week.

- An Extreme Heat Watch has been issued for the Phoenix metro for next Monday through Wednesday.

SHORT TERM /TODAY THROUGH FRIDAY/

Showers continue to wind down across eastern portions of Arizona as drier air slowly works northeastward into southwest and south- central Arizona. This activity is expected to end just before sunrise with little to no additional rain chances the rest of the morning. The main culprit for the widespread storm coverage yesterday was a Pacific trough moving into California. The trough has even briefly formed a closed low just south of San Francisco, but it is anticipated to open up as it moves northeastward into Nevada later today. The broader upper level trough is forecast to remain stationary through Friday with the main trough axis along and just west of the California coast. This will keep much of the Desert Southwest under the east side of the trough allowing for the below normal temperatures to persist with highs from the upper 90s to the low 100s across the lower deserts.

Despite the drier air already starting to work in from the southwest, guidance shows enough lingering moisture for another round of afternoon isolated to scattered showers and thunderstorms today. A mostly worked over atmosphere will make it a bit difficult for storms to fire this afternoon, but with a decent amount of sunshine, some recovery is anticipated. HREF guidance does show a narrow strip of 1000 J/kg of MUCAPE this afternoon across the Lower CO River Valley into the central Arizona high terrain which could provide enough energy for a few stronger storms to develop. Also can't rule out some isolated late afternoon showers and weak storms near or within the Phoenix metro today.

Additional dry air is forecast to move in on Friday with low level mixing ratios dropping to 8-10 g/kg across the area. The lack of moisture shows in the forecast MUCAPE dropping to 250-500 J/kg over much of the area to at most 500-750 J/kg across the Lower CO River Valley. As of right now, we are only expecting a few widely isolated showers or weak storms around the Lower CO River Valley and over the AZ high terrain on Friday.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

The weather pattern will shift over the weekend into early next week as the subtropical ridge once again becomes the dominant weather feature across the Southwestern U.S. The Pacific trough will somewhat stick around into Saturday, but it is shown to quickly lift to the north by Sunday as the ridge moves back in from the east. Forecast H5 heights rise from 588-590dm on Friday to 591-594dm Saturday before peaking around 594-596dm during the first half of next week.

The dry air and increasing subsidence this weekend should keep all but a few eastern Arizona high terrain showers and storms from developing. Temperatures will warm up quickly over the weekend with the expansion of the ridge with NBM forecast highs reaching 103-106 degrees Saturday and 106-109 degrees Sunday. The drop in moisture will at least push surface dew points well into the 50s this weekend to as low as the upper 40s early next week. This will help to keep overnight lows close to normals through the weekend before we heat up further next week. Ensembles eventually show the ridge center shifting directly over Arizona by around Monday, likely staying in place for a few days. The increased subsidence from the ridge with no meaningful moisture return happening early next week should keep lower desert PoPs below 10% to at most 10-20% over the far eastern Arizona high terrain.

As the modestly strong (90-97th percentile of climatology) ridge settles over Arizona during the first half of next week, temperatures are likely to heat up enough to result in Major HeatRisk across portions of the lower deserts. The highest likelihood for Major HeatRisk looks to impact the Phoenix metro with daytime highs around 110 degrees and lows around 90 degrees starting Monday. More rural lower desert areas should benefit from overnight lows within a couple degrees of normals, but with similar daytime highs. Due to the higher confidence, we have issued an Extreme Heat Watch for the Phoenix metro, valid from Monday through Wednesday.

AVIATION

Updated at 1000Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation concern will be the potential for isolated convection this afternoon into the early evening. VCSH, with perhaps isolated VCTS, may pop once again by 21-22Z this afternoon, but chances to see anything at the terminals is only around 20%, so nothing more than a mention of VCSH in the TAFs felt warranted at this time. Outside of any remaining rainfall influences, and periods of VRB conditions overnight, winds are expected to follow familiar diurnal trends with perhaps an earlier than usual southeasterly switch this evening.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected over the next 24 hours. Outside of periods of VRB conditions, winds will follow familiar diurnal trends. Some isolated convection may pop around the region this afternoon, but no direct impacts are anticipated at or in the vicinity of the terminals at this time. BKN to even OVC skies will be common through much of the period.

FIRE WEATHER

Elevated humidity levels, below normal temperatures, and lingering chances for showers and a few thunderstorms are expected today before much drier air filters into the region Friday into the weekend. Afternoon MinRHs of 30-50% today will drop to 20-30% by Saturday and into the teens across the lower deserts starting Sunday. Overnight recoveries will remain good to excellent. Winds will remain predominately out of the south southwest much of the period with overnight light easterly downsloping winds over the eastern districts. Temperatures will warm back to normal by Saturday and then above normal starting Sunday.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Watch from Monday morning through Wednesday evening for AZZ537-540-542>544-546-548-550-551.

CA...None.