textproduct: NWS Phoenix

KEY MESSAGES

- Shower and thunderstorm chances will continue today, especially over the Arizona high terrain. Isolated storms and gusty storm outflow winds can also be expected into the south-central Arizona lower deserts.

- Drying conditions starting Wednesday will mostly end monsoon storm chances by Thursday.

- Extreme Heat Watches are in effect for all of the lower deserts for the latter half of the week as hot and humid conditions will develop over the next couple of days.

SHORT TERM /TODAY THROUGH WEDNESDAY/

Active monsoon weather will continue for one more day this week as a distinct easterly inverted trough continues to track northwestward through New Mexico early this morning and then through the northeastern 1/3 of Arizona this afternoon/evening. The atmosphere will however have to recover a bit after the complex of showers and storms that affected the Arizona high terrain before dying as it moved into the Phoenix area. Currently, there is little available CAPE, but lingering ascent from an MCV just north of Phoenix and broad scale upper level ascent from the incoming inverted trough may be enough for some isolated elevated showers through sunrise.

For this afternoon and evening, hi-res CAMs are painting a similar picture to what occurred yesterday. Widespread showers and thunderstorms are expected to develop across eastern Arizona this afternoon with heavy rainfall and localized flooding being the main threats. A Flood Watch for this afternoon through mid evening covers much of the Arizona high terrain where localized amounts of greater than 2" will be possible. Another potential line of storms or even a more organized MCS is quite likely to approach the lower deserts this evening. However, much more limited instability over the south-central Arizona lower deserts is expected to cause the storms to diminish in strength and coverage as they attempt to push through Pinal and Maricopa Counties. Gusty outflow winds of 35-50 mph seem quite plausible for the Phoenix area this evening with at least some areas seeing some measurable rainfall, though most areas are not likely to see more than 0.25".

Starting Wednesday, drier air will be advecting into the area from the south southwest while the inverted trough exits northward into Utah. Shower and storm chances Wednesday will be much more limited compared to today with only 15-30% PoPs over the Arizona high terrain to less than 10% across the lower deserts. Temperatures will become the main forecast concern beginning Wednesday as the subtropical ridge shifts southward into the Southern Plains while also nudging more into our region, pushing our H5 heights to 592-594dm. Although forecast highs Wednesday are right around normal, it will be a fairly muggy day for our area with surface dew points still in the 60s in most areas. Extreme Heat Watches are in effect for southeast California starting Wednesday with afternoon heat indices (feels like temperatures) topping 110 degrees in many spots.

LONG TERM /THURSDAY THROUGH MONDAY/

Extreme Heat is expected to impact much of the area late this week through at least the weekend as the subtropical ridge strengthens while the center gradually shifts over the region Friday into Saturday. Record or near record climatological H5 heights are forecast across New Mexico starting Thursday, eventually moving into Arizona Friday into Saturday with H5 heights potentially reaching 598-600dm over Arizona. Major HeatRisk is expected to impact all of the lower deserts starting Thursday, becoming even more widespread Friday into the weekend as localized areas of Extreme HeatRisk may develop. The forecast highs of 110-116 degrees Thursday through next Monday (below records) is masking the potential severity of this heat wave as dew points and humidities are expected to remain elevated. Heat indices are likely to reach 113-120 degrees over the lower deserts starting Friday with the highest readings across southwest Arizona and southeast California. Overnight lows will also become very warm with readings not likely falling much below the upper 80s. Lower 90s can be expected for much of the urban core of Phoenix starting Thursday night. If we do end up mixing out more of the boundary layer moisture, the higher end of NBM guidance shows highs potentially reaching 116-118 degrees in the Phoenix area to as high as 117-120 degrees for portions of southeast California.

The hottest days are likely to be Friday-Sunday, but there is more forecast uncertainty by Sunday as some guidance indicates some moisture return this weekend and the potential for more convective activity. For now, we are not expecting much if any high terrain showers or thunderstorms Thursday and Friday, but that could change starting Saturday or Sunday. Any more extensive convective activity may eventually disrupt the extreme heat, at least over eastern and possibly into central Arizona by Sunday or Monday. If we don't get relief from storm activity, guidance at least heavily favors some weakening of the ridge early next week while also shifting it farther to the east.

AVIATION

Updated at 1100Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Gusty outflow winds and modest chances for TSRA this evening are the primary weather issues under periods of mid/high cloud decks. Confidence is good that prevailing east winds will switch to W/SW by noon with only some minor afternoon gustiness. TSRA forming over higher terrain to the east should send gusty outflows and an abrupt wind shift towards the Phoenix terminals. Confidence is fair that wind speeds will be weaker than Monday, yet still possibly 25-35kt. The chance for direct TSRA impacts on any given aerodrome is also lower than Monday, but still nearly a 30% chance of some SHRA behind the outflow boundary.

Southeast California/Southwest Arizona including KIPL and KBLH: No major weather issues will exist through Wednesday morning under a few passing mid/high cloud decks. Winds will generally favor a south direction at KBLH, while directions should be more varying at KIPL between south and west. There may be sundowner gusts over 20kt at KIPL this evening, however confidence in expanse and duration of gusty winds is low.

FIRE WEATHER

Much of the shower and thunderstorm activity will be over the Arizona high terrain today while gusty storm outflow winds will be the main concern today for the south-central Arizona lower deserts. Chances for showers and storms will quickly drop off Wednesday with little if any activity expected Thursday as hotter and drier conditions settle into the region. Humidities will remain elevated today with MinRHs of 30-45% across the eastern districts, while the western districts will dip to 20-25%. MinRHs will then dip to 15-20% across all of the lower deserts starting Thursday.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Watch from Wednesday morning through Monday evening for AZZ530-532.

Flood Watch from 11 AM MST this morning through this evening for AZZ556-560-562-563.

Extreme Heat Watch from Thursday morning through Monday evening for AZZ531-533>555-559.

CA...Extreme Heat Watch from Wednesday morning through Monday evening for CAZ562>570.

Extreme Heat Watch from Thursday morning through Monday evening for CAZ560-561.