textproduct: NWS Phoenix

KEY MESSAGES

- Extreme heat will persist for at least the first part of the week with high temperatures topping out in the 110 to 116 degree range over much of the lower deserts.

- Shower and thunderstorm chances will increase at least across eastern Arizona today with gusty winds and blowing dust a better possibility into the south-central Arizona lower deserts this evening.

- Day to day mostly eastern Arizona high terrain storm chances are expected through the rest of the week with hot and humid conditions persisting.

SHORT TERM /TODAY THROUGH TUESDAY/

A conditional and uncertain forecast is presenting itself for today into tonight as moisture continues to increase across southern Arizona and instability improves. The strong high pressure ridge remains in place across the Southwestern U.S., but it will continue to weaken over the next 24 hours. The heat dome is still in place which will continue to bring areas of Major to Extreme HeatRisk through at least Tuesday, but it will also help to produce steep mid-level lapse rates and modestly high elevated CAPE of around 1000 J/kg today and potentially even higher CAPE Tuesday. Extreme Heat Warnings remain in effect through Tuesday with highs lowering slightly, but still staying above 110 degrees, while increasing humidities help to keep afternoon heat indices between 110-115 degrees.

Convective potential today is a challenging forecast as many factors will come into play. Low and mid level moisture has been lacking over the past several days, but that is now changing as increasing southerly flow in the low levels will pull decent moisture northward into southeast California and southern Arizona over the next 48 hours. Surface dew points have been gradually improving since Saturday with mid to upper 50s now across much of the area to as high as the upper 60s to lower 70s in Yuma. Steady moisture advection is forecast, keeping dew points mostly in the mid 50s to around 60 even through this afternoon.

The decreasing influence of the high will lessen subsidence a bit today, while the increasing moisture will improve instability. 00Z CAMs showed scattered storm coverage forming over the White Mtns and even more so across southeast Arizona, tracking southeastward late this afternoon and evening. However, a strong capping inversion is still expected across the lower deserts, likely prohibiting convection to form. The main threats for today should be strong gusty winds and blowing dust as moisture will still be fairly marginal and the flow aloft is very weak. A storm or two could still end up becoming anchored to a high terrain feature leading to localized heavy rainfall, but the threat for any flooding is still quite low. CAMs also mostly agree on a decent northward propagating outflow moving into Pinal and Maricopa Counties this evening potentially bringing 35+ mph winds and blowing dust.

The 06Z that came out just after midnight has now introduced some showers and storms developing over the high terrain just to the northeast of Phoenix mid to late evening before progressing through Phoenix. There had been hints of this potential within the 00Z runs, but the CAMs only showed weak showers. This storm potential will be very conditional and of low probability (10%) as it will be difficult to get through a rather strong CAP. It looks like the HRRR is trying to pick up on a little batch of showers and weak storms drifting south southwestward across northern Gila County before potentially interacting with the expected northward progressing outflow mid evening. It is possible colliding outflows may be enough to tap into the elevated CAPE over the lower deserts to spawn some additional storms near or even over Phoenix late this evening. We have introduced a slight chance for this potential from around 9 PM through just after midnight tonight.

Tuesday's forecast is also quite conditional as it will largely depend on what happens this evening. If storms end up being quite extensive today, it should cool things off some or even slightly overturn the atmosphere to decrease instability. Right now, this scenario probably has a 20% chance of occurring. Of higher likelihood is a more modest storm coverage today, mainly affecting higher terrain areas. This would not affect instability for Tuesday much leading to another decent chance of showers and storms across east-central and southeastern Arizona Tuesday afternoon/evening. In fact, the 00Z HREF shows even higher instability for Tuesday as moisture continues to increase with similar levels of convective inhibition. CAMs for Tuesday show decent storm coverage from the White Mtns southward through far southeast Arizona with another potential outflow progressing into the lower deserts in the evening. However, the chances for any further development into the lower deserts seems lower than today.

LONG TERM /WEDNESDAY THROUGH SUNDAY/

Hot and humid (at least for this region) conditions are forecast for the latter half of the week as the subtropical ridge will strengthen once again with peak H5 heights likely reaching 597-599dm over northern Arizona, southern Nevada, and southern Utah. The strength looks to be just shy of what just occurred this past weekend, but guidance shows the ridge center to our north instead of directly over central Arizona.

The current Extreme Heat Warning goes through Tuesday with the HeatRisk decreasing to very localized Major on Wednesday. However, this decrease in HeatRisk is largely due to the increase in moisture as forecast heat indices really do not go down. Forecast highs Wednesday range from 107-110 degrees in Phoenix to 108-111 degrees across the western lower deserts, however heat indices should stay between 110-115 degrees as dew points surge into the 60s to as high as the lower 70s across far southwest Arizona and southeast California. Beyond Wednesday, a resurgence of the ridge is expected to begin to raise temperatures again with highs drifting a degree or two higher by Friday. Unless things change, it seems likely the Extreme Heat Warning will be extended for a good portion of the area through the rest of the week. However, any bigger convective day could disrupt the atmosphere leading to a slightly cooler day.

Monsoon storm potential for the rest of the week looks to be fairly low to at most mid grade with 20-40% chances most days over the eastern Arizona high terrain to 10-20% into the Arizona lower deserts. There are some hints at a potential increase in convective activity maybe by the weekend into early next week, but we do not put much faith in the long range models for monsoon activity. Although, as long as the high center ends up staying positioned far enough to our north, it at least seems quite plausible any minor easterly wave could march on through providing enhanced chances at some point.

AVIATION

Updated at 0800Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The primary aviation weather concern and potential impact will be strong thunderstorm outflow winds moving through the Phoenix area late this evening from the south. There is moderate to high confidence that an outflow will move through, currently favored between 03-06Z, but there is some uncertainty on how strong winds will be. There is model support for gusts as high as 35-40 kts and winds of this magnitude may also bring a wall of dust into the area, enough to reduce visibility to 5SM or less. Storms are expected to stay well east and south of the area, but there is still a 5-10% something pops up or drifts into the metro.

Prior to the outflow, winds will continue to follow typical diurnal tendencies, with some gusty (20-25 kt) westerly winds this afternoon. There is some uncertainty with wind directions later tonight, following the outflow, but easterly winds generally favored to develop before dawn Tuesday. Mid and high clouds will be common through the TAF period, with bases dropping down to around 10K ft AGL Monday evening.

Southeast California/Southwest Arizona including KIPL and KBLH: Gusty southerly winds, primarily at KBLH Monday afternoon and evening, will be the main aviation weather issue throughout the TAF period under mostly clear skies. At KIPL, winds will generally be out of the southeast, with an increase in speeds between the south and south-southwest. Overall wind speeds will fluctuate between 8-13 kts, with gusts near 20-25 kts in the afternoon at KBLH that increase further in the evening upwards of 25-30 kts. Localized dust streamers and widespread lofted dust may be generated by the winds Monday night, which could impact slantwise visibility.

FIRE WEATHER

Hot temperatures will continue all week, but with increasing low level moisture and improving humidities over the next couple of days. Shower and storm potential will be quite good across far southeast Arizona today, while the activity is likely to stay mostly east and southeast of Globe. Gusty southerly outflow winds are likely this evening across south-central Arizona and potentially again on Tuesday. MinRHs will improve to 15-20% today and 20-25% on Tuesday with overnight recoveries between 40-60%. Outside of storm outflows, winds will mostly follow diurnal patterns, but west southwesterly winds will be dominant much of the time with afternoon gusts of 15-25 mph at times. Similar low- grade monsoon storm conditions are expected through the rest of the week with humidities staying fairly elevated and temperatures above normal.

CLIMATE

Record high temperatures:

Date Phoenix Yuma El Centro ---- ------- ---- --------- Aug 3 116 in 2024 116 in 1998 117 in 2021

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Tuesday for AZZ530>544-546- 548>551-553>555-559.

Extreme Heat Warning until 8 PM MST this evening for AZZ545-547- 552-556>558-560>563.

CA...Extreme Heat Warning until 8 PM PDT Tuesday for CAZ562-563- 565>567-569-570.

Extreme Heat Warning until 8 PM PDT this evening for CAZ560-561- 564-568.