textproduct: NWS Phoenix

KEY MESSAGES

- Extreme heat will persist through at least the end of the workweek with increasing moisture pushing heat indices to around 110 to 115 degrees for the latter half of the week.

- Shower and thunderstorm chances will favor eastern and southeastern Arizona through the rest of the week with gusty outflow winds and blowing dust a possibility each day into the south-central Arizona lower deserts.

SHORT TERM /TODAY THROUGH WEDNESDAY/

An elongated high pressure ridge with H5 heights just shy of the 90th percentile of climatology currently stretches from off the coast of California through the Desert Southwest into Oklahoma. There is no well defined high center at the moment, but it is loosely found over central Arizona. The extreme heat remains the main forecast concern for at least the next few days as even though temperatures will dip some, the increased low level moisture will make it feel just as hot.

The ridge went through a weakening stage Sunday and Monday, but now has stabilized. Despite the weakening, high temperatures yesterday barely budged with 112-116 degree readings across the lower deserts. Thanks to the widespread convection that impacted southeast Arizona last evening, temperatures across south-central Arizona are running at least 10 degrees cooler than this time last night. The cooler start to the day today will help to bring daytime highs down today with highs of 107-110 degrees in the Phoenix area to 108-112 degrees across the western lower deserts. Unfortunately, the increased low level moisture has allowed for surface dew points to climb into the 60s areawide and we anticipate this moisture to persist through midweek. As a result, forecast afternoon heat indices will stay above 110 degrees across the lower deserts to as high as 118 degrees across southeast California. Even though the HeatRisk, which doesn't factor in humidity, drops to Moderate in many locations today and Wednesday, the combination of temperatures and humidity will still be enough to bring extreme heat conditions. The increased humidity will also help to keep overnight lows very warm with some locations staying above 90 degrees. However, just like late last evening, daily chances for storm outflows may end up bringing some minor relief from the worst of the heat across south-central Arizona.

Shower and thunderstorm chances today and likely through the rest of the week are expected to mainly be confined to the eastern Arizona high terrain. Moisture has improved to a degree, but not enough across the lower deserts to expect to overcome the subsidence inversion created by the heat dome in place. CAMs continue to point toward afternoon storms firing from the White Mtns through southeast Arizona, but due to a northeasterly steering flow the bulk of the convection is expected to track outside of or along the outer edges of our CWA over the next few days. However, the environment remains well suited for storm outflows to form with the convection which will have a decent chance of progressing into Pinal and Maricopa Counties each evening. The latest HRRR for this evening basically mirrors what occurred on Monday with gusty south southeasterly outflow winds and most likely fairly widespread dust eventually reaching the Phoenix metro late this evening. It would not be impossible for some showers or even thunderstorms to develop into the lower deserts today and Wednesday, but due to the strong capping inversion it would take fairly strong colliding outflows for any redevelopment to occur over the lower deserts. PoPs for the Phoenix area are only around 5% today and 10% Wednesday due to the conditional environment. However, if that 5-10% chance involves a couple strong storms that manage to go up, that could also create a cascading scenario where many more pulse type showers and storms impact the area due to the high amount of available elevated instability that will be in place.

LONG TERM /THURSDAY THROUGH MONDAY/

Little will change with the weather pattern for the rest of this week and likely well into next week as the high pressure ridge is expected to remain centered over the Desert Southwest. The center of the ridge is forecast to somewhat shift farther to the north by Friday which could help to bring better rain chances, but guidance shows the ridge also strengthening which will likely reinforce the subsidence over much of our area. As of now, Thursday and Friday are expected to follow the same pattern as today and Wednesday with mostly eastern Arizona high terrain storms and very conditional 5-15% PoPs into the south-central Arizona lower deserts. As stated in the short-term discussion, even though it will be a day-to-day low chance environment for the lower deserts, it would not be surprising to have a day where the elevated instability is managed to be tapped into causing some scattered storms to fire. Hot and humid conditions will continue to prevail with daytime highs likely nudging upward again late week as the ridge restrengthens. The Extreme Heat Warning currently goes through Friday which is very reasonable, but it will be possible to have a day in there where it fails to reach criteria over south-central Arizona if convection ends up being more widespread one evening/overnight.

For the weekend into early next week, ensembles still favor the ridge basically staying stationary with the center somewhere just to our north. Some weakening of the ridge is likely to occur by early next week and depending on how much moisture will be available we could be looking at a further uptick in monsoon activity. The GEFS is more optimistic with rain chances by early next week with activity expanding into central and northern Arizona, but the EPS holds off until later next week. If convection does eventually become more widespread and impacts the lower deserts, temperatures are expected to dip closer to seasonal normals.

AVIATION

Updated at 0920Z.

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

The main aviation concern during the TAF period will be gusty outflow winds and reduced VIS due to blowing dust late this evening into early Wednesday morning. In the meantime, winds are expected to follow diurnal trends, with and early switch out of the west. Winds will eventually become breezy during the afternoon hours, with gusts around 20 kt. Most of the evening will be fairly calm, until an outflow boundary arrives from the south/southeast sometime between 06-10Z based on current forecast trends. Associated gusts may reach upwards of 30 kt, and with the orientation of the boundary, another round of blowing dust is certainly not out of the question. However, this outflow will be heavily conditional upon how distant convection over southeast Arizona evolves, so adjustments and amendments should be expected.

Southeast California/Southwest Arizona including KIPL and KBLH: :

Breezy conditions, mainly at KBLH, will be the main aviation concern during the TAF period. There, winds will favor a S'rly direction through the forecast, with gusts during the afternoon around 20-25 kt. Another around of gusts is expected to develop late in the period, with readings approaching 25-30 kt. Winds at KIPL will be much weaker, generally ranging 7-15 kt out of the SE. FEW to occasionally SCT high level clouds will be present through the majority of the forecast window.

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 late this evening. MinRHs will continue to improve, climbing to around 25-30% by Wednesday, while overnight recoveries range between 40-70%. Outside of storm outflows, winds will mostly follow diurnal patterns, but southwesterly winds will be dominant much of the time with afternoon gusts of 15-25 mph at times.

PSR WATCHES/WARNINGS/ADVISORIES

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

CA...Extreme Heat Warning until 8 PM PDT Friday for CAZ560>570.