textproduct: NWS Phoenix

KEY MESSAGES

- Hot temperatures will continue this week resulting in areas of major Heat Risk and Extreme Heat Warnings through Thursday across lower desert locations.

- Chances for showers and thunderstorms will continue over higher terrain areas of eastern Arizona today and to a lesser extent Thursday and Friday.

- The weather pattern will become very favorable for moisture import and more widespread thunderstorm activity by early next week.

SHORT TERM /Today through Friday/

Elongated subtropical ridging in a retrogressive phase continues to be the dominant feature across the SW Conus with twin 596dm H5 anti- cyclone centers over NE Arizona and just off the southern California coast respectively. This retrogression will persist the next 48 hours as shortwave energy and westerly jet energy break over the ridge into the Great Basin. This evolution will also act to dampen midtropospheric heights with H5 levels falling closer to 592-594dm by Friday before the high pressure center builds back northeast behind the departing northern stream shortwave. Despite these subtle shifts in positioning and intensity, confidence is excellent that temperatures 5F-10F above normal will be common across the region the next several days with today and Thursday the hottest days. With areas of major HeatRisk persisting, Extreme Heat Warnings continue through Thursday, and may need a small expansion in future forecasts across parts of SE California Friday juxtaposed with the core of the high pressure center.

Objective analysis indicates a marked increase in moisture across much of south-central Arizona since yesterday with 7-9 g/kg low level mixing ratios enveloping most of the Arizona CWA with total column PWATs in excess of 1.00". Farther south closer to the international border, mixing ratios 10-11 g/kg, H8 dewpoints greater than 10C, and PWATS near 1.50" were analyzed, though this more thermodynamically favorable airmass will only be transported north with aid of strong outflow boundaries. With better instability in place and very favorable stretching and divergence aloft, mountain storms over Gila County should be more robust this afternoon with DCAPE better than 1500 J/kg promoting strong outflow boundaries. Additional outflows from convection over southern Arizona may also surge north into the forecast area this evening importing additional moisture to the area. While colliding outflow boundaries would be beneficial for new thunderstorm development over lower elevations, forecast soundings and HREF output are not particularly bullish on this outcome, and have maintained POPs under 10% (though not impossible for an isolated storm to erupt this evening given the otherwise excellent synoptic setup).

Thunderstorm coverage and impacts will decrease Thursday and Friday as the aforementioned upper jet progressing through the Great Basin imparts greater subsidence and midlevel warming into the CWA while shifting the better synoptics towards the international border. Isolated storms are still possible over mountains of eastern Arizona (and certainly into southern Arizona), however despite near steady state moisture profiles, stronger inhibition over lower elevations and lack of effective boundaries should preclude any notable storm chances. However, there is convincing evidence from model output that a large convective complex over northern Sonora Friday evening will send a pool of greater theta-e into central Arizona with mixing ratios above 10 g/kg and PWATs near 1.50" setting the stage for a more convectively active period of the monsoon over the weekend and next week.

LONG TERM /Saturday through Tuesday/

The weather pattern will undergo a noticeable shift through the weekend as the high pressure ridge quickly tracks northeastward with the center reaching Utah Saturday. The ridge is then forecast to expand, strengthen, and become the main driver of strong moisture advection back into the Desert Southwest next week. As the high center finally shifts to our north into Saturday, it should first open up eastern and southern Arizona to an easterly moisture fetch. Moisture is still likely to be fairly limited on Saturday with afternoon convection currently favored more across southeast Arizona, but once the steering flow shifts out of the east moisture advection and storm chances are expected to spread westward through the rest of Arizona starting Sunday.

Although forecast uncertainty remains fairly high going into next week, ensembles generally agree we will enter a period of active monsoon weather. The subtropical high is favored to stay positioned over the Northern Plains into the Central Rockies for much of next week which will allow for fairly persistent moist easterly flow into the Desert Southwest. Ensemble member forecast PWATs show a wide spread, but also a meaningful average increase likely pushing past 1.3-1.4" starting Sunday. Forecast PoPs increase into a 20-40% range starting Sunday and generally stay there through the first half of next week. Any notable easterly waves moving through the flow will surely help drive our convective potential, but pinpointing those this far out is a dubious proposition. Outside of the expected increase in monsoon storm activity, temperatures should slip further toward seasonal normals.

AVIATION

Updated at 1730Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Breezy outflow winds during the evening will be the main aviation concern during the TAF period. Best timing to see a sudden switch out of the E/SE from one of these boundaries looks to be between 02- 04Z, with gusts 20-25 kt in accompaniment. In the meantime, W'rly winds will prevail through the afternoon, with typical gusts in the mid-teens to around 20 kt. The E'rly turn mentioned above with the evening outflow will stick through Thursday morning. Other than distant convective CU over eastern Arizona, skies around the terminals will be mostly clear.

Southeast California/Southwest Arizona including KIPL and KBLH: No major aviation concerns are expected during the next 24 hours. Winds at KIPL will generally be out of the SE, with perhaps a very brief switch out of the SW this evening. At BLH, familiar diurnal trends will prevail as winds swing between SSE and SW, with gusts this afternoon around 20-25 kt. Periods of VRB conditions may also be observed during the nighttime hours, especially at KIPL. Clear skies will be common across the region through Thursday.

FIRE WEATHER

Gusty and erratic thunderstorm winds with isolated lightning over the far eastern districts the next few afternoons will be the main fire weather concern. Outside of any potential outflows, winds should favor light and diurnal trends with typical afternoon breeziness around 20 mph. Afternoon RH values between 10-15% will be common across the region. Overnight recoveries will offer only limited to modest relief as MaxRH values range generally between 20- 45%. Temperatures are expected to run well above normal through the middle of the week, with the lowest elevations reaching between 110- 115 degrees. A decrease in monsoon activity is likely late week, before picking up again and becoming more widespread by early next week. Moisture levels and humidities should also improve considerably by early next week, alleviating much of the fire weather concerns.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...Extreme Heat Warning until 8 PM MST Thursday for AZZ530>534- 536>551-553>555-559-560-562.

CA...Extreme Heat Warning until 8 PM PDT Thursday for CAZ562-564>570.