July 25th: Hotter and Mostly Dry through Thursday, Significant Rainfall Possible this Weekend

 Synopsis

The mid level ridge will be centered near west-central New Mexico through Thursday leading to less coverage of convection as well as excessive heat conditions across the lower deserts. Scattered afternoon and evening showers and thunderstorms can still be expected mainly across the higher terrain and near the International Border; however, a few storms cannot be ruled out over some valley locations in southeastern Arizona (including Tucson) over the next few days. By Friday models forecast the mid level ridge to expand northward and an inverted trough is expected to impact our region this weekend, possibly providing Arizona with its first significant rainfall event of the monsoon season.

 Current Conditions

    At 9AM MST, water vapor satellite displayed a mid level ridge centered over central New Mexico.  

GOES-16 mid level water vapor imagery courtesy of College of Dupage.

Surface observations this morning measured dew points in the upper 50s to lower 60s across the valleys of southeastern Arizona, but GOES derived precipitable water only shows about an inch or less over and east of Tucson. The 12z TUS sounding was unavailable today so am using the 12z WRF-HRRR model sounding which shows pwat around an inch, MUCAPE near 300 J/kg, and southeasterly steering flow between 15 and 25 kts.

12Z KTUS WRF-HRRR Sounding at 12Z today.

 Today through Thursday (Convective Outlook)

    The 6Z WRF-GFS depicts an upper level trough moving into the Pacific Northwest today which has slightly squashed the ridge and displaced the center over central New Mexico. This synoptic scale setup is likely to persist through Thursday leading to an unfavorable monsoon pattern. 

6Z WRF-GFS 500mb Temps and Winds for today at 18Z.

Visible imagery overlaid with GOES derived precipitable water this morning displays a few features of interest for today. There is a significant west-east oriented gradient in precipitable water between far southwestern AZ and southeastern AZ. Pwats of 1.25-1.50 inches to the south and west of Tucson, and pwats of 1 inch or less between Tucson and the AZ NM border are observed. With southeasterly steering flow today, drier air aloft will continue to advect from the east throughout the day which should maintain a drier and more stable environment for most of southeast Arizona. An MCV propagating to the north is also shown over the lower Colorado River Valley this morning which could aid in convection this afternoon over the higher terrain in northwest Arizona. 

Visible Imagery overlaid with GOES derived pwats courtesy of College of Dupage.

  In general, minimal amounts of moisture and CAPE should confine scattered showers and thunderstorms to the higher terrain of northern Arizona and near the International Border this afternoon and evening. A couple stray, short-lived storms cannot be ruled out over the valleys of southeastern Arizona (including Tucson) which is reflected in the 12z WRF-HRRR simulated maximum reflectivity. 

12Z WRF-HRRR Simulated Maximum Reflectivity for this evening at 5:30 MST.

    For tomorrow, the WRF-GFS forecasts the ridge to build slightly to the west, centered over the AZ NM border which has been a familiar position this summer. This will lead to an even drier and more stable air mass tomorrow as well as an increase in synoptic scale subsidence. 

6Z WRF-GFS 500mb Temps and Winds for tomorrow at 2:00PM MST.

This pattern will likely lead to delayed convective initiation as well as less coverage of storms which is reflected in the 12Z WRF-HRRR simulated maximum reflectivity tomorrow afternoon and evening.

    On Thursday, the pattern remains nearly unchanged with the ridge remaining centered over the AZ NM border. Models do forecast a slight increase in available precipitable water (slightly greater than an inch in Tucson), but with the close proximity of the center of the ridge limited instability and synoptic scale subsidence should prevent any significant, widespread convection from occurring. Afternoon and evening convection should be mainly confined to the mountains of Northern Arizona and near the international border tomorrow and Thursday, but once again a few weak storms cannot be completely ruled out over the valleys of southeastern Arizona. 

Today through Thursday (Temperature Outlook)

    Unfortunately with the drier, more stable air mass influencing our region through Thursday, the focus shifts back to extreme heat. Models forecast the center of the high to max out at 600 dm over the next few days which will once again lead to excessive heat conditions across the lower deserts of Arizona. Today, high temperatures should remain at or slightly below 110 degF in Tucson with areas between Phoenix and Yuma reaching between 110 and 112 degF. Tomorrow looks to be the hottest day due to the closer proximity of the mid level ridge. The 12z WRF-HRRR forecasts temperatures tomorrow to be near 110 degF in Tucson, and temperatures between 110 and 115 degF between Phoenix and Yuma. A few isolated locations could even reach near 117 degF!

12Z WRF-HRRR 2-meter temps for Wednesday at 5:00 PM MST.

Thursday appears to be a couple degrees cooler due to the slight uptick in moisture, but still am expecting temps around 110 degF across the lower deserts. 

Friday through this Weekend

    On Friday, the deterministic GFS and ECMWF as well as their ensemble members

forecast the 500mb ridge to increase in amplitude and be centered closer to the Four Corners.

Also, pwats are expected to increase (~1.25 - 1.50 inches) which should lead to slightly cooler temperatures and an increase in thunderstorms across southeastern Arizona. 

12Z GFS Total Precipitable Water on Friday courtesy of Maiowx.com

12Z GFS 500mb Absolute Vorticity on Friday courtesy of Maiowx.com

    By this weekend, models are hinting at an inverted trough propagating along the southern

periphery of the ridge which should help lower heights aloft over Arizona. 

12Z GFS 500mb Absolute Vorticity on Saturday courtesy of Maiowx.com

12Z GFS 500mb Absolute Vorticity on Sunday courtesy of Maiowx.com

This feature could provide Arizona with its first significant monsoon rainfall event of the season! There are still some differences in the models with the exact timing and impacts it will have on precipitation here. Will need to wait until CAMs can get ahold of it to nail down the exact details. Regardless, we can at least expect an increase in moisture and thunderstorm coverage as well as some relief in extreme heat across the lower deserts this weekend.