Summary

Today, Arizona has very dry air at the surface beneath these morning clouds, with much of southern Arizona forecast to experience excessive heat after the clouds dissipate. While some mid-level moisture is present, it is not sufficient for significant rainfall due to a lack of instability and a dry boundary layer. The monsoon high remains stationary near El Paso, keeping the atmosphere stable and hot.

Limited moisture is being transported into western Arizona, and while afternoon thunderstorms are expected to develop in the mountains of eastern Arizona and western New Mexico over the next few days, these storms will be mostly weak and localized, producing little rain. This pattern is expected to persist, with slightly increasing boundary-layer moisture and surface dewpoints by Tuesday, but significant rain will remain confined to mountainous areas, leaving valleys hot and dry.

Yesterday’s Weather

The combined satellite, radar, and lightning strike imagery below shows clouds across Arizona: mainly remnants from former Tropical Storm Douglas. Thunderstorms were confined to eastern New Mexico.

Precipitation indicated by radar in Arizona and western New Mexico was likely virga. Still, the clouds provided some relief from the heat. 

The water vapor satellite imagery below shows four low-pressure systems: two in the westerlies and two Tropical Upper Tropospheric Troughs (TUTT) in the easterlies south of the monsoon ridge. Either one of these TUTT systems could kick off the monsoon for Arizona. The most likely scenario is the flow aloft provided by the low labeled “L2,” combined with thunderstorm activity along the spine of the Sierra Madre mountains, aided by divergence aloft from the TUTT labeled “L3.”  The question is, when? And it’s possible that another system, not yet seen here, will trigger the monsoon. 

Analysis

There is no significant surface moisture in Arizona, and even the moisture in eastern New Mexico has decreased. Much of Southern Arizona is under an excessive heat warning.

The observed weather balloon sounding for Phoenix at 5am (below left) shows nearly an inch of precipitable water mostly contained in the layer from 650 mb to 420 mb. This moisture is causing the cloud layer to appear on the radar plots, but it is not usable for convective precipitation due to both a lack of instability and the very dry layer below. Note that the wind’s direction through the atmosphere is mainly from the west. The corresponding HRRR-UA WRF 12Z forecast initialization (below, left) is overestimating the precipitable water (27mm in the model, 20mm in the obs), and the mid-level moisture is somewhat smoothed in the model vertical profile. Though the near-surface dry air and the wind shear profile are well initialized in the model. The lack of instability and dry boundary layer are the dominant factors limiting convection and precipitation in Arizona today.

At 250mb, is it a polar or subtropical jet? Not too sure, and as mentioned yesterday, the subtropical jet tends to go away during the summer. A look at upper-air data for other locations indicates this is the subtropical jet, as the polar jet has retreated towards Canada. The two troughs are associated with lows L1 and L2 shown on the satellite image above.

Monsoon high is near El Paso at 500mb this morning and hasn’t moved much over the last few days. Warm air aloft is moving into Arizona, making the atmosphere more stable and allowing surface temperatures to get hot. 

At 700 mb, the flow around the monsoon ridge in southeast Arizona and the remnants of Tropical Storm Douglas in California are bringing meager amounts of moisture into western Arizona. As we go through the week, thunderstorms developing each afternoon in the Sierra Madre mountains will increase available moisture. This moisture will shift slowly northward and help kick off afternoon thunderstorms in the mountains of eastern Arizona and western New Mexico. 

Today

The afternoon sounding from Truth or Consequences, NM, shows enough Convective Available Potential Energy (CAPE) for thunderstorms in the mountains, about 200 [J/kg]. But unlikely enough CAPE for strong cells to propagate. As storms move away from the mountains, they will dissipate with only light precipitation and wind gusts of less than 30 knots.

The afternoon thunderstorms in New Mexico do not produce much rain, but they will increase boundary-layer moisture, which then spreads westward and southward. This process is commonly called moisture recycling. The 24-hour accumulation totals at the end of the animation below (right) are about 1″ in the mountains of New Mexico and less than an inch in the valleys.

Tomorrow

Thunderstorms form again in the afternoon, mainly in the mountains, spreading as far west as the mountains of eastern Pima, Pinal, and Santa Cruz counties. On the HRRR-UA WRF 12Z forecast sounding for 6:30 pm below, there isn’t any CAPE with easterly winds at 580mb.

So, any storms that form here will be on the mountains, as indicated by the forecast radar reflectivity (below, left). Any measurable rain will be confined to the mountains, and this pattern would likely produce dry lightning. 

Outlook

By Tuesday, there is a little more CAPE in the Tucson sounding, and flow is more from the northeast.

The thunderstorms form further west, but remain near and over the mountains. 

How about surface dew points? From today through Tuesday, Tucson will increase by 17°F. Not high enough for thunderstorms to move into the valleys much, but the mountains should be firing storms each afternoon. 

The long-range ensemble forecasts from GEFS are shown below. The scale makes it look like more rain is falling than you might think; it’s only 0 to 0.25 inches. And the 2-meter dewpoint and precipitable water forecasts keep us in a low-grade monsoon into next week. Phoenix is not any better.

But it’s not just the North American Monsoon! The 2026 Southwest Monsoon season in Asia has been described as “highly erratic, with an unusually weak and delayed start linked to El Niño conditions, which initially suppressed rainfall”. This does not bode well for recharging Lake Mead anytime soon.

Discussion written by Pat Holbrook and Patrick Bunn