Summary
Tropical Storm Iselle remains the only active tropical system in the eastern Pacific, and a Tropical Upper Tropospheric Trough (TUTT) low is just west of Puerto Vallarta in the Pacific Ocean. Westerlies at 250 mb signal that the dynamic end of the monsoon season is close. Still, the subtropical high-pressure (monsoon ridge) is over New Mexico at 500 mb, but moisture at 700 mb remains much further south into Mexico. Arizona and western New Mexico are experiencing dry surface conditions, while eastern New Mexico has higher humidity from a back door cold front moving north to south.
Today, thunderstorm activity is expected to be scattered and mostly light, focusing on central and eastern Arizona and New Mexico, with the strongest storms near Roswell. Tomorrow, thunderstorms will become more widespread across Arizona, but are not expected to be severe; rainfall will be heaviest in mountainous areas. The pattern will persist into the weekend, bringing more storms and cooler temperatures, followed by a drier and warmer trend next week, though not as hot as the current week.
Yesterday’s Weather
The visible satellite animation shows only small storms along the Mogollon Rim at 5 pm, with larger thunderstorms in eastern New Mexico. A cold front pushed into northeast New Mexico, significantly increasing thunderstorm intensity, with many high-wind reports in the Texas Panhandle and northeast New Mexico. Arizona has southwest winds in the upper levels signaling the end of the 2026 monsoon season. On occasion, a tropical system or short-wave troughs in the westerlies will travel near enough to tap monsoon moisture to the south and provide a few days of thunderstorms. Global-scale models have had a difficult time forecasting tropical activity, so 3- to 10-day tropical forecasts are too uncertain to make any robust comment.
The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) are shown below. There was more thunderstorm activity in northeast New Mexico than forecast due to the frontal zone in that area. Only a few stations along the Mogollon Rim reported any rainfall yesterday; otherwise, Arizona was dry.
Analysis
Tropical Storm Iselle is the only tropical system in the eastern Pacific, and a broad, high-level anticyclone sits above its low-level cyclonic circulation, the typical structure for tropical systems. A Tropical Upper Tropospheric Trough (TUTT) low is just west of Puerto Vallarta in the Pacific Ocean, and another one is in the northwest Caribbean Sea.
A broader view of the central and eastern Pacific shows two tropical storms, LaLa and Iselle, and several areas with potential for tropical development.
The visible satellite image at 8 am shows mostly debris clouds from last night’s convection. Some high-level moisture from Iselle’s outflow is over far southwest Arizona.
Surface moisture remains low in Arizona and the western third of New Mexico. The higher dewpoint temperatures in eastern New Mexico are associated with the front.
The HRRR-UA WRF 12Z initialization (left) and observed (right) sounding for Phoenix at 5 am are shown below. It’s really dry, although there is a layer between 550 and 500mb on the observed sounding where clouds will form this afternoon.
Westerlies dominate the atmospheric circulation at 250 mb. The subtropical ridge is displaced well to the south, with one center over the spur of the Baja Peninsula and another south of the boot heel of New Mexico. The TUTT low is a little further south and west, as annotated.
At 500mb, the monsoon high is over southern and southeastern New Mexico with a broad anticyclonic flow between the high and low pressure off the West Coast and a trough in the Great Plains. The monsoon circulation is stretched across northern Mexico.
At 700mb, the monsoon high is over the Four Corners region, and easterly flow is in place in Mexico, south of the ridge. No significant moisture is associated with the monsoon flow in Arizona, but there’s just enough to keep thunderstorms active over the spine of the Sierra Madre. The moisture in the Midwest is behind the front.
Today
The forecast sounding below for Nogales (left) at 2:30 pm shows both Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) are low. The minimal amount of moisture available would make any storm that does form have bases above 15,000 feet (4.5 km). The forecast for Carlsbad at 5pm has too much CIN, and surface-based thunderstorms will need outflow from nearby storms developing over the mountains to form in the valleys.
The HRRR-UA WRF 12Z radar reflectivity forecast for this afternoon shows thunderstorms in central Pima County with a few showers near Tucson. To the north, thunderstorms are in eastern Arizona extending east into New Mexico with the strongest storms northwest of Roswell. The strongest wind gusts will be associated with the storms near Roswell.
Tomorrow
Thunderstorms increase in areal coverage across Arizona, though they’re forecast to be relatively unorganized. Still, each storm would be capable of producing wind gusts up to 40 mph. Most of the rain in the valleys will be light, with heavier rain totals near mountains.
The total precipitation forecasts through 24 hours (left) and through 48 hours (right) show mostly light precipitation over the next two days, with the only >0.5″ totals in the mountains, especially in central New Mexico.
Outlook
A similar pattern continues on Friday, with storms forming in eastern Arizona and moving northwest into the low deserts, likely into the Phoenix area.
Storms increase into the weekend, and daily high temperatures will drop back to normal. Next week, moisture decreases and temperatures warm again, but not as hot as this week.
Discussion written by Pat Holbrook and Patrick Bunn