Summary

A low pressure system in the Midwest is pushing the subtropical high southwest and bringing cooler mid-level temperatures over New Mexico that will promote thunderstorms. Mid-level warmth will limit strong storms in Phoenix and Tucson, while Truth or Consequences has steeper lapse rates due to cooler air aloft, making it the key area for activity today. With drier surface dewpoints across the region, microbursts are expected given the significant downward convective available potential energy (DCAPE). Dust storms are possible along I-10 due to gusty winds and dry conditions. Outflows moving southwest will increase surface moisture and cause light showers overnight in SE Arizona.

Tomorrow, isolated high-based thunderstorms are possible in Tucson and southeast Arizona, and outflow from storms in northwest Mexico may bring additional moisture into southern Arizona by Monday. The outlook calls for increased thunderstorm activity midweek, with a Gulf Surge expected to bring more moisture into Arizona by Tuesday.

Yesterday’s Weather

The infrared satellite animation for yesterday evening shows an area of thunderstorms extending from southeast Arizona to El Paso and from central New Mexico into the Texas Panhandle at 9 pm. Strong winds were recorded with these storms, with the highest of 77 mph at Deaf Smith County, Texas. The El Paso area had wind gusts over 60 mph, but wind gusts were not as high westward towards the Arizona border, where wind gusts were less than 40 mph. The strongest storms in Texas were associated with a cold front moving southward, which will help push moisture into Arizona.

Thunderstorm activity in northwest Mexico moved into the Gulf of California and pushed an outflow northward up the Gulf of California to the Yuma area. The Velocity Azimuth Display Wind Profile (VWP) below shows the southerly thunderstorm outflow extending to a height of 2000 feet (600m). Unfortunately, westerly winds aloft will mix in dry air and offset the moisture from the outflow. 

The HRRR-UA WRF 12Z 24-hour accumulated precipitation forecast (left) and corresponding observed precipitation from MRMS (right) are shown below. The forecast was in good agreement with observations. 

The visible satellite combined with composite radar reflectivity for 8 am this morning shows remnants of the storms in New Mexico moving southward across Arizona, east of a line from Phoenix to Tucson, then Nogales. The clouds/showers will dissipate this morning, with a few drops of rain falling in Tucson, Sierra Vista, and Nogales. 

Analysis

The 60°F isodrosotherm is only in southern Arizona and far eastern New Mexico. Drier air is in northern Arizona and New Mexico. The front that dropped south into eastern New Mexico and West Texas (annotated in black) will be the focal point for thunderstorms again today.

The HRRR-UA WRF 12Z initialization (left) and observed (right) sounding for Phoenix at 5 am shows the forecast does a better job with the lowest layer. In contrast, the observed sounding has a better handle on the moisture at and above 500 mb. Both have the same convective temperature of 117°F, and while it will be a hot day, we won’t reach convective temperatures in either location.

At 250mb, the monsoon high-pressure ridge is in southwest Arizona along a west-east ridge axis. The trough that dropped into the Midwest has a trough axis extending into northern New Mexico. 

The monsoon ridge has been displaced west to the Las Vegas area at 500 mb. Cooler air associated with a low in the Midwest is moving southward into eastern Arizona and New Mexico and will make the atmosphere unstable with steep lapse rates. 

At 700mb, the monsoon high is over southern Arizona and has established a monsoonal (easterly) flow south of the ridge in northern Mexico. There is not a lot of moisture present yet, but enough to combine with the colder air aloft for thunderstorms. 

Today

The forecast soundings for Phoenix at 1 am (below, left) and Tucson (below, right) are very similar. The layer above 550 mb at both sites is unstable and will support showers overnight. In the afternoon and evening today, the mid-level temperatures are still too warm to allow for sufficient lapse rates and enough Convective Available Potential Energy (CAPE) for strong thunderstorms.

The Truth or Consequences sounding for 2 pm this afternoon shows an atmosphere in which the storms will form today. Only an inch of precipitable water is present, but the rain falling into this layer will support a microburst of 50+ mph, given 1778 [J/kg] of Downward-CAPE. These storms will expand south and west and will be the source of moisture for the Tucson and Phoenix showers after midnight.

The thunderstorms and wind gusts forecast by the 12Z HRRR-UA WRF show I-10 will be prone to dust storms between Willcox and Las Cruces at about 4 pm. Precipitation will generally be light as the lower atmosphere will be dry.

Tomorrow

The Tucson and Phoenix forecast soundings at 4 pm on Sunday afternoon and evening are similar, with some high-based thunderstorms more likely for Tucson and southeast Arizona.

The radar reflectivity forecast for tomorrow shows the formation of thunderstorms and potentially an MCS in northwest Mexico. The outflow from these storms will push moisture into southern Arizona on Monday morning, increasing thunderstorms along the International Border Monday afternoon. New Mexico will have fewer storms in the wake of the front that has moved into the state.  

Outlook

The higher CAPE to the south and west of Arizona means storms will move southwest. One thing that is missing is any significant CAPE in the valleys, so storms will only initiate in the valleys with outflow initiation. Still, any storms that form in the valleys will have to compete with surrounding dry air in the low deserts. Another MCS in Mexico will provide surface moisture that will be advected along the pressure gradient set by the front in New Mexico. A resulting Gulf Surge is predicted to move into Arizona on Tuesday morning.

The Tucson and Phoenix global model ensemble forecasts show the effect of increased moisture, with precipitation chances increasing from Wednesday in Phoenix and Monday in Tucson.

Discussion written by Pat Holbrook and Patrick Bunn