A strong ridge of high pressure — shown here as 500 mb height anomalies — is expected to build over the Southwest U.S. later this week. Temperatures in the Phoenix area could challenge all-time records. (weatherbell.com)

All-time records for high temperatures could be broken over the weekend in parts of Arizona and Southern California as a mega-ridge of high pressure strengthens over the Southwest. Afternoon temperatures could reach 120 degrees in the hottest locations, and forecasters are asking people to have a plan for how to stay cool.

An excessive heat watch is in effect for over 55,000 square miles of Arizona and southeastern California, including Phoenix, Tucson and Yuma. High temperatures are expected to reach 105 to 110 degrees Friday, 110 to 115 degrees Saturday and peak heat — 115 to 120 degrees — Sunday.

Five days out, the Phoenix office of the National Weather Service is already using strong language to describe the looming heatwave. In the Phoenix area, the Weather Service is cautioning that the magnitude of this heat is “rare, dangerous, and deadly.”

It’s also being referred to as a high-confidence forecast. One of the weather models that the National Weather Service relies on “remain at or above the 99th percentile from Saturday through Monday,” which means this model is forecasting the ridge to be at the uppermost echelon of mega.

Given what we know now, daily records are certainly in jeopardy, and some all-time high-temperature records could also fall. At the least, Sunday’s temperatures could surge into the five hottest temperatures on record in Phoenix and Yuma. Phoenix has only hit 120 degrees three times since records began in the city in 1895. The record warmest temperature for the location is 122 degrees set on June 26, 1990.

Top 5 hottest temperatures in Phoenix:

1. 122 on June 26, 1990
2. 121 on July 28, 1995
3. 120 on June 25, 1990
4. 119 on June 29, 2013
5. 118 on several days

Top 5 hottest days in Yuma:

1. 124 on July 28, 1995
2. 123 on Sept. 1, 1950
3. 122 on June 26, 1990
4. 120 on Aug. 27, 1981
5. 120 on June 24, 1957

Things should start to cool down a little after Monday, but models are in disagreement on whether or not this ridge will stick around. If it does, the heat could last into at least the middle of next week.