The heat dome is migrating. After baking the American Southwest for a week with temperatures that reached 108 degrees Fahrenheit — a reading that should not exist in March — the massive high-pressure system is creeping east, pushing summer-grade heat into the Midwest and threatening to rewrite temperature records from Minnesota to South Carolina [1].

As we reported in One Hundred Eight Degrees in March Was Not Supposed to Happen, the initial readings from Arizona and California were already extraordinary. What has happened since is worse. Fourteen states have now set all-time March temperature records [2]. Almost 180 locations with data dating to the 1960s or earlier have tied or set new March highs, from California to Pennsylvania [3]. Over 1,500 individual daily records have fallen in the past seven days [4].

To understand what is happening, it helps to understand what a heat dome actually does. It is not simply hot air. It is a circulation pattern — a slow-moving high-pressure ridge in the upper atmosphere that acts like a lid on a pot. Air descends within the dome, compressing and warming as it sinks. The dome blocks weather systems that would normally bring cooler air. Beneath it, temperatures rise day after day with no mechanism for relief.

This particular dome is unusually large and unusually stubborn. The World Weather Attribution group, which conducts rapid climate analysis, published findings describing the event as "virtually impossible without climate change." Temperatures have been running 11 to 17 degrees Celsius — that is 20 to 30 degrees Fahrenheit — above normal across much of the affected region [5]. To put that in perspective: a 20-degree anomaly in March is like experiencing July in the middle of spring. Except that actual July does not typically produce 108-degree readings in North Shore, California.

The eastward expansion is what makes this week different from last. The dome's western edge is weakening slightly as it encounters normal Pacific weather patterns, but its eastern flank is spreading into Kansas, Nebraska, Iowa, and Minnesota — regions where March temperatures typically hover in the 40s and 50s. These areas are now seeing readings in the 80s and, in some locations, the low 90s [1].

The US-wide temperature record for March was tied or beaten on four consecutive days last week, according to Yale Climate Connections [2]. That statistic is worth pausing over. Not one record day. Four consecutive record days. The nation's average temperature exceeded the previous March record by a margin that climate scientists described as "outside any reasonable natural variability."

The human consequences are accumulating. Emergency rooms across Arizona and Nevada reported a 40 percent surge in heat-related admissions during the dome's peak, many of them among elderly residents and outdoor workers [1]. Power grids in the Southwest are straining under air conditioning loads that utilities had not anticipated until June. Agricultural impacts are harder to quantify but potentially severe: spring planting schedules in the Midwest depend on predictable temperature ranges, and the sudden heat is accelerating soil moisture evaporation at a critical moment.

There is something vertiginous about watching the atmosphere do something it has never done before. The instruments that measure these things — thermometers, barometers, radiosonde balloons — are precise and indifferent. They do not editorialize. When 14 states break all-time March records in a single event, the instruments are not expressing an opinion. They are reporting a measurement. The measurement says that the climate system has shifted into a configuration that produces outcomes that were, until this week, outside the range of recorded experience.

The dome is expected to weaken by the weekend as a trough of low pressure moves in from the Pacific. Temperatures in the Southwest should return to merely above-average by Saturday. But the records that were broken this week will stand in the data for decades, and the pattern that produced them — the slow-moving, high-amplitude ridges that climate models have long predicted would become more frequent and more intense — shows no sign of becoming less frequent or less intense.

"Basically the entire US is going to be hot," said Victor Gensini, a meteorologist at Northern Illinois University, in a quote that captures the scope of the event with admirable economy [1].

The atmosphere is not making an argument. It is making a measurement. The measurement is that March, as we understood it, no longer exists.

-- KENJI NAKAMURA, Los Angeles