Miami just experienced its hottest week on record, rounding out its warmest first half of the year ever observed. Two out of every three days this year have featured a broken record of some sort somewhere in South Florida.
Temperatures have climbed into the 90s each of the past 11 days, unremarkable by itself since Miami’s average late-June high is 90 degrees. But more impressive have been the overnight lows — refusing to fall below 80 degrees for nearly a week. Miami’s hottest weather typically comes in late July or early August.
How air from the Sahara is supercharging Florida heat
The record-setting heat has been exacerbated by a layer of arid desert air wafting westward from Africa at the mid-levels of the atmosphere. That has acted to suppress Florida’s characteristic thunderstorm activity, a staple in cooling the afternoons.
After breaking a "warmest low" record this morning, Miami just broke a "warmest high" record this afternoon at 94°. For Miami, that's 6 records in 5 days. For South Florida that's 12 records in 4 days. 2020 so far, 116 records or a record 2 out of every 3 days. @johnmoralesnbc6 pic.twitter.com/klkKtxzpkR— Steve MacLaughlin (@SteveMacNBC6) June 28, 2020
Without that temperature moderation, there’s been nothing to stop South Florida from baking beneath the summer sun.
“The heat is unprecedented … in terms of prolonged extreme warm temperatures, daily average temperatures, and one week and counting with a 90-degree-plus heat index,” wrote Brian McNoldy, CWG’s tropical weather expert, in an email.
Miami’s warmest week on record by the numbers
Insane heat in #Miami for any time of the year, let alone June. The heat index has not dropped below 90° since last Tuesday morning, and the daily peak heat index has been 103°+ for the past 7 days, with more coming up this week. #MiamiHeat #heatwave #summer pic.twitter.com/lWK1MPbB0E— Brian McNoldy (@BMcNoldy) June 29, 2020
Over the past week, the mean temperature in Miami was 88.1 degrees — a number arrived at by averaging seven days’ worth of highs and lows. That marks Miami’s all-time warmest week in recorded history, but the record could grow in the coming days. Monday was anticipated to climb into the mid-90s for highs, with lows forecast to drop only into the low- to mid-80s overnight.
Four of the past five days have been among the city’s 10 warmest on record. Nine of Miami’s 10 hottest days since 1937 have occurred in the past three years — a symptom of a climate swiftly heating up. Sunday’s high hit 94 degrees, also a record.
“When you don’t get the showers and thunderstorms, it continues to heat,” said Steven Ippoliti, a meteorologist at the National Weather Service in Miami. “[Thunderstorms] mix the atmosphere, so it brings the cooler air. But without the [showers and thunderstorms], it causes the heat to really be there.”
Ippoliti said that even weather models are struggling to get a handle on the wicked heat.
“Models don’t like anomalies all that much,” he explained. “They’re not compensating enough [for the dry mid-level air]. They’re thinking we should get some rain.”
Despite “excellent” dynamics for storms, not a drop of rain has fallen in Miami all week. The city normally sees closer to nine inches of rain during the month of June.
Joseph Prospero, a professor emeritus in the department of atmospheric sciences at the University of Miami, analyzed data from an on-site lidar device used to profile the atmosphere. He noted that dust marking the Saharan Air Layer was present between about one and 2.5 miles in altitude over the weekend.
Other long-term factors at work
The heat isn’t solely due to the presence of a Saharan air layer, although that has certainly helped. Exceptional humidity has made it tougher for the atmosphere to rid itself of heat at night. That has been most prominently reflected in overnight minimum temperatures, affected even more dramatically than the daytime highs.
That humidity stems from Gulf of Mexico water temperatures. As they warm, the air gets more moist. Water temperatures off Miami are in the mid-80s.
With a low of 84 degrees, Miami tied its all-time record for warmest nighttime low temperature last Thursday, and again on Saturday. Wednesday, Friday and Sunday came close, missing the mark by just one degree.
“The average low temperature during the past week was 82.1 degrees,” wrote McNoldy. “The previous record for that same week was 81.1 degrees.”
In fact, Miami has tied or broken records for warm nighttime “low” temperatures a staggering two dozen times already in 2020. Miami set 34 such records in 2019.
Since 2015, warm nighttime temperature records have been tied or broken 107 times. Records date back to 1895.
An unprecedented year for heat
This year has already been a wild one for heat in Miami, even long before the arrival of the Saharan Air Layer. “Brutal” heat in April brought the city’s earliest heat wave on record, with heat indexes in the triple digits. Easter felt more like the Fourth of July.
Heat earlier in the year was jump-started by a stubborn ridge of high pressure that warded off clouds and cold fronts earlier in the spring. That allowed Gulf of Mexico water temperatures to heat up, intensifying South Florida’s humidity and already tropical climate. Since then, the warm weather has become self-reinforcing, bearing the fingerprint of climate change.
The first half of 2020 has been Miami’s warmest on record, preliminarily beating out the previous record holder — 2017 — by more than half a degree. That period has warmed about 3.5 degrees since the 1930s, in line with how much annual temperatures have warmed during the same span. A portion of observed warming may be due to the urban heat island effect, but human-induced climate change due to the buildup of greenhouse gases in the atmosphere is also contributing.
McNoldy, himself a resident of Miami experiencing the skyrocketing temperatures firsthand, says that, so long as climate change continues affecting the weather, there is no end in sight for Floridians dealing with the more extreme heat.
“It is undoubtedly getting easier and easier to break heat-related records,” he wrote.