The month of January is historically the coldest month of the year across the lower 48. This certainly stayed true to course back in January of 1977, as a bone-chilling air mass enveloped much of the eastern US. Even the tropical beaches of South Florida would not be spared this time.
In January, the average high temperature in Miami is in the mid 70's, while lows of around 60 can be expected. South Florida has many barriers that keep its temperatures relatively warm during the middle of the winter season. One barrier to the cold is that the southern part of the state is surrounded by warm waters in the 70s on both the Gulf and Atlantic coasts. This warmth that resides on all sides makes it difficult for arctic air masses to make it to the southern part of the sunshine state. The high angle of the sun is another obstacle as bitter cold air masses will modify and warm fairly quickly when they head into the southern reaches of the peninsula. Unfortunately, these safeguards were incapable of holding off the onslaught of bitter cold that occurred back on January 19th and 20th of 1977.
The provided surface chart depicts what brought on this historic event. On the morning of the 19th, a strong arctic high pressure system was centered over Louisiana. The clockwise surface winds around the high brought gusty northwest winds across the Florida peninsula. This propelled a series of arctic cold fronts southward, eventually reaching the southern coast. As freezing temperatures enveloped the state, an upper level trough headed in from west, resulting in snow showers and a rain/snow mix over areas that had never experienced it since weather records began in the late 1800's. Most areas saw a trace of snow, but a reporting site just east of Tampa measured a whopping 2 inches. Eventually, even the Miami region saw the flakes flying by midday on the 19th.
Agricultural impacts were devastating on the citrus crop, as subfreezing temperatures blanketed the entire region. On the morning of the 20th, Naples and Fort Lauderdale saw frigid lows of 26 and 28 degrees respectively. Meanwhile, Miami Beach bottomed out at 32. These locations each achieved all-time record lows which still stand today.