Black ice and heavy frost contribute to major rush hour headaches

Nearly ideal conditions for heavy frost and black ice developed early this morning, resulting in massive traffic problems, particularly on bridges and overpasses. So what were the meteorological conditions that resulted in extra long commutes, and plenty of damaged vehicles?

Screenshot of Google traffic data from 7:50am Wednesday

There are several potential causes for black ice on the roadways. You're likely familiar with most of them, but in general you simply need moisture on road surfaces that drops below freezing. The factors that result in the freezing on the roads are the tricky part, as major thoroughfares are warmer than the air temperature just above them.

Causes of black ice

Clearly falling wintry precipitation on the roads will result in icy surfaces, but black ice occurs in the absence of precipitation - and it appears to be wet - which is why it can catch us off-guard. Black ice can form when winter precipitation melts during the day, leaving wet surfaces as temperatures fall below freezing overnight. This is particularly true for puddles and shady areas. Standing water from previous rainfall can also clearly freeze, resulting in icy surfaces, especially on bridges and overpasses. Neither of these were the primary cause of icy roads this morning though, though there were some patchy areas of wetness from Tuesday that simply froze over.

What happened this morning

The other scenario that can occur, and what likely caused most of the issues early this morning, was an ideal scenario for heavy frost that in this case also affected elevated roadways. Precipitation over the previous couple of days resulted in saturated ground, providing ample ground-based moisture to be susceptible to a freeze. As clouds moved out right before sunset Tuesday afternoon, there was no time for evaporation to take place, so the low-level moisture remained in place. In addition, wind was very light - another favorable scenario for frost, dew and fog depending on the temperature and moisture profile.

Rainfall on Monday into early Tuesday that contributed to frost formation Wednesday (NWS via WxBell)

What made it worse?

Finally, on most nights the frost process doesn't start until well into the night, providing only a handful of hours in which frost forms before warming begins after sunrise. Yesterday, temperatures were already near freezing by about 6pm after a high only in the 30s. If you went out last evening, you may have noticed frost already forming not long after sunset, by 8-9pm! That meant that the frost formation process continued all night long, about 12 hours. The longer frost is allowed to form, the thicker and heavier it becomes.

The effects of heavy frost today

So, with little traffic overnight to keep roads warm and frost formation occurring all night long, not only was everything white when we awoke this morning, but the elevated roadways (that don't have warmer earth just below them to keep them warm) also started accumulating frost. Frost is ice! With a bit of traffic early this morning, the tires warm the frost enough to melt it, but the air temperature freezes it up again - creating black ice! As more and more unaware drivers travel on the thin layer of ice as rush hour approaches, the probability of slipping and sliding increases with traffic congestion and wrecks start happening, which leads to chain reactions.

So, our clues to potential issues as early as last night were the early formation of frost and the forecast of nearly ideal conditions (light to calm wind, clear skies, and temperatures below freezing) all night long. Those conditions could manifest themselves in either heavy frost, freezing fog, or a combination of the two. Either can create real headaches for commuters! In fact, we called it out last night at 9pm, though we didn't expect it to be as bad as it got!

Temps are currently in the low 30s under mostly clear skies. Overnight lows are expected to reach the mid to upper 20s w possible patchy freezing fog by morn. Be cautious on your morning commute w/ potential for some slippery bridges for morn rush hour. Aftn temps in the 40s /AS

— MemphisWeather.net (@memphisweather1) December 18, 2019

Looking ahead, we'll need to watch Thursday morning carefully as well. Though sunshine today will help warm temperatures into the 40s and get the evaporation process going, meteorological conditions again favor heavy frost tonight. I don't expect to see a repeat of this morning though as we'll have fewer hours below freezing and a little less ground-based moisture.

Erik Proseus
MWN Meteorologist

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