A mashup of webcams from AccuWeather showed the fog enveloping many major cities east of the Rockies, including Cincinnati, Nashville, Atlanta, Pittsburgh, Dallas, Oklahoma City, New York City, Washington and Chicago.
Thursday marked the third day in a row of this record-breaking fog outbreak, largely the result of a storm drawing tremendous amounts of moisture from the Gulf of Mexico overtop a shallow layer of cooler air left behind by last week’s Arctic blasts. This same storm has unloaded as much as 10 inches of rain along Gulf Coast and triggered serious flooding.
Dense fog, according to the National Weather Service, drops visibility to a quarter mile or less for an extended period of time. This week’s fog has caused numerous travel troubles, including flight delays and cancellations as well as accidents:
- A pedestrian was fatally struck by a vehicle in Indiana early Thursday amid the fog.
- The Lake Pontchartrain Causeway Bridge in Southeast Louisiana was closed for a time Wednesday after a multivehicle accident.
- In Kansas, a power outage occurred when a truck driver ran off the road and hit a utility pole.
Chicago has been particularly hard hit by flight issues. On Tuesday, 8 percent of flights out of O’Hare International Airport were canceled and 37 percent delayed, according to the tracking website FlightAware. A similar numbers of flights were affected Wednesday and Thursday morning.
Around Cleveland, some schools were delayed for a second straight morning Thursday because of the low visibility.
Advection fog is the cause. Unlike radiation fog, which typically forms overnight when skies are clear and winds are calm in the spring and fall, advection fog develops when warm, moist air is transported over a layer of cold air near the ground.
This week’s advection fog episode is a textbook case as warm and moist air from the Gulf of Mexico has been pumped over what was a record snowpack last week across the Lower 48 by a zone of high pressure off the Mid-Atlantic coast.
“The setup for the ground surface will be a snow-covered ground or a saturated ground that has been chilled by cold temperatures before the winds shift back from a southerly type direction,” wrote meteorologist Jeff Haby in an online tutorial published by the Weather Service.
If winds aren’t too strong and the ground remains cool, advection fog can persist for days, as it has this week.
Tuesday, Wednesday and Thursday mornings set records for the number of dense fog advisories nationally, according to Daryl Herzmann, a systems analyst who manages a weather hazard database at Iowa State University. Each day surpassed the record set the day before. The fog advisory database dates back to January 2005.
Herzmann told the Capital Weather Gang that more than 1,500 Weather Service forecast zones, analogous to counties and sometimes sub-counties, were under a fog advisory early Thursday.
Thursday’s fog advisory tally topped the 1,341 zones under advisories on Wednesday and the 1,107 zones Tuesday, according to Herzmann.
Fog coverage should diminish as a storm system sweeps from the Mississippi Valley to the East Coast by the weekend, increasing winds to mix out the soupy air.
Snow melt in the Mid-Atlantic and Midwest has also been rapid over the past couple days because of the warm air surging northward. As the snowpack disappears in more places, cold temperatures near the ground will moderate, lowering…
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