His agency already has a winter storm watch in place or all of the northwest Iowa counties it serves, including the Spencer, Spirit Lake and Storm Lake areas.
Projections often change as a storm approaches but, 16 to 18 inches of snow are forecasted for the Spencer, Spirit Lake and Sioux City areas. Storm Lake may get closer to 20 inches.
"The way it looks right now, northwest Iowa appears to be the target area for the heaviest snow," Harmon said. "This is going to be a very juicy, wet storm so there's upwards of 15 to 20 inches possible across portions of northwest Iowa, including the Spencer area."
Harmon said the early winter storm watch was triggered by confidence in the forecasts and the timing of the holiday season.
"Christmas Eve and Christmas Day, travel across the whole region is probably going to be very difficult," he said. "Even the snow that's likely to develop on Wednesday will probably slow travel to some degree. The other issue, and the reason the watch came out as soon as it did is, the forecasters are developing quite a bit of confidence that this particular storm system is going to come together like we're thinking. That degree of confidence is growing as we move forward."
Here's the expected time line:
* Tuesday looks quiet and mainly cloudy, with a bit of freezing drizzle possible.
* The first band of snow from the major storm is expected to arrive before daylight Wednesday morning. Harmon is expecting 3 to 5 inches of snow during the day on Wednesday.
* A lull in the storm is possible late Wednesday afternoon into Wednesday night.
* Blustery conditions may develop overnight Wednesday as winds increase and the snow starts to pick back up before sunrise on Christmas Eve, Harmon said.
"The bulk of the snow looks like it will probably fall during the day into the evening hours on Thursday," he said.
* The storm will start to wind down Christmas Day.
"Again, there is a potential for anywhere from 10 to 20 inches of snow across southeast South Dakota into northwest Iowa," Harmon said. "There is a very, major storm potential with this system and a lot of wind, especially Thursday and Friday."
* Saturday will be dry but still windy. A cold snap is expected to follow on Sunday.