Getting Ready for Winter: The basics of winter weather

SIOUX CITY, Iowa (KCAU) — Some areas in Siouxland have already seen their first measurable snow (at least 0.1′′) as we quickly approach the winter season. Meanwhile, other communities across the area have yet to see their first flakes.

So now is the time to refresh your winter weather knowledge and review some of the basics.

Forecasting Winter Weather: Early forecasts and when to trust the forecast

First, precipitation types. Snow isn't the only wintry weather experienced here in Siouxland, especially as we transition from fall to winter and then from winter to spring.

Most often in the winter, snow is the main type of precipitation we usually see. But there's also sleet, freezing rain, graupel, and just plain rain.

As you could probably guess, rain is liquid as it falls through the atmosphere and at the surface level. All levels are above freezing (32° F).

Then there's freezing rain. It's essentially the same concept as rain. It begins frozen at the very top of the atmosphere and melts as it falls through a deep, warm layer aloft, allowing for full melting. Precipitation falls through most levels aloft as raindrops. However, the surface level is at or below that freezing point of 32°F. As a result, those raindrops freeze on contact once they reach the surface. This could be on anything from trees and cars to sidewalks and roadways, leaving a slick coating of ice.

Next up is sleet. Sleet, like freezing rain, is initially completely frozen, beginning as snowflakes. As that frozen precipitation falls though, it travels through only a shallow warm layer, resulting in only partial melting. Then, as it approaches the surface, the partially melted precipitation refreezes and falls to the surface as small ice pellets. These pellets can accumulate like snow on roadways, sidewalks and other surfaces.

Sleet and freezing rain tend to develop in the transition zone of a storm system as temperatures go from above to below freezing and precipitation begins to fall as all snow.

And then, of course, there's snow. Snow starts off frozen, all layers of the atmosphere are at/below freezing and the snowflakes never melt at all during their descent to the surface.

Occasionally, we also see something called graupel, also known as snow pellets. This happens when temperatures aloft are very cold and surface temperatures are above freezing. This results in supercooled droplets aloft (liquid below the freezing point but still in liquid state). These supercooled droplets freeze onto the falling snowflakes and form a soft, white pellet (these look like Dippin' Dots ice cream!)

First Snow in Siouxland: When to expect it and how to get winter-ready

Now, with snow, you may hear your local meteorologist use a few different terms to describe snowfall. A few examples of this are "Snow flurries possible the next few days" or "Snow showers fill in through the evening hours."

So here's a handy guide to exactly what those descriptors mean:

And oftentimes, when wintry weather is in the forecast, there may be watches, warnings and/or advisories issued in advance of the onset of precipitation.

In Siouxland, there are three different National Weather Service (NWS) offices that cover the area: Sioux Falls, Omaha/Valley, and Des Moines.

For less impactful systems, Winter Weather Advisories are commonly issued by the local NWS office. In Siouxland, these are typically issued when 3-6′′ of snow is expected over a 12-hour period. In the Sioux Falls NWS county warning area (CWA), a Winter Weather Advisory may also be issued if 5-8′′ of snow is expected over a 24-hour period.

In addition to being issued to snowfall, Winter Weather Advisories may also be issued if less than half an inch of or less than a quarter (0.25′′) inch of ice/freezing rain is anticipated. An Advisory may be issued for blowing snow that occasionally reduces visibility to a quarter mile with winds less than 35 mph.

Preparing for Winter: Getting your home & car ready before inclement weather hits

For higher impact systems, Winter Storm Watches and Warnings are typically issued. These are issued if one or more of the following criteria are met:

If significant ice accumulation is expected, the National Weather Service will issue an Ice Storm Watch/Warning instead.

And when sustained winds of 35 mph or higher are expected with falling and/or blowing snow and will frequently reduce visibilities to less than a quarter mile for three or more hours, a Blizzard Warning will be issued.

Another key aspect of winter weather is the bitter cold that often accompanies winter storms, from single-digit highs to sub-zero wind chills.

In past winters, the NWS has typically issued Wind Chill Advisories, Watches, and Warnings.

New for this winter, you won't see Wind Chill alerts issued by NWS offices. Instead, these have been replaced with Extreme Cold Advisories and Extreme Cold Watches/Warnings.

What to expect in Siouxland as NOAA releases 2024-25 winter weather outlook

Thresholds across all three NWS offices' county warning areas (CWA) are relatively similar with just a few degrees of difference for issuance criteria throughout Siouxland.

For Extreme Cold Advisories, much of the NWS Sioux Falls and NWS Des Moines CWA falls within the -25° to -34° apparent temperature range for advisory issuance. However, some of the NWS Des Moines CWA may fall within the criteria for issuance by the NWS Omaha/Valley office, which is slightly lower than through northern portions of Siouxland, with a general threshold for issuance of -20° to -29°.

And in the Sioux City metro and surrounding areas: for Dakota, Dixon, Ida, and Woodbury counties, the criteria for issuance of an Extreme Cold Advisory is between -20° to -24°. However, since Woodbury and Dakota counties (which include Sioux City and South Sioux City, respectively) fall within different criteria for issuance than Union County (which includes North Sioux City), these temperature thresholds within the NWS Sioux Falls CWA may not always be adhered to when issuing these alerts to avoid splitting the Sioux City metro up based off just a couple of degrees of difference.

After the storm: Tips for staying safe after a winter storm

This holds true for Extreme Cold Watches and Warnings as well. Apparent temperatures of -35° or below in northernmost portions of Siouxland, including all of southeast South Dakota, will warrant an Extreme Cold Warning issuance or an upgrade from Extreme Cold Advisory to an Extreme Cold Warning.

Through most of northeast Nebraska and the southern portion of northwest Iowa, this threshold is a bit "warmer" at -30° or below.

And like the Extreme Cold Advisory criteria, Dakota, Dixon, Woodbury and Ida counties have a higher ("warmer") threshold than the rest of Siouxland, at -25° or below. Again, these temperature thresholds may not always be adhered to when issuing a warning to avoid splitting the Sioux City metro up based off just a few degrees of difference.

NWS Sioux Falls Winter Preparedness Page

NWS Des Moines Winter Preparedness Page

NWS Omaha Preparedness Page

And for the latest weather any time in Siouxland, visit the KCAU 9 weather page .