Here a few "big" misnomers we'd like to address and "clear the air," so to speak.
1. Give or take an inch
First, no matter how much we'd like to, we cannot forecast the amount of snowfall for your back yard. While weather model data has advanced significantly in the last 10-20 years, meteorology is one of the youngest sciences and we still have much to learn about the upper atmosphere and how it interacts with the lower atmosphere and more localized (meso- and micro-scale) weather features. For snowfall events with the data that is available now, snow totals can be predicted within a 1-to-2-inch range accurately.
2. The forecast should stick
Second, human forecasts do not (or should not) change several times per day. If you're seeing this happen, you're most likely looking at a free weather app which changes every 6 hours with new GFS (American weather model) data. For the record, this data is at a lower resolution and is not nearly as accurate as other available data. You should NEVER make business decisions based on free weather apps or straight model data.
Human forecasters sift through the data, throwing out models that initialized badly or are not picking up on certain atmospheric features that will impact the system and resulting snowfall. Generally, 1-2 forecast "touch ups" are all that are needed by human forecasters as a wintry system approaches the 24-hour time frame to the start of the event. Additionally, we know you want the latest information as soon as possible, but we only have one or two models that come in each hour. The rest only come in every 6 hours, so we generally won't be able to give you much new information with confidence until that next model run. New data is available at roughly 9:30am/pm, and 3:30am/pm.
3. Temperature is a factor
Third, when a mix of snow, sleet, and/or freezing rain is expected (Generally, this occurs along warm-front scenarios), temperature fluctuations of 0.5-1.0 degrees aloft or at the surface can have MAJOR impacts on the observed precipitation types at the surface.
For example, earlier this winter in central Indiana, initial thoughts of 3 to 5 inches and even 5 to 8 inches of snowfall were expected even 12-24 hours ahead of the start of the event. As the event unfolded, temperatures aloft remained only 1 degree too warm for snow to fall, and freezing rain persisted for hours on end. This turned into an ice storm with up to 0.40 inches of freezing rain, and only 1 to 3 inches of snow for most locations. With these types of events, it is imperative for clients and the general public to closely follow forecast updates from trusted weather sources. Meteorologists call these "now cast" situations, as this is sometimes not able to be determined by weather models or even humans ahead of the event.
If you have any winter weather questions, or snow and ice topics you'd like addressed, just email them to firstname.lastname@example.org and we'll address them in future articles.
Beth Carpenter is a co-founder and meteorologist at Thermodynamic Solutions, based in Indianapolis. You can reach Beth at email@example.com.