Birmingham’s Winter Storm Watch, Warning, & Advisory Criteria for Alabama

The National Weather Service has sole responsibility for issuing weather watches and warnings.  When it comes to winter weather, there are numerous advisory types ranging from Wind Chill Advisories to Ice Storm and Blizzard Warnings.  This page is devoted to the four main winter weather products:  Winter Storm Watches, Winter Storm Warnings, Winter Weather Advisories, and Ice Storm Warnings.  As always, be sure to follow credible sources for weather information, and beware of winter-weather hype created by unreliable sources that tends to spread rapidly on social media.

Schematic of how freezing rain, sleet, and snow form as a function of the temperatures aloft
Schematic of how rain, freezing rain, sleet, and snow form as a function of the temperatures aloft

Winter Storm Criteria Simplification and Consistency

In the wake of the infamous “snow jam” of 2014, it became clear that many people are confused by the various winter weather alert types and the criteria used for each.  Since that event, there local NWS forecast offices in the Southeast have worked to simplify and blend the criteria.  This simplification will not only facilitate office to office forecast coordination, but it should also reduce the advisory-type differences historically seen at CWA boundaries (CWA is an abbreviation for “County Warning Area” and is the area of forecast responsibility for your local National Weather Service office).  While the criteria changes will help improve forecast consistency, some discrepancies from office to office are inevitable. The forecasters at each office may interpret the meteorological data and model simulations differently, resulting in some variance in the predicted precipitation types and amounts.

Which Winter Weather Alert is “Worse”?

I’m often asked, “which is worse, a watch, warning, or advisory?”  To answer that question, here is the hierarchy:

Winter Storm Watch < Winter Weather Advisory < Winter Storm Warning

In other words, both an “advisory” and a “warning” are upgrades to a watch. An advisory is “worse” than a watch and a warning is “worse” than an advisory. Ultimately, the difference between an advisory and a warning lies in the predicted accumulation and expected impacts. Sometimes, the impacts of a small and seemingly insignificant accumulation can be nearly the same as those of a large accumulation.

Flow chart of the winter weather alert hierarchy, indicating which type is worse

When you see an accumulation map, don’t focus on the fact that surrounding areas may get more precipitation.  Oftentimes, the public interprets the forecast graphics to mean that “the worst is going to miss us.”  It’s important focus on the impact that the predicted snow or ice will have on your community (even it’s just an inch of snow). Don’t focus on the fact that adjacent counties may be under a “worse” advisory type or that adjacent counties may be expected to receive higher accumulations. If your county is under a Winter Weather Advisory for a snow accumulation of less than two inches, what will be the impact of that amount in your community?  With all of that said, let’s discuss the winter storm criteria used by the National Weather Service office in Birmingham when issuing winter-weather alerts.

Winter Storm Criteria for Central Alabama

Birmingham's Winter Storm and Ice Storm Criteria (Central Alabama)
Birmingham’s Winter Storm and Ice Storm Criteria (Central Alabama)
  1. A Winter Storm Watch is issued when significant winter weather (i.e. 2 inches or more of snow, ½ inch or more of sleet, ¼ inch or more of freezing rain, or a combination of these events) is possible, but not imminent. A watch is typically issued 12 to 72 hours before the possibility of winter weather when the forecasters at the National Weather Service in Birmingham have at least 50% confidence that the event will occur.  A wide range of expected weather events can prompt a Winter Storm Watch (freezing rain, sleet, snow, or a combination of all three, in various amounts, per the chart above).  When a watch is issued, make sure to pay attention to the specific forecast for your community and the possible impacts.
  1. A Winter Weather Advisory means that winter weather is imminent, but the expected amounts do not rise to “Warning” criteria.  As indicated in the hierarchy above, an advisory is an upgrade to a watch; but sometimes a winter weather event develops when a “Watch” wasn’t previously in effect. In those cases, a Winter Weather Advisory may be issued to cover the event, assuming that the precipitation may pose a hazard to the pubic in the area of concern.  A Winter Weather Advisory is typically issued up to 36 hours before an event if there is an 80 percent chance or greater that winter precipitation will develop (i.e. snow, freezing rain/drizzle, sleet, blowing snow, or any combination thereof), but the predicted accumulations do not meet the warning criteria (see the warning criteria below).  Note: If freezing rain is expected to be the predominant precipitation type, but the anticipated amounts do not meet “Ice Storm Warning” criteria (described below), a Freezing Rain Advisory may be issued in lieu of a Winter Weather Advisory.  An example would be light freezing rain that could cause a very thin glaze, but not cause tree damage or widespread power outages.
  1. A Winter Storm Warning is issued when a significant winter storm (i.e., two inches or more of snow, ½ inch or more of sleet, ¼ inch or more of freezing rain, or any combination thereof) is imminent and is expected to be a dangerous threat to life and property. A Winter Storm Warning can also be issued at forecaster discretion when significant impacts are expected, but official warning criteria are not met.  A Winter Storm Warning can be issued at any time, typically up to 36 hours in advance, when there is an 80 percent chance or greater that a significant winter-weather event will occur.
  1. An Ice Storm Warning is issued up to 36 hours before an event if there is an 80% chance or greater that freezing rain will accumulate to ¼ inch or more.  Large amounts of freezing rain can cause tree limbs and power lines to snap, as well as significant travel disruptions.  It doesn’t take a lot of freezing rain or freezing drizzle to create hazardous driving conditions, even if an Ice Storm Warning is not issued.  As always, focus on the predicted impacts for your community rather than the amounts for adjacent areas.

✏️ Remember: A warning is “worse” than an advisory, and an advisory is “worse” than a watch. Focus on the impacts that the predicted precipitation type and accumulation will have on your community. Do not focus on whether or not the counties adjacent to yours are under a “worse” advisory type or if those counties are expected to receive greater accumulations. As we learned in during “Snow Jam ’14), even an inch or two of snow with temperatures in the 20s can have an enormous impact.

Product Criteria from the NWS in Birmingham
Product Criteria from the NWS in Birmingham

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