Tuesday, February 3, 2015

U.S. Winter Weather Advisories and Warnings

How wintry is it where you live? Well, that is a tough question to answer. As with many measures of comfort, it really is in the eye of the beholder. Ever since I discovered the NWS forecast product archive, I have wanted to look at NWS forecasts as a proxy measure of wintry-ness.


In 2005, the National Weather Service (NWS) made a significant change to the way they issue watches and warnings. They introduced the VTEC coding system to standardize the procedure for issuing these products. The VTEC code allows various software packages to quickly decode the forecast product and display it on a map or transmit the information to subscribers and the public. Figure 1 shows a sample forecast product with the VTEC code outlined in red. The VTEC line notes that it is operational, a new issuance, from the Fairbanks NWS Office, a winter weather advisory, and the start/end times are noted.

Figure 1. Winter Weather Advisory issued by the Fairbanks, Alaska, NWS office on January 1, 2015. The VTEC is outlined in red.

The University of Iowa generously logs all NWS forecast products from every office in the country. Their archive site is quite handy for analysis of historical data. Unlike many data archive sites, they are perfectly happy to let you grab data using your own scripts and other tools. They even provide some sample Python scripts for you to use. Thank you Daryl Herzmann!

Winter Weather Products

In this analysis, I am interested only in winter weather products issued by various NWS Offices. Each NWS Office is responsible for all forecasts within their zones (except for severe thunderstorm and tornado watches). Figure 2 shows the NWS Offices in the U.S. and Figure 3 shows the forecast zones for the U.S. For most of the U.S., forecast zones share boundaries with counties. However, this is not always the case.

Figure 2. Map of all NWS Offices in the U.S.

Figure 3. Map of all NWS forecast zones in the U.S.

Local Criteria

With the exception of Blizzard Warnings, the criteria for every winter weather product is defined by the local NWS office. For example, the Wind Chill Advisory criteria for most of Alaska requires wind chill value of -40° or lower with a sustained wind of 15 mph or higher for three or more consecutive hours. In southern Florida, a wind chill of +35°F is sufficient for issuance of a Wind Chill Advisory. Therefore, southern Florida has seen more Wind Chill Advisories than most of Alaska! Unfortunately there is not a single repository for local advisory criteria. The NWS Central Region and NWS Alaska Region have winter criteria summaries but the other regional offices no not. Therefore, a comparison of the number of advisory and warning products is an apples to oranges comparison – but it is fascinating nonetheless. 


In this section we present eleven (11) maps of winter weather advisory products. For all maps, we use the 2009-2014 time period. The reason for this is because significant changes were made to the suite of forecast products in 2008. For example, Snow advisories and Freezing Drizzle Advisories were consolidated into Winter Weather Advisories. Due to these changes, I thought it best to start in 2009 instead of parse out the pre-2009 data. Please keep in mind that these are forecast products. No attempt has been made to assess the accuracy of the forecasts.

And here are the maps .....

Figure 3. Combined number of Frost Advisories, Freeze Warnings, and Hard Freeze Warnings between 2009 and 2014.

Figure 4. Number of Freezing Fog Advisories between 2009 and 2014.

Figure 5. Number of Freezing Rain Advisories between 2009 and 2014.

 Figure 6. Number of Ice Storm Warnings between 2009 and 2014.

Figure 7. Combined number of Wind Chill Advisories, Wind Chill Warnings, and Excessive Cold Warnings between 2009 and 2014.

 Figure 8. Number of Winter Weather Advisories between 2009 and 2014.

Figure 9. Combined number of Lake Effect Snow Advisories and Lake Effect Snow Warnings between 2009 and 2014.

Figure 10. Number of Winter Storm Warnings between 2009 and 2014.

Figure 11. Number of Blizzard Warnings between 2009 and 2014.

Figure 12. Total number of winter precipitation-related advisories (Winter Weather, Lake Effect Snow, and Freezing Rain) between 2009 and 2014.

Figure 13. Total number of winter precipitation-related warnings (Winter Storm, Lake Effect Snow, Ice Storm, And Blizzard) between 2009 and 2014.

There are far too many patterns in the maps to mention. The patterns are a result of climatology, forecast criteria, and forecaster judgement. Is there anything that surprises you?

Note: While I attempted to accurately portray the data depicted above, I cannot guarantee it is error-free.


  1. Very interesting maps. I expected to see more breaks along the NWS boundaries. One suggestion, your maps would be easier for readers to interpret if you used sequential color pattern instead of the diverging colors. Initially, the oranges and reds made me think they were higher in numbers. You can see some great color schemes here : http://colorbrewer2.org/

    1. Thank you for the comment. Indeed, choosing colors is one of the most difficult decisions in cartography. I spent quite a while playing with various color combinations. In the end, I used the one in the maps for a couple of reasons. First, since these are winter maps, I wanted to have "cold" colors represent the most wintry conditions. I have a lot of data for other advisory types too (e.g., heat advisories, dense fog advisories, etc.) and wanted the winter maps to look different than the other map set(s). Second, my experience when using graduated colors (e.g., light blue to dark blue) for the entire range is that it becomes difficult to distinguish between consecutive color values. As much as possible, I wanted to allow readers to identify the forecast zone they live in and match it with a legend category. For those readers that have had a cartography class, the first page of most textbooks describes cartography as "the art and science of map making." In a lot of ways, the art part is the most difficult to nail down.


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