One of the primary roles of the National Weather Service in fulfilling their mission to “protect life and property” is to issue warnings during severe convective (thunderstorm) events. One of our roles in the media is to pass along that information on the larger scale to help keep you safe.
In full disclosure, I may not always agree with the warnings issued by the NWS. Occasionally there are storms that my analysis indicates should have a warning, or there are times when a warning is issued, and it appears to me that the storm is just under severe levels. That said, I’ll always pass that warning information on, and those times I disagree tend to be few and far between, as meteorologists with similar degrees and experiences will often reach a similar conclusion. It’s those borderline times when a storm is on the cusp of being severe that agreement may not always occur.
That said, taking a look at where warnings are issued can be very eye-opening as to the climatological pattern of severe weather in southern Colorado.
The maps I’m showing you below are from a great data visualization website at Iowa State University. I’m displaying the shape of severe thunderstorm and tornado warning polygons (that’s what we call them in the weather community) on top of each other since the beginning of the year.
Doing this reveals some interesting patterns about our thunderstorm formation and intensification zones and in a roundabout way how surface moisture (water vapor) flows into Colorado.
Take the severe thunderstorm warnings below. If we focus on southern Colorado we see a couple areas of higher warning frequency. Area 1 is over the Palmer Divide in Elbert County. This area sits on the “hill” that separates the South Platte River Valley coming out of Denver and paralleling Interstate 76 from the Arkansas River Valley that runs from Cañon City to Lamar and Kansas along US Highway 50.
This hill you drive over between Colorado Springs and Denver sits at about 7,600′ at it’s highest point. The extra elevation compared to downtown Colorado Springs (~6,000′) and Denver (5,280′) provides upslope lift on its southern slopes most summer afternoons. This air moving from SE to NW from the Arkansas River to the crest of the Palmer Divide is being lifted as it moves. This lift is often enough to help produce the beginning of the convective process that produces thunderstorms. This area typically has higher moisture available at the surface than surrounding areas as the moisture will pool along the southern flank of this important hill. The combination makes the area a little more likely to experience severe thunderstorms than other areas, even on days when all other atmospheric variables are the same.
Area 2 is in Baca County in the southeast corner of Colorado. This area has a couple pockets that have been under a severe thunderstorm warning between 11 and 15 times so far in 2019. There are a couple reasons for this. The area is closer to the Gulf of Mexico than any other part of the state. Closer proximity means it is often the area to have the deepest surface moisture, or the area to receive an increase first. Combined with the Raton Mesa a little farther to the west (an area that tends to initiate thunderstorms similar to the Palmer Divide Area).
Area 3 is east of the average location of the dry line, which separates the dry desert air to the west from more moisture-laden air that sits over the Great Plains. Areas east of the dry line often experience more wind shear too, thanks to a strong southerly wind that develops in the late afternoon and evening from eastern Colorado all the way east to Missouri. This wind, called the low-level jet, helps spread Gulf of Mexico moisture north to fuel the storms.
This graphic displays the most reported tornado warnings for Colorado. The yellow area is north of the Arkansas River but some of the tributaries of the Arkansas often funnel moisture into this area. The rivers help funnel low-level moisture into eastern Colorado and cause strong updrafts that are necessary for tornado formation. In general, you see that most tornado warnings tend to be over the eastern 1/4 of the state. This is pretty typical too. In general, the farther east you go in our state the more moisture for storms there is. The more moisture, the stronger the updraft.
The lighter blue area farther to the west in Elbert County is located over the higher elevation of the Palmer Divide. The region often experiences tighter wind shear in the lower portions of the atmosphere and can help to produce funnel clouds and tornadoes. So far in 2019, the area around the town of Elbert has been under four tornado warnings.