10.3: Tracking Mid-Latitude Cyclones

What are Mid-Latitude Cyclones?

Often, when people think of “cyclones,” Tornadoes, Hurricanes, and Typhoons come to mind. However, cyclones are common almost anywhere on Earth and impact large swaths of land on a near-daily basis. The rainfall that occurs across California in wintertime is caused by a type of cyclone called a “Mid-Latitude” cyclone (also known as an "Extratropical cyclone"), which lies beyond the tropics. These cyclones then travel across the United States and Canada, giving rain, snow, ice, and even thunderstorms to most of the region.

What are the Mid-Latitudes?

When we discussed El Niño in Investigation 9, we focused on the planet's Equatorial region, often referred to as the Tropics. The mid-latitudes, on the other hand, are further North (or South in the southern hemisphere) of the tropics. There are various definitions of the Mid-Latitude, but for this class, we’ll describe them as the region of the world between latitudes 30° North/South and 60° North/South. The mid-latitude region is prone to severe storms because it serves as a battleground for warm tropical air from the equator and cold polar air from the poles. These regions are highlighted in Figure 10.3.1.

Figure \(\PageIndex{1}\): General location of the mid-latitudes on Earth. (Public Domain; Titoxd via Wikimedia Commons). Alternative description of the image. 

What is a Mid-Latitude Cyclone?

The definition of a Mid-Latitude Cyclone sounds just like the name, a cyclone (low pressure), in the mid-latitudes. The big deal with them, however, is that cyclonic motion pulls warm air up from the tropics and pushes cold air down from the polar regions. These movements of air create fronts, and a set-up similar to that shown in Figure 10.3.2

Figure \(\PageIndex{2}\): The United States with a mid-latitude cyclone over Iowa. (CC-BY-SA 3.0; Phoenix2 via Wikimedia Commons). Alternative description of the image. 

15. From the hand-twist model for cyclonic flow, we know that winds will travel _____________ around the low pressure system over Iowa.
    1. clockwise and out of
    2. counterclockwise and into

You can print out or download and open Figure 10.3.2. Draw arrows on the map indicating how air is flowing around the low-pressure system. This will help you answer the following questions.

16. Warm air from the south is being pushed northward along which side of the mid-latitude cyclone? (Hint: where are the winds primarily coming from the south)
    1. The northwest
    2. The southwest
    3. The southeast
    4. The northeast

The region from question 16 is called the “Warm Sector” of the mid-latitude cyclone, and is responsible for bringing warm, moist air into the storm. This acts as the storm’s fuel.

17. Because much of the warm air is coming from the humid Gulf of Mexico, dew point temperatures in the warm sector should be relatively _______.
    1. high
    2. low
18. Cold air from the north is wrapping around the ______ side of the cyclone:
    1. the east-southeastern
    2. the western
    3. the southern
    4. the northern

The two black lines represent fronts. which act as boundaries between the warm sector and cold air wrapping around the back of the cyclone. The cold front, where cold air is moving in and warm air is moving out, extends southwest from the low-pressure system through Kansas, Oklahoma, and Texas. In contrast, the warm front, where warm air is moving in and pushing cold air away, extends East from the low pressure through Illinois, Michigan, and Pennsylvania.

Now, let’s assume the low-pressure cyclone is traveling due east, which is common for mid-latitude cyclones following the Jet Stream. As the storm travels east, the two fronts spin counter-clockwise like a pinwheel (or a propeller) with the low pressure at the center. As the fronts rotate around the low-pressure system, the warm front will likely be traveling north of its current position. As a result, Michigan and Wisconsin can expect warming over the next few hours as the warm front passes, while Colorado and Wyoming can expect cooling as the cold front passes. As the cold front advances, it forces warm, moist air from the warm sector to rise, producing tall storm clouds and precipitation.

19. While both the cold and warm fronts spin around the low-pressure system, the cold front actually moves faster than the warm front. As a result, the warm sector of the storm will _______ overtime.
    1. grow
    2. shrinks
20. Eventually, the cold front catches up to the warm front, a process known as occlusion. This cuts the storm off from warm, moist air, causing the storm to:
    1. Become stronger
    2. Weaken and eventually dissipate.

Figure 10.3.3 is a satellite image of the Continental United States, taken at 1651 UTC (9:51 am Pacific Time, 12:51 pm Eastern Time) on July 29, 2019. Notice the long white band of clouds draping across the Upper and Middle Mississippi valleys, with a comma-shaped spiral over the Wisconsin-Minnesota border.

Figure \(\PageIndex{3}\): GOES-East Visible Satellite for the Continental United States, taken at 1651 UTC on July 29, 2019. (Public Domain; GOES-East). Alternative description of the image.

An animation of the storm’s movement can be found here. An animation zoomed in over the Great Lakes can be found here: Close-up of animation

21. The cold front, or the long line of clouds extending from Michigan down to Oklahoma was mainly moving _________ during the time of the animation.
    1. east   
    2. west

Figure 10.3.4 is a map of surface observations taken at 1200 UTC on July 29, several hours before the satellite picture and animations from Figure 10.3.3.

Figure \(\PageIndex{4}\): Surface Weather Conditions for the United States at 1200 UTC on July 29, 2019. (CC BY-NC 4.0; American Meteorological Society via Unidata). Alternative description of the image. 

While the mid-latitude cyclone being investigated will not have major impacts on St. Louis’ temperatures, it should be noted that this cyclone is present during summertime, when the difference between the warm air ahead of a cold front and the cold air behind one isn’t large.

22. At 1200 UTC, the low-pressure center of the mid-latitude cyclone we are investigating was over:
    1. Southern Texas   
    2. Southern Canada and Lake Superior
    3. The southeastern United States
23. The bulk of the precipitation associated with this cyclone was located:
    1. Ahead of the warm front
    2. In the warm sector
    3. Behind the cold front

Saint Louis, in Eastern Missouri, directly under the large swath of rain, has an air temperature of 79°F and a dew point of 68°F. While it’s hard to read, at 1200 UTC, winds out of St. Louis were blowing from the Southwest at 10kts. Because these storms bring heavy rainfall and sudden temperature changes, weather forecasters want to predict their movement. Figure 10.3.5 is a map of the 500 mb winds and contour lines, derived from radiosonde (weather balloon) launches at the same time as Figure 10.3.4 (1200 UTC on July 29, 2019), with a red "L" superimposed to indicate the location of the mid-latitude cyclone. Think of the solid blue lines as the edges of a river, and the wind barbs as a guide to where the current is coming from.

Figure \(\PageIndex{5}\): 500 mb Winds, isotherms, and height contours taken at 1200 UTC on July 29, 2019. (CC BY-NC 4.0; American Meteorological Society via Unidata). Alternative description of the image. 

24. The approximate surface location of the center of the mid-latitude cyclone has been indicated by a red 'L'. Based on the stream's flow around the cyclone, and by the wind barb direction over northern Michigan, the storm is likely to travel _______ over the next 12 hours.
    1. northeast   
    2. southwest   
    3. southeast   
    4. northwest

Figure 10.3.6 is a map of surface weather conditions taken at 0000 UTC, 12 hours after Figures 10.3.4 and 10.3.5.

Figure \(\PageIndex{6}\): Surface Weather conditions at 0000 UTC on July 30, 2019. (CC BY-NC 4.0; American Meteorological Society via Unidata). Alternative description of the image. 

25. Over the past 12 hours, the center of the mid-latitude cyclone ______ move in the direction predicted in question 24.
    1. did not
    2. did 

While the mid-latitude cyclone being investigated did not have major impacts on St. Louis’ temperatures, it should be noted that this cyclone is present during summertime, when the difference between the warm air ahead of a cold front and the cold air behind one isn’t large.