Chapter 8 TRACKING A STORM ACROSS THE COUNTRY Goals: To use surface pressure and temperature maps, 500-mb height and absolute vorticity maps, and satellite images to follow the development and maturation of a mid-latitude low pressure system and attendant fronts across the United States.

Data:

Satellite images and surface and upper-air maps from an extratropical cyclone that moved across the United States from April 30, 1997 to May 3, 1997.

The "Sfc Pressure/Temp" maps show isopleths of both sea-level pressure (in millibars) and temperature (in degrees Fahrenheit). Isobars are the solid lines, with labels on the left side, while isotherms are dashed, with labels on the right side. Maxima and minima of both fields are also labeled. Some additional labels are also shown to aid in answering some questions.

The "500-mb Height/Vorticity" maps show isopleths of both 500-mb height (in meters) and absolute vorticity (in 10^-5 per second). Contours are the solid lines, with labels on the left side, while "isovorts" (lines of equal absolute vorticity) are dashed, with labels on the right side. Maxima and minima of both fields are also labeled. Some additional labels are also shown to aid in answering some questions.

To help students complete the exercise efficiently, the questions associated with a particular time are also shown below the maps for that time.

Procedure:

Though there is an impressive cyclone over the Great Lakes in late April (see 18Z April 30 visible satellite image and 00Z May 1 infrared image), this exercise will focus on a system that traversed the nation from the Western states to the East during the first few days of May 1997.

1. On the 500-mb height/vorticity map at 00Z on May 1, a trough of low pressure has pressed inland over Washington and Oregon from the Pacific Ocean. It is marked by a closed center of maximum absolute vorticity (maximum value 17x10^-5 sec^-1). Note that the trough is not easily detectable just by looking at the 500-mb heights - the lines do not sag equatorward in the typical way we normally think about troughs. Keeping this in mind and also recalling the definition of relative vorticity, which effect do you think is more important in generating this vorticity maximum at this time - curvature or horizontal wind shear (see Figure 8.20)?

2. Where would you look to find the surface low pressure system associated with this absolute vorticity maximum - to its east or to its west? Briefly explain.

3. Because of the disruptive nature of the Rocky Mountains and the inherent problems involved in correcting station air pressures to sea-level pressure, the surface low is not readily apparent on the surface map at this time. So let's try to find one! First note the 1010-mb isobar ridging east into Idaho from the high pressure center just off the coast from the California-Oregon border. Also note the 1009-mb pressure reading along the Utah-Colorado border. This barometric reading is slightly higher than the 1008 isobar just to its south. So there's a relative high pressure in Idaho and a relative high pressure along the Utah-Colorado border. So it seems logical that there's relative low pressure between them. Near what large city in northern Utah would you place this relatively weak surface low? Is this low in the general region you thought it would be in the previous question?

4. As a check to your answer in the previous question, look at the isotherms sagging slightly south over northern Nevada (the 55^oF, 60^oF, and 65^oF isotherms) and the same three isotherms bulging slightly north over Utah on the surface pressure-temperature map. These waves in the isotherms, although subtle, suggest cooler, more northerly winds over northern Nevada and milder, more southerly winds over Utah. Would such a circulation of winds be consistent with a surface low near the city in the previous question? Briefly explain your answer.

1. On the surface map, the surface low is now apparent. On the 500-mb height-vorticity map, the trough is also now apparent (there is a definite sag equatorward in the 500-mb heights over the Rockies). Do you think curvature has become more or less important in generating relative vorticity associated with the absolute vorticity maximum (of magnitude 16x10^-5 sec^-1 in eastern Idaho?)

2. Before we leave the 500-mb height-vorticity map, there's another vorticity center taking shape over the West, but it's not marked yet! Let's see if we can find it! What is the value of the isovort going through northwestern Arizona? Is the value of the absolute vorticity near the juncture of the Nevada, Utah and Arizona borders larger or smaller than the value of this isovort? Thus, conclude whether the region in northwestern Arizona that is bounded by the dashed line is a relative maximum or relative minimum in vorticity. Keep this in mind as we go to the next set of maps.

3. Use the surface map to determine where the largest temperature gradient is in Nevada and Utah: in the northern or southern half of these states? Recall that cold fronts are typically placed at the leading edge of the cold air (that is, on the warm side of the zone of largest temperature gradient). Is the linear cloud formation on the infrared satellite image at this time consistent with where the cold front should be placed?

1. First, let's look at the surface map. The low that was along the Wyoming-Colorado border on the surface map for 12Z May 1 has lost some of its identity as it moved into the Nebraska panhandle (it's no longer a closed low, but is merely part of a surface trough extending over the southern high plains). But there is another more impressive low taking shape over the Texas panhandle that has apparently become dominant. In what state is the new low's associated vorticity maximum? What is the maximum value of absolute vorticity of this vorticity maximum? Does this result follow logically from your answer to the second question of the previous section?

2. The low over the Texas panhandle developed rapidly. Look at the visible satellite image at 18Z on May 1. Does the weather appear to be active over Oklahoma (that is, do you see any evidence of thunderstorms)? Then take a look at the infrared satellite image at 00Z on May 2 (just 6 hours later). Is the weather now more active? Briefly explain your answer.

3. Can you make a broad generalization about a "weather symptom" of rapidly deepening lows over the southern Plains in spring?

12Z May 2

1. Having moved out of the Rockies and onto the Plains, the storm has now consolidated into a single center of surface low pressure with one, definitive vorticity maximum at 500 mb. Where would you place the center of this vorticity maximum (give a specific location)? Give a reasonable value of the absolute vorticity for this maximum.

2. Are the relative positions of the surface low and 500-mb trough consistent with the theory that you've learned in this chapter? Briefly explain.

3. On the infrared satellite image, look at the bright white clouds over the western Great Lakes region. Keep in mind that the surface low and the warmest air associated with the system lie far to the south. What type of clouds do you think exist over this region: cirrus or cumulonimbus? What lifting mechanism do you think is primarily responsible for generating these clouds: overrunning, cold-frontal lifting, or surface-based convection (recall the lifting model of a low pressure system in Figure 8.15)?

4. What type of clouds (cirrus or cumulonimbus) do you think have formed in the extreme northeastern corner of Oklahoma and also southern Missouri? What type of front is likely playing a role in generating these clouds? Is this cloud type consistent with the availability of moist air from the Gulf of Mexico? Briefly explain, using the direction of surface winds over northeastern Oklahoma and southern Missouri to aid in your answer.

1. Recall that fronts are typically found in surface troughs. Thus, the cold front at this time extends from the surface low in northeastern Missouri to ___________ Arkansas and ____________ Texas, and then southwestward to the central Rio Grande Valley of Texas (fill in the blanks with a geographical reference for each of the states; for example, northeastern or southwestern or central).

2. On the infrared satellite image, the vee-shaped thunderstorms in northern Louisiana are a signature of severe weather. Such potent thunderstorms derive energy from fast jet-stream winds. When updrafts of air rising within such storms reach the tropopause (the boundary, or layer, between the troposphere and the stratosphere, characterized by temperature remaining nearly constant with height) and start to spread out horizontally, they have relatively slow momentum compared to speedy high-altitude winds. Like a "Sunday driver" on an interstate, the horizontally spreading air that was once part of an updraft acts like a roadblock to the faster high-altitude winds. So these fast jet-stream winds divert around the Sunday driver, forming a vee-shape that forecasters often use as a clue to detect severe thunderstorms. Now note that the vee-shaped thunderstorms have erupted AHEAD of the cold front (see cold-front placement in problem #1). If you also look at the 12Z infrared image on May 2, you will see that northeastern Louisiana received some sunshine earlier in the day. Also note that warm (and moist) air was available near the ground (see surface temperature at 00Z for verification). With these factors in mind, give an upper-air mechanism that would have provided favorable lifting for warm, moist parcels to rise high into the atmosphere (hint: consider where the thunderstorms have developed in relation to the approaching 500-mb trough).

3. If you were to draw a dashed line through the 500-mb trough STARTING with the middle of the sag in the 5520-meter height line over eastern Kansas and EXTENDING to north-central Texas (use the sag in the isovorts to help you as well), the trough axis would slope from the _____________ to the ________________ (give compass directions here: choose your answers from N, NNE, NE, ENE, E, ESE, ... , WNW, NW, NNW). Such a slope is said to have a "positive tilt". Later, we will see that a "negative tilt" in the 500-mb trough axis is associated with rapid deepening of the surface low. Stay tuned.

4. In the meantime, we don't want to ignore the low's warm front. Use the surface map to answer the following: the warm front associated with the low passes near what large city in southern Ohio?

1. Now look at the 500-mb trough. STARTING with the middle of the sag of the 5460-meter height line, mentally draw a line through the trough axis into northern Alabama (again, use the sag in the isovorts to help you place the trough axis). Now the trough axis tilts a bit from the ______________ to the ______________ (use the same directional headings as in question #3 in the previous section). Such a slope in the 500-mb axis is called a "negative tilt".

2. Note the lower central pressure of the surface low over the eastern Great Lakes (compared to 00Z on May 3). Obviously, the low has deepened by a few millibars. It seems that the change in tilt of the 500-mb trough axis went hand-in-hand with the surface low's intensification. Apparently, the "negative tilt" of the 500-mb axis provided increased _________________ aloft that helped the low to deepen.

3. Based on what you've learned previously, and using the infrared satellite image, determine where the most potent thunderstorms are likely occurring at this time. What mechanisms (both surface-based and aloft) are helping to drive these thunderstorms?

4. Note the classic "comma shape" to the clouds associated with this system on the infrared satellite image. The "comma head" over the Great Lakes region is primarily formed by the cold-conveyor belt (see text) wrapping counterclockwise around the low. Based on the infrared image, how far to the south and west of the surface low do the low clouds associated with the cold conveyor extend (just give the name of a state for your answer)?

1. The first thing to note is that the large 500-mb trough once associated with the surface low (which is located along the New York-Canada border at this time) is hanging too far back over the Great Lakes to provide much in the way of upper-level divergence for the surface low. This begs the question: how can this 996-mb low exist without divergence aloft? The answer, of course, is that the low HAS some upper-level support. Where is the vorticity maximum that is providing some divergence aloft for the surface low (give the vorticity maximum's location and its maximum value of vorticity)?

2. Is there a clear-cut trough in the 500-mb height lines associated with this vorticity maximum? If not, which term (curvature or horizontal wind shear) must be hard at work generating the necessary cyclonic vorticity?

3. Are the surface low and this 500-mb vorticity maximum nearly "vertically stacked"? That is, does it appear that this vorticity maximum is getting close to being directly above the surface low?

4. Based on the relative positions of the surface low and this 500-mb vorticity maximum at this time, do you think that divergence aloft over the low has become less or more optimal over the last 12 hours?

5. Keeping your answers to these last two questions in mind, do you expect this surface low to weaken over the next 12 hours (that is, would you expect its central pressure to rise)?

1. At this time, the surface low is centered near the St. Lawrence River Valley to the north-northwest of Maine (the center of the low lies off the map, but you should be able to answer the following question based on the value of the last isobar partially shown in the upper-right corner of the map). Determine whether the central pressure of the low is less than, greater than, or about the same as (defined for our purposes as within 2 mb of) the central pressure that was observed at 00Z on May 4?

2. Looking at the 500-mb height/vorticity fields at this time, can you explain why the low didn't weaken as much as you might have thought it would (recall your answer to #5 at 00Z May 4.) Hint: Base your discussion on the main 500-mb trough, which, 12 hours earlier, appeared to be hanging far west of the surface low. Unlike previously brief answers, your explanation will likely take a couple of sentences to describe what happened and why.