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March-April 2015

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Weather Queries

In winter, TV meteorologists often point to an equatorward dip in the jet stream on a map and say, “Here comes the cold air,” implying that this configuration of the jet stream is about to cause a cold air outbreak. I have consulted articles that say the jet stream is the result of differential pressures aloft, which are, in turn, caused by a large contrast in surface temperature over a short distance. This seems to imply that surface fronts cause the jet stream. I sense a contradiction here.

Ralph S. Brown

Terre Haute, Indiana

You are to be forgiven for being confused. Atmospheric fronts, characterized by rapid temperature changes over short distances, and jet streams, ribbons of high-speed air found in the high troposphere, go hand in hand, but the fronts need not be at the surface. Moreover, fronts and jet streams exert mutual influence over each other.

I first dealt with this topic in the February 1987 issue of Weatherwise, but will repeat one of the diagrams here. Suppose the existence of a long, east-west cold front, for which the figure gives a north-south vertical crosssection. The distance between A and B need not be very large, say 100 kilometers or less. The cross-section assumes that the surface pressure is constant (1000 mb) across the front—an approximation, but not a bad one because surface pressure is usually at a minimum along the front. In the colder air north of the front, the pressure decreases with altitude more rapidly than in the warmer air south of the front because the colder air is denser. That means that the 700-mb pressure surface cannot be horizontal; rather, it slopes downward from south to north. This also implies that, on a horizontal surface that cuts through the 700-mb surface, there will be a pressure gradient. The wind tends to blow toward lower pressure (toward the north), but in the Northern Hemisphere, the Coriolis force deflects the wind toward the right, generating a wind from the west.

The diagram supposes that the front exists throughout all three layers and that it is oriented vertically. Consider the next higher layer from 700 mb to 500 mb. This layer is deeper on the warm side of the front and thinner on the north side, again, because the air is colder and denser north of the front. Consequently, the 500-mb pressure surface has a greater slope than the 700-mb surface. The greater the pressure gradient, the stronger the wind, and so the west wind at 500 mb will be stronger than at 700 mb.

By the same argument, the west wind at 300 mb will be strongest of all, ultimately because of the north-south temperature gradient at all levels below. If the front is strong throughout the troposphere, the wind at 300 mb can easily exceed 150 mph, which is a strong jet stream. If the front is strong only at the surface and weaker aloft, or if there is a strong front in the mid-troposphere but not above or below, the jet stream near the top of the troposphere will not be as strong because the pressure surfaces there will not slope so steeply.

But this is not the whole story. At the outset, we assumed a long front stretching east-west. If the temperature contrast across a front becomes great enough, an instability develops along the front, resulting in the formation of a major bend or kink in the frontal position. Aloft, a wave trough forms in straight westerly flow and amplifies with time. A low pressure center forms at the surface, warm air rushes north ahead of the low, and cold air plunges south behind it. (This discussion pertains to the Northern Hemisphere.)

Troughs aloft and strong surface fronts do not always travel together, but when a vigorous trough in the westerlies overtakes a strong low-level frontal zone, a low can develop and deepen rapidly. In this case, traveling disturbances in the jet stream greatly influence the evolution and movement of surface fronts.

To summarize, atmospheric fronts, not just those strongest at the surface, are generally associated with strongly sloping pressure surfaces aloft, and the steepness of the slope at a given altitude determines the wind speed to a good approximation. Thus, strong atmospheric fronts are usually associated with a jet stream. But not all jet streams are associated with surface fronts. (The subtropical jet stream of lower latitudes is a prime example.) Short-wave troughs, traveling in the jet stream, can pass over low-level frontal zones and cause development of surface lows, and winds spiraling around the center of the low then rearrange the surface frontal positions.

I've noticed that often a storm will track across the country. Then, when it reaches the East Coast, it makes a left turn and heads up the coast toward New England. Why does this happen?

Bob Hall

Lexington, Massachusetts

You've described the classic Nor'easter, a low pressure system that can originate over the northeastern Gulf of Mexico, the southeastern United States, or along the North Carolina Coast. The low often deepens rapidly and develops into a powerful winter storm that either skirts the East Coast or occasionally moves inland. The storm is so named because it brings strong northeast winds north and west of its center. It affects large populations with coastal erosion, flooding, and sometimes heavy snow. But why does the track of these storms make a left turn when they near the coastline?

Most significant East Coast storms feature a trough (wave-like feature) in the jet stream that amplifies as it approaches the coast. In advance of most troughs, divergence is common in the high troposphere. In an imaginary column of air, divergence at high levels removes atmospheric mass. Provided that convergence at lower levels does not add as much mass as is being removed aloft, the surface pressure drops. A surface low forms, inducing low-level convergence. The high-level divergence and low-level convergence then travel as a couplet ahead of the trough.

If an upper tropospheric trough and its attendant surface low pass over a strong surface front, rapid storm development often occurs. In winter, there is a semipermanent front along the Atlantic seaboard. Warm Gulf Stream water lies near the coast from Florida to Cape Hatteras, North Carolina, and thence east-northeastward into the Atlantic Ocean. Cold air masses from Canada frequently sweep offshore bringing sharp land-sea temperature contrasts, sometimes reaching 40–50°F. Such contrasts are possible because the land surface temperature adjusts to the air temperature within hours, but sea-surface temperature adjusts very slowly, taking from days to weeks. The warm ocean is an immense heat reservoir that very effectively conveys heat to the overlying air, but itself cools little in the process.

Upper air divergence, a strong coastal front, and steep, low-level lapse rates (decrease of temperature with altitude) developing as cold air sweeps across warm water and is heated from below all contribute to rapid storm development. The storm track closely follows the coastal front, which parallels the Gulf Stream, hence the leftward curving storm track as the surface low reaches the coast.

An East Coast storm that developed on February 13–14, 2014, provides a good example. The figures, from NOAA's Storm Prediction Center, show conditions on the 925-mb and 250-mb pressure surfaces. The 925-mb pressure surface is roughly 2,000–2,600 feet above sea level; the 250-mb surface is roughly 33,000–35,000 feet above sea level. I chose these surfaces because the 925-mb charts show the strong low-level front, which the storm track followed, and the 250-mb charts show the position and strength of the jet stream.

At the initial time, 0000 UTC (Universal Time Coordinated—the time in Greenwich, England), February 13, 2014, a low pressure center is over the Florida Panhandle. (See Figure 1.) Twelve hours earlier, it lay to the west over southern Louisiana. Note the strong temperature gradient to the northeast of this center, stretching to the Virginia coast. This is the direction the storm will take. At 250 mb (Figure 2), a high-amplitude trough lies from Lake Michigan to southern Louisiana. Strong jet stream winds greater than 100 knots are common on both sides of the trough. Twelve hours earlier, the trough line lay from Minnesota to eastern Texas. This trough is on the move, and strong divergence at jet stream levels precedes it.

Figure 1:  Height in meters of the 925-mb pressure surface at 0000 UTC, February 13, 2014. Solid black lines are height contours at 30-meter intervals. Observed winds are plotted in blue, temperature in red, and dewpoint in green. The dashed lines contour the temperature at intervals of 2°C. Red contours are for temperatures above freezing; blue contours are for temperatures below freezing. Solid green lines contour moisture values where moisture is high. Few data are plotted in the West because most reporting stations lie above the 925-mb surface.

Figure 1. Height in meters of the 925-mb pressure surface at 0000 UTC, February 13, 2014. Solid black lines are height contours at 30-meter intervals. Observed winds are plotted in blue, temperature in red, and dewpoint in green. The dashed lines contour the temperature at intervals of 2°C. Red contours are for temperatures above freezing; blue contours are for temperatures below freezing. Solid green lines contour moisture values where moisture is high. Few data are plotted in the West because most reporting stations lie above the 925-mb surface.

Figure 2:  A 250-mb flow pattern at 0000 UTC, February 13, 2014. The soild black lines with small arrows are streamlines, which follow the wind. The closer the spacing of streamlines, the stronger the wind. Wind reports are plotted in blue with the staff pointing into the wind. Flags on the staff represent 50 knots; long barbs, 10 knots; short barbs, 5 knots. Wind speeds over 75 knots are color-coded as indicated in the color bar at the extreme left of the map below center. Observed temperature (°C) is plotted in red, dewpoint (°C) in green.

Figure 2. A 250-mb flow pattern at 0000 UTC, February 13, 2014. The soild black lines with small arrows are streamlines, which follow the wind. The closer the spacing of streamlines, the stronger the wind. Wind reports are plotted in blue with the staff pointing into the wind. Flags on the staff represent 50 knots; long barbs, 10 knots; short barbs, 5 knots. Wind speeds over 75 knots are color-coded as indicated in the color bar at the extreme left of the map below center. Observed temperature (°C) is plotted in red, dewpoint (°C) in green.

By 1200 UTC, February 13, the surface low has moved northeast, intensified, and is in eastern North Carolina (Figure 3). Near the coast, the surface temperature is in the 30s (not shown). The temperature of the Gulf Stream, just tens of miles offshore, exceeds 70°F. With such a strong temperature gradient along the coastline and the approaching upper level trough, the storm is sure to intensify further. At 250 mb (Figure 4), the trough line has moved over Georgia, Tennessee, and Kentucky, and has developed a negative tilt (like a backslash), in part, because a second trough and an accompanying surface low, over Minnesota, is catching up with the first one. This juxtaposition of troughs is not uncommon; it helps to deepen a coastal low more rapidly.

Figure 3:  925-mb chart for 1200 UTC, February 13, 2014. See Figure 1 for plotting details.

Figure 3. 925-mb chart for 1200 UTC, February 13, 2014. See Figure 1 for plotting details.

Figure 4:  250-mb chart for 1200 UTC, February 13, 2014. See Figure 2 for plotting details.

Figure 4. 250-mb chart for 1200 UTC, February 13, 2014. See Figure 2 for plotting details.

By 0000 UTC, February 14, the coastal low is centered along the Maryland–New Jersey coast and has deepened considerably (Figure 5). At the center, the height of the 925-mb pressure surface has dropped from 650 meters to 538 meters in 12 hours. The trailing surface low over Lake Superior and Wisconsin is catching up with the primary surface low and is beginning to merge with it. The 250-mb trough now has a pronounced negative tilt (Figure 6) as the short wave responsible for the Wisconsin surface low (admittedly hard to find) rushes southeastward toward the major trough position. A 150-knot jet maximum lies just off the mid-Atlantic coast, almost over the Gulf Stream.

Figure 5:  925-mb chart for 0000 UTC, February 14, 2014.

Figure 5. 925-mb chart for 0000 UTC, February 14, 2014.

Figure 6:  925-mb chart for 0000 UTC, February 14, 2014.

Figure 6. 925-mb chart for 0000 UTC, February 14, 2014.

By 1200 UTC, February 14, the coastal low has reached maximum development. The minimum height at 925 mb has fallen rapidly again to about 410 m (Figure 7). (The corresponding sea-level pressure is 975 mb.) A broad trough of low heights extends westward from the coastal low to the Great Lakes, the only remnant of the second low. At 250 mb (Figure 8), a broad trough dominates the United States east of the Mississippi River. The most pronounced curvature in the flow is over New England and is directly coupled to the surface low.

Figure 7:  925-mb chart for 1200 UTC, February 14, 2014.

Figure 7. 925-mb chart for 1200 UTC, February 14, 2014.

Figure 8:  250-mb chart for 1200 UTC, February 14, 2014.

Figure 8. 250-mb chart for 1200 UTC, February 14, 2014.

Well illustrated by Figures 7 and 8 is the coupling between the surface and upper level disturbances as they interact and deepen. The isotherm pattern in Figure 7 has deformed, with the cold air wrapping to the south around the low, and the warm air wrapping around to the north. So, although the surface low may approximately track parallel to the low-level isotherms, these isotherms are themselves being dramatically deformed by the circulation around the cyclone, even as it moves.

Storms this intense occur once or twice every winter. They are almost always associated with a fairly narrow, high-amplitude trough in the jet stream digging toward the southeast United States at a time when relatively cold air lies along the eastern seaboard, and air warmed by the Gulf Stream lies just offshore. The combination of a strong temperature gradient at low levels and a vigorous trough with divergence aloft approaching the frontal zone usually means a left turn and intensification of the surface low. If the trough in the jet stream is broad and has low or moderate amplitude, the accompanying surface low is more likely to move eastward over the Atlantic Ocean and not deepen appreciably, rather than make a left turn and deepen sharply.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut St., Suite 850, Philadelphia, PA 19106.

I thank John M. Brown of NOAA's Earth System Research Laboratory for help in answering these questions.

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