Multi-level analysis

Low pressure systems often form, mature, and dissipate predictably. On occasion, though, they form and grow in unexpected ways that catch meteorologists and sophisticated supercomputer models by surprise. This unpredictability is attributable to the Earth’s dynamic atmosphere, where the interaction of weather elements is complex and highly volatile.

Understanding lows takes practice, and a good way to build experience is by regularly studying weather charts and examining low development and movement. This will give you a feeling for the typical life span of a low pressure system. Below are some general rules for low behavior.

· Surface low pressure systems generally form beneath, and to the east of, an upper level (jet stream) trough, as this is where the two ingredients (dry cold air and warm moist air) needed for a low to grow are brought together and mixed.

· Low pressure system development is affected by both the strengths of upper level winds and the amplitude (north-south development) of upper level troughs. Similar to a fairground roller coaster ride, high trough wind speeds and steep amplitudes produce well-developed systems.

· Surface lows follow a path and move at speeds influenced by jet stream flow. Forward motion for lows is one-half to one-third of jet stream speed, with a low’s path parallel to the jet stream. For example, 60 knots of jet stream speed will move a surface system at 20 to 30 knots.

· On 500-mb charts the 5,640 meter height contour line is considered the “storm track” and represents the prevailing path followed by strong (gale and storm force) surface lows.

Two types of weather charts are used for basic marine weather analysis and forecasting: 1) upper-level charts (500 mb), and 2) surface charts (SFC).

Upper-level charts show the speed and direction of jet stream flow and show the location of upper-level troughs and ridges. Surface charts provide the location of lows (L); highs (H); fronts (cold, warm, stationary, occluded); wind (speed and direction); cloud cover (type and precipitation); and barometric pressure (millibars). When viewed together, upper-level and surface charts afford a dimensional perspective to weather systems.

In the following four-day sequence of weather charts, multiple low pressure systems form and move across the U.S. East Coast. These lows are typical of spring and fall systems and the type likely to be experienced by voyaging sailors Monday, November 3, 1997. 1) 500-mb Analysis 03 November 1997 (1200Z): A well-defined upper-level trough (dashed line and two large Ls) sits over the eastern U.S., extending from the Great Lakes to the Gulf of Mexico. A slightly northeast/southwest tilt indicates it is a new trough, as troughs move from an NE/SW orientation to NW/SE as they develop. Maximum wind speed shown in the jet stream is 105 knotsoccurring just off the East Coast over the Gulf Streamwith a minimum of 55 knots. Jet stream speeds greater than 100 knots are significant, as surface lows that form under strong jet stream influence often develop to gale or storm strength (Force 7 and 8). Moisture is also needed to fuel a low, which makes areas over warm ocean currentssuch as the Gulf Streamworth examining closely.

2) Surface Analysis Chart 03 November 1997 (1200Z): Several low pressure systems sit under the upper-level trough shown on the November 3 500-mb chart. Note the “new” 1012-mb gale forming near 35° N and 70° W. This new low is almost directly under the 105-knot jet stream flow. Note also the wind arrows in the vicinity of the new low, showing counterclockwise flow, with wind speeds of 30 knots on the low’s east side. Barometric pressure is noted in an abbreviated form using three digits located just to the right of each wind arrow. For example, 155 indicates a barometric pressure of 1015.5 mb, and 959 represents a pressure of 995.9 mb. Wind direction and speed change dramatically across frontal boundaries, with wind gusts often being 50% higher than the average wind speed. A 30-knot average wind could easily gust to 45 knots. Strong clockwise flow around the 1039-mb high centered in the middle Atlantic (48° N and 45°W) is adding to wind strength around the new low as is seen by the tight isobar lines found near 40° N and 62°W.Tuesday, November 4, 1997

3) 500-mb Analysis 04 November 1997 (1200Z): Note how the trough described for November 3 has swung a bit to the east and now lies in a more north-south orientation. Its amplitude and wind strength have decreased slightly from November 3, indicating that surface lows under the trough may not develop to the strength suggested by yesterday’s jet stream flow. Strong jet stream winds110 knotsare now over Newfoundland, with winds over the mid-Atlantic coast at 75 knots.

4) Surface Analysis 04 November 1997 (1200Z): A defined surface low centered near 38° N and 70° W is sitting under the jet stream’s main flow. A warm front extends to the northeast from the low’s center and a cold front arcs to the south. Wide separation of these two fronts indicates that this low is new and still developing. Note also the second 1013-mb low just to the west of the 1014 low. This second low is also supported by the jet stream trough over the East Coast. Central pressure in the 1014-mb low is predicted to drop three mb, to 1011, during the coming 24 hours, as shown by the arrow and underlined numbers 11. Words DVLPG GALE indicate this low will be developing to gale strength (Force 7) as it moves to the NE. A three-mb change over 24 hours is a slow drop, so this low should not rapidly intensify. (A pressure drop, for example, of one mb per hour signifies a very strong and rapidly developing low likely to develop to storm-force strength in 12 to 24 hours.) A high pressure system in the mid-Atlantic has weakened slightly since yesterday, now showing a 1036-mb central pressure, but it is predicted to rise again to 1037 mb. A second high pressure system to the west of the developing 1014-mb low is predicted to grow in strength over the next 24 hours, going from 1024 mb to 1033 mb. Both highs, with their clockwise flow, enhance the counterclockwise flow around the 1013- and 1014-mb lows. Note sharp bends and tight isobar lines in areas where H’s and L’s abut. These areas will experience significant shifts and increases in wind direction and strength.Wednesday, November 5, 1997

5) 500-mb Analysis 05 November 1997 (1200Z): Having weakened, the trough that yesterday was over the East Coast has moved to the NE and is now over Nova Scotia. An area of zonal (flat) jet stream flow dominates the East Coast. Wind speed aloft has diminished significantly, with a maximum of 50 knots and minimum of 35 knots. However, a new trough presently sitting over Texas (shown by a dashed vertical line) will be working its way east, propelled by the jet stream.

6) Surface Analysis 05 November 1997 (1200Z): As anticipated, the 1014 surface low has moved quickly to the northeast, with its cold front overtaking the warm front, forming an occluded front that is wrapping counterclockwise behind the low’s center. Surface wind speeds range from 25 to 35 knots. Note the close isobars and overcast conditions along the warm and cold fronts. These are areas where winds are normally strong and gusty, and precipitation heaviest. Wind shifts are also experienced across these fronts, shown by bends in isobar lines. The weaker 1010-mb low is being absorbed (ABS) by the dominant 1011-mb low. As predicted, this 1011-mb low has developed to gale strength, with winds shown at 35 knots. In the next 24 hours this low is predicted to mature into a storm (DVLPG STORM). A 1032-mb high pressure area sits over the East Coast, and, as expected, a new 1014-mb low is forming under the Texas trough.Thursday, November 6, 1997

7) 500-mb Analysis 06 November 1997 (1200Z): The trough over Texas has moved east and grown, with winds to 60 knots, and significant north-south amplification. This trough stretches from the Great Lakes to the Gulf of Mexico, resembling the trough seen on November 3. Notice, though, this trough does not have strong winds north of the 5640 contour line. There is actually an area of light jet stream winds over the Great Lakes region. Light winds are often a precursor to the formation of a “cut-off” low, which is an area in the jet stream that separates from the main flow. Cut-off lows often cause surface systems to stall or become quasi-stationary. Also shown on this chart is a continued strong zonal flow north of 40°N, between 30° W and 70° W, signifying rapid west-to-east movement of surface weather systems.

8) Surface Analysis 06 November 1997 (1200Z) Two low pressure systems are developing under the upper level trough shown on the 500-mb chart. A 1012-mb low near 30° N and 75°W is shown moving north and developing to a gale (DVLPG GALE) in the next 24 hours. Its central pressure is forecast to drop six mb, from 1012 to 1006. Note kinked isobar lines around this surface low as counterclockwise flow develops. A second, 1014-mb low to the west is moving southeast and predicted to deepen to 1012 mb within the next 24 hours. Both lows have distinct warm and cold fronts, counterclockwise circulation, and paths that follow jet stream flow. Jet stream winds above these lows are approximately 35 knots, so the lows will move at 12 to 17 knots, and cover 348 miles in the next 24 hours: (12 knots + 17 knots)/2 = 14.5 knots x 24 hours = 348 miles. When these lows have moved off to the east, a ridge of high pressure will build in, bringing clear, sunny, and cooler weather, followed then by another upper trough and associated surface lows.

This pattern of alternating troughs and ridges, with accompanying lows and highs, is the nature of the Earth’s atmosphere. Each ridge and trough are a variation on a theme, and so time spent studying and comprehending weather patterns, particularly lows, is an investment certain to pay off in accurate analysis and forecasts.

By Ocean Navigator