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Old 10-23-2003
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Power from Above—the Jet Stream


The jet stream influences the weather that most sailors experience, and understanding its patterns can help you predict its behavior.
Ocean waves, gales and storms, high-pressure systems, and hurricanes, all have a common bond: each derives energy from an undulating, magic carpet of air flowing around the earth at an altitude of just a few miles. This carpet of air, known familiarly as the jet stream, carries tremendous energy and uses it to develop and move surface weather features. Where the jet stream flows is where the weather flows, and so answers to our weather analyses and forecast questions truly lie blowing in the breeze.

In marine weather, meteorologists represent jet-stream flow using a graphic product called the 500-millibars chart. Where does this often-intimidating name come from? Well, pressure on the earth's surface is 1,013 millibars and as you depart the earth's surface and move up into the atmosphere, jet-stream winds are felt when you reach an altitude of approximately 18,000 feet or 3.5 miles. When you arrive at this altitude, barometric pressure is 500-millbars and so weather maps showing wind flow at this altitude are called 500-millibar charts.

An altitude of 3.5 miles is not very high when it comes to moving energy from the jet stream down to the earth's surface and so momentum aloft translates to events on the earth's surface. Now, here is a fascinating aspect of the jet stream: it undulates north and south as it moves around the earth. Where it undulates toward the North or South Pole, it induces a clockwise (northern hemisphere) and counterclockwise (in the southern hemisphere) flow pattern, which support the clockwise or counterclockwise flow of high-pressure surface systems.


The bold line here represents the flow of the jet stream over North America.
Where the jet stream undulates toward the equator, its counterclockwise (northern hemisphere) and clockwise (southern hemisphere) flow support surface low-pressure systems. The term "ridge" is given to poleward undulations which support surface high pressure, and "trough," describes those undulations that parallel the equator supporting surface low-pressure systems.

Patterns in jet-stream flow change slowly, taking days and often weeks. And it is this slowly changing pattern that allows its flow to be used as a planning tool for routing. By noting where ridges and troughs are forming and moving, you can anticipate the formation and movement of surface highs and lows and their associated fronts. When jet-stream winds are flowing primarily west to east, which is called zonal flow, surface weather systems also move west to east. But when jet-stream flow is more north to south, or south to north, which we call meridional flow, then surface weather features likewise move in this more longitudinally oriented direction.

Why is this worth noting? Because by identifying jet-stream flow as either zonal or meridional you are able to track and forecast movement of weather features such as low and high pressure. For example, meridional flow approaching the US East Coast from the west will cause low-pressure systems to move south to north, extending a low's duration of encounter with the East Coast and bringing extended coastal exposure from east and northeast winds. Severe storms, such as The Perfect Storm and Storm of the Century were supported by highly developed, upper-air troughs.


This recent satellite image of the jet stream depicts meridional flow since the stream is oriented north to south.
Large blizzards along the East Coast are always associated with meridional flow, which brings warm, moist air over the cold, dry land producing large amounts of condensation in the form of snow. Zonal flow, on the other hand, moves low-pressure systems directly west to east, producing a track perpendicular to the East Coast and thus minimizing the duration of exposure and precipitation.

Jet-stream flow alternates between zonal and meridional, transitioning over days and often weeks, responding to the atmosphere's need for movement of heat and cold in maintaining a stable atmosphere.

The Weather Channel produces satellite images that show jet-stream flow each hour, emphasizing areas of ridging and troughing, as does the New York Times weather page. For mariners, the best source of jet-stream wind information is the Marine Prediction Center's (MPC) 500-millibars charts. Twice a day, at 00Z and 12Z (midnight and noon in Greenwich, England), 500-millibars analysis charts are produced by meteorologists at the MPC and then broadcast by the US Coast Guard over Weatherfax and placed on the MPC website http://www.mpc.ncep.noaa.gov).

"Patterns in jet-stream flow change slowly, taking days and often weeks."

In addition to 500-millibars analyses, the MPC uses the powerful computing power of the National Weather Services IBM Deep Blue computers to produce 500-millibars forecasts out to 120 hours (five days) in 24-hour intervals.

Another impressive jet-stream product produced by the MPC is the seven day to 10-day surface Storm Track chart. This chart uses 500-millibars jet-stream energy information to determine areas on the earth's surface where winds of 30, 40, and 50 knots will be found during the next seven to 10 days. Wind direction is not shown, since this can be determined when specific weather features are examined. What is of importance is identifying those areas where gale and storm-force surface winds are likely to develop.

Areas where winds of 50 knots or greater, i.e. storm-force conditions, are indicated with hatched markings. Flow direction for low-pressure systems, which contain these high winds, are shown using a dark undulating arrow line (see accompanying seven to 10 day Storm Track chart for Pacific Ocean).


Wind and name analyses like this one can be used to quantify the effect of the jet stream on the Gulf Stream.
In addition to the Storm Track chart, the MPC is capable of producing a Wind/Wave Forecast chart annotated with Gulf Stream location and, if appropriate, location of any North Wall events (see attached sample chart). North Wall events are defined as conditions where winds are, or have been, blowing against the Gulf Stream's flow, leading to formation of exceptionally high, steep, breaking waves.

Since the strongest current flow in the Gulf Stream is found along its North Wall (western edge), this is where the largest, and shortest, period seas are found when winds are bucking the current flow. There is one dilemma though, both the seven to 10 day Storm Track chart and Gulf Stream annotated Wind/Wave chart are experimental products, meaning they are produced "in-house" and not routinely made available to the public. However, they could be turned into a daily operational product if mariners feel these charts would be useful and request their dissemination.

If you would like to see either or both the seven to 10-day Storm Track and Wind/Wave Gulf Stream products made available to you, send an e-mail to James Hoke, Director of the Marine Prediction Center, (James.Hoke@noaa.gov) and request these products be placed on the MPC website.

Remember, the upper air wind flow is given to mariners daily on 500-millibars charts. These charts are produced as both analysis and forecast products and should be consulted and examined in detail when planning a departure and monitoring weather en route. Jet-stream charts are your answer in the wind!

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