The 300-millibar prog is located at or near the core of the jet stream. Winds within the jet stream pattern are commonly over 100 knots. A 100-knot wind does not have the same force as a 100-knot wind at the surface because the density of the air is about 70% smaller at 300 millibars than at the surface. The two jet streams forecasters are interested in at this level are the polar jet and the subtropical jet. The winds associated with the polar jet tend to be stronger than the subtropical jet because there is a larger temperature gradient between polar and mid-latitude air than between tropical and mid-latitude air. The forecaster also looks for jet streaks. These are high velocity segments of wind within the jet stream. The 300-millibar prog is available on the UNISYS weather website at: http://weather.unisys.com/nam/300.php The colors on the chart represent wind velocity in knots using 10-knot wind speed intervals. A significant jet streak has winds over 100 knots. A jet streak itself may only move 20 to 30 knots. It is the winds WITHIN the jet streak that are moving at the speeds portrayed on the prog. Jet streaks help carve the trough and ridge pattern. If a jet streak is on the left side of a trough, that trough will tend to become more amplified with time and "dig". If a jet streak is on the right side of a trough, that trough will tend to become less amplified with time and "lift". Jet steaks tend to develop at those locations in the atmosphere that have the greatest horizontal temperature gradient. A cold dome of air next to a warm dome of air (synoptic scale) will cause the pressure gradient to flow from the warmer toward the colder air. The Coriolis force then turns the airflow to the right of the path of motion. If cold air is to the north and warm air to the south, the airflow will be south to north then turned to the east by the Coriolis force. Therefore, the jet stream tends to flow from west to east. Jet streaks, as you will learn or have learned in synoptic meteorology, have regions of divergence and convergence. Upper level convergence leads to sinking air while upper level divergence leads to rising air. Rising air is favorable in the right rear and left front quadrants of a jet streak. |