Margaret Lama and Matthew Parn

Monday, 27 September 2004

Assignments:

Guest Lecturer -- In Class Exercises Monday, 4 October and Friday, 8 October. Note that class has been canceled for Wednesday, 6 October.

Midterm has been moved to 11 October.

Read—Gilliam (all); "Normals, Means and Extremes" link on class website.

Writing Assignment is due on 13 October.

Discussions:

Hurricane Jeanne and Typhoon Meari

San Francisco current weather conditions

National weather conditions with emphasis on Florida

Hurricane Jeanne:

Hurricane Jeanne was the fourth hurricane this year to hit the same region. This is unusual in terms of frequency, but it’s not unusual in terms of total number for the region. Jeanne is now a tropical storm and still producing severe weather.

Jeanne’s eye is hitting almost the exact same spot as Hurricane Frances did. After looking at a radar loop of Jeanne we noted the eye approaching the East coast of Florida and containing green and yellow coloring which indicates heavy rainfall except in the Southern tip of Florida. We observed the cumulonimbus (storm) clouds moving counterclockwise due to them traveling along counterclockwise winds. Jeanne is weakening as she crosses overland and is expected to continue northward weakening further and then move out into the Atlantic.

After looking at rainfall maps, we noted that Florida has had over 22 inches of rainfall since 1 September. We also noted that a tornado crossed over Cape Canaveral.

Typhoon Meari is in a direct path with the Southern coast of Japan and currently has sustained winds of 80 knots. This is called a typhoon because it is in the Western Pacific Ocean. Meari is much weaker than both Ivan was and Jeanne is.

California’s Current Weather:

The National Weather Service divides California into Northern, Southern, and Central regions. San Francisco is situated in the northernmost tip of Central California, which extends to Monterey. Incidentally, Northern California begins halfway across the Golden Gate Bridge.

After looking at the current visible satellite image of California we noted that San Francisco is covered with low clouds, which are the beginning of our autumn fog.

Fog is caused by warm air cooling to the dew point temperature while traveling across lower ocean temperatures. This horizontal motion of the atmosphere is called advection. Last week we had warmer winds blowing across the continent from the East, but today the winds are from the West and, therefore, significantly cooler. We noted that air, in the absence of other factors, moves from higher to lower pressure, so we believe that the East Bay must have lower pressure than us.

To verify the above statement, we examined a surface plot weather map. We noted the wind direction in San Francisco to be West/Northwest at 5mph. The pressure here was within the 1016.0 mb isobar heading towards a 1012.0 mb isobar, which confirms that winds move from greater to lower pressure. We also observed a low pressure trough in the Sacramento Valley area.

There was also a noticeable low pressure trough in Mexico known as the North American Thermal Low. It occurs all through the spring and summer due to the extensive heat of the area.

National Weather:

After looking at the radar, we noted several cumulonimbus clouds in Florida stacked at over 50,000 feet. We also noted severe weather on this map, which we could not see on the surface map because the labeled weather stations did not report any. This shows the value of radar in meteorology. We also noted a moderate risk of severe thunderstorms along the East coast, especially in South Carolina.

Winds:

We viewed a weather map that shows weather at 18,000 feet, which is where 500mb generally occurs. We noted winds of 45 knots at this elevation compared to 5 – 15 knots at ground level due to friction with the terrain. We also noted a jet stream was beginning to move south from Canada and into the U.S., which will eventually bring winter rain.