Chapter 9 introduced how planetary-scale ridges (associated with high pressure aloft) and troughs (low pressure) are associated with warm and cold air masses respectively (see Figure 1). Ridges are also associated with a poleward shift in the jet stream, and troughs with an equatorward shift in the jet. Because large planetary waves in the jet stream tend to remain stationary for several days, they can be used to make predictions about long term temperature trends and and the track of storm systems.

The Geophysical Fluid (GFS) model is a numerical weather prediction (NWP) model (i.e. a huge computer program) used by the National Weather Service (NWS) to predict large-scale weather patterns out to 14 days (usually, forecast accuracy goes down after 2 or 3 days and nosedives after 5 to 7 days). Figure 2 shows the 300 mb GFS model analysis for 12Z (8 AM EDT) Monday, April 9, 2007. Note that the blue shading indicates wind speed in knots as you plotted in Investigation 9A, NOT temperature. This is the data used to initiate the model.

The map shows a large trough over all of eastern North America. You will note that this is so pronounced that the trough has cutoff into closed cyclones in places. Meteorologists call this a cutoff-low (clever, aren't they?) The jet stream is well to the south of the northeastern U.S., looping around the Great Lakes and out over the Atlantic.

This trough has been with us for about a week and is associated with the cold weather we are having. Persistent troughs like this are also associated with damp, humid weather. At this time of year, it makes it feel like winter is never going to leave. Going to the course weatherpage and clicking on the Summary link near the top of the page gives climate data for Monday and shows a high temperature of 36.3 F compared to a normal high if 48.0 F, 12 degrees colder than normal. So if you think it's cold for April, you're right.

Will winter leave us soon? Let's check the 5-day forecast that the model produced Monday morning (Figure 3). This forecast is a 120-hour (exactly five days) valid 12Z (8 AM EDT) Saturday morning Apr. 14. This is a very different pattern. A wavy jetstream snakes its way across the continent. Over the northeastern U.S., there is a ridge that pushes the jetstream to the north.

Weatherblog Assignment 3:
1) Make a prediction for the high temperature for Saturday based on Figure 3. Discuss your reasoning. You may include references to web or broadcast forecasts to support your prediction. The NWS forecast for Lyndon Center can be found here.
2) Update the temperature and 300 mb wind forecast before Wednesday evening. New forecasts for Saturdays are issued every 24 hours at the GFS model Upper air website. Here, new forecasts run at 12Z (8 AM local time) are posted everyday at about 11 AM.
3) Plot the verified 300 mb map for 12Z Saturday and comment on the quality of Monday's original forecast. Discuss the position of the ridge relative to previous forecasts and whether temperature forecasts were accurate. I will post the verification on this blog.

The 300 mb Wind forecast is best for monitoring the polar jet stream. The forecast maps are given every 6 hours. You can loop them, or look at them individually. The numbers represent forecast times: 24 represents a 24 hr (1 day) forecast, 048 a 48 hour (2 day) forecast, and so on. You can check the valid forecast date at the bottom of each chart.

Remember that the Saturday forecast hour will change every day. On Tuesday it will 096 hr, Wednesday 072 hr, Thursday 048 hr, and Friday 24 hr. It will verify on Saturday as the 000 hr forecast.