Help with reading the difax maps

What does a 12 hour (or 24 hour, 36 hour, 48, etc) forecast mean? If one were to click on a 12 hour forecast product of a Difax map, one would notice a long label on the bottom of the map.

12 hour forecast {type of map} Valid {time, day, date, month, year}.

An example
12 hour forecast 500mb Heights/Vorticity Valid 12Z THU, 30, OCT 1997

12 hour forecast: Conditions expected 12 hours into the future, or 12 hours from the time the model was run. The NGM, ETA, and SPM-AVN are run at 00:00 Zulu (00Z) and 12:00 Zulu (12Z). If a model is run at 00Z, a 12 hour forecast would be the forecast for 12z, a 24 hour forecast would be for 00Z of the next day, etc, etc.
Examples and Definitions of the type of maps are all explained further along in this help document.
Valid 12Z, Day/Date: The conditions (The highs, lows, vorticity, etc) that are shown on this map are forecast to occur, or be present, at the time/date shown here. The models believes that the atmosphere will be set up to look like this at this time.

Ok, but how do you explain a 00 hour forecast?

A 00 hour forecast is essentially the models idea of what the current conditions are as it begins it's forecast. If the model is run at 00z, the 00hour forecast is valid for the same time, 00Z. A FORECASTER SHALL NEVER CONFUSE ACTUAL CONDITIONS AT 00Z AND A 00 HOUR FORECAST FOR 00Z TO BE THE SAME THING!!. Say that over and over because they are different. In fact, a forecaster should make it a habit to compare the actual conditions at 00Z, and the 00 hour forecast. IF they are very similar, one can at least know that the model started off on the right foot. However, if there are differences in the two, then one must beware in using the model for a forecast. If a model does not start, or initialize, correctly, then there are certain to be errors in all the models forecasts. It takes a few hours for a model to inialize, hence the 00hour forecast is not acually the conditions experienced at that time, but rather what was believed to occur at that time. Becuase the forecast is for a time very close to when the model actually was run, it is generally quite accurate. With each progressive forecast hour, more and more mistakes are likely to occur which is why it is essential that it at least is correct in it 00hour forecast because the more errors it starts off with, the more it will encounter down the road.

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The VORTICITY map
Reference: Graphical Guidance (1994) pp. 24-26

The VORTICITY link brings up the 500 mb heights and Vorticity chart.

500mb heights: Solid lines
Vorticity: Dashed lines

the white numbers, embedded in a black box, that you see, either in the 400's or 500's, are the heights in decameters pronounced (deka-meters). Simply add a zero to get the height in meters. 500 Dm = 5000 m The heights are contoured at an interval of 30 meters.

H's and L's are also shown on this chart. These are areas of High/Maximum and Low/Minimum heights. Near and H or L, there will be a tiny circle with an X through it, along with a white, black outlined, number. This is the maximum or minimum height for the area, measured in dekameters.

The little numbers that go along with the vorticity are values of 10 to the minus 5 radians per second. It is absolute vorticity, which takes into account the earth's rotation, thus all the numnbers will be positive. The higher the number, the stronger the vorticity, or spin of the air. Vorticity is a bit too involved to be explained here, but the Graphical Guidance gives a good description of it. ES115, 155-157, and expecially ES205 go into vorticity much deeper. Understand that it is a way of finding shortwaves embedded in the longwave (jetstreams) pattern. These shortwaves are disturbances in the flow. Shortwave troughs tend to dig, or buckle, the jet stream and allow for disturbed weather, normally downstream, or ahead. We consider shortwaves to be a form of lift. Here is an example of a shortwave trough. The red depicts the trough axis. An "x" depicts the center of maximum vorticity while an "n" depicts an area of minimum vorticity.

Practice:

Get the 500 mb/Vorticity 12 hour forecast map.
Mark the significant shortwave troughs with a red line, extending from the center, through the axis, to the end. For help, see the example above.

Denote the jet stream(s). If there is a split jet, mark a blue arrow for the northern stream and a red arrow for the southern stream. You may also want to simply trace over a specific height line instead.

Note any areas of diffluence, where the air literally flows apart, with a purple, squiggly, line. An example below will help explain this.

Repeat these steps for the 24 and 36 hour charts. Note any changes to the jet. Have the shortwaves strengthend or weakend. How much have the shortwaves moved? Look at the height's over Chicago at 24 hours. Have they changed much since the 12 hour forecast?

Here is an example of a simplified 500 mb/Vorticity forecast map analysis. Students learning to forecast should also look at the map board to see how these maps are analyzed by COD forecasters daily.

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