Jimbo's Weather School: How to Read the Models: 500MB Height Anomalies

What are the 500MB height anomalies? They are how much higher or lower the 500MB heights are compared to normal. How is this important? Well, the lower the 500MB height is, that usually means there is more troughiness. If they are higher than normal that means there is ridging taking place. The jet stream usually resides between the troughiness and ridging. So you can tell if the pattern is favorable for a storm track. For example, on the map above, it shows a pretty favorable track for a strong Nor'easter. You can see a ridge of high pressure in the western US. That causes the jet stream to be more amplified, meaning it is running more vertical (or north to south), which can bring down more cold air behind (or north) the jet stream, and more warm air north in front of the jet stream, which can be more favorable for a storm to develop because extremes fuel storms. And a more amplified weather pattern often allows troughs with storms to really dig, allowing them to strengthen and not move quickly off the east coast. You can also see some ridging over the north Atlantic, or blocking. This will allow the storm from going straight out to sea (because it would block the storm). You don't want too much blocking, because the storm would just go around it and stay away from the northeast. You can also see ridging over the Arctic, which often forces cold air south from the arctic. You can also determine the three important teleconnections (AO, NAO, PNA) by looking at these maps...the amount of ridging over the Arctic determines the AO. Troughiness over the arctic creates a negative AO which often allows colder air to spill southward. Ridging over northern Canada creates a positive AO which keeps arctic air bottled up. So for a negative AO and cold air spilling down you want troughiness over northern Canada and the arctic and no ridging over Canada or the arctic. The amount of ridging over the Atlantic determines the NAO. Ridging over the Atlantic creates a negative NAO. The ridging over the western US determines the PNA. Ridging over the western US equals a positive PNA.