Aside from the day-to-day forecast model data, something we meteorologists like to look at are indicies. What are these? They are indicators of the large scale, global weather pattern that we can use to correlate with various types of weather. For example, all winter long we've been locked in something called a negative NAO (North Atlantic Oscillation)/postive PNA (Pacific-North American Index) that favors generally cold/stormy weather in the eastern part of the country. Factor in our El Nino pattern, which favors an active subtropical -- or southern branch -- of the jet stream.. and all of the ingredients were there for a cold/extra stormy winter.
But time's are changing. Below, I've included links to the various indicies that we look at. A brief summary:
North Atlantic Oscillation (NAO) -- Deals with pressure patterns in the North Atlantic Ocean. When this is in a negative phase, strong high pressure is present over Greenland which leads to low pressure being locked over the eastern part of the US. In the positive phase, this is reversed.
Pacific-North American (PNA) -- In the positive phase, a strong area of low pressure is centered south of the Aleutian Islands. This leads to a ridge of high pressure over the western half of the country and a trough in the east -- or a dip in the jet stream.
Arctic Oscillation (AO) -- When this is negative, colder weather is usually present over the Lower 48.
Above, you'll see how each index has been over the past several months... and generally, the NAO/AO have been strongly negative and the PNA has been strongly positive. Notice, though, at the edge of each graph.. the forecast part of the graph.. how the NAO and AO indices are now trending opposite of what it was over the winter (or at the very least, toward a more "neutral" pattern). This is what likely will lead to a "thawing out" of our much colder than average winter in SWFL.. just in time for the start of spring! Now, to feel real confident about even warmer weather.. I'd like to see the PNA changing more strongly toward a neutral pattern -- that is not forecast... so it's not altogether impossible that we could see another cooler period or two.. but the extra cold stuff should be done with!
Brian
