Phil Pugh Asks .... What Happened?

Steve: I am curious. Yesterday the 10 day forecast had virtually no chance of rain for the next 9 days. Today that same forecast has 5 of 9 days with 30% or greater chance of rain. What happened to make such a dramatic change in such brief period of time? I would find such explanations an interesting addition to the weather forecast. You could have the “Inside the forecast” segment and the “What happened we didn’t expect” segment!

Well Phil ... here is the answer ....




Courtesy weather.cod.edu

 

At jet stream level a "cutoff" low is going to form above us.  It is called a cutoff low because it becomes isolated from the main branches of the jet stream and because there is no wind to push it along it wobbles around over an area for days sometimes.

Because it has a core of air colder than the surrounding area the atmosphere under it is unstable and showers pop up mostly during afternoons. And because it is a low pressure system air within it is rising creating the cloud cover that can lead to showers.

But because the amount of moisture is limited and the low is fairly weak the rains are generally light, brief and scattered.

Now .... how did the forecast change? 

Models generally do a good job upstairs in the atmosphere. The chart above is from about 18,000 feet above sea level and shows the "cutoff" low Sunday at 2PM.  Sometimes the model takes a while to evaluate how the jet stream interacts with the Rocky Mountains. 

In this case models this past weekend showed the forecast weather pattern to be what meteorologists call a "progressive" pattern. Systems move in and move out fairly quickly.

Once the models got a handle on the amount of energy at jet stream level and how the energy would interact with the mountains the forecast changed from a "progressive pattern" to a "closed" or cutoff low. All cutoff lows are closed lows but some closed lows can be embedded in the jet stream flow and move quickly so are not "cutoff lows".

Because they move only slowly the forecast went from a couple chances of rain in 10 days to the possibility of a shower nearly every day.