Monday, January 24, 2011

7-Day Acurracy

Mark Nelsen, known around work as "The Chief", grades his forecast accuracy each week. Daily high/low temperatures and sky/precip conditions all go into his grading scale. He churns out a pretty high accuracy rate. His report card got me accurate can we forecast seven days out? A lot of people think that meteorologists can't even forecast 1 day out so how accurate could we be seven days ahead?

In all reality, forecasting is becoming much more accurate overall. When it comes to you certain aspects of forecasting though, things can be more challenging. Take for instance winter weather. Everyone gets upset at the weatherman when says it is going to snow and then it never does. Why does it always seem that way? First, we live in a geographical area that doesn't often produce snow where we live. Being in a valley, temperatures really have to sink down to the floor in order for snow to survive. The Pacific Ocean also acts as a moderator and will warm an otherwise cold air mass that could produce snow. Water has a high heat capacity which means that it take much more energy to warm the water. Because of this, the winter time is actually when water temperatures in the Pacific are WARMEST. Another reason why forecasting winter weather is so tough? We just do not know what will happen until right before it actually happens. So many dynamics go into winter weather and it only takes the difference of a few degrees that would give us snow, rain or even freezing rain. So next time you see snow in the forecast, just be patient and keep in mind that things can (and probably will) change.

Now onto my little experiment. I've decided to track my forecast for next Sunday, the 30th, for the next seven days. We will see how much it will change and see how accurate it is. Let's look at day 1:

Ok, so seven days out I see us staying dry. Things cloud up after a decent stretch of sunny conditions. The average for January 30th is 47 degrees. I am shooting for 48 degrees. This many days out, climatology can be a decent way to forecast a high and low temperature. A lot of hi-resolution weather models do not go out 7 days so my confidence on this as of today isn't all that high. But I am, as of this moment, fairly certain we will stay dry Sunday. Here is why:

Looking at the weather charts, a good place to start is the 500mb. This is about half way up the atmosphere and gives a good estimate of where weather systems will go.

This is the 0Z (+8 hours) 500 mb model run, valid Mon. Jan 31. This is our 4 p.m. forecast for Sunday. Two things to point out. Notice the white contour lines over Oregon and Washington. They form a ridge in the overall flow. A ridge indicates a region of high pressure and often results in fair conditions. That is a pretty well-established ridge over our heads. But look to the south, right around the Oregon-California border. That is a shortwave disturbance, kind of a kink in the overall flow. Shortwaves often result in unstable weather. So we have a ridge overhead, but a shortwave of energy to the south. What will this lead to?

The map on the left is a relative humidity chart and the map on the right is what is expected at the surface. Again, these maps are all forecasting for next Sunday. On the relative humidity map, treat the lighter areas as clouds. Now look at the that bulls-eye area at the Oregon-California border. Showing a pretty good area of high relative humidity, meaning the air is close to saturation and is a stronger area of potential rain. Now turn to the surface map. There is some precipitation near the border. But in our area, we look dry. As these models update we can get a better idea of the flow and where this shortwave may actually land. It is possible this area moves a bit further north and impacts us. But as of right now, all I see are some clouds from this shortwave that could filter into our area, thus the cloudy yet dry forecast.

What do other forecasts look like for the 30th? The Weather Channel has a high of 48, low of 42 with a few showers. The National Weather Service also has a forecast high of 48 under mostly cloudy skies. So at least other people are seeing what I am seeing. I'll be back with an update on the forecast tomorrow.

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