Friday, January 28, 2011

Day 5

Alright, we are closing in on Sunday. With only two days to go we have reached the time where forecasts are now becoming much more accurate. On average, forecasting accuracy drops significantly when you get 4-5 days out, yet several weather services produce 10-16 day forecasts! There are no excuses at this point with close to 48 hours to go! Today, I have completely taken out the chance of rain for Sunday leaving us with a partly cloudy afternoon. We should get some rain today and tomorrow though. Sunday begins another stretch of precipitation free conditions that we have been enjoying for the last week or so. After analyzing today's maps I will include a bit on how I forecast temperatures.

Most of the action remains to our south on today's models. The development of a second low near Alaska has also helped drag moisture further to our north that will follow that establishing ridge that shows up quite well on the 500mb map. Look at the big high pressure region to the north of Montana. That is really cold arctic air. Temperatures look to be in the single digits over the weekend with lows in the negatives for Montana, Wyoming, North and South Dakota (I just looked at the forecast temp for Lemon, SD...the HIGH temp is 3 for Sunday, ouch!).

We see the same thing happening at the 850mb level that we see close to the surface. The further up in the atmosphere you see features, the "deeper" they are considered. In this case, the lows are fairly deep, although it is not uncommon to see features all the way up through the 300-200mb level, like hurricanes. We see little hope for some cold air advection, which would stabilize the atmosphere. Nonetheless, we still are looking dry.

That big ridge is still lingering out near the coast. It acts as a wall that forces energy (PVA) up and over the ridge. This ridge also impacts our temperatures at the surface. Under them, locations can expect warmer than average temperatures. We will take a look at how this impacts our temperatures in a bit with the numerical forecast.

The weak jetstreak that we are dealing with looks more harmless today. It is pushing south, we call this "digging". Notice the little dip in the overall flow to our south. That is a small trough, caused by that shortwave energy. As long as the jetstreak stays on the LEFT side of the trough axis, we can expect that energy to "dig" and deepen that trough. That should send the shortwave even further south. In fact, it will impact southern California this weekend. For us, we will be enjoying dry and partly cloudy conditions.

I haven't really spoken much about temperatures for Sunday yet. There are a couple ways I go about forecasting temperatures. As I noted in the into, forecast accuracy declines when you look to predict 4-5 days out. I typically do not use these maps for high and low forecasting. It is possible, however. A lot of forecasters use the air temperature at the 850mb (in degrees Celsius) that corresponds with a temperature at the surface (in degrees Fahrenheit). For Sunday's forecast, I have used two sources. One is the 2m surface temperature model that the University of Washington provides. It forecasts 84 hours out and uses 3 hour increments. This is about as high-res as we can get in forecasting for our area.
Forecasting a high temperature is easy enough using this method, assuming you have confidence in this model. Over a year's worth of forecasting has lead me to notice some trends (flaws, in my opinion) within the models. Every model is great at handling some aspects and struggle with others. As the map shows, we are right at the threshold of 48 degrees. I am holding onto hope that this model keeps us right there since that was my initial forecast temp!
Another method used to forecast is model output stats, or MOS. MOS is great for the Pacific NW, Oregon and Willamette Valley forecasting. Why? MOS has a strength in handling local effects really well. Local effects include the Pacific Ocean, Coast and Cascade mountain ranges and the Willamette Valley. All these things are taken into consideration. Here is what MOS looks like:
A lot of categories there! Some include dewpoint temperature, cloud cover, probability of precipitation for 6 hours, probability for 12 hours, type of precip, ect. What we are concerned with is temperature. You'll have to forgive the skewness of the graph but the first few days mostly line up correctly. Look at the row labeled "X/N". That is the nighttime/daily max temperature. Go to Sunday, 72 hours out (this was yesterday's model, they re-run at 0z, which is 4p.m. our time). MOS is expecting a high of...48! A great way to re-enforce a forecast! This particular MOS has several perturbations on it, about 20. The above is just one of those. I averaged all the perturbations and the temperature came out to about 47.5 degrees. I'm sticking with 48, why change based on half a degree. You got to own your forecast!!

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