This 10 part series will detail forecasting tricks that can be used to try to outforecast MOS. Outforecasting MOS is an important skill for a forecaster. MOS stands for Model Output Statistics and they are used as a guide for temperature prediction and precipitation prediction by forecasters. Model consensus is the average of the high temperatures, low temperatures or precipitation amount predicted by several forecast models. PART 1: ADVECTION ALOFT AND SURFACE TEMPERATURE Warm Air Advection and Cold Air Advection aloft will have an important contribution on surface temperature. In many thermal advection situations the temperatures aloft will change more rapidly than at the surface. When thermal advection aloft is significantly stronger than at the surface some of this air aloft will mix down to the surface. You can get a fairly good idea of how thermal advection is changing with height (non high elevation regions) by studying the surface temperature (or 1000 mb) and 850 mb temperature model panels and note how much the temperature changes over time at the surface as compared to at 850 mb over the forecast area. Current surface panel forecast: http://weather.unisys.com/nam/sfc.php Current 850 model panel forecast: http://weather.unisys.com/nam/850.php Low temperatures: Suppose the MOS for the primary forecast models have lows of 23 F, 25 F and 27 F. This gives a model consensus of (23+25+27)/3 = 25 F. If there is Cold Air Advection occurring aloft such as winds blowing in colder air from the northwest, the actual low will tend to be colder than what MOS consensus indicates. The models may not pick up on cold air mixing down to the surface. This forecast trick works best when the surface wind speed and direction continues to advect colder air in at night and there is even more significant Cold Air Advection moving in aloft at 850 mb. In other words, Cold Air Advection is increasing with height. Suppose the MOS for the primary forecast models have lows of 37 F, 39 F and 40 F. If there is Warm Air Advection occurring aloft such as winds blowing in warmer air from the south, the actual low will tend to be warmer than what MOS consensus indicates. The models may not pick up on warm air mixing down to the surface. This forecast trick works best when the surface wind speed and direction continues to advect warmer air in at night and there is even more significant Warm Air Advection moving in aloft at 850 mb. In other words, Warm Air Advection is increasing with height. High temperatures: Suppose the MOS for the primary forecast models have highs of 70 F, 69 F and 71 F. If there is Cold Air Advection occurring aloft such as winds blowing in colder air from the northwest, the actual high will tend to be colder than what MOS consensus indicates. The models may not pick up on cold air mixing down to the surface. Study the models for Cold Air Advection increasing with height during the day. Suppose the MOS for the primary forecast models have highs of 78 F, 75 F and 80 F. If there is Warm Air Advection occurring aloft such as winds blowing in warmer air from the south, the actual high will tend to be warmer than what MOS consensus indicates. The models may not pick up on warm air mixing down to the surface. Study the models for Warm Air Advection increasing with height during the day. With experience you will recognize when cold or warm air aloft will have a chance to mix down to the surface. This mixing can throw the model high and low temperatures off by several degrees. This knowledge will help you outforecast MOS in these situations. |