LIMITATIONS Forecasting the surface high using the 850-mb temperature has been a popular forecasting technique. First I will go over the limitations of using this method: 1. Method does not work on cloudy days or days with afternoon precipitation 2. There is a tendency that temperature will be higher than predicted on days the wind is light and will be lower than predicted on days the wind is strong. This is because the low level wind effects the depth of mixing. 3. Method assumes air is mixed only between the surface and 850 mb. If the air mixes to a height significantly above or below 850 mb the technique will not work accurately. 4. Method does not account for elevation. High elevation regions have a greater chance of mixing air that is above 850 mb. 5. Daylight hours effect accuracy. There is a significant difference in daylight hours between the warm and cool season. 6. Method only works in a barotropic atmosphere. Fronts or differential advection will contaminate technique. 7. Method does not work well in regions with complex topography or near mesoscale temperature gradients such as coastal areas, very hilly areas, and areas near large lakes. The method works best in locations near sea level, in the warm season, on barotropic days, with flat topography, on moderate windy cloud-free days. If any of these conditions are not met then take that into account on the temperature prediction. THE METHOD The method itself is very easy. Method needs to be done on a Skew-T to ensure accuracy. From the morning sounding, note the 850-mb temperature. Take a parcel of air at 850 mb using the 850-mb temperature and bring it dry adiabatically to the surface. The temperature of this parcel after it is brought to the surface will estimate the high temperature for the day. IS THIS METHOD BETTER THAN USING MOS? The only way to know is to try the method each day for a year and compare it to the MOS values and actual high. With experience in using the 850-mb technique you will be able to "tweak" the value you attain each day to take into consideration the limitations given at the start of this essay. On days with active weather, the 850-mb technique and MOS have problems. Your greatest skill in outforecasting MOS will be on days with active weather (frontal passages, precipitation). |