WHY IS THE SLP DIFFERENT BETWEEN AWIPS AND AFOS?
The sea level pressure found is AFOS uses the Shuell reduction method, (PMSL) which is a function mainly of low level temperature. This is quite different from the Mesinger method, which can be found on AWIPS (EMSL). This uses a horizontal Poisson equation to smoothly interpolate from above-ground to underground temperatures. This results in a smoother field, particular over the Rockies. PMSL has been added to AWIPS grids for the Eta, but they are not distributed yet. A change request must be filed with with the AWIPS DRG before that can happen. Some further comments on this subject: Let me attempt to convey a couple of my thoughts & feelings on sea-level pressure. Remember that the Eta’s initial field of sea-level pressure is not a result of performing a sea-level pressure analysis. It, like at all the other forecast output times, is computed diagnostically via the reduction procedure using the free atmosphere values of the state variables in the model. I admit to being unemotional ab
