May 10, 2013

Preliminary Outlook

Here's a preliminary look at things.   I am using an ensemble approach to judge the probability of the patterns we are likely to encounter.   This is about a week out.  
The ensemble approach involves looking at about 20 different forecasts that the model (in this case, the Global Ensemble Forecast System , which is basically the Global Forecast System).   Small tweaks to the original observations are made to see how sensitive the model is to the initial conditions.   Beyond about 5 days, the average of these conditions is often a better to much better forecast than the model run with just the observations.

First, the model brings a weak to moderate trough into the west, with southwesterly flow impinging on the Plains starting about Thursday evening.   The best flow occurs Saturday and Sunday afternoons.  However, the direction of the flow is good with marginal wind speeds even Thursday and Friday afternoon in the western High Plains, if this pattern verifies.

The surface pattern is shown on the next panel.   I annotated it and added an isobar to make the pattern clearer.   This forecast suggests a wave cyclone just east of the Rockies with a lee side/thermal trough extending southward, within which a dry line should be found.   Somewhere near the KS/Nebrasksa border should be a warm front.   South of that boundary should be a flow of fairly warm and moisture rich air.
The final two panels are the ensemble mean sbCAPE and probability of CAPE > 4000 J/kg.  Meteorologists looking at the previous chart would probably guess that there would be adequate CAPE, possibly a lot with this pattern.
The bottom line, integrating all this information, is that this far in advance, the weekend of the 18-19-20 is looking possibly pretty active for severe weather in the west-
central Great Plains.