This is a discussion concerning the December 20th Winter Storm.
The above image portrays an average predictability of something happening by combining all the NCEP ensembles into one average. The more oranges and reds you see indicates a higher predictability the ensembles believe that atmospheric feature will come to happen. There are some very important factors involved here.
•Ridge in the Southeast
•Disturbance in the South Plains
On the image above, there is a spot of orange in Florida, where the 500mb height isobars are positioned at an arcing angle. This is a typical position a ridge of high pressure takes. It appears that the NCEP ensembles are more open to this ridge in the Southeast, as per the darker colors around the arcing isobars. The consequences of this ridge, should it be strong enough, would direct the jet stream north, along with other disturbances following the jet stream.
The second atmospheric feature we are looking at is a disturbance in the South Plains. Ultimately, as this disturbance ejects from the Southwest, this would be our storm system for December 20th. The ensembles are latching onto this disturbance's presence, characterized by a valley formation in the 500mb isobars. This is a feature you will find is associated with troughs, or disturbances. The placement of this disturbance appears to be somewhat debated, but there is an indication that the system will be present in the Texas Panhandle region.
The third feature that is worth watching is the jet stream. The jet stream can be thought of as a highway- low pressure systems follow the jet stream as they make their way west to east. The jet stream can be disrupted and moved around by disturbances and ridges. The ridge in the Southeast, if present, would direct the jet stream more northward, likely where the isobars on the image get really tight in the Northeast. An interesting feature that may be enhancing this jet stream is a proposed other atmospheric feature farther north, judging by the continued tight isobars into Canada. The jet stream will be a big player in this storm, as will the ridge- the ridge's placement/presence essentially determines the storm track, among other factors.
Side notes: The ECMWF did come back south, determining the low placement will be in Central Illinois with the 12z run. This would bring the heaviest snowfall to Chicago north to around a Madison-Milwaukee line westward.