December 20 Snow Event Discussion (Updated 12/16)

Confidence Level: Medium

System that will eventually become the Dec. 20 storm system.
Defined as the dark blue circle of increased Water Vapor.

The system that will eventually turn in to the December 20th storm for the Midwest and Ohio Valley is currently just offshore Baja California. The system is defined above as the dark blue circle of increased water vapor. The circular shape is similar to that of a cutoff low, which this storm system actually is. A cutoff low basically is disconnected from the jet stream, or cut-off, and can spin around for days in a single area before eventually being nudged somewhere else by a different atmospheric feature (another disturbance, for example).
This cutoff low will be progressing eastward into the US at some point in the next couple days. From that point on, the system should sit by itself, possibly losing some of its strength to the storm-ravaging Rocky Mountains. The cutoff low should be moved eastward by another disturbance sliding southeast-ward from the Pacific Ocean. It can be compared to a 'one leaves, another takes its place' sort of pattern.
This first cutoff low should be nudged far enough eastward to be influenced by the jet stream and be pulled back into the flow. One question is just how strong the second system that will be moving southeast will be. It is possible that a stronger system would move the first low farther east, thus moving the track farther east. We want to find out what the models think will happen, but models are known to not be good with systems in the Pacific due to lack of upper air observation data. So we turn to the ensembles.

Measure of Predictability using NCEP Ensembles for the December 20 period.
Darker colors indicate more confidence in a certain atmospheric event happening.

As the system ejects eastward out of the Southwest, the NCEP ensembles are believing that a strong ridge (high pressure system) will be in place in the Southeast. This would do two things. One, it would stop eastward progression of the disturbance due to the ridge being in the way. Two, the ridge would take the disturbance and pull it northward, moving the disturbance around the ridge. This would ideally take the system through the Midwest and Ohio Valley before entering the Northeast.
Something we are monitoring, however, is how strong the ensembles take this ridge. The ensembles appear to be really pushing this ridge northwestward, farther northwest than previous ensemble predictability images have seen. This increased northwest ridge presence would alter the disturbance's track, presumably taking the storm more northward.
The big concern with this is how the ensembles may not be picking up the system. We can see an area of light orange colors in the Texas region, around where the disturbance is projected to be by models. However, the shade of colors would indicate that 50-70% of the ensembles are predicting this disturbance to be present, with more than 80% predicting that the ridge will be present. We do believe that the disturbance will be in place, but with the ensembles not agreeing, the final solution may be altered due to the disagreement.

Models are Junk Today

We don't have much to share with you today, other than the fact that the models are junk with the Dec. 20 storm. Winter forecast probably won't play out as expected, and we are debating a revision.

Model Checks: Dec. 14 Evening

Concerning Dec. 20 storm.
North Camp: storm into Wisconsin
South Camp: storm into Ohio Valley/Northeast

North Camp: GGEM
South Camp: NOGAPS, ECMWF, GFS ,ECMWF ENSEMBLES

Preferred Model Blend: ECMWF/NOGAPS/ECMWF ENSEMBLES

Models will know Dec 20 storm by Sunday

As the storm responsible for the Dec 20 is being tracked, keep in mind that the system will be 'sampled' when it comes onshore the US. This means weather balloons will be able to gather data and thus greatly improve the models.

December 20th Major Snow Event Discussion (Updated 12/14)

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

•Jet Stream

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.