Friday, December 16, 2011

ECMWF Ensembles Picking Up on Pattern Change with Positive PNA

The ECMWF Ensembles are picking up on the long-awaited pattern change by displaying the typical PNA index in its positive phase, as shown below as well in 500mb heights.
Typical Positive PNA 500mb heights in January.
A positive PNA features a trough in the East and a ridge in the West. Putting that in simple terms, a positive PNA means many low pressure system occurrences in the East and many high pressure system occurrences in the West. As the stratosphere warms, it is only fitting that the PNA, AO and NAO (all 'essential' for cold air and snow for the East) will be turning into much more favorable phases. January could be, and likely will be if everything goes right, THE month for this winter. February could be snowy as well, but January will really start cranking the snow machine.

Side note: For those seeing my exaggerated feelings on this winter or this storm, I am a snow lover, and, no matter how hard I try to avoid sending those feelings on here, they do get on here. Winter is typically a turbulent time for my emotions and patience with storms and winter overall, so y apologies in advance if I start to go overboard.

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.