Friday, December 27, 2013

January 2-6 Potentially Significant Winter Storm

There is the chance for a potential winter storm around the January 2-6 timeframe.

Model guidance has been firm on the idea of a strong storm system passing through Japan tomorrow morning, on December 27th. This is shown well by the ECMWF 500mb height anomaly projection for that timeframe above. If we utilize the correlation that states a storm in East Asia then results in a United States storm 6-10 days later, we find a potential winter storm hitting the nation around the January 2-6 timeframe.

Here is the 12z ECMWF 500mb vorticity forecast for January 4th, where we see two pieces of energy impacting different portions of the nation. There is energy coming down from Southwestern Canada, as well as a storm system over southern Texas and into northern Mexico. It seems to be that these two pieces of energy would be involved in this potential winter storm. If you have been watching the models, you will recall that the ECMWF showed a triple phase (three pieces of energy phasing/merging) that then created a monster storm system. While that storm was forecasted to occur in the outlined January 2-6 period, and it was deemed possible at the time, the ECMWF no longer shows such a solution, and the likelihood of such an event happening is low.

This is the ECMWF Pacific North American (PNA) index forecast. We can see that the PNA is projected to be negative for when the possible storm timeframe comes about.

During a negative PNA, we tend to see big high pressure form across the Gulf of Alaska. Because we have a big rise in height anomalies over that body of water, there is a response of a dropping in height anomalies over the West Coast, meaning stormy weather forms in that region, as the graphic above from NCSU illustrates. Thanks to that stormy weather in the West, high pressure likes to form in the Southeast, and the two arrows (which illustrate the wind pattern in the area) combine over the Plains to show the storm track of systems during a negative PNA. Now usually, negative PNA storms like to go up into the Plains because the Southeast ridge is usually very strong. In this case, however, the positive NAO helps to keep the overall atmospheric pattern very progressive, meaning we don't see any persistent high or low pressure systems in any given area. There is some conflict over the predicted phase of the PNA, with the NCEP Relative Measure of Predictability charts supporting a positive PNA, and the ECMWF forecast here going with a negative PNA. The vorticity chart further up on this post more resembles a positive PNA, but I guess this sort of variability is to be expected with the event still nearly 10 days away.

Forecasts for the North Atlantic Oscillation (NAO) are also a bit messy, most likely due to the impending weather pattern shift. The GFS Ensemble set shows the NAO descending from its moderate positive state to a more neutral position by the time January 2nd rolls around. From there, the ensemble mean takes the NAO back into weak positive territory, and I actually think we see an intensification of the positive NAO more than what is forecasted for the 2nd and 3rd. A piece of the polar vortex will be rather close to Greenland, and it's possible that the ensembles are underestimating the +NAO that will arise as a result.

Regardless of how these teleconnections end up, let's go ahead on the basis that the ECMWF will be correct in its 500mb vorticity forecast above. The model doesn't really phase the two systems together in the vorticity forecast above (though on the most recent run, it nearly does-- remember I made this post last night to be released today), and I don't think that lack of phasing is correct. Based on the seemingly-positive PNA, I would expect the Canadian energy to dive further south and at least come close to interacting with the southern energy. As far as the iffy NAO, the chart immediately above suggests a weak positive NAO or neutral NAO, but some 500mb charts I looked over hinted at more of a negative NAO, further adding to the confusion. I'm not ready to say if this storm will definitely cut north to ride the East Coast, but I do feel comfortable saying that there does look to be a chance for a potentially significant winter storm during this timeframe. Until we know more details on teleconnections, we won't know much more on track.


1 comment:

Anonymous said...

Is this the same system that looks to be headed for eastern VA and NC?