Sunday, March 9, 2014

March 11-14 Significant Snowstorm

I'm still looking at the idea of a significant snowstorm on the March 11-14 period.

An upper air analysis at the 500mb level of the atmosphere shows our storm system as the depression in the contour lines over the northeast Pacific, which tells us the system isn't onshore yet. The storm not being onshore means that model guidance will continue to change until the storm actually gets onshore. The reason being that National Weather Service offices release weather balloons twice a day, and the data those balloons gets goes into the models to enhance their forecasts. Thus, when the balloons are sent into the storm when it comes onshore, the models get a better handle on the storm, and that's why we tend to see a model consensus come about when the storm system in question comes onshore. We also see suppressed ridging over the Western US, which will impact the eventual track of this event, as we'll go over later.

Shown here is the GFS 500mb vorticity projection for the evening of March 11th. On this image, it seems our storm system is separated into two storms, located over Nevada and Kansas. In this case, the GFS model wants to take the westernmost portion of the storm and retrograde it into the ridge located along the West Coast. This piece of energy then closes off and separates from the other piece of vorticity that continues progressing eastward. This move is a bit suspect to me. It is well known that model guidance has the tendency to hold energy in the Southwest for too long, and this could very well be one of those situations where the GFS is too eager to bring a piece of the storm into the Southwest US.

By the time we get to the evening of March 12th, about 24 hours later from the image we just discussed above, we see a lot has changed. We now see that the storm has indeed separated into a closed system along southern Nevada and California, which is linked to its parent storm by an elongated lobe of positive vorticity draped across the Plains. We'll get to that parent storm in a second, but first let's go over the storm in the Southwest. The system has retrograded directly into the West Coast ridge, and has closed off in the process, leading to a Rex Block-style pattern, where we have a ridge directly north of a storm system. While the typical Rex Block produces a zonal flow (west-to-east flow) synoptic set-up across the areas downstream of the block (to the east of the block), the GFS prefers to initiate a northwest flow regime, with the West US ridging leading to deep troughing across the East US. This leads us into the parent storm, which is in its own category here. The aforementioned northwest flow has led to a phasing (merge) of the easternmost storm we saw in the second image of this post (the first GFS image we discussed) with a weak system originating from northern Canada. This comes as no surprise, with the northwest flow regime being very supportive of phasing should the opportunity arise. I'm a little skeptical on this idea, as models are notorious for phasing storm systems too eagerly. This could mean we actually see two weak, unphased systems when the March 11-14 timeframe comes around, but with guidance supporting this phasing more and more, I find that the positives for phasing outweigh the things going against it.

This is the ECMWF 500mb vorticity forecast for the evening of March 11, the same timeframe as the first GFS image we analyzed. In this model's forecast we see the two systems are still trying to split up, but are nowhere near as elongated as the GFS model portrays them to be. Rather, we see the two systems fairly well defined, with one over Utah and the other over Kansas.

The above graphic shows the ECMWF 500mb vorticity forecast for the morning of March 13th, about 6 hours after the second GFS image we analyzed. Here, we see a significantly differing view as to what happens. The ECMWF takes the westernmost system and does retrograde it into the Southwest, but it does so as an incredibly weak system, so the system does not become a closed low and the Rex Block cannot form. We see a strong lobe of positive vorticity extending across the southern Plains and Gulf Coast, before we arrive on the parent storm, which has phased with the Canadian storm. This solution also results in a snowy solution for much of the Northeast, and it is a viable idea. However, because this is a northwest flow regime, and the ECMWF model has performed poorly in northwest flow situations this winter, I'm not ready to buy in on this solution just yet. It will probably take another day or so before we can at least refine the solutions we have now to try and come across a more solid consensus.

Here's an overview of current model projections for snowfall.