Saturday, February 2, 2013

Revised 2013 Severe Weather Season Outlook

This severe weather outlook has been revised for details and a reconstructed forecast using the best methods I am able to confidently use. The following information is put forth in the utmost confidence with recognition that this forecast does indeed have potential to make some disappoints for verification.

"Severe Weather Hotspot Eyes Midwest, East Plains This Spring"

Hello everyone, I am here to bring you the detailed forecast for this severe weather season. We'll get right on it with my analogues.

We start with geopotential heights at the 500 millibar level for years that had a La Nina early in the preceding year turning into a neutral or slight El Nino winter, again in the preceding year. Also included were years following solid negative Pacific Decadal Oscillation (PDO) years. Looking at the geopotential height anomalies for these analogue years, we find a gigantic swath of below normal height anomalies across the entire West US, including the West Coast and Rockies. The Upper Midwest is also included. More significant than this is the presence of that light green color in the Southeast. This is known in the weather world as the 'Southeast Ridge'. The Southeast Ridge is literally a ridge of high pressure that forms in the Southeast and provokes storms to stay further north in the southern Midwest rather than going through the Gulf Coast. Seeing this in place enhances my initial thoughts that storms will indeed take this track that sends storms (and severe weather) up into the Midwest and Plains.
This analogue image involves the same years as were used in the first image, and for the same spring months. This time, however, the image displays zonal wind anomalies. Zonal wind anomalies just means wind anomalies from west to east. These zonal wind anomalies are valid for the 300 millibar level, which is where the jet stream lies. For those unfamiliar with the upper wind dynamics, when you have the jet stream over you, severe weather potentials are enhanced. However, there is a very special incident that happens when the two main jet streams cross. You see, there are actually two separate jet streams- the Pacific Jet Stream is the one that determines how far south cold weather goes, and directs storms that come from the Pacific. The second jet stream is the subtropical jet stream, and this is the one that really gets the Gulf moisture flowing and pumps up severe weather in the South US. These two jet streams can actually merge into a single, very powerful jet stream in response to a strong storm system. To show just how powerful these merges can be, some of the worst severe weather outbreaks have happened in the midst of merged jet streams. Now imagine if these jet streams merged multiple times during the spring, and you can guess what that means. I pointed out both streams in the above image, but then circled an area of very high wind anomalies in the Midwest and Northeast. Inside this circle is where the jet streams have merged, and thus where I would expect the highest instances of strong to severe weather. Not necessarily the most tornadoes, but a general higher chance of more severe weather events.

Another item to closely watch is the sea surface temperature anomalies in the Gulf of Mexico. As we all know, an important item in all severe weather situations is ample Gulf moisture. Naturally, in above normal water temperatures, moisture is increased as air is inspired more frequently to rise in the face of warmer than normal waters. In a similar aspect, with above normal moisture in the Gulf, storm systems will then drag more moisture north onto land, that then aids in providing instability for severe weather. What this higher moisture also does is enhance heavy rain potentials, as higher moisture saturates the air, thus allowing more rain to fall in weather systems. 

Finally, we have to address the ongoing severe drought that is hitting much of the country. The ongoing drought does exactly the opposite to the effects the additional Gulf moisture has. The drought makes the land so dry that evaporation cannot occur. Thus, when storm systems enter the region highlighted in reds and maroon colors, they will not be able to grab moisture from the ground and cannot produce as strong of storms as would have been produced if the drought was not in place. Also, the drought creates warmer than normal weather over the Plains. Because little evaporation is occurring, clouds struggle to form. This lack of cloud cover enables the sun to shine on in a vicious cycle that supports long-term droughts. The warmer than normal temperatures can then create something called a baroclinic zone, which just means a temperature gradient. That is, the warmer, drier air can create its own air mass that can collide with the more temperate air mass out east. When storm systems plow through, it would not be uncommon to see these conflicting temperature masses enhance severe weather potentials.

I anticipate the best risk of severe weather to be over the Southern Plains, Midwest and Great Lakes. This comes as a result of my analogue years predicting strong merging of the jet streams over this region, as well as the presence of the infamous Southeast Ridge. I did mark the Southeast Ridge with an H in its respective region. Now, if you're wondering how often these outbreaks could be and how often we could have them, that's a whole other story. There will certainly be a period of a few weeks where some strong storm systems could very well bring about some solid severe weather outbreak potentials, but that's all that is known at the moment.


1 comment:

Anonymous said...

How is the northeast looking this summer in terms of thunderstorms/severe weather?