Wednesday, March 13, 2013

Severe Weather Risk Increases Next Few Weeks

I anticipate the risk of severe weather to increase across the South Plains in the next week or more.

Shown above is a forecast chart of 850 millibar dewpoints. In areas of elevated dewpoint values, the atmosphere is considered richer and more supportive for convection in those areas. The American model predicts a nearly constant flow of moisture from the South and into the Plains to allow for more than one shot at a severe weather event across states such as Texas, Oklahoma, Arkansas, and more. The formation of this flow from the Gulf into the Plains may be enhanced by predictions of high pressure formation in the Plains in the medium range forecast time period. My personal expectation on this potential is we should see the beginning of severe weather season in the next 7-10 days. Granted, we have already seen a fairly active start to the severe weather season, but the official season may very well begin in the next 7-10 days if a storm system is able to intercept this ample moisture in the southern Plains.


March 18-20 Potential Winter Storm

The March 18-20 timeframe is being watched for a potential winter storm.

Shown above is a forecast for 500 millibar heights just 5 days from today. We see a sharp dip in the height contours and a cooling of temperatures over the Northern Plains, and it is fron this that we gather where the storm system is. A couple days ago, I was telling of how models would most likely shift back  south and give snow to the heart of the Midwest. Right now, model guidance is in good agreement that the storm track will give the heaviest snow to the Upper Midwest and North Plains.

The reason for this track change is the persistent high pressure in the Southeast. If you look closely, you may be able to see how the height contours (in solid black) begin to spread out when you get into the Northeast and Mid-Atlantic. This is symbolic of high pressure in the area, and we know how high pressure deflects low pressure. The problem with this storm system is that it will not be able to dive deep enough south to move in a track favorable for the heart of the Midwest. If high pressure in the West Coast was stronger, the storm would be persuaded to dive further south into the central Plains. If the high pressure in the East was weaker and not as spread across the Eastern Seaboard as it is projected to be, the storm would have an easier time moving more to the east than northeast. The northeast movement is preferred if there is dominant high pressure spread across the East US, as we will see with this storm system.


Long Range Overview - March 13, 2013

6-10 Day 500mb height anomalies from European (left) and American (right) model guidance.
Discussion: Medium range model guidance differs moderately in how the pattern will take shape over the Northern Hemisphere in the next couple of weeks. Both model systems agree in unusually strong high pressure over the Northern Hemisphere. This strong dominance of high pressure will allow low pressure anomalies to form and remain stagnant in perts of the globe. European model differs from American model in potential formation of low pressure in Canada. The American model is much more enthusiastic and allows for low pressure encompassing Canada and bringing about a very zonal (jet stream is west-to-east, no big cold blasts or warm shots) pattern. The European model prefers two centers of considerably stronger low pressure in the Gulf of Alaska and New England. This sort of pattern would allow high pressure to attempt to form in the Plains and Midwest, possibly able to connect to the dominant high pressure over the Arctic Circle.

Negative West Pacific Oscillation index will be provoked in either scenario with strong high pressure in the Bering Sea. The tendency of high pressure in the Bering Sea allows a pattern more supported by the European model, with lower pressure placed in a portion of Canada and not elongated throughout the entire region. Additionally, high pressure instigated by the negative WPO in the Southwest US is more enhanced by the European model, and it is this solution I favor for the next little while. Regardless of which model guidance prediction is correct, a weakly amplified pattern appears to be in store for the United States, and this should allow for a more stagnant pattern- not many sudden temperature swings due to a wild jet stream.