As you can tell by the current Drought Monitor map and the Drought Outlook map, much of the Southern Plains is currently experiencing very dry conditions, with drought expected to worsen in most areas and spread even farther outward during the weeks leading to our late May and early June storm chases.
Southern Plains drought similar this is a common occurrence following a “La Nina” winter, when the cooling of equatorial Pacific sea surface temperatures leads to an atmospheric pattern often devoid of strong southern-stream winter storm systems that spread moisture across the southern tier of the nation.
A few considerations brought on by drought:
* High pressure systems. Droughts tend to exacerbate heat waves, which then exacerbate droughts. This vicious cycle can lead to a high pressure dome that can suppress convective development. Storm systems would be more likely to move along the edge of the high pressure system. If this transpires, it would definitely favor the Northern Plains over the Southern Plains for more activity this spring.
* Local moisture. While a strong flow off the Gulf of Mexico is the optimum means of moisture transport for severe weather in the central U.S., local moisture can be important in where storms fire, especially later in the season. This can come from recent storms dumping moisture in a local area, long-term patterns leaving soaked soils and standing water on the surface, and even cornstalks or other vegetation releasing moisture into the air through transpiration. Long-term drought areas would have drier soils and less standing water, meaning daily evaporation would be much less likely to aid in the atmospheric moisture needed for storm formation. Contrasted with the dry Southern Plains, the Northern Plains are very wet from two years of bonus rain and snow, so that region may have more moisture to work with in a marginal severe weather situations than farther south.
* Dryline. A larger region of dry soil can lead to the “dryline” shifting farther east than it typically does. That could take some of the severe weather out of the open Plains into the more forested/hilly regions of the central U.S.
* Dust. If there are storms in regions suffering drought, they can become choked with visibility-reducing dust. We experienced this during our hectic 2008 tornado frenzy in Kansas — we saw lots of tornadoes, but most of them weren’t very distinct. On the flip side, dusty situations can make outflow boundaries and inflow jets more obvious, and led to the development of visible “gustnadoes,” usually short-lived, rotating surface eddies created by outflow winds. (At right, a gustnado from eastern Colorado in 2006)
Of course, an individual chase situation is based on that specific day’s setup, not on long-range trends. We have chased high-precipitation supercells right smack in the middle of extreme drought zones. Large-scale patterns can change to overcome and change local drought conditions.
But these dry trends are worth monitoring and considering as we count down toward the 2011 Virginia Tech storm chases.
— Kevin Myatt