Ag Weather Forum

It's All About The Block

Bryce Anderson
By  Bryce Anderson , Ag Meteorologist Emeritus
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We have had almost a tale of two Octobers this month. The first half of the month had its share of storms and rain across much of the central U.S. with associated issues with harvest. That round of difficult weather was replaced this past week, however, by a huge round of warm and dry conditions. In fact, we will likely see quite a few record high temperatures in the southern Plains this weekend; values to the low 90s are even possible in southwestern Oklahoma and northern Texas.

The genesis for this switch in weather fortunes can be traced to a major development in northern Canada--the weakening and fading of blocking high pressure in the northern Canadian territories. As we have described before, the occurrence of upper-atmosphere high pressure over northern Canada has been around to some extent for most of the past 18 months. Its influence was noted last spring with the rain and cool temperatures; a stormy late spring-early summer round of conditions--and of course the very cool conditions in July. That blocking high--whether it was located over Alaska, northern Canada or Greenland--forced the polar jet stream south and kept things cool and unsettled.

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But in the past two to three weeks, the weather maps have been empty over the northern latitudes. Blocking high pressure has not been around. And the result has been a more zonal air pattern, with a pronounced west-to-east tendency. That allows milder conditions to form--and at harvest time, those drier conditions are very useful for harvest progress. And so, we have two far different scenarios on the ag weather scene going into late October compared with earlier in the month.

If one is looking for a "magic bullet" for the winter forecast, the key variable will be if we have this blocking high decides to return or not--and this may well be independent of El Nino development. If blocking returns, we could see a quite cold and stormy winter--but if not, the typical El Nino tendencies are more likely to rule the scene. A big caveat here is that orecast models, as noted by NOAA, are not at this point able to discern many details about the formation of blocking high pressure, either forecasting its development, or its demise. This quirky entity will drive the fortunes the rest of this fall and into winter.

Bryce

Twitter @BAndersonDTN

(ES)

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JERRY KOCHANIUK
10/31/2014 | 8:18 AM CDT
How much warning could there be if this blocking pattern starts to set up? How is this being monitored?