Even in a year when El Nino has commanded the weather news headlines, there is still occasion for blocking high pressure to elbow its way into the forefront. We're seeing that now at the end of March with the pattern over the Corn Belt set to be on the cold and stormy side over the next couple weeks. That means March will go out roaring -- like a lion.
What's happening to cause this is the formation of a large stationary high pressure ridge in the upper atmosphere over western Canada and Alaska. This high is a stout critter. We haven't seen this very much during the winter season 2015-16; but, it's around now, and serves to wall off -- block -- Pacific air from migrating into North America.
The Alaska ridge is complemented by the development of a trough -- or, low pressure -- in eastern Canada near Hudson Bay. Remember the Polar Vortex? This trough is the manifestation of that feature. (After all, the Polar Vortex never really goes away; it just "is", so to speak.)
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As a result, with circulation patterns going clockwise around the Alaska ridge, and counterclockwise around the Hudson Bay trough, the corridor is set for a healthy dose of chilly air to move on a beeline south from the Northwest Territories of Canada to the Midwest. And, there you have it -- a reminder of the primary weather newsmaker of the 2013-14 and 2014-15 winter seasons -- the Blocking High/Polar Vortex combo -- just in time for fieldwork.
But, but, but -- what happened to El Nino? Fear not -- it's still around. There is a fairly robust southern-branch jet stream indicated on the forecast models to run west-to-east out of the Pacific Ocean toward the southern tier of states. That jet is courtesy of the still-robust El Nino in the central Pacific. And, its influence may be profound, because the energy from that El Nino-source jet is set to interact with Gulf of Mexico moisture, bring that wet air farther north, and combine with the energetic northerly flow in the Midwest to bring frequent rounds of showers to the Corn Belt -- and thus, delay fieldwork -- during at least the early part of the month.
So, take the northern ridge/trough duet, toss in the strong El Nino-related southern jet stream, and stir in the usual contrasts that we have in spring as a matter of course, and you can see why the idea of some disruptions to fieldwork and planting is being talked about. Forecasts for a cool and wet spring are so far verifying.
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