Ag Weather Forum
Bryce Anderson DTN Ag Meteorologist and DTN Analyst

Monday 12/09/13

Blocking High Helped Bring Winter Storm

This past weekend's ice and snow storm is well-documented. All sorts of problems--travel, livestock stress, human safety--have been catalogued and talked about. And we will see more of this, at least the cold, during much of this week before some--emphasis some--moderation develops by the end of the week through next week.

A big reason for the harsh cold that moved in is the development of powerful upper-air high pressure in Alaska, which impeded air flow from the Pacific Ocean into the interior U.S. This feature is known as a "blocking high", and it can be a big, big weather-maker. We also saw this feature earlier in 2013, from late winter clear into midsummer. In that time frame, blocking high pressure set up in northern Canada, steered the jet stream (along with associated colder temperatures and rainfall) south through the Midwest. And we all know what happened; copious rain and cool temperatures caused many, many delays in corn planting progress during the spring, and then allowed the late-planted corn to go through pollination in unusually mild conditions.

What's tricky in the forecast world with these blocking highs is that--as influential as they can be--they don't show very well on forecast weather models. Here is the NOAA definition of blocking highs and the agency's interpretation of that issue. I have added some further details and highlights of interest.

Bryce

Twitter @BAndersonDTN

"Atmospheric blocking is commonly referred to as the situation when the normal zonal flow is interrupted by strong and persistent meridional (north to south) flow. The normal eastward progression of synoptic disturbances is obstructed leading to episodes of prolonged extreme weather conditions. On intra-seasonal time scales the persistent weather extremes can last from several days up to a few weeks, often accompanied by significant temperature and precipitation anomalies. Examples of the 500 hPa height (500 millibar constant pressure height) and anomaly fields (are) associated with mature blocking episodes over the northeastern Atlantic and the North Pacific. These two regions are preferred areas for atmospheric blocking during the northern hemisphere cold season.

A common finding among scientific studies is that these long-lived weather extremes are associated with recurrent atmospheric flow anomalies. Numerous studies have found that the poor forecast skill beyond a few days results principally from the inability of numerical weather prediction models to simulate the onset and evolution of blocking flows.

(CZ)

Posted at 7:31AM CST 12/09/13 by Bryce Anderson
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