Record blocking patterns fueling extreme weather: detailed look at why it’s so cold

Posted by Rick Grow on March 21, 2013 at 11:20 am

Many a March has featured wild gyrations in temperatures from start to finish, but the extremes of this month and a year ago have given “springtime variability” a completely new meaning. The meteorology behind our current bone-chillingly cold weather is every bit as impressive as that which drove a stunning heat wave exactly one year ago.

Upper level pressure pattern late March 2013 (WeatherBell.com)

These hot and cold events arose from two large atmospheric “blocks,” last year’s – which brought the heat – was anchored close to home while this year’s formed 2,000-plus miles away over Greenland in recent days. It’s this second blocking pattern that is steering sub-zero cold out of the Polar region into the U.S. While much of this chilled air mass over the U.S. is not historically cold, 63 percent more daily snowfall records have been set so far this month compared to March 2012. Additional opportunities for record-setting snow will greet the Midwest and parts of the East in coming days.

514 daily snowfall records were broken in March 2012, primarily across the Western U.S. Through the first two thirds of this month, 847 new records have been set, with the lion’s share of these record-breakers occurring over the Midwest.

Meanwhile, Washington, D.C. – presently running one degree colder than average and near normal precipitation-wise – can expect temperatures to feel winter-like through month’s end. A near-record low value of the Arctic Oscillation (AO) – the climate index that measures the difference in relative pressure between the Arctic and mid-latitudes – is partly responsible for the unseasonable chill.
Just how low has the AO tanked? The AO index plummeted to -5.6 on Wednesday, a historically low value. It’s possible that the measurement taken today, tomorrow or over the weekend will be even more extreme, nearing or surpassing the most negative daily values appearing in the top half of the table below.

The daily AO index has decreased to -5 or lower on 12 separate occasions since 1950. Four, or one third, of these periods occurred during the winter of 2009-10. Prior to the December 20-25, 2009 period, it had been nearly 25 years since daily AO values dropped below -5.

The flow pattern roughly 18,000 feet above sea level (at the 500 mb level) serves as another manifestation of this record-challenging Arctic block. Forecasters at the Weather Prediction Center had called for positive height anomalies (which imply warmer than normal temperatures) between 350-400m to the north and west of Greenland. Underneath this exceptionally strong ridge or bump in the jet stream, an air mass supporting high temperatures up to 20 degrees below normal has plunged into the Washington, D.C. area today.

Blocking ridge over Polar region traps cold trough over continental U.S. Black “X” marks denote positive height anomalies of 375m or greater. Red “X” mark near Washington, D.C. indicates region where the trough is strongest relative to normal (negative height anomalies of 250m or greater).

As stated, a blocking pattern like this ranks among the strongest ever; to further illustrate that fact, I have plotted the 500 mb height composite anomaly from the 12 separate dates on which the AO plunged to its most negative values in recorded history. The composite, shown below, reveals positive anomalies in excess of 350m near Greenland – neatly matching the current block’s intensity.

This plot shows the average 500 mb height anomaly field over the Polar region on the days that the AO index dropped to its lowest 12 values of all time: 11/18/1959; 1/28/1966; 2/13/1969; 3/5/1970; 12/29/1976; 1/15/1977; 2/5/1978; 1/19/1985; 12/21/2009; 1/3/2010; 2/6/2010; and 2/14/2010.

One more remarkable aspect of this major league block: observations over Greenland are threatening to break the worldwide record for highest barometric pressure of 1083.3 mb, set on Dec. 31, 1968 in Siberia. NCEP’s Ocean Prediction Center analyzed the surface map (from Tuesday night) below, which features a high pressure center of at least 1074 mb over Greenland.

While another snow storm was raging in New England, tranquil conditions dominated under the massive, record-challenging high pressure system in Greenland.

Last March’s heat wave was also impressive from a meteorological standpoint, but certainly more extreme from a temperature record-setting perspective. A dominant block had developed over eastern North America, though positive anomalies (250m) were considerably lower. The more southerly position of the ridge and the clockwise flow pattern around it, which circulated very warm, moist air from the southwest Atlantic and Gulf of Mexico into areas from the Plains east – coupled with antecedent ground warmth from the winter of 2011-12 and an utter lack of snow cover over the northern tier – mostly explain why March 2012 was so warm.

The period from March 12-21, 2012 featured a different sort of “blocking” ridge (left image), one that steered storms and cold fronts well to the north of the U.S.-Canadian border. Underneath this heat dome, temperatures averaged around 9 degrees Celsius (~16 degrees Fahrenheit) warmer than normal in Washington, D.C. and around 15 degrees Celsius (~27 degrees Fahrenheit) above average over the Great Lakes region.

Though it fell short of matching the meteorological extremities of the March 2013 blocking event, the March 2012 heat wave set far more (over 36 times as many) temperature records. Consider that 6,216 daily warmest maximum temperature and 6,237 warmest minimum temperature records had been established in March 2012; by comparison, just 223 coldest maximum temperature and 120 coldest minimum temperature records have been broken to date in March 2013.
In short, the last two Marches have forced Washingtonians to embrace the extreme, ranging from an early summer to a late winter, scorching spring heat to a freakin’ cold vernal equinox, mega-stateside ridge to an all-time great Arctic block.