Introduction to Blocking By Richard H. Grumm National Weather Service State College PA With contributions from Joshua Watson ER/SSD

Introduction What is blocking –Definitions –Locations –Time of year Some known blocking patterns –The Rex Block –The Omega block Forecast Implications of Blocking Most blocking research has focused on mid-latitude blocking systems Higher latitude stationary cut-off anticyclones are not as well understood.

Blocking Normally anticyclones and cyclones flow west to east. –Normal geostrophic mid-latitude flow is west-east –Therefore, most mid-latitude systems move west to east The thermal wind steers most systems Alterations of these steering winds is considered a form of blocking –Get easterly geostrophic flow Warm cut-off anticyclones in the middle and upper troposphere will alter the normal flow of cyclones –Storms either go over the block or, –They cut under the block, creating –Split flow, a well known characteristic of blocking is “split flow” The impact is statistically known and validated Easterly geostrophic flow as a result of a closed low, with no longitudinally co-located ridge or anticyclone, is not considered a blocking system.

Rex Defined Criteria Basic zonal flow split into two branches Each branch must move an appreciable amount of mass Each branch must extend over at least 45 o of longitude A sharp zonal to meridional flow must exist in the region of the split This pattern must persist for at about 10 days (note short-lived patterns are not considered to be blocking episodes)

A few more bits The warm anticyclones are often accompanied by cold- cut off cyclones or troughs Blocking has preferred locations of occurrence and is far more frequent in the northern hemisphere –North Atlantic 30E to 60 W peak 30W-10W –North Pacific 150E to 120W peak 170W-140W Clear preference off the west coast of continents –More blocks of Europe than North America –Atlantic blocks last about 8-16 days on average –Pacific blocks last about 10 days –The 40-year September - May climatology showed 35 blocking days in the Atlantic and 30 days in the Pacific. –More of a cold season phenomena.

Seasonality Minimum Jul-Oct (summer minimum) Maximum is reached in winter and drops off fast April and May. Newer data suggest Maximum Dec-Jan Rapid increase in November peaking in over winter, Cold season is best time of year with blocking present in the North Atlantic about 40% of the time. Summary Blocks tend to form most often over the eastern Pacific and eastern Atlantic oceans in the Northern Hemisphere, just west of North America and northern Europe. They form most often in winter and spring. They are most pronounced near at higher altitudes of 500 to 300 hPa but affect the flow of all weather systems. Some more recent work suggests Atlantic block is closer to the Prime Meridian and near the dateline in the Pacific

The Rex Block Has two adjacent highs and lows. Impressive Rex Blocks will have a strong low pressure next to a strong high pressure. The high pressure will be located in a generally north direction from the low pressure. The low associated with the Rex Block is not completely cut-off from the upper level flow. Strong ridging north of the low causes the airflow to move from high latitudes to low latitudes with little west to east movement. Typical block has close high poleward of the cut-off low. The airflow pattern follows a backward "S" trajectory.

Jan 1996 Example Note there was blocking in Atlantic Jan 1996

Record Block The blocking pattern and anomalies with the blocking in the winter were large. This may be the most impressive blocking season in the 50-year records according to Josh Watson’s research. See figures on next slide 

The flow

The Anomalies

Pacific Block formed 01 Feb-5 Feb 1996

The Omega Block  The shape resembles the Greek Letter Omega (  ) The region under the omega block experiences dry weather and light wind for an extended period of time while rain and clouds are common in association with the two troughs on either side of the omega block. Omega blocks make forecasting easier since you can pinpoint areas that will be dominated by dry or rainy weather for several days. The right side of the omega block will have below normal temperatures while the region to the left will have above normal temperatures in this case.

29 APR 1999 to May 2

Cut-off lows Cut off lows commonly occur when the upper level winds shift to a higher latitude and leave a circulating low pressure behind. Several height contours encircle the low at upper levels. A cut- off low is more dramatic than the one shown in the Rex block example in the Southwest US. They can persist for several days, bringing several days of rainfall underneath the areas they spin-down. They occur commonly off the coast of California but can develop anywhere.

Cut-off low of 1 Nov 1999 our  had two this one was transient!

Cut-off low of 1 Nov 1999 our  had two!

Split Flow Split flow occurs when the jet stream branches into two separate branches. Weather systems flow quickly through each branch of the jet but the weather pattern can become slow to change in the region between the two jets. Normally these patterns result in or are the result of a more significant blocking pattern.  blocks produce split flow.

Blocking Examples - Atlantic (Watson and Colucci 2002 WAF: Dec 1995) Retrogressing Atlantic Blocks are a difficult forecast challenge!

Upper Ridge Retrogresses (has cut-off low / Rex block at times too!)

Blocking Examples – Pacific (Watson and Colucci 2002 WAF: 1-5 Feb1996)

Evolves to Rex Look 29 Jan

Strong Cut-off High and lows see any  ‘s

Block was 1-5 Feb

Forecast of Blocking Use ensembles and calibrate them Compute a blocking index along a longitude –High latitude area and mid-latitude area –GHGS = (Z 60 -Z 40 )/(I 60 -I 40 ) –GHGN = (Z 80 -Z 60 )/(I 80 -I 60 ) If Index is 1 for potentially blocked areas. If over about 20 degrees of longitude the area is considered blocked if GHGS greater than 0 and GHGN is less than 10 m per degree of latitude. Values are actually taken over -4,0, and 4 degree increments of the specified latitude due to resolution of EC re-analysis data being 4x4 grid

Forecast of Blocking

Forecast Implications of Blocking Pattern slow to change –Nice weather under the upper ridge –Can be cool and persistently wet under the low Block has to spin down or break down Models do not do well –Forecasting the formation of a block –Forecasting the end or decay of a block Retrogressing Atlantic blocks are the most difficult block forecast challenge (JW)

Conclusions What is blocking Basic zonal flow split into two branches Each branch must move an appreciable amount of mass Each branch must extend over at least 45 o of longitude A sharp zonal to meridianol flow must exist in the region of the split This pattern must persist for at about 10 days Some known blocking patterns Preferred locations: North Atlantic/Pacific Types of blocks: –The Rex Block –The Omega block Forecast Implications of Blocking –Beware the retrograding North Atlantic block! –Not to mention….the block to the back!

Further Study New studies suggest: –There are pre-conditions to blocking due to planetary scale mechanisms that may set-up the blocking episode –Colucci 2001: Journal of Atmospheric Sciences discusses this concept of preconditioning. –Blocking Indices vary from organization to organization. The Tibaldi and Molteni index from 1990 was updated and improved in 1995.

Links Blocking defined: htmlhttp:// html NCEP Blocking index and forecasts: ecip/CWlink/blocking/index/index.nh.htmlhttp:// ecip/CWlink/blocking/index/index.nh.html Forecast of blocking: locking/gifs/030113/forecast.htm locking/gifs/030113/forecast.htm

Acknowledgements Software: Robert Hart Joshua Watson –Blocking case of with images –Blocking references –Editorial assistance and comments to improve the text