Blocking Highs and Bad-Call Bricks
Once the most recent snowstorm to impact New England moved off the East Coast on Thursday, March 7, the low went nowhere fast. To see what I mean, check out the meteogram (below) at Boston's Logan Airport (KBOS), and note the protracted period of snow and northerly winds from 20Z on March 7 to 21Z on March 8 (Thursday to Friday afternoon).
The meteogram for Boston's Logan Airport (KBOS) from 20Z on March 7 to 21Z on March 8 (Thursday to Friday afternoon). Courtesy of the University of Wyoming.
Obviously, the low-pressure system became nearly stationary during this period (off the Northeast Coast). For confirmation, check out the GFS model analyses of mean sea-level pressure at 18Z on March 7, 00Z on March 8, 18Z on March 8 and 00Z on March 9.
On Friday, television forecasters attributed the nearly stationary nature of the storm system parked off the Northeast Coast to a "blocking high" over eastern Canada. I didn't react very well to this explanation...I threw "bad-call brick" at our television when I heard the status of the high over Canada being casually elevated to a block (the brick is made of foam, so no worries). I usually reserve the bad-call brick for when I'm watching sports, but, in this case, I just couldn't help myself, especially when forecasters talked about blocking in the setting of the surface high (the context for blocking is almost always 500 mb, not the earth's surface...stay tuned).
Even so, the surface high (check out the 00Z surface analysis on March 7 below) over Canada didn't really interrupt the natural west-to-east progression of weather systems across the Atlantic Ocean.
The GFS model analysis of mean sea-level isobars at 00Z on March 7, 2013 (7 P.M. EST on Wednesday, March 6). At this time, a low had moved off the Middle Atlantic Coast and was heading slowly northeast, but it's eastward progress was eventually blocked by a 500-mb ridge centered near Iceland (not shown...stay tuned). It was NOT blocked by the surface high over eastern Canada. Courtesy of Penn State.
Before I show you the real blocking high in last week's stagnant weather pattern, I first want to bring you up to speed on blocks. For starters, I'll list the formal criteria for a Rex block, which is the real McCoy when it comes to blocking highs. I'm sure some of you have heard of a Rex block before, but, in case you haven't, allow me to introduce Daniel F. Rex, who hailed from the University of Stockholm. For the record, he wrote two seminal papers on blocking highs in 1950 and 1951. I've always considered Rex the original guru on blocking highs, but I admit that my reverence toward him is probably not a universal sentiment in the meteorological community.
According to Rex, "blocking action at 500 mb must exhibit the following characteristics:
a) the basic westerly current must split into two branches,
b) each branch current must transport an appreciable mass,
c) the double-jet system must extend over at least 45 degrees of longitude,
d) a sharp transition from zonal type flow upstream to meridional type downstream must be observed across the current split, and
e) the pattern must exist with recognizable continuity for at least ten days."
The standards for blocking sometimes vary from author to author, but the term, Rex block, has persisted for a very long time. The last criterion, which deals with the longevity of the blocking anticyclone, is pretty restrictive. As a professional forecaster, I sort of combined the "spirit" of Rex's first four standards with all the criteria set forth by Treidl et al in their study published in 1981:
a) The 500-mb high must be closed (at least one closed height line),
b) The westerly current at 500 mb must split into two branches around the high,
c) The high must occur poleward of Latitude 30 degrees North (or South), and
d) Conditions a) and b) must persist for at least five days.
Obviously, the longevity requirement in the Treidl checklist is much less restrictive than the corresponding Rex standard, and I found the former to be more suitable for discussion related to short-range forecasting.
Unfortunately, over the years, some meteorologists have watered down the standards for blocking, much like the definition of a blizzard. In my view, blocking criteria, whether they be from Rex or Treidl list, have been diluted so much that even subtropical high-pressure systems have casually been elevated to blocking status (subtropical highs usually don't block mid-latitude weather systems because the main westerly 500-mb current typically doesn't dip equatorward of Latitude 30 degrees for any significant period of time).
The 500-mb height pattern at 12Z on February 25, 2004, displays a classic blocking high over the North Atlantic Ocean. Larger image. Courtesy of Earth System Research Laboratory.
So what does a real blocking high look like? To answer this question, check out the 12Z analysis of 500-mb heights on February 25, 2004 (above; larger image). This configuration of 500-mb heights is a classic Rex block! I annotated the image to help us go through the checklist, the most important two of which are the presence of a split flow and the fact the southern stream associated with this split flow carries "appreciable mass." We can make a qualitative assessment about the southern stream of the split flow by noting the relatively large gradient in 500-mb heights, which implies that 500-mb winds were blowing moderately fast (thus carrying "appreciable" mass of air). The split flow comes together again around Longitude 10 degrees West, so the "double-jet system" extends more than 45 degrees longitude. There's no doubt that there is a sharp transition from zonal flow (roughly west to east) to a more meridional flow (the uppermost arrow on my annotated image).
What I'm suggesting to you here is that the closed, 500-mb subtropical high centered over the western Caribbean yesterday morning was NOT a blocking high (check out the 12Z GFS model analysis of 500-mb heights on March 12). Indeed, there wasn't even any split in the westerly current. Moreover, the flow on the southern flank of the high was easterly, not westerly (here's 12Z GFS model analysis of 500-mb heights and 500-mb streamlines on March 12).
The 18Z GFS model analysis of 500-mb heights over part of the Northern Hemisphere on March 8, 2013. Larger image. Note the blocking high centered near Iceland and the closed low off the New England Coast. Courtesy of Penn State.
Okay, let's finally examine the 500-mb pattern during the period March 7-9, when protracted snow and northerly winds repeatedly buffeted the New England Seaboard (please revisit the Boston meteogram at the beginning of the blog). The 18Z GFS model analysis of 500-mb heights on Friday, March 8 (above; larger image) is a cropped polar view of the Northern Hemisphere (the North Pole lies at the center of the uncropped version, and Africa, Europe and Asia lie in the upper half of the image).
The real blocking high responsible for the stagnant weather pattern from New England and across the Atlantic Ocean was the real blocking high centered near Iceland. It satisfies the criteria for a blocking high...note that the westerly current splits near Spain, the southern stream carries appreciable mass over Africa. Note the log jam of weather systems across the Atlantic, including the closed 500-mb low associated with the storm system that produced protracted snow and northerly winds along the New England Seaboard.
In my view, the criteria for a blocking high are somewhat subjective, but there must be a set of standards that at least encapsulates prior scientific research. It just seems to me that blocking comes cheaper nowadays. Maybe I'm just an old-timer who winces when I hear ordinary high-pressure systems casually elevated to blocking status.