A GUIDE TO BAROCLINIC DISTURBANCES AFFECTING NORTH AMERICA; Section 1A; Thursday, November 4, 2021

[Larry Cosgrove]

1) TYPES OF UPPER LEVEL CYCLONIC SYSTEMS

 

TUTT SIGNATURES

Upper-level cyclones normally have convergence into their centers producing downward motion and clear skies. Divergence normally occurs along the periphery of the cyclone with rising motions causing cloudy skies and precipitation. Over land, however, convective activity is often present near the centers in the afternoon when enough instability is produced by a combination of daytime surface heating and the cold core of the upper-level low. The strongest convection normally occurs along the western periphery ahead of westward-moving cyclones (Source: NCEP).
 
TUTT signatures can serve to either shear apart nearby tropical features, or act to intensify the warm-core disturbance (if the center of the impulse drifts into the difluent upper right pocket of the TUTT). These baroclinic systems can also influence the steering of tropical waves and cyclones, depending on the vertical structure of the warmer feature. There are also cases of merger between the cold upper lows and the warmer counterparts, which may ultimately result in a larger circulation and a greater threat for hurricane formation. In a few scenarios, TUTT lows can give rise to subtropical or tropical cyclones (where the upper low warms and convection wraps around the diminished cold core, creating both vorticity and a surface circulation.
 
A TUTT can form at any time of the year. Greatest frequency of occurrence is in early and late summer, between 5 deg and 35 deg N Latitude in the Northern Hemisphere. The upper lows are easily discernible from concurrent warm-core disturbances, exhibiting a comma shape in satellite views with non-symmetry of radar echoes. Symptoms of apparent weather are heavy to severe thunderstorms in a mainly diurnal time frame.

 

 

MESOSCALE COLD POOLS AND DISTURBANCES (From: NOAA NWS Glossary)

 

See Also:

CIMSS Case Study Of MCC

Mesoscale Convective Complex

(abbrev. MCC)- MCC - Mesoscale Convective Complex. A large Mesoscale Convective System (MCS), generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs: * Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. * Duration: Size criteria must be met for at least 6 hours. * Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

Mesoscale Convective System

(MCS): A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.  If the grouping takes on a perfect spiral array with an eye (like a hurricane), it may be termed an MCV (Mesoscale Convective Vortex). Should convection group together in a linear fashion, projecting mostly in an eastward direction and maintain intensity for more than 750 miles and/or twelve hours duration, the term derecho is used as a descriptive expression.

 

TRANS-RIDGE WEAKNESSES

 

In some cases during the warmer months, the gap or cleavage between two existing subtropical highs can form an alley of cyclonic shear or even circulation. Termed a weakness, this corridor can serve to steer smaller features, such as an MCC or a tropical cyclone. Often in summer, the weakness will be home to pockets of cold air that increase the potential for diurnal and/or seabreeze convection. Continental thunderstorm groupings may drop into discontinuity (a phenomenon seen many times in late spring and summer over the lower Great Plains in 2007). The best way to view a weakness is by examining water vapor imagery. There are cases of breaching, however, where a weakening anticyclone is broken apart by upstream or lower latitude cyclonic systems, thus opening up a new pathway for disturbances to gravitate to higher latitudes. 

 

 

SHORTWAVES

 

A shortwave is the smallest type of upper level disturbances in the synoptic scale. These impulses are also the fastest moving at the 500MB level, at least until encountering blocking features and amplification trends aloft. Energy tends to be distributed in a linear fashion, accounting for precipitation being limited to a narrow stretch in the cold and overrunning sectors of the cyclone (although convective displays can and do occur close to the frontal structure). Distortion of the 500MB height contours is usually in a flattened "U" signature embedded within the westerlies. An example of this type of disturbance can be found in association with the February 3 1976 Arctic outbreak in the U.S., where two distinct impulses are found rotating through a mean trough complex.

 

In a few situations, the upper system may be quite intense, displaying a closed core about a small geographic area. These cyclones are termed mega-shortwaves, and are in essence major storms confined to a more compact coverage. A classic case was the February 19 1979 "President's Day Storm" which buried sections of the Virginias and Mid-Atlantic states with as much as three feet of snow.

 

 

TROUGH-TYPE CYCLONES  

 

Areas of low pressure that form along a non-centered upper disturbance are termed trough-type cyclones. If the surface system accompanies a full-latitude trough (or complex) and has sufficient moisture and instability to work with, widespread intense convection can result. This was the case with the Nov 11 2001 tornado outbreak across the Midwest, which occurred without a singular 500MB vorticity maximum but rather in association with a linear spread of energy stretching from northern Mexico to Ontario. 

 

 

HYBRID-TYPE STORMS

 

"Hybrid-type" storms are so called because they combine the progressive nature of a trough with the energy and abundant upper dynamics associated with closed 500MB lows. There may or may not be a closed contour in the upper level height display. But the extent of these systems is always synoptic-scale, with precipitation and cloud arrays covering a wide geographic area that at some point can cover most of the U.S. as well as portions of Canada and Mexico.

 

A hybrid disturbance can produce astounding amounts of severe weather, as evidenced by such events as the Nov 15 1989 (see graphic above), April 3 1974, and April 11 1965 outbreaks. Frozen precipitation tends to occur over a lesser time frame, given the tendency for linear advection of vorticity at the upper levels. The hallmark of these storms is the well-defined satellite signature of the cold front associated with the low. 

 

 

LONGWAVE LOWS

A storm falls into the "longwave" category when its representation at 500MB is closed, semicircular and alters the atmospheric height field to its north and east (that is, creates ridges through warm advection aloft). Another characteristic of a longwave low is slow motion and concentrated areas of precipitation within the cold and overrunning sectors. The core of vorticity is very intense, yet compact, and is more supportive of stratiform rain and snow to the immediate north and west of the surface low center. As an example, the Feb 23 1977 storm shown above had blizzard conditions and thundersnow in Goodland KS, while Hays KS 100 miles to the east saw sunshine and 60 deg (albeit with very strong straight-line winds).

 

2) COMMON STORM TRACKS OF MIDLATITUDE CYCLONES

 

BORDER TRACKERS

 

Region Of Cyclogenesis: British Columbia

 

Lowest Range Of Central Pressure At Surface: 988 to 1000MB

 

Forward Speed: Rapid; coast to coast sequence within 72 hours

 

Season of Occurrence: Any

 

500MB Structure: Shortwave or Hybrid; transitional from semizonal to amplifying trough; one of the most common types of storm track

 

Associated Sensible Weather: High winds along and north of 40 N Latitude; very strong warm advection along and south of storm track; precipitation usually sparse within the warm sector, except in cases during the "January Thaw"; Border Tracker storms in mid-January may draw in deep tropical moisture from the Gulf of Mexico, resulting in widespread overrunning and convective rainfall along and to the right of the trajectory of lowest surface pressure falls. Locations close to the actual storm track may see mixed or icing precipitation, with snowfall relegated to Canadian communities affected by the disturbance. Storms of this track type can be prolific severe thunderstorm producers in southern Canada and the northern Great Plains during the summer months.

 

Classic Case: The severe weather outbreak of June 22 - 25, 2007 across the Prairie Provinces and Ontario (see footage of EF 5 cell at Elie MB June 23, 2007 here) was in relation to a northern variant of a Border Tracker cyclone.

 

 

 

ALBERTA CLIPPERS

 

Region Of Cyclogenesis: Alberta Or Montana

 

Lowest Range Of Central Pressure At Surface: 982 to 1018MB

 

Forward Speed: Rapid; cross-continent sequence within 72 hours

 

Season of Occurrence: November 15 to April 1

 

500MB Structure: Shortwave in conjunction with vortex over Hudson Bay or James Bay

 

Associated Sensible Weather: One of the most misunderstood of all storm tracks. A true Alberta Clipper exhibits a rapid and parabolic motion. Accompanied by sudden deepening and intense winds at most levels of the atmosphere to the right of the Rocky Mountains. While moisture may be lacking, a typical "Clipper" can generate ground blizzards with rapid advection of cold air along and in the wake of its passage. Cold and overrunning sector snowfall is usually about 1 - 3 inches, although maximum amounts of 8 inches are not unheard of. Unlike many mid-latitude cyclones in North America, the heaviest snow is generally seen along and just north of the path of maximum vorticity at 500MB.

 

Some systems of the Clipper type may produce sleet and freezing rain in the overrunning sector, especially those storms that take the traditional track through MO into S QC. Prolonged wedge episodes may result with anywhere from 1 to 6 hours of icing or glazing in parts of Appalachia and the Interstate 95 corridor above Washington DC.

 

Variations In The Storm Track: There can be a wide range of deviations from the traditional Alberta Clipper trajectory. Some "clippers" imitate the "Border Tracker" type after formation. Another subfamily may take a further south and east path, with potential for center jumping or secondary cyclogenesis in the vicinity of the VA Tidewater. This option may present risks of moderate or heavy snowfall to the Mid-Atlantic and New England states, using the Atlantic Ocean as a moisture source while cA values infiltrate the northwest quadrant of the low. All disturbances of the Alberta Clipper family have two things in common: the lows are produced by leeside redevelopment and the upper level component is steered in some way by a cPk, cA, or cAk gyre over ON, MB, or Hudson Bay.

 

Classic Case: Occasionally the southern variant of the Alberta Clipper can produce near-blizzard conditions with heavy snows. Witness the January 22 - 24, 2005 storm that formed over S AB before taking an east-southeast path into the VA Capes. As for a "traditional" Clipper track, the January 28, 1977 blizzard affecting the lower Great Lakes stands out as an example of this type that can trigger extreme lake-effect and lake-related snowfall, escorted by bitter cold air from northern Canada.