A GUIDE TO BAROCLINIC DISTURBANCES AFFECTING NORTH AMERICA, Part 2; Revised/Edited Thursday, November 4, 2021

[Larry Cosgrove]

GREAT PLAINS/CORN BELT DUSTERS

 

Region Of Cyclogenesis: Northern High Plains, often in WY or MT

 

Lowest Range Of Central Pressure At Surface: 978 to 1004MB

 

Forward Speed: Initially slow, with a rapid eastward motion as energy ejects from the Intermountain Region; sequence may last as much as six days

 

Season of Occurrence: Between September 15 and July 1; most often seen in autumn or spring months

 

500MB Structure: Varies; most common scenarios are ejecting longwave transforming to a shortwave or a large-scale open impulse that deepens into a vortex

 

Associated Sensible Weather: This type of low is noted for excessive production of tornadic supercells. Snow and ice can be a factor within the cold sector, and strong winds from the associated pressure gradient may kick up dust clouds (and in a few cases, ground blizzards) 

 

Classic Case: An example of a Great Plains Duster is the April 11 1965 "Palm Sunday Outbreak" which produced 78 tornadoes, killed 271 and injured 1500 people. 

 

PENINSULA STORMS 

 

Region Of Cyclogenesis: eastern Gulf of Mexico

 

Lowest Range Of Central Pressure At Surface: 988 to 1012MB

 

Forward Speed: Rapid; most effect seen in less than 48 hour period.

 

Season of Occurrence: Between September 1 and May 1

 

500MB Structure: Varies; most common scenarios are ejecting shortwave or a subtropical disturbance interacting with a trough further upstream

 

Associated Sensible Weather: There are two viable scenarios for lows tracking from the eastern Gulf of Mexico into the Gulf Stream. One is subtropical, wherein the lower latitude disturbance is kicked northeastward across the neck of the FL Peninsula (resulting in severe weather); the other is a cold solution with a wintertime low moving across S FL, resulting in an ice storm across the central and northern counties of the Sunshine State (as in the Christmas Eve 1989 storm). 

 

Classic Case:  A disturbance with subtropical characteristics triggered severe weather and tornadoes with damage across Pinellas County FL on Oct 3 1992.

 

PIEDMONT CYCLONES (slang: Inland Runner)

 

Region Of Cyclogenesis: Florida, Georgia, or South Carolina

 

Lowest Range Of Central Pressure At Surface: 968 to 1000MB

 

Forward Speed: Varies between 48 hours (shortwave) to as much as six days (longwave or tropically influenced systems).

 

Season of Occurrence: Usually October 1 through May 1; but can also occur in the summer months.

 

500MB Structure: All types are seen in range of storms taking this type of path

 

Associated Sensible Weather: There are three critical areas of interest with Piedmont Cyclones. One is heavy rainfall, with the deep fetch of moisture from the Gulf of Mexico, Gulf Stream and (sometimes) the northern Atlantic Ocean. Another is heavy snow across portions of the Appalachian Mountains. Still another issue is wind, often generated by the deepening pressure gradient associated with the rapidly ejecting form of this type. 

 

Classic Case: You can make an argument that the storm and related flooding event associated with Hurricane Agnes in June 1972 fits the description for this feature. And even a small case can be spoken for that the famed March 13 1993 "Superstorm" was a Piedmont Cyclone (although many would postulate that monster was a Miller A Hatteras Low). Perhaps a strong candidate for this designation was the April 8 2000 bombogenesis event that brought heavy snow, thunderstorms, strong winds and power outages to communities from Virginia to Maine.

 

 

 

CHATTANOOGA CHOO-CHOOS

 

Region Of Cyclogenesis: Northern Alabama, northern Georgia, eastern Tennessee

 

Lowest Range Of Central Pressure At Surface: 988 to 1008MB

 

Forward Speed: Normal to rapid; between 1 and 3 days.

 

Season of Occurrence: October 15 through May 1.

 

500MB Structure: Shortwave

 

Associated Sensible Weather: Heavy stratiform rain (overrunning) and convection form about a frontal wave over lower Appalachia, then spread north-northeast before the low exits the coast over the Delmarva Peninsula. If cold air is present in New England or Quebec, changeover to frozen types may occur. The name of this storm track type was coined by (then) WABC-TV (New York City NY) weatherman Tex Antoine in 1970. 

 

Classic Case: Small storm which developed over Chattanooga TN on New Year's Eve 1975 moved rapidly north-northeastward. Heavy rainfall within the cold sector of the low turned to a heavy wet snow impacting holiday revelers in the Philadelphia PA metro area early on New Year's Day. This event was called 48 hours in advance by Dr. Joel Myers of Accu-Weather in State College PA (when most sources went for light rain and not the verified 2-4 inches of snowfall).