A GUIDE TO BAROCLINIC DISTURBANCES AFFECTING NORTH AMERICA P.3

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HATTERAS LOWS (Nor'easters)

MILLER "A" and MILLER "B" Scenarios

 

Region Of Cyclogenesis: Miller "A" has origins along the coastline of the Gulf of Mexico; Miller "B" type may form as a secondary to cyclones of the Alberta Clipper, Great Lakes Backdoor, or Colorado/Trinidad Cyclone (see:

J. E.

of

the United States,

Department of Meteorology,

College of Engineering,

York University,

1945,

77 PP

J. E.

of

the United States,

York University,

1945,

77 PP

J.E. Miller, Cyclogenesis in the Atlantic Coastal Region Of The United States; Dept. Of Meteorology, New York University, 1945)

 

Lowest Range Of Central Pressure At Surface: 968 to 1014MB

 

Forward Speed: Normal to slow; sequence may last as much as six days, especially with the southern variance of this track 

 

Season of Occurrence: Any

 

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: The prime concern with the formation of both types of Hatteras Lows is the production of snow and ice within the northern quadrants of the storm.

 

Classic Cases: The all-time heaviest snowfall in the Philadelphia PA metro, January 7-9, 1996 (31 inches, 6 foot drifts) occurred as a result of a Miller"A" type of Hatteras Low. The February 5-8, 1978 Blizzard Affecting the northern Mid-Atlantic and New England states is an excellent example of the Miller "B" form of Nor'easter.

 

 

 

3) VORTICES AND SLOW-PROGRESS CLOSED LOWS

 

ALEUTIAN LOW

 

The Aleutian Low is known for four important variations. The classic position, over the central portions of the Aleutian Islands, often results in a semizonal flow aloft across the northern U.S. If the gyre drifts southward to a position between Unalaska and the Hawaiian Islands, a -EPO or +PNA styled blocking signature (or at least a more amplified ridge) is likely to arise. This worst-case scenario offers the threat for plentiful cold air drainage AND storm threats in the lower 48 states. Westward, or northward relocation into the Kamchatka Peninsula or Bering Sea is almost always accompanied by widespread warmer and drier conditions below 55 N latitude in North America.

 

 

GULF OF ALASKA VORTEX 

The formation of a Gulf of Alaska Vortex can have many ramifications on weather forecasts in the lower 48 states and southern Canada. For instance, the presence of an mP or mA closed cyclonic circulation in this position during the winter months is usually not conducive to the formation of blocking ridges. But at other times of the year, the presence of the gyre may boost +PNA ridging (see satellite image below of May 13 1997 as an example).

 

Greatest impacts in terms of apparent weather are excessive precipitation along the coastlines of Alaska, British Columbia, Washington and Oregon. In some cases, associated frontal structures with shortwaves ejecting from the Gulf of Alaska vortex can trigger high wind events. This is especially true when the low occurs in a southward variation from its climatological position. A northward displacement of the gyre results in abnormally cold, stormy weather across Alaska with showers and milder/warmer conditions in the Pacific Northwest.

 

HUDSON BAY VORTEX

In the classic position, the presence of this semipermanent feature can be viewed as a virtual guarantee of a cold intrusion across the eastern half of the U.S. If a southern variation is observed (such as James Bay or the Great Lakes), then Arctic air may have a particularly extreme presence over the most of the lower 48 states to the right of the Rocky Mountains (example: mid-January 1977). A rightward drift into the Ungava Peninsula means prolonged cold only for the Great Lakes and Northeast. Westward reformation of the gyre over the Prairie Provinces brings the harshest cold into the Upper Midwest while allowing for warming through the Eastern Seaboard. Should the motherlode relocate into northern Nunavut AR, then the most intense cold stays above the Canadian border.

 

 

GRAND BANKS VORTEX (Slang: 50/50 Low)

The presence of an Arctic motherlode over the Atlantic Ocean north and east of Newfoundland can present a critical role in winter storm and cold maintenance over the Eastern Seaboard. A gyre which is very large, closer to the shoreline of NL, and associated with a plentiful reservoir of cA and mA values can act to suppress coastal cyclogenesis (or merely take the path of any low far from the shoreline, thus resulting in a "miss" of precipitation chances). But a small, perhaps compact core of 500MB heights below 530dcm can help to hold a colder profile along the Coastal Plain while also act to merge with the oncoming storm. This action may result in heavy snowfall for the Interstate 95 corridor. An example of this scenario was the December 24-25 1966 "thunderblizzard", which interacted with a cold closed low just north of Atlantic Canada.

 

 

KONA LOW

An excellent description of the processes and characteristics of a Kona Low, with respect to the Hawaiian Islands, is SYNOPTIC STRUCTURE AND EVOLUTION OF A KONA LOW by Ian Morrison and Steven Businger of SOEST at the University of Hawaii.

 

Kona Lows evolve as disturbances which dig southeastward from the vicinity of the Kamchatka Peninsula, and become cut off from the polar westerlies. For as much as seven days (usual lifetime is about 96 hours), the cold low will entrain deep tropical moisture from the equator and, through mesoscale vorticity maxima ejecting from the center, trigger widespread heavy rainfall and thunderstorms through much of the "Chain of Pearls". High elevation snowfalls are common, and the dense cloud cover lowers temperatures in the island group into the much below normal range.

 

Teleconnections on Kona Lows are very favorable for ridging in the EPO and/or PNA positions, with resultant tendency for cold intrusions across Canada and the lower 48 states.

 

 

BERMUDA LOW

While semi-stationary closed 500MB lows in this position are a rarity, the formation of an upper level cyclone near Bermuda often produces severe impacts on oceanic as well as continental weather. In terms of affecting the jet stream configuration, a gyre in this position may be associated with the negative phase of the North Atlantic Oscillation. Rising heights north and east of the storm can produce blocking in the NAO position, as evidenced by the Rex signature that formed concurrent with the October 31 1991 "Perfect Storm". That block, in turn, forced amplification of a trough in the Great Plains, resulting in an unusual early November blizzard in portions of MN and ND. Of course, the cutoff circulation also drew in and absorbed Hurricane Grace. In its end sequence, the warming core produced the Unnamed Hurricane Of Nov 1 1991 over the Gulf Stream between the NC Outer Banks and Bermuda.

 

A more recent example of a Bermuda Low giving rise to a gale center and then a subtropical storm was the "Ana" sequence of April 20 2003.

 

 

 

 

CIRCUMPOLAR VORTEX

 

The circumpolar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America. It is important to note that the polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometers).

 

A true circumpolar (or polar) vortex is a closed cold circulation that is located within the Arctic Circle. The gyre is (by far) the largest cyclonic feature at higher latitudes and shows a dominant effect in teleconnection with the Arctic and polar westerlies. The cA motherlode is very slow moving, with 500MB depths of at least 500dcm. This vortex may be situated very close to the North Pole, at a time when the Arctic Oscillation is said to be positive.

The Arctic Oscillation

The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and high latitudes.

The oscillation exhibits a "negative phase" with relatively high pressure over the polar region and low pressure at about 45 degrees North, and a "positive phase" in which the pattern is reversed. In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase, as illustrated below.

Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.

Lower latitude jet stream flow tends toward semizonal, but in cases of a moderate or strong La Nina can exhibit great amplification with intense surface low pressure systems and frontal structures. (Credit: National Snow And Ice Data Center)

 

 

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