A GUIDE TO BAROCLINIC DISTURBANCES AFFECTING NORTH AMERICA P.1C

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GALVESTON BAY SPIN-UPS

 

Region Of Cyclogenesis: Western Gulf of Mexico, East Texas or Louisiana

 

Lowest Range Of Central Pressure At Surface: 980 to 1008MB

 

Forward Speed: Normal to rapid; sequence may last as much as four days 

 

Season of Occurrence: November 1 to April 15

 

500MB Structure: Shortwave, sometimes evolving into a longwave low over Quebec

 

Associated Sensible Weather: Cyclogenesis occurs along a stationary cP/mT boundary along the TX Gulf Coast; baroclinicity increased by warm SSTs over the western Gulf of Mexico may be a contributing causal factor.

 

Impulse is usually picked up by a broad 500MB trough and transported north-northeastward, along or west of the spine of the Appalachian Mountains. Realm of very high dewpoints follows the storm, concentrated in the overrunning and warm sectors. In this manner, heavy rainfall and perhaps thunderstorms can occur. When very cold values are dislodged, considerable icing may occur; snow is usually relegated to the upper left quadrant. In some cases, exceptionally deep storms of this type may combine with lake-related and lake-effect mechanisms to produce truly extreme snowfall and blizzards.

 

Classic Cases: January 9-10 1977;  Blizzard in much of Ohio Valley and lower Great Lakes (Buffalo 12" snow); January 24-27 1978; "The Great Blizzard" which set records for snow (36" South Bend IN); wind (Sandusky OH 111 mph gust); barometric pressure (958MB Cleveland OH); "The Ice Storm of 1998": devastating swath of sleet and freezing rain from E ON through S QC, N NY, VT, NH, W ME).

 

 

 

GULF COASTERS

 

Region Of Cyclogenesis: Northern Mexico, Southern or Central Texas

 

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

 

Forward Speed: Normal to slow; sequence may last as much as seven days 

 

Season of Occurrence: October 15 to May 1

 

500MB Structure: Shortwave which deepens to a longwave low

 

Associated Sensible Weather: Cyclogenesis occurs along a stationary cP/mT boundary over northern Mexico, South Texas, or along the TX Gulf Coast; baroclinicity increased by warm SSTs over the western Gulf of Mexico may be a contributing causal factor.

 

Superficial similarity to some storms of the "El Nino Express" type, but with much colder presentation in its northern quadrants. As a result of the cold thermal profile, may deliver significant snow and/or ice to very low latitudes (with severe weather in the warm sector over the central and southern Florida Peninsula). Passage of a Gulf Coaster storm can deliver true Arctic (cA) values as far south as the Bay Of Campeche.

 

Classic Case: February 12 - 15, 1960 system delivered heavy snow, ice and wind from the Rio Grande River Valley in Texas through most of the Old South, Appalachia, into New England and Quebec. Intrusion of cAk values brought 0 deg F isotherm into S TX, with all-time low of -23 F at Tulia in the TX Panhandle.

 

 

COLORADO - TRINIDAD CYCLONES (Northern "A", Southern "B" Variants)

 

Region Of Cyclogenesis: Northern New Mexico or Southern Colorado

 

Lowest Range Of Central Pressure At Surface: 978 to 1010MB

 

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

 

Season of Occurrence: November 1 to April 15

 

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

 

Associated Sensible Weather: The prime concern with the formation of both types of Colorado-Trinidad cyclones is the production of snow and ice within the northern quadrants of the storm. These systems tend to run along lines of extreme baroclinicity, and thus introduce warm, moist values of Gulf of Mexico origin into rarified cold air from Alaska and northern Canada. In a few cases of the northern path, a triple threat emerges of overrunning, synoptic and lake-effect snowfall. The southern variety of Colorado-Trinidad storm can generate an impressive transition spectrum involving rain with thunder, freezing rain, sleet and heavy snowfall ranging across the track line of the low pressure center. Reformation of the surface low near the Virginia Capes may result in a tap of deep tropical (mTw) moisture from the Sargasso and Caribbean Seas, with resultant excessive snows along the northern third of the Interstate 95 corridor.

 

Classic Cases: The all-time heaviest snowfall in the Chicago IL metro, January 26-27, 1967 (23 inches, 6 foot drifts) occurred as a result of a Colorado-Trinidad "A" cyclone. The February 16-17, 2003 President's Day Blizzard Affecting the Mid-Atlantic and New England states is an excellent example of the southern variant "B" form.

 

 

COLORADO - LIMON STORMS

 

Region Of Cyclogenesis: Southern Wyoming or Northern Colorado Front Range

 

Lowest Range Of Central Pressure At Surface: 960 to 994MB

 

Forward Speed: Very slow; sequence may last as much as six days

 

Season of Occurrence: February 1 to May 1

 

500MB Structure: Longwave (closed at 500MB, vertical structure); usually starts as a shortwave digging southeast from the vicinity of the Puget Sound

 

Associated Sensible Weather: Because this system occurs over the High Plains, orographic effects can influence intensity (through difluence, rapid lee cyclogenesis), wind (funneling effect into the Front Range) and snowfall (upslope enhancing precipitation output). Characterized by vertical (or nearly so) structure with deep pressure falls, Colorado-Limon lows can produce outlandish amounts of snow while also exhibiting sharp delineations in weather about the center. For instance, communities in W KS may experience warmth and sunshine while less than 100 miles away in CO, blizzard conditions with lightning and near-freezing temperatures are the rule.

 

There can be variations in track, and snows from these longwave lows have been known to reach as far south as east as Wichita KS and Omaha NE. Normally, however, blizzard conditions are only a concern for N CO....NW KS....NE Panhandle....SD Black Hills....E WY. If cold air is present north and east of the center of the cyclone, icing may be a complication affecting portions of E SD and MN. A typical Colorado-Limon low winds down after 48 hours of life as a filling process begins, with the low center undergoing cyclolysis over ND....N MN....W ON....S MB about six days after formation.

 

Classic Cases: The monster blizzard that struck the CO Front range with as much as 6 feet of snow on March 18-19, 2003 is a worst-case example of the Colorado-Limon storm. It should be noted that severe weather is rarely a major concern with longwave features, as the most intense dynamics are concentrated within the cold sector and PVA is rarely impressive in conjunction with the attendant cold front.

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