SHW Ch. 10 - Extratropical Cyclones Forming Along the East & Gulf Coasts

Coastal Extratropical Cyclones

• Preferentially form along the central East Coast (near Cape Hatteras, NC) and just off the Gulf Coast (centered near the TX-LA border); can, and do, form at other points along each coast; Figure 10.2

• East and Gulf coast cyclones are often more intense that their Rocky Mountain counterparts, due to the following four major factors:

  • 1) Latent heat released during condensation in the clouds contributes more to storm intensification; local moisture source in the warm Gulf of Mexico and in the warm Gulf Stream current.

  • 2) Sensible heat from the ocean (gulf) surface acts to heat the atmosphere, contributing more to storm intensification; heat energy transferred directly from water to air through conduction.

  • 3) Strong thermal contrasts between the ocean (gulf) and land enhance and maintain a sharp thermal boundary (i.e., baroclinic zone) along the coastline; most pronounced in winter.

  • 4) There is often more than one jetstreak acting to create upper-level divergence; the jetstreaks are found within two separate jetstreams (polar and subtropical) and their "phasing" creates the strongest storms.

East Coast Cyclones

• East coast cyclones typically develop after an earlier (primary) cyclone developed to the lee of the Rockies and tracked near or through the Great Lakes or Ohio Valley regions and into Canada, thus setting up the necessary environment for the secondary low to form.

  Environment Prior to Development:

  • Figure 10.3A: The primary low moves cold air southeastward across the continent behind its cold front, spilling over the Appalachians and reaching the east coast, thus creating the necessary baroclinic zone.

  • Figure 10.3D: The polar jetstream is now flowing across the central Atlantic coast, with a jetstreak (labeled J1 in the figure) on the west side of the trough, approaching the east coast.

  • Figure 10.4: If high pressure is located over New England or southeastern Canada, it will set up an easterly flow of warmer, maritime air into the east coast, thus creating a "coastal front" at the leading edge of the baroclinic zone; this also creates "cold air damming".

  • The "coastal front" becomes the boundary on which the secondary low will develop.

  • In the development of a strong east coast cyclone, a subtropical jet stream will merge with the polar jet off the coast coast and provide a second jetstreak (labeled J2 in Figure 10.3D), that acts with J1 to enhance the upper-level divergence.

    Initial Development:

  • Figure 10.3E: The jetstreak on the west side of the polar jet (J1) migrates toward and into the base of the trough; the left-exit region of J1, in addition to the change in curvature east of the trough, maximizes upper-level divergence on the east side of the trough over the "coastal front."

  • Figure 10.3E: The jetstreak in the higher altitude subtropical jet (J2) also contributes to the divergence region, as its left-exit region superimposes with J1.

  • Figure 10.3B: A surface low pressure will rapidly form and develop directly underneath the region of maximum upper-level divergence, enhanced by latent heat release and sensible heat transfer from the ocean by the action of surface wind circulations.

  • A "bomb cyclone" has a central pressure drop of 24 mb/24 hours, normalized at 60° N Latitude.

    (Continued: SHW Ch. 10 - Extratropical Cyclones Forming Along the East & Gulf Coasts)

    Storm Evolution:

  • Figure 10.3C: East coast cyclones (called Nor'easters) track northeast up along the coast, typically reaching their most intense stage 24 to 48 hours after initial development.

  • Dry air descends from the upper troposphere (Figure 10.5) to create the prominent "dry slot".

  • "Conveyor Belt Model" (discussed in Ahrens Ch. 13) describes the air mass flow into and around the low.

  • Figure 10.3F: Sometimes, the polar jetstreak (J1) and subtropical jetstreak (J2) merge ("phase"), creating a structure to the northwest of the cyclone center, similar to the "trowal" associated with the strong Rocky Mountain cyclones; can result in the greatest blizzards.

  • The upper-level low continually deepens until if becomes a "cut-off low"; the surface low and upper low align vertically to create a deep vortex.

  • Figure 10.3F: As the merged jetstreak propagates out of the trough and into the downstream ridge, and the flow around the "cut-off low" consists of counterclockwise flow (i.e., no longer any curvature change), the upper-level divergence ceases and the surface low "fills" (i.e., weakens).

  • Surface friction aids in the "filling" process, as it causes air to flow into the low, but is slower over water than land due to reduced friction over water, thus oceanic cyclones can remain strong for several days as they move up along and off the coast.

Gulf Coast Cyclones

• Gulf coast cyclones develop most frequently during years when the subtropical jet is a persistent strong feature in the upper troposphere over northern Mexico and the Gulf of Mexico (such as during "El Nino").

• Gulf coast cyclones typically follow one of two tracks (Figure 10.2), either first along the Gulf coast and then northeast up the Atlantic Seaboard (east of the Appalachians), or from the Gulf coast, inland along the Mississippi and Ohio River Valleys (west of the Appalachians).

1: East Coast Storm Track

• Figure 10.6A: Cold (or Arctic) front moves across the U.S. And arrives at the East and Gulf coasts, thereby setting up a strong barolclinic zone between the cold air and the warmer waters.

• Figure 10.6E: A large upper-level trough is often present over the entire eastern U.S., usually as a result of an earlier Rocky Mountain cyclone; there is usually a jetstreak (labeled J2 in this figure) to the west of the trough axis.

• Figure 10.6E: A subtropical jet flows from off the Pacific Ocean, across Mexico and over the Gulf coast; the subtropical jetstreak (labeled J1 in this figure) is the one that triggers storm formation.

• Figure 10.6B & 10.6F: the Gulf coast surface low develops in response to the left-exit region jetstreak effects of J1; this begins the low-level wind circulation pattern that drives warm air northward and cold air southward, creating "barolclinic instability"; latent heat and sensible heat transfers enhance this development process.

• Note: as the cold front is forced south over the Gulf of Mexico, it can trigger a line of strong T-storms.

• Figure 10.6C: the surface low will move eastward to remain under the divergent left-exit region of J1.

• Figure 10.6G: sometimes the approach of J2 can superimpose its left-exit region effects, leading to explosive deepening; this is not a necessary factor in development, but will affect the magnitude of the upper-level divergence and hence, the ultimate lowest central pressure of the cyclone.

• Gulf coast cyclones will continue to intensify as long as the upper-level divergence associated with the jetstreaks, the changes in flow curvature and latent heat release in the clouds (due to precip) all together EXCEED the convergence at the surface due to friction.

• Figure 10.6D: the fully matured cyclone has developed the "comma-shaped" cloud pattern associated with the "trowal-like" structure to the northwest of the low (where the heaviest precip falls) and the "dry slot" from the subsiding upper tropospheric polar jet; the surface system is occluded at this point.

• Figure 10.6H: the two jetstreaks, J2 & J1, have merged to the east of the cyclone; "cut-off low" forms.

• The dissipation stage is the same as with other previously discussed cyclone development.

  (Continued: SHW Ch. 10 - Extratropical Cyclones Forming Along the East & Gulf Coasts)

2: Mississippi-Ohio River Valley Storm Track

• Figure 10.7C: this track is more likely when the upper-level trough is farther west over the central U.S. Prior to formation, with the airflow more southerly over the eastern third of the U.S., the subtropical jet merges into the polar jet to the east of the trough axis.

• Figure 10.7A: a surface low develops in response to the divergent left-exit region of the subtropical jetstreak (labeled J1 in this Figure).

• Figure 10.7D: the single jetstreak, J1, remains over land, flowing northward over the eastern U.S.

• Figure 10.7B: the surface low tracks to the north and east, remaining under the maximum upper-level divergence, passing between the Mississippi River Valley and Appalachians, up to the Ohio Valley; the heaviest precip typically occurs in advance of the warm front and in the "wrap-around" precip band extending to the northwest of the surface cyclone.

• Note: strong southerly winds in the southeast sector of these storms transport warm, moist air from the Gulf of Mexico and over the Appalachians, which can lead to flash flooding.

• The dissipation stage is the same as with other previously discussed cyclone development.

Forecasting and Assessing the Impact of Coastal Cyclones

• Coastal cyclones affect the heavily populated corridor between Washington DC and Boston, MA.

• A forecast of significant snowfall from Nor'easters sets into motion an expensive and disrupting chain of public actions designed to protect people and property (DOT snowplows to clear roads, extra police & paramedics for accidents and stranded people, school and business closings, closed airports or canceled flights, etc.).

• Millions of dollars (spent or lost) are tied to the accuracy of storm forecasts!

• Numerical Weather Prediction (NWP) models are getting better, but determining the exact location of the "rain/snow line" remains a problem for the coastal cities, due to current "resolution" (i.e., grid point spacing) of the models; a distance of only 20 km (~12 miles) can mean the difference of crippling heavy snow or just another rainy day.

• Professor Gregory Zielinkski (University of Maine) proposed a winter storm classification scale (similar to the Saffir-Simpson Scale for hurricanes) from 1 to 5 that is based on: a) central low pressure of the cyclone; b) its 12-hour deepening rate; c) the maximum pressure gradient between the cyclone and its nearest high pressure. While this scale has the advantage of rating a storm in progress and disseminating the information to the public, it does not communicate information about expected snowfall.

• Dr. Paul Kocin (TWC) and Dr. Louis W. Uccelini (NCEP Director) proposed a index called "Northeast Snowfall Impact Scale" (NESIS), which takes into account both the snowfall distribution and amounts, as well as the population distribution and density (based on the 2000 Census). The NESIS Index (from 1 to 5) classifies storms on their impact of snowfall on major population centers. While this index has the advantage of classifying storms on their impacts on society, the disadvantage is that it cannot be applied while the storm is in progress.