04-29-2008, 01:07 AM
OUTLOOK FOR SUMMER SEASON WEATHER: JUNE 1 Through SEPTEMBER 30, 2008
INFLUENCES OF WANING LA NINA SST ANOMALY AND MADDEN-JULIAN OSCILLATION ACROSS EQUATORIAL PACIFIC BASIN
It must be said from the outset that while ENSO episodes have an undeniable effect on atmospheric and seasonal patterns, each event has its own distinct outcome. For instance, the moderate-to-strong La Nina of 2007-2008 featured a volatile, often cold character over the U.S., with a highly active subtropical jet stream, much like what has happened in a number of El Nino winters! So I feel it is unwise to make a blanket forecast for the summer season based solely on ENSO climatology.
Over the past six weeks, the La Nina configuration has weakened tremendously to what may be termed a "weak" signal. However, with pronounced warming seen in waters near the Galapagos Islands and a strong Kelvin wave evident over the Indian Ocean and in the vicinity of Indonesia (which may act to induce upwelling of warmer subsurface waters in the western Pacific Ocean), an argument can be made that a slight El Nino episode could be in place by June. Numerical modeling of the ENSO status has been awful in recent months, with many equations (and human interpretations of same) calling for a continuation of La Nina into next winter. A middle ground seems the best option to use here: if one accepts this argument, then the above normal SSTs in the eastern equatorial Pacific Basin may be transitory and not a good indicator of potential tropical cyclone development in the Atlantic Basin. My suspicion is that what may appear to be a budding El Nino signature will drift westward and be replaced by a more chaotic hydrothermal pattern between Ecuador and the Philippines. While early signals, including activity associated with the Madden-Julian Oscillation and below-surface warm pools, might favor a +ENSO formation, keep in mind that such developments are more apt to occur as autumn approaches.
In compiling this forecast, I generally ignored the other (and increasingly numerous) "atmospheric indices" that have been populating discussions of forecasting in recent months. Using the past year as an example, outlooks considering Quasi-Biennial Oscillation (QBO), Pacific Decadal Oscillation (PDO), Global Atmospheric Angular Momentum (GLAAM) were of little or no use in figuring out the course of seasonal weather. The much-abused MJO is only useful in forecasting out to perhaps 30 days, and even then with the proviso that one ignores the "diagrams" and "phase charts" and concentrates, instead, on satellite and SST information which can tell the forecaster just what is happening in the tropics, and what bearing this activity may have on actual weather that occurs in North America and other locations.
Summing up, if ENSO-based climatology is applied to temperature and tropical cyclone forecasts for the upcoming summer, perhaps the best analogues would consider those seasons where a stronger La Nina episode is followed by a neutral ENSO signature over the equatorial Pacific Ocean. Given the unique nature of 2008 so far, which has vague resemblances to 1976 and 1999, I will use a blend of those two seasons as a reference point, but not an exact analogue, for temperature and tropical cyclone predictions for this summer.
500MB LONGWAVE PATTERN AND JET STREAM CONFIGURATION
Since November 2007, the polar westerlies have had the biggest impacts on sensible weather across North America. Fed by constant interaction with tropical moisture and energy sources (MJO overcoming La Nina influences), a parade of storms with wild temperature fluctuations produced a statistically normal temperature display in winter with extremely heavy total snow output over Canada and the northern third of the U.S. As we move deeper into the high-sun period, the mean storm track and jet stream will translate northward, allowing the formation and latitude increase of heat ridges.
In the past three months, formation of the strongest ridge signatures have been over central Mexico and over Cuba into the Bahamas. Extrapolating on these two positions (figuring on north to west relocation, and expansion, over the next three months), the end result should be near-classic Sonoran and Great Smokies heat ridges. Since the latter subtropical high sometimes undergoes merger or phasing with the former, a scenario emerges where hot temperatures dominate the western and central two-thirds of the nation, with occasional inroads into the Eastern Seaboard. Note below the continued dry soil associations over the Southwest, Highs Plains, and portions of the Old South.
There has been a tendency in the 500MB longwave pattern for -NAO ridge complexes. Should this alteration prove long term, there will be potential for cold air pooling over Ontario and Quebec, with energy from the west riding along a boundary from the Great Lakes into the Mid-Atlantic region. In later summer, a weakness may set up along the Atlantic Coastal Plain and Appalachian Mountains which could serve as a corridor for motion from tropical features moving out of the Sargasso Sea.
TEMPERATURE FORECAST
The possibility for widespread heat this summer seems centered over the western two-thirds of the nation. While deep mean easterlies beneath the two ridge complexes should enable lowering of temperatures along the southern tier of the U.S., the development of this moist flow (with its attendant clouds and convection) may be later in the season (that is, after July 15). The latitude increase in the heat ridges could mean a very warm start to summer from California into the Gulf States.
Anticipating periods of -NAO styled ridging, much of eastern Canada should have a cooler than normal summer. But I will caution that in many cases, the formation of positive height anomalies above Newfoundland is NOT accompanied by cP and mP regimes. So it is entirely possible that Ontario and Quebec could see a strong warm intrusion or two, as could sections of New England and the Great Lakes states. In the West, frequent northward lurches by the Sonoran heat ridge may bring portions of the Pacific Northwest, Black Hills and Prairie Provinces into the searing heat of summer.
PRECIPITATION FORECAST
It would be easy to fear that the recent trend of excessive rainfall across much of the Corn and Tobacco Belts will continue into and through the upcoming summer. Truth be told, however, the pattern of heavy rains is associated with passage of midlatitude cyclones and attendant frontal structures. As the high sun period progresses, the northward relocation of the storm track will surely mean a shift of the heavier precipitation chances to the Great Lakes and the Northeast. There may be an extensive period of northwest-flow convection, one of the characteristics of the 1976 analogue.
A suppressed monsoon season is also a possibility in the western states, especially in California and the lower Colorado River Valley. Conjunction of the two heat ridge complexes should also cut down on rainfall in the Great Plains and much of the Old South, and critical drought could result in some areas that just last year saw constant flooding (such as Dallas TX).
SEVERE THUNDERSTORM POTENTIAL
Typically in summer, the most intense convection occurs in areas where orographic effects, thermal conflicts and tropical weather processes (i.e. waves, depressions, etc.) are most prevalent. On the argument that the strongest upper dynamics will occur in the Great Lakes, Mid-Atlantic and southern New England regions (with northwest-flow climatology coming into play), then the chance for derechoes, supercells and LEWP signatures will be highest in a belt from western Ontario through Michigan into the New York City NY and Philadelphia PA metro areas. Climatology alone argues for a significant convective presence in parts of Florida, the Rio Grande Valley, and the central/southern Rocky Mountains, some of which may follow periodic "summer monsoon" intrusions into the Desert Regions. With a cool layer intruding from the Pacific Ocean, there may also be an increase in thunderstorm potential in the coastal ranges of northern California, Oregon, Washington, and British Columbia.
HURRICANE SEASON OUTLOOK
There has been speculation that the warming of waters adjacent to the Galapagos Islands (and with it, the demise of La Nina) would favor a diminished tropical cyclone output in the Atlantic Basin this summer and fall. While it is true that ENSO 1,2 sector warmth is tied to a more shearing profile aloft that is not supportive of warm-core cyclone growth in the eastern Caribbean Sea and the vicinity of the Lesser Antilles, there is no real proof that the current moderation of SSTs west of the Ecuadorian coastline is a lasting characteristic of the high-sun period. The waters may cool as the initial westward surge of warming moves into the central and western sections of the equatorial Pacific Basin. And the possibility of a moderate or strong Great Smokies heat ridge taking shape may mean more hospitable conditions for warm-core cyclogenesis in the Gulf of Mexico as well as the western Sargasso Sea.
Of greatest interest is the outcome of the ITCZ-born disturbances which emerge off of the western shoreline of Africa. After a very dry winter season, the number, intensity, and size of tropical waves adjacent to the equatorial shear zone has increased dramatically. Countering this renewal of impulse presence has been the large, powerful presence of the Saharan heat ridge. Impartation of dust has been very noticeable, spreading out in all directions from the center of the desert (see the visible METEOSAT image at the top for proof of this phenomenon). The subtropical high will likely expand northward into the Mediterranean Sea and southern Europe this summer, which may cause the advection of particulates and dry air to diminish between July and mid-September. And with waters in the corridor of the Atlantic Basin off to a warmer than normal start (and considering that the current cool patch adjacent to the Lesser Antilles should moderate in the coming months), there should be perhaps a two month window for "Cape Verde" type storms to organize and move westward. With potential threats to the major islands and sections of North America.
The rapid warming of the Gulf of Mexico (almost a given in spring and summer) may also interact with TUTT signatures or spin-off convection groups that advance southward and become stranded from the westerlies. Since there have been two incidents so far this spring of gale center formation in the westernmost Atlantic Basin above 20 N Latitude, a possibility looms for a subtropical system to tale shape with at least a slight threat for impact on the Gulf Coast or shoreline of the Southeast early in Hurricane Season, or maybe slightly previous. A more likely risk of cold-origin disturbances will come in October in marine areas between the Bahamas and Bermuda, or near the Azores chain.
When you factor in the forecast of a neutral ENSO environment, a heightened warm water presence over the Gulf of Mexico and western Atlantic Ocean, against negatives such as a reduced presence of a classic Bermuda High, prevalent heat and dust from the Sahara Desert, and a possible shearing wind band and cool patch across the eastern Caribbean Sea, the overall outlook begins to resemble that of the previous tropical cyclone season in 2007. Potential exists for systems to spring to life after a weakening phase, with effects on Mexico and Central America. Other landfall options are Florida and Texas in the middle or late part of summer, and an outside risk to the Eastern Seaboard in September and October, when the polar westerlies may occasionally contort into a favorable west-of-Appalachia trough complex.
SUMMARY
Based on drought signatures, an increasingly neutral ENSO signature in the Pacific Basin, early indicators of heat ridge formation and weak analogues to the summers of 1976 and 1999, the call here is for a very hot summer across much of the U.S. Critical heat and drought may envelop sections of the West, Great Plains, and Old South, with more normal temperature indicators in coastal sections of the Pacific Northwest and the Deep South. A cool summer season is a likely outcome for much of eastern Canada, the lower Great Lakes and the Northeast. Threats from tropical cyclones look to be slightly above normal, with greatest potential for a North American landfall in Florida and Texas (with some chance for a warm-core system affecting the East Coast or Appalachia during the first part of autumn, as the 500MB longwave pattern shifts into a weakness signature over the eastern third of the nation.
INFLUENCES OF WANING LA NINA SST ANOMALY AND MADDEN-JULIAN OSCILLATION ACROSS EQUATORIAL PACIFIC BASIN
It must be said from the outset that while ENSO episodes have an undeniable effect on atmospheric and seasonal patterns, each event has its own distinct outcome. For instance, the moderate-to-strong La Nina of 2007-2008 featured a volatile, often cold character over the U.S., with a highly active subtropical jet stream, much like what has happened in a number of El Nino winters! So I feel it is unwise to make a blanket forecast for the summer season based solely on ENSO climatology.
Over the past six weeks, the La Nina configuration has weakened tremendously to what may be termed a "weak" signal. However, with pronounced warming seen in waters near the Galapagos Islands and a strong Kelvin wave evident over the Indian Ocean and in the vicinity of Indonesia (which may act to induce upwelling of warmer subsurface waters in the western Pacific Ocean), an argument can be made that a slight El Nino episode could be in place by June. Numerical modeling of the ENSO status has been awful in recent months, with many equations (and human interpretations of same) calling for a continuation of La Nina into next winter. A middle ground seems the best option to use here: if one accepts this argument, then the above normal SSTs in the eastern equatorial Pacific Basin may be transitory and not a good indicator of potential tropical cyclone development in the Atlantic Basin. My suspicion is that what may appear to be a budding El Nino signature will drift westward and be replaced by a more chaotic hydrothermal pattern between Ecuador and the Philippines. While early signals, including activity associated with the Madden-Julian Oscillation and below-surface warm pools, might favor a +ENSO formation, keep in mind that such developments are more apt to occur as autumn approaches.
In compiling this forecast, I generally ignored the other (and increasingly numerous) "atmospheric indices" that have been populating discussions of forecasting in recent months. Using the past year as an example, outlooks considering Quasi-Biennial Oscillation (QBO), Pacific Decadal Oscillation (PDO), Global Atmospheric Angular Momentum (GLAAM) were of little or no use in figuring out the course of seasonal weather. The much-abused MJO is only useful in forecasting out to perhaps 30 days, and even then with the proviso that one ignores the "diagrams" and "phase charts" and concentrates, instead, on satellite and SST information which can tell the forecaster just what is happening in the tropics, and what bearing this activity may have on actual weather that occurs in North America and other locations.
Summing up, if ENSO-based climatology is applied to temperature and tropical cyclone forecasts for the upcoming summer, perhaps the best analogues would consider those seasons where a stronger La Nina episode is followed by a neutral ENSO signature over the equatorial Pacific Ocean. Given the unique nature of 2008 so far, which has vague resemblances to 1976 and 1999, I will use a blend of those two seasons as a reference point, but not an exact analogue, for temperature and tropical cyclone predictions for this summer.
500MB LONGWAVE PATTERN AND JET STREAM CONFIGURATION
Since November 2007, the polar westerlies have had the biggest impacts on sensible weather across North America. Fed by constant interaction with tropical moisture and energy sources (MJO overcoming La Nina influences), a parade of storms with wild temperature fluctuations produced a statistically normal temperature display in winter with extremely heavy total snow output over Canada and the northern third of the U.S. As we move deeper into the high-sun period, the mean storm track and jet stream will translate northward, allowing the formation and latitude increase of heat ridges.
In the past three months, formation of the strongest ridge signatures have been over central Mexico and over Cuba into the Bahamas. Extrapolating on these two positions (figuring on north to west relocation, and expansion, over the next three months), the end result should be near-classic Sonoran and Great Smokies heat ridges. Since the latter subtropical high sometimes undergoes merger or phasing with the former, a scenario emerges where hot temperatures dominate the western and central two-thirds of the nation, with occasional inroads into the Eastern Seaboard. Note below the continued dry soil associations over the Southwest, Highs Plains, and portions of the Old South.
There has been a tendency in the 500MB longwave pattern for -NAO ridge complexes. Should this alteration prove long term, there will be potential for cold air pooling over Ontario and Quebec, with energy from the west riding along a boundary from the Great Lakes into the Mid-Atlantic region. In later summer, a weakness may set up along the Atlantic Coastal Plain and Appalachian Mountains which could serve as a corridor for motion from tropical features moving out of the Sargasso Sea.
TEMPERATURE FORECAST
The possibility for widespread heat this summer seems centered over the western two-thirds of the nation. While deep mean easterlies beneath the two ridge complexes should enable lowering of temperatures along the southern tier of the U.S., the development of this moist flow (with its attendant clouds and convection) may be later in the season (that is, after July 15). The latitude increase in the heat ridges could mean a very warm start to summer from California into the Gulf States.
Anticipating periods of -NAO styled ridging, much of eastern Canada should have a cooler than normal summer. But I will caution that in many cases, the formation of positive height anomalies above Newfoundland is NOT accompanied by cP and mP regimes. So it is entirely possible that Ontario and Quebec could see a strong warm intrusion or two, as could sections of New England and the Great Lakes states. In the West, frequent northward lurches by the Sonoran heat ridge may bring portions of the Pacific Northwest, Black Hills and Prairie Provinces into the searing heat of summer.
PRECIPITATION FORECAST
It would be easy to fear that the recent trend of excessive rainfall across much of the Corn and Tobacco Belts will continue into and through the upcoming summer. Truth be told, however, the pattern of heavy rains is associated with passage of midlatitude cyclones and attendant frontal structures. As the high sun period progresses, the northward relocation of the storm track will surely mean a shift of the heavier precipitation chances to the Great Lakes and the Northeast. There may be an extensive period of northwest-flow convection, one of the characteristics of the 1976 analogue.
A suppressed monsoon season is also a possibility in the western states, especially in California and the lower Colorado River Valley. Conjunction of the two heat ridge complexes should also cut down on rainfall in the Great Plains and much of the Old South, and critical drought could result in some areas that just last year saw constant flooding (such as Dallas TX).
SEVERE THUNDERSTORM POTENTIAL
Typically in summer, the most intense convection occurs in areas where orographic effects, thermal conflicts and tropical weather processes (i.e. waves, depressions, etc.) are most prevalent. On the argument that the strongest upper dynamics will occur in the Great Lakes, Mid-Atlantic and southern New England regions (with northwest-flow climatology coming into play), then the chance for derechoes, supercells and LEWP signatures will be highest in a belt from western Ontario through Michigan into the New York City NY and Philadelphia PA metro areas. Climatology alone argues for a significant convective presence in parts of Florida, the Rio Grande Valley, and the central/southern Rocky Mountains, some of which may follow periodic "summer monsoon" intrusions into the Desert Regions. With a cool layer intruding from the Pacific Ocean, there may also be an increase in thunderstorm potential in the coastal ranges of northern California, Oregon, Washington, and British Columbia.
HURRICANE SEASON OUTLOOK
There has been speculation that the warming of waters adjacent to the Galapagos Islands (and with it, the demise of La Nina) would favor a diminished tropical cyclone output in the Atlantic Basin this summer and fall. While it is true that ENSO 1,2 sector warmth is tied to a more shearing profile aloft that is not supportive of warm-core cyclone growth in the eastern Caribbean Sea and the vicinity of the Lesser Antilles, there is no real proof that the current moderation of SSTs west of the Ecuadorian coastline is a lasting characteristic of the high-sun period. The waters may cool as the initial westward surge of warming moves into the central and western sections of the equatorial Pacific Basin. And the possibility of a moderate or strong Great Smokies heat ridge taking shape may mean more hospitable conditions for warm-core cyclogenesis in the Gulf of Mexico as well as the western Sargasso Sea.
Of greatest interest is the outcome of the ITCZ-born disturbances which emerge off of the western shoreline of Africa. After a very dry winter season, the number, intensity, and size of tropical waves adjacent to the equatorial shear zone has increased dramatically. Countering this renewal of impulse presence has been the large, powerful presence of the Saharan heat ridge. Impartation of dust has been very noticeable, spreading out in all directions from the center of the desert (see the visible METEOSAT image at the top for proof of this phenomenon). The subtropical high will likely expand northward into the Mediterranean Sea and southern Europe this summer, which may cause the advection of particulates and dry air to diminish between July and mid-September. And with waters in the corridor of the Atlantic Basin off to a warmer than normal start (and considering that the current cool patch adjacent to the Lesser Antilles should moderate in the coming months), there should be perhaps a two month window for "Cape Verde" type storms to organize and move westward. With potential threats to the major islands and sections of North America.
The rapid warming of the Gulf of Mexico (almost a given in spring and summer) may also interact with TUTT signatures or spin-off convection groups that advance southward and become stranded from the westerlies. Since there have been two incidents so far this spring of gale center formation in the westernmost Atlantic Basin above 20 N Latitude, a possibility looms for a subtropical system to tale shape with at least a slight threat for impact on the Gulf Coast or shoreline of the Southeast early in Hurricane Season, or maybe slightly previous. A more likely risk of cold-origin disturbances will come in October in marine areas between the Bahamas and Bermuda, or near the Azores chain.
When you factor in the forecast of a neutral ENSO environment, a heightened warm water presence over the Gulf of Mexico and western Atlantic Ocean, against negatives such as a reduced presence of a classic Bermuda High, prevalent heat and dust from the Sahara Desert, and a possible shearing wind band and cool patch across the eastern Caribbean Sea, the overall outlook begins to resemble that of the previous tropical cyclone season in 2007. Potential exists for systems to spring to life after a weakening phase, with effects on Mexico and Central America. Other landfall options are Florida and Texas in the middle or late part of summer, and an outside risk to the Eastern Seaboard in September and October, when the polar westerlies may occasionally contort into a favorable west-of-Appalachia trough complex.
SUMMARY
Based on drought signatures, an increasingly neutral ENSO signature in the Pacific Basin, early indicators of heat ridge formation and weak analogues to the summers of 1976 and 1999, the call here is for a very hot summer across much of the U.S. Critical heat and drought may envelop sections of the West, Great Plains, and Old South, with more normal temperature indicators in coastal sections of the Pacific Northwest and the Deep South. A cool summer season is a likely outcome for much of eastern Canada, the lower Great Lakes and the Northeast. Threats from tropical cyclones look to be slightly above normal, with greatest potential for a North American landfall in Florida and Texas (with some chance for a warm-core system affecting the East Coast or Appalachia during the first part of autumn, as the 500MB longwave pattern shifts into a weakness signature over the eastern third of the nation.