Table of Contents

Executive Summary

1. Overview

2. Southern U.S. heat wave and drought

    a. When and where?

    b. How bad is it?

    c. Agricultural and livestock impacts

    d. Fires

    e. Short-term outlook

    f. Seasonal outlook

3. Storminess in the central and northeastern states

    a. When and where?

    b. How bad is it?

    c. Short-term outlook

    d. Seasonal outlook

4. Analysis of prevailing atmospheric circulation features

 

 

 

 

Executive Summary

Special Climate Summary

Atmospheric Conditions Affecting the United States during April - June 1998

Overview: By the end of June, widespread drought covered the southern part of the United States from eastern New Mexico to Florida. The drought was most severe in Texas and Florida, where it adversely impacted crops, ranges and pastures, and contributed to the burning of nearly one-half million acres of Florida land. This drought resulted from well below-normal rainfall in the southern Plains and Gulf Coast states during April - June 1998, and from much above-normal temperatures across the region during May-June 1998. The April - June 1998 period also featured abnormally wet and stormy conditions across much of the central and northern United States east of the Rockies. The surplus rainfall was particularly evident in June, when near-record totals were reported in the Midwest and Northeast.

April - June 1998 was the driest such period in 104 years of record in Florida, Texas, Louisiana, and New Mexico (Fig. 1), the seventh driest such period in Arkansas, and the eighth driest in Oklahoma. Similarly, May-June 1998 was the warmest such period in 104 years of record in Florida, Texas, Louisiana, and Arkansas (Fig. 2).

In Florida, the drought intensified during June in response to record heat and near-record dryness. These conditions, combined with excess ground cover resulting from abundant January - March rainfall, contributed to numerous uncontrolled wildfires during June and early July.

Atmospheric Circulation: The abnormal temperature and rainfall patterns observed across North America during April - June were essentially manifestations of a persistent atmospheric circulation pattern (Fig. 3). Primary features of this pattern included: 1) a pronounced amplification of the subtropical ridge and high-pressure zone across the eastern North Pacific, Mexico and the south-central United States; 2) increased jet stream winds [1.5 - 2 times normal] and increased storminess across the central and northern United States; 3) a strong low-pressure area over the western United States, which acted as a continued source region for the storms; and 4) above-normal pressure and weaker-than-normal jet stream winds across Canada which helped to concentrate the jet stream and storminess over the central United States.

The interaction of two primary factors contributed to the persistence and amplitude of the above circulation pattern, and to the details of the atmospheric flow which ultimately helped to determine the precise regions of hot, cold, wet and dry. These are 1) the 1997/98 El Niņo, and 2) internal atmospheric dynamics which contributed to large-scale changes in the atmospheric circulation across the North Pacific and North America.

Despite the decay of the El Niņo in late May in the east-central tropical Pacific, abnormally warm ocean waters and enhanced tropical rainfall over the eastern tropical Pacific during June contributed to a continuation of the strong subtropical high-pressure zone across the southern United States. Thus, El Niņo's impact on the atmospheric circulation had not yet dissipated by late June.

Long-range outlook: The seasonal outlook for July-September 1998 indicates warmer than normal conditions across the Southwest, Texas and southern Florida, and a continuation of above-normal rainfall in the Ohio valley and mid-Atlantic states.

The extended seasonal outlook indicates drier-than-normal conditions for the Southwest from September 1998 through May 1999. The outlook also suggests the likelihood that drier-than-normal conditions will expand eastward this winter to encompass Texas, the Gulf Coast, Florida and the Southeast.

1. Overview

This Special Climate Summary examines the atmospheric conditions accompanying the southern U.S. heat wave and drought (section 2), and the increased rainfall and storminess in the Midwest and Northeast (section 3). In section 4, the large-scale atmospheric conditions that have controlled these exceptionally persistent conditions are identified and examined. The discussion and analyses in section 4 are oriented toward individuals with a working knowledge of atmospheric dynamics and atmospheric processes. A synthesis of this information is presented in the Executive Summary and in schematic form (Fig. 3).

2. The southern U. S. Heat wave and drought

a. When and where

Below-normal rainfall covered the southern part of the United States from eastern New Mexico to Florida during April - June (Fig. 1). April-June 1998 was the driest such period in 104 years of record-keeping in New Mexico, Texas, Louisiana, and Florida, and among the 10 driest in Oklahoma and Arkansas. During the period, many locations across Florida and Louisiana received less than half of normal rainfall (generally under 6 inches) and broad sections of Texas and New Mexico received less than 25% of normal rainfall (1 - 4 inches) (Fig. 4). This dryness was a dramatic change from the surplus rainfall observed in much of these areas from late 1997 through March 1998.

The April - June 1998 dryness was accompanied by above-normal surface temperatures since early May, which exacerbated drought conditions considerably. Much of Texas and parts of Florida, Georgia, and Alabama reached or exceeded 95°F on more than 50% of the days during May - June (Fig. 5), with temperatures averaging 4°-6°F above normal across most of the region (Fig. 6). In Florida, the heat became particularly intense by early June, when temperatures routinely exceeded 95°F and new daily record high temperatures became commonplace. Overall during 1998, Florida recorded its warmest May and June periods on record dating back to 1895. Other states which also recorded their warmest May-June period on record with temperatures averaging 3.5°-4.5°F above normal included Texas, Louisiana and Arkansas. Each of these states also recorded significantly below-normal rainfall during the period.

b. How bad is it?

Numerous temperature records were tied or broken across Texas and Florida during May - June 1998. For example, daily high temperatures in Amarillo, Texas reached 100°F on thirteen days during June, and averaged at least 100°F for eight consecutive days between 22 - 29 June, thus tying a record set in 1924. Amarillo also tied its all-time high temperature record of 108°F on 27 June. In Brownsville, June 1998 was the warmest June in their historical record dating back to 1880. During this month the city recorded a record 17 days in which minimum temperatures failed to drop below 80°F. Midland, Texas reported a record 14 consecutive days from 16 - 29 June with high temperatures reaching 100°F, thus breaking the previous record of 13 consecutive days observed in both 1964 and 1981.

In Florida, numerous temperature records were also broken during June 1998. For example, daily high temperatures at Melbourne either tied or broke records 22 times during the month, and record daily highs at Daytona Beach and Orlando were tied or broken 14 times and 13 times, respectively. Daytona Beach, Melbourne, Orlando, and Miami each recorded their hottest June on record, while Tampa recorded its hottest June in over 100 years. Elsewhere, the extreme heat also affected Kansas during 23-29 June, when Wichita recorded seven consecutive days of 100°F temperatures.

With respect to the region's dry spell, rainfall in Midland, Texas during January-June totalled only 1.48 inches, their second driest such period on record. Farther east, April - June rainfall at Corpus Christi (0.31 inches) was the lowest in the historical record. This rainfall deficiency has led to widespread moderate to severe drought from eastern New Mexico eastward to Florida, as indicated by both the short-term drought Crop Moisture index (Fig. 7) and the long-term Palmer Drought Index (Fig. 8).

c. Agricultural and livestock impacts

The heat and dryness impacted crops and pastures in Texas and Florida, and some negative impacts were also felt in parts of New Mexico, Oklahoma, Arkansas, Louisiana, Mississippi, Alabama, and Georgia.

The Texas Agricultural Statistics Service (TASS report) indicated that non-irrigated crops in Texas such as cotton and soybeans were severely short of moisture. Livestock were generally in fair condition with the help of supplemental feeding, but 66% of range and pasture were in poor to very poor conditions. Irrigation kept corn fields in good condition in the Plains of Texas, but much of the crop in other areas was in very poor condition. Statewide, 41% of the Texas corn crop was in poor to very poor condition. Also, 43% of the state's sorghum crop and 40% of the cotton crop was rated in poor to very poor condition.

In Georgia, approximately 58% of the corn crop was in poor to very poor condition at the end of June (Fig. 9). Corn in many non-irrigated fields has already been damaged beyond repair.

d. Fires

The June - early July 1998 wildfires in Florida have become almost infamous. According to Florida's Emergency Operations Center (EOC site) all 67 Florida counties have been affected by fires. As of 5 July, 483,000 acres and 356 structures had been consumed by fires, resulting in an estimated $276 million in damages. Over 100,000 people had been ordered to leave their dwellings by early July, including the entire population of Flagler County. Also, long stretches of Interstate-95 and other roads were closed because of dense smoke and sharply reduced visibility. Fortunately, increased rainfall and humidity over the 4 July weekend allowed firefighters to make progress toward controlling several large wildfires. Additional control over the fires, particularly those in Flagler county, came on 6-7 July as widespread rainfall covered central Florida.

In Texas, the Texas Emergency Response Division reported that wildfires during May-June 1998 had burned 143,000 acres. Elsewhere, by early July, large fires in Colorado, New Mexico and Arizona had been contained, but new fires were reported in Utah.

e. Short-term and intermediate-term outlook

Latest 1-5 day rainfall forecast issued by the National Weather Service.

Latest 6-10 day forecast issued by the National Weather Service.

f. Seasonal outlook:

Long-range outlooks issued at the Climate Prediction Center (Fig. 10) suggest a continuation during July of warmer and drier than normal conditions across Texas, and a continuation of warmer than normal conditions in Florida and portions of the southeast. The seasonal outlook for July-September also indicates warmer than normal conditions across the Southwest, Texas and southern Florida.

The latest long-range seasonal outlook issued by the Climate Prediction Center (Fig. 11) suggests drier-than-normal conditions for the Southwest from September 1998 through May 1999. The outlook also suggests the likelihood that these drier-than-normal conditions will expand eastward during the 1998/99 winter season to encompass Texas, the Gulf Coast, Florida and the Southeast. This outlook indicates the potential for serious long-term drought in these states if summertime rainfall remains deficient.

3. Storminess in the central and northeastern states

a. When and where

Persistent heavy rains and frequent severe thunderstorms affected much of the northern half of the country east of the Rockies during June. Rainfall totals of 5 inches to locally 22 inches were observed across most of the central and northeastern states (Fig. 12), with totals exceeding 200% of normal across the Ohio Valley, New England and the upper Mississippi Valley.

Although wetness was especially notable during June, parts of the Midwest have experienced above-normal rainfall since April, and surplus moisture dates back to late 1997 in portions of the Northeast. During April - June 1998, total rainfall reached 18 to 27 inches in most of the primary cities in the Ohio Valley, Tennessee Valley, central Appalachians and lower Northeast (Fig. 4). The rains frequently fell from strong to severe thunderstorms, especially during May and June, thus leading to abnormally frequent episodes of tornadoes, hail, damaging winds and flash floods. According to preliminary data from the National Severe Storm Prediction Center (NSSPC site), 372 tornadoes were recorded during June over the nation as a whole, which is nearly 200 more than average.

b. How bad is it?

Numerous new rainfall records were established during June across the northern half of the country, with record amounts reported in Paducah, Kentucky (10.98 inches), Nashville, Tennessee (11.95 inches), and Elkins, West Virginia (10.05 inches). Statewide, June 1998 was the second wettest June in 104 years in Massachusetts, New Hampshire, Rhode Island, and Vermont, and the third wettest June in Iowa and Maine. For the April-June period as a whole, rainfall totals were the highest in the historical record dating back to 1895 in Rhode Island and Massachusetts, the third highest in Tennessee, and the fourth highest in Iowa. As a result of several months of generally above-normal rainfall, combined with localized extremely heavy downpours, rivers in 17 states were near or above flood stage as of 6 July (Fig. 13).

Unseasonably wet conditions also set records in the West during April-June 1998, with California, Idaho, and Nevada recording one of the wettest April-June periods in the past 104 years. Since this is typically a dry time of the year, surplus precipitation west of the Rockies does not normally pose a problem. However, this year's unseasonably wet and cool conditions have retarded the typical seasonal snowmelt at some higher elevations across the West, most notably in the Sierra Nevada range where an exceptional late-season snowpack has increased the potential for flooding.

c. Short-term outlook

Latest 1-5 day rainfall forecast issued by the National Weather Service.

Latest 6-10 day forecast issued by the National Weather Service.

d. Seasonal outlook:

The official Climate Prediction Center monthly outlook for July indicates above-normal rainfall and cooler than normal temperatures in the northern Plains and upper Midwest (Fig. 10), and near-normal rainfall and temperatures across the eastern and northeastern states.

The outlook for July-September indicates an increased likelihood of above-normal rainfall from the Ohio Valley to the mid-Atlantic region, and cooler than normal temperatures in the central Plains. The extended seasonal outlook for the 1998/99 winter season (Fig. 11) indicates the increased likelihood of drier-than-normal conditions in the mid-Atlantic states and near-normal precipitation in the Midwest.

4. Analysis of prevailing atmospheric circulation features

The abnormal temperature and rainfall patterns observed across the United States during April - June are essentially manifestations of a persistent atmospheric circulation pattern (Figs. 14, 15). Primary features of this pattern included: 1) a pronounced amplification of the subtropical ridge across the eastern North Pacific, Mexico and the south-central United States; 2) increased jet stream winds [1.5 - 2 times normal] and increased storminess across the central United States in the region just north of the subtropical ridge; 3) an abnormally strong upper-level trough over the western United States; and 4) above-normal heights and weaker-than-normal jet stream winds across Canada.

Each of these features has been a prominent aspect of the atmospheric circulation since January (Fig. 16), and their extreme persistence is likely linked to the 1997/98 El Niņo. In fact, one of the most significant differences in the atmospheric circulation features between January - March and April - May has simply been a slight poleward shift of the anomaly centers in the Northern Hemisphere in association with the northward seasonal progression of the sun. Accompanying this northward shift has been a corresponding movement in 1) the region of hot and dry conditions from central America and southern Mexico to northern Mexico and the Gulf Coast States, and 2) the areas experiencing increased storminess, rainfall and severe weather from the southern tier of the United States to the central and northern sections of the country.

The link between the above circulation features and the 1997/98 El Niņo are highlighted in Figs. 17-21. First, an enhanced subtropical ridge is typically observed over the eastern subtropical North Pacific in April-May during El Niņo (Fig. 17), although there is considerable variation in the precise location, structure and duration of this feature from one El Niņo to the next. Second, during April - June 1998, this enhanced subtropical ridge was associated with a large-scale pattern of above-normal heights that spanned the entire tropics and subtropics of both hemispheres. These areas of above-normal heights were consistent with the pattern of enhanced tropical rainfall that prevailed across the eastern half of the tropical Pacific for most of the period (Fig. 18). During June, the continuation of a strong subtropical ridge across Mexico and the southern United States was also consistent with a large area of enhanced tropical rainfall and abnormally warm sea surface temperatures (Fig. 19) over the extreme eastern tropical and subtropical North Pacific. Thus, although the El Niņo weakened considerably in late May, its impact on the atmospheric circulation had not yet dissipated by the end of June.

A third link between the El Niņo and the strong subtropical ridge over the southern United States is evident through the strong inter-hemispheric symmetry of atmospheric features during the period (Figs. 14, 15). In addition, this subtropical ridge was accompanied by a remarkably coherent and symmetric pattern of temperature and wind anomalies in the subtropics and extratropics of both hemispheres throughout the entire period, as highlighted in a north-south cross-section at 115°W of mean atmospheric conditions during April - June 1998 (Fig. 20).

Another factor associated with the increased storminess and very focused jet stream winds over North America during April-June 1998 was a persistent pattern of above-normal heights across Canada and reduced westerly winds across southern Canada. These conditions developed during the 1997/98 Northern winter as one important component of the extratropical atmospheric response to strong El Niņo conditions. They then persisted through the April - June period, consistent with a continued El Niņo influence on the atmospheric circulation. It is also plausible that the conditions over Canada during June were partly associated with a large-scale evolution toward the development of above-normal heights across most of the higher latitudes of the Northern Hemisphere. Additional modeling and diagnostic studies are required to resolve these issues.

An additional important contributing factor to the temperature and rainfall departures over North America during April-June 1998 was an abnormally strong upper-level trough over the western United States, which represented a continued source region for the storms throughout the period. This feature also developed initially during the 1997/98 Northern winter as another well-known component of the extratropical atmospheric response to strong El Niņo conditions. The subsequent persistence of this trough through April-June 1998 was therefore also likely associated with the continuation of El Niņo conditions during the period. However, there is also some indication that large-scale circulation features extending across the North Pacific and associated exclusively with internal atmospheric dynamics may have helped to perpetuate this trough. In particular, April-May 1998 featured a persistent pattern of above-normal heights and reduced jet stream winds across the central North Pacific. These conditions are often associated with an amplified trough and increased cyclonic flow over the west coast of the United States, similar to that which was observed. Collectively, these features often occur in association with a large-scale pattern of internal atmospheric variability known as the negative phase of North Pacific (NP) Oscillation (Fig. 21). April and May 1998 featured the strongest negative phase of the NP pattern in the historical record dating back to 1950 (Fig. 22). A similar examination for June 1998 indicates that the western trough was more expansive and extended across most of central North America. This evolution occurred in association with the development of a large-amplitude ridge over the Gulf of Alaska, a feature which is probably linked to internal atmospheric dynamics.