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Table of Contents

1. Overview

2. Heat and drought in Texas

3. Increased rainfall and continued heat in Florida and the Southeast

4. Dryness in the Middle Atlantic and Great Lakes states

5. Excessive July heat in the West

6. Decreasing wetness in the Midwest and northeastern states during July

7. Analysis of prevailing atmospheric circulation features

Special Climate Summary 98/2

Atmospheric Conditions and Impacts Affecting the United States during July and early August 1998

1. Overview

Weather patterns during July and early August were notably different than those observed during the previous three months. Across Florida (cooler and wetter) and much of the central and northeastern states (fewer severe thunderstorms, and drier in many areas), these changes in the atmospheric circulation led to substantially improved conditions from those observed in April - June 1998. However, they also contributed to an intensification of hot and dry conditions in the south-central states, to the development of exceptionally hot weather across the West, and to short-term dryness in portions of the Great Lakes and Middle Atlantic states. The heat and drought in Texas and the south-central states is discussed in section 2. The cooler and wetter conditions in the Southeast, which resulted in reduced wildfire activity in Florida, are discussed in section 3. Incipient dryness in the Great Lakes and Middle Atlantic States is discussed in section 4, and the excessive July heat in the West is discussed in section 5. Section 6 features the decreased wetness in the central and northeastern states, and section 7 contains analysis of the prevailing atmospheric circulation features during July 1998.

2. Heat and Drought in Texas

(a) July and early August Temperatures

July temperatures averaged at least 3oF above normal across most of the southern Plains and western Lower Mississippi Valley (Figure 1, top), with several locations reporting monthly departures of +5oF to +7oF. July 1998 was the warmest in 104 years of record for Texas, the 2nd-warmest in Louisiana, and the 5th-warmest in both Oklahoma and Arkansas. The July 1998 temperature rankings by climate division are depicted at the top of Figure 2.

Numerous July temperature records were set at individual locations. At least 4 sites including Del Rio, TX; Austin, TX; San Antonio, TX; and Shreveport, LA) reported their warmest month on record, and several locations experienced a large number of consecutive days with high temperatures reaching at least 100oF. These included 29 at Dallas, TX (2nd to 42 consecutive days in 1980), 30 at College Station, TX (a new record), 29 at Waco, TX (the record is 42 in 1980), 16 at Oklahoma City, OK (the record is 22 in 1936), and 11 at Houston, TX. By early August, the total number of days in a year with highs at or above 100oF was approaching or had exceeded record levels at many locations (Figure 3), according to the Joint Agricultural Weather Facility. New monthly extreme maximum temperature records were noted at Shreveport, LA (107oF on July 31) and Alexandria, LA (108oF on August 1), and several locations set all-time high daily minimum temperature records, including Midland, TX (82oF), Texarkana, AR (83oF), and Del Rio, TX (87oF). By early August, Dallas already eclipsed their previous record for number of days with minimum temperatures at or above 80oF in a year (36 through August 13; 22 occurred in 1980).

(b) The May - July Heat Wave

May - July 1998 was the warmest such period on record in most climate divisions across Texas, central and southern Oklahoma, much of Arkansas, and all of Louisiana (Figure 2, bottom). Statewide, Texas, Louisiana, and Arkansas all experienced their warmest May - July since records began in 1895, while Oklahoma recorded its 2nd-warmest. During this 3-month period, temperatures averaged 3oF to 6oF above normal across most of these regions.

(c) July and early August Rainfall Deficits

In addition to the extreme heat, abnormally dry weather during July also dominated central and southern Oklahoma, the lower Texas panhandle, and southern and northeastern Texas. Most of these areas received less than 25% of normal precipitation for the month (Figure 1, bottom). July 1998 was the 6th-driest on record for Louisiana and the 10th-driest for Texas. A number of locations across Texas, including Galveston, Dallas, San Antonio, Waco, and Corpus Christi, measured less than one-third of an inch of rainfall during July.

Although rainfall totals for July and early August averaged well below normal, a few regional episodes of moderate to locally heavy rain kept most areas from setting short-term dryness records during the period. Overall, record July dryness was restricted to parts of Oklahoma, where some sites observed no rain in July, and southern Texas, where Brownsville and Del Rio received only a trace of precipitation. Locally moderate to heavy rains finally fell on much of south-central and southwestern Texas during August 16 - 17, with isolated sites in Webb County near Laredo receiving almost 9 inches.

(d) Severe April - July Drought

Record April - July dryness was observed across central and eastern Texas, southwestern Oklahoma, and northwestern Louisiana (Figure 4, bottom). Louisiana and Texas both received their lowest April - July precipitation totals since the start of regionally-averaged records in 1895, while Oklahoma experienced its 3rd-driest such period, and Arkansas recorded its 4th-driest. Several individual locations also observed record or near-record April - July rainfall deficits, according to the Joint Agricultural Weather Facility (Figure 5). Scattered areas, including Midland and Brownsville, TX, have recorded subnormal precipitation since the beginning of the year. The heavy mid-August rains in south-central and southwestern Texas had only a minimal impact on most of the region=s long-term rainfall deficits, and some of the driest areas in far southern Texas (such as Brownsville) missed the bulk of these rains.

(e) Agricultural Impacts

By far, the heat and drought across Texas and adjacent areas accounted for most of the negative drought-related impacts affecting the country during July. According to the Department of Agriculture, the proportion of areas with short or very short topsoil moisture remained very large through early August (Figure 6), and both the Crop Moisture Index (short-term) and the Palmer Drought Index (long-term) indicated that extremely dry conditions were widespread as of August 8 (Figure 7). All 254 counties in Texas were declared Federal Agricultural Disaster areas by the end of July, with crop losses in Texas and Oklahoma estimated at $2.0 to $4.0 billion, according to the state governments= agricultural offices. As of August 9, 52% of Texas cotton was described as being in poor or very poor condition, and the state=s total cotton output is expected to be 40% lower than for 1997 even if conditions improve before harvesting is completed, according to the National Agricultural Statistics Service. In addition, 45% of Texas corn, 36% of Louisiana soybeans, and 33% of Arkansas soybeans were in poor or very poor condition as of August 9. Between 45% and 80% of pastures and ranges were in poor or very poor condition across New Mexico, Texas, Oklahoma, Arkansas, and Louisiana, according to the National Agricultural Statistics Service (Figure 8). Livestock were increasingly affected by the deteriorating pasture and range conditions, and the proportion described as being in poor or very poor shape steadily increased through the period.

(f) Increased Wildfire Activity

Wildfire activity increased during July in response to persistently hot and dry conditions across Texas and adjacent areas. During May 1 - August 10, over 344,000 acres had been consumed in Texas, according to the Texas Forest Service. The wildfire risk for the region remains moderate to high through at least mid-August, according to the National Interagency Coordination Center.

(g) Hydrologic and Miscellaneous Impacts

A number of significant non-agricultural impacts were caused by the heat and drought in the south-central states. Of the 173 heat-related deaths that had occurred nationally by August 3, 108 were in Texas, according to media sources. By August 11, the Texas figure increased to 127.

As of August 14 1998, hydrologic impacts from the severe drought in Texas and adjacent areas had been limited, primarily in response to the wet 1997-98 winter. Some water rationing was briefly imposed in late July and early August before a period of somewhat cooler and damper weather reduced water and energy demands. Odd/even day water rationing was imposed in Oklahoma City. Also, a 90-inch demand-strained water pipeline that supplies one-third of the water to Fort Worth and Arlington, TX burst as July ended, forcing water usage restrictions on the region until the pipe was repaired August 3. Farther east, the water level in the Toledo Bend reservoir near Shreveport, LA dropped 6 feet since April and was 2 feet below normal in early August.

Also, heat-warped railroad tracks derailed two freight trains, each comprised of more than 100 cars, within 60 miles of Dallas/Ft. Worth. As a result, speed reductions were mandated throughout the region.

(h) Short- and intermediate-term outlooks

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

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

(i) Long-term outlooks

The monthly and seasonal outlook issued by the Climate Prediction Center indicates that the Southwest and the southern Plains states are at risk of experiencing prolonged dryness into the 1998-99 winter. The developing La Niņa conditions in the equatorial Pacific Ocean favor reduced precipitation across these regions from mid-autumn through the winter. Since many of these areas have already been dry for several months, significant rainfall in September (when the impact of La Niņa on the region=s rainfall is typically weak) will be important for reducing the risk of further drought intensification

3. Increased Rainfall and Continued Heat in Florida and the Southeast

(a) Increased July and early August Rainfall

Florida and the central Gulf Coast states experienced substantial relief from the hot and dry conditions observed in late spring and early summer (Special Climate Summary 98/1 contains detailed information on the heat, dryness, and wildfires that affected the Southeast from April through early July). Most of Mississippi, Alabama, southern Georgia, and Florida recorded 6 to 15 inches of rain during July, which was more than 150% of normal across parts of central and northern Florida and the southern half of Georgia (Figure 1, bottom). Significant July dryness was restricted to the southern Appalachians, northern Georgia, and the east-central Carolinas, where totals were generally 50% to 75% of normal. In early July, short-term drought indices, such as the Crop Moisture Index, indicated that severe to extreme dryness covered central and southern sections of both South Carolina and Georgia, southeastern Alabama, extreme southeastern Louisiana, and most of Florida. However, by August 8, severe to extreme dryness had become restricted to sections of Alabama and Georgia (Figure 7, top).

(b) April - July Dryness

Despite increased July rainfall, Florida experienced its 4th-driest April - July period on record in 1998. The dryness was particularly evident across the Florida peninsula, where rainfall amounts were among the 5 lowest observed since 1895 (Figure 4, bottom).

(c) July and early August Heat

Temperatures in the Southeast averaged above normal for the month, but departures were not as large as those measured in May and June. Temperatures during July 1998 generally averaged 1oF to 3oF above normal, with scattered locations averaging 4oF or 5oF above normal (Figure 1, top). The heat was particularly prominent near the Florida/Georgia border, where July temperatures averaged among the 5 highest since 1895 (Figure 2, top). Temperatures across the rest of Florida, southern Alabama, and central South Carolina all ranked among the top 10 of the last 104 years.

(d) Record May - July Heat Wave

May - July 1998 was the hottest on record across broad sections of the Southeast, primarily because of the extreme heat during May and June (Figure 2, bottom). Statewide, both Florida and South Carolina experienced the hottest May - July on record, while Georgia, North Carolina, Alabama, and Mississippi endured the 3rd, 4th, 6th, and 7th hottest such period on record, respectively.

(e) Agricultural Impacts

According to the Department of Agriculture, the proportion of areas with short or very short topsoil moisture decreased significantly in the Southeast by early August (Figure 6), easing some of the region=s drought-related agricultural impacts. Unfortunately, the rains came too late in the growing season to benefit some crops. As of August 9, the National Agricultural Statistics Service indicated that many crops in Georgia were in poor or very poor condition, including 42% of the state=s cotton, 58% of their soybeans, and 67% of their corn. Other states with a significant proportion of certain crops in poor or very poor condition on August 9 included South Carolina=s peanuts (46%) and soybeans (40%) and North Carolina=s corn (38%).

(f) Wildfires

The severe wildfires that ravaged parts of Florida were generally contained by mid-July and remained contained as increased rainfall persisted through early August. Between May and early July, wildfires scorched 500,000 acres and destroyed 270 homes and businesses in the state, primarily in Brevard, Seminole, Volusia, and Flagler counties, according to the Federal Emergency Management Agency.

(g) Short- and intermediate-term outlooks

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

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

(h) Long-term outlooks

The Climate Prediction Center=s long-range outlook favors near-normal rainfall during August - October across the Southeast and adjacent areas. The outlook also indicates an increased likelihood of below-normal precipitation from November through March or April in response to the expected strengthening of La Niņa conditions. Since this is a relatively dry time of year for the region, substantial drought-related impacts are not anticipated. There is an increased likelihood that wildfires will be larger and more numerous than normal across Florida and adjacent areas in late 1998 and early 1999, but severe wildfire outbreaks like the ones in June & July 1998 are rare, and the chances for similarly severe conditions during winter and/or spring 1998-99 are low.

4. Dryness in the Middle Atlantic and Great Lakes States

(a) Rainfall Deficits

By early August, developing dryness was evident across much of the middle Atlantic states, and the northern and western sections of the Great Lakes region. Most of these areas recorded less than half of normal July rainfall (Figure 1, bottom), resulting in short-term dryness (as reflected by the Crop Moisture Index) across Michigan, Wisconsin, much of Maryland, northeastern Virginia, Delaware, southeastern Pennsylvania, and New Jersey (Figure 7). July 1998 ranked as one of the 5 driest such months on record for many sites in these areas (Figure 4, top), and April - July precipitation totals were between the 5th- and 10th-driest on record for most of Michigan (Figure 4, bottom). During August 5 - 10, moderate to heavy rains (1 to locally 7 inches) abruptly interrupted the dryness in Michigan and Wisconsin, and even caused some localized urban and flash flooding.

(b) Impacts

According to the Department of Agriculture, the proportion of areas with short or very short topsoil moisture increased during July across the Great Lakes region and the middle Atlantic states (Figure 6), resulting in some crop stress.

(c) Short- and intermediate-term outlooks

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

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

(d) Long-term outlooks

The Climate Prediction Center=s outlook favors near or above normal rainfall during September in Delaware, southern New Jersey, and eastern sections of Maryland and Virginia, but the likelihood for subnormal precipitation increases across eastern Virginia and southern Maryland during November - February. Across the Great Lakes region, there is an enhanced probability of surplus precipitation during early autumn and during December 1998 - March 1999.

5. Excessive July Heat in the West

(a) Temperatures

During July 1998, temperatures averaged above normal over most of the western United States (Figure 1). This heat was in marked contrast to the unseasonably cool conditions that had prevailed during April - June. In July, the largest departures from normal covered the Northwest (Figure 1, top), where Washington experienced its 3rd-warmest July on record while Nevada, Idaho, and Montana experienced their 5th, 7th, and 9th warmest July, respectively (Figure 2, top). Farther south, July 1998 temperatures averaged closer to normal, but a few periods of exceptional heat were observed. Several locations tied or established new all-time record highs during July 1998, including Rawlins, WY (99oF), Ely, NV (101oF), Milford, UT (107oF), Dugway, UT (109oF), and Organ Pipe Cactus National Monument, AZ (118oF). In the California desert, Death Valley reached 129oF (Figure 9), the highest temperature recorded in North America for at least 38 years.

(b) Impacts

According to the Department of Agriculture, the July heat induced above-normal evapotranspiration rates, causing the proportion of areas with short or very short topsoil moisture to increase (Figure 6) despite near-normal July rainfall (Figure 5). The heat actually benefitted crops in California, where unseasonably cool conditions during May - early July slowed crop development.

(c) Short- and intermediate-term outlooks

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

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

(d) Long-range outlook

There is an increased likelihood of surplus autumn and winter 1998-99 precipitation in the Northwest and an enhanced probability of below-normal precipitation during the same period in the Southwest. Both of these features are anticipated in response to the expected further development of La Niņa conditions.

6. Decreasing Wetness in the Midwest and Northeastern States during July

(a) July and early August 1998 Precipitation

Most of the upper Midwest and Northeast received normal to below-normal rainfall during July 1998 (Figure 1, top), allowing long-term moisture surpluses to decline. Parts of the western Corn Belt, upper Ohio Valley, and lower Northeast recorded less than half of normal rainfall for the month. Several areas in the Northeast recorded one of the 20 driest Julys since 1895 (Figure 4), and Waterloo, IA received only 0.35" of rain during the month, setting a new July dryness record. Moderate to locally heavy rains (1 to 5 inches) returned to the western Corn Belt and Northeast during August 4 - 12, but drier conditions again prevailed by mid-August.

There were a few notable exceptions to the July drying trend. For example, exceptionally wet weather extended from the east-central Rockies eastward through part of the central Plains and the Mississippi/Ohio Rivers= confluence during July, with 6 to 12 inches reported at many sites. July 1998 was the wettest such month on record for Colorado, the 5th wettest for Missouri, and the 6th wettest for Kansas. Memphis, TN recorded 9.96" for the month, setting a new July record. In addition, locally heavy July rains of 6 to 8 inches fell on central and southern Indiana.

In addition to the decreased rainfall in many previously-wet areas, severe weather incidents also declined dramatically. According to preliminary estimates from the Storm Prediction Center, only 59 tornadoes touched down in the United States during July 1998 (32% of the month=s average for the previous 3 years). This suppressed tornadic activity contrasted with conditions during June, when 372 tornadoes (208% of the 1995 - 1997 average) were observed.

(b) Longer-term Wetness

For April - July 1998 as a whole, many areas from the western Corn Belt southeastward through the Middle Mississippi, Lower Ohio, and Tennessee Valleys experienced one of their 10 wettest such periods on record (Figure 4, bottom). In the Northeast, some areas experienced one of the 10 wettest such periods on record even though well-below-normal rainfall was observed in July.

(c) Short-term and intermediate-term outlooks

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

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

(d) Long-term outlooks

The monthly and seasonal outlook from the Climate Prediction Center indicates that above-normal precipitation is periodically likely across the Upper Mississippi Valley and Corn Belt from mid-autumn through early spring. Farther east, winter and early spring have an increased chance of being wetter than normal in the Northeast, especially along and west of the Appalachians.

7. Analysis of Prevailing Atmospheric Circulation Features

Many features of the atmospheric circulation pattern that had generally persisted from last winter through at least part of June broke down during July, signaling an end to the direct influence of the 1997-98 El Niņo on North American weather patterns.

A wave-like upper-level flow pattern prevailed during July, with ridges and troughs taking on a north-south orientation. An amplified ridge covered the South-Central States and the West while an anomalously-strong upper-level Hudson Bay Low was evident across eastern North America. These circulation features appeared to reflect internal atmospheric variability, and did not appear to reflect any influence from either lingering El Niņo or developing La Niņa conditions (Figure 10).

This upper-level circulation pattern was in sharp contrast to that which prevailed from April through much of June, when a strong upper-level ridge covered Mexico and the south-central United States, a trough prevailed near California, and the Hudson Bay Low was weaker than normal. An unseasonably strong jet stream and increased storminess across central and northern sections of the lower 48 states accompanied this circulation pattern. These conditions were strongly influenced by the 1997-98 El Niņo, as detailed in Special Climate Summary 98/1.