July 2001
Temperature and Precipitation Anomalies
July 2001 was a month of contrast in the Midwest, starting very dry in most areas in the first half of the month and ending with lots of precipitation in the southern part of the region during the second half of the month. The totals for the month reveal that more than 6 inches of rain fell in much of Missouri, southern Illinois, Indiana, and eastern Kentucky (Figure 1); however, much of central Iowa and Illinois, northeastern Ohio, and most of Minnesota, Wisconsin and Michigan were far below normal in precipitation (Figure 2). A wide area of abnormally dry conditions was formed in these areas, with parts of northeastern Michigan declared to be in moderate drought status at the end of the month by the U.S. Drought Monitor (Figure 3). Because of the combination of northern dryness and southern wetness, the Midwest as a whole ended up in the middle, experiencing its 42nd wettest July in 107 years. Indiana had its 7th wettest July, while Wisconsin and Michigan had their 15th and 16th driest July, respectively. Temperatures also oscillated up and down during the month, with no one state averaged more than 1°F above or below normal, although the western edge of the Midwest was a bit warmer (Figure 4). July 2001 was the 48th warmest on record for the region. No individual states were in the top 20 rankings for temperature in July.
On July 1 a strong cold front swept through the Midwest, producing a line of thunderstorms that extended from eastern New York back into eastern Illinois. As the front stalled south of the Ohio River on July 2, thunderstorms developed on the periphery of the cooler air in western Iowa and Minnesota. The dry weather was especially welcome in western Missouri, Iowa, Minnesota, and Wisconsin, areas which have received too much rain recently. However, showers and thunderstorms developed ahead of a warm front on July 3, producing moderate rainfall over parts of Iowa, Missouri, Illinois, and Indiana. This was the first significant rainfall in two weeks over parts of Illinois and Indiana, and came at an opportune time for corn and soybean development (Figure 5). Temperatures were well below normal for most of the central and northern Midwest during the first three days of July, due to the dominance of the Canadian high (Figure 6).
The week of July 4-10 was very dry in the northern tier states of the Midwest, and quite wet in a region of active weather arcing across the central and southern states of the region. Rain amounts were less than 0.25 inches across the north, while some places to the south received 2-4 inches (Figure 7). A narrow band passing through La Crosse, WI, received no measurable rain; in fact, La Crosse had experienced 19 consecutive days with no measurable rain, starting June 22. Most of eastern Illinois and Indiana received more than 200% of normal during the same period (Figure 8). The departures in Iowa, Minnesota, and Wisconsin exceeded an inch just for a single week (Figure 9), and, combined with previous dryness, that area began to have serious soil moisture shortages. A month earlier, these areas were saturated or under standing water. Ironically, the corn and soybeans that did make it through the wet period formed shallow roots, making them more susceptible to the incipient dry period. A quasi-stationary boundary to the south of the Midwest pushed back to the north on July 7 and 8, bringing the ring-of-fire convection to the central Midwest. Temperatures increased after the passage of a warm front on July 6, but it stalled short of the northeastern half of the region. The southeast Midwest was 3-5°F above normal, while the northeast Midwest was a few degrees below normal (Figure 10). Rain was spotty on the 7th and 8th, but some very strong mesoscale convective systems started rolling along the boundary from northern Nebraska, through central Iowa, into northern and eastern Illinois, the southern two-thirds of Indiana, and southward into much of Kentucky. From July 8 to 10, at least 3 separate mesoscale systems traveled on nearly the same trajectory (Figure 11, UIUC Department of Atmospheric Science).
The week of July 11-17 was very dry in most of the Midwest, with a large swath through southern Minnesota, eastern Iowa, north and central Illinois, southern Indiana, and central Kentucky receiving no rain (Figure 12). This meant that cumulative precipitation anomalies increased from 0.5 to 1.5 inches in just one week in these areas (Figure 13). Only in Missouri and the northern Midwest were there heavy rains. The western two-thirds of Missouri exceeded 200% of normal (Figure 14), due to strong convective activity at the beginning of the period on July 11 and 12. Missouri, in fact, was the exception to the rule this week, and was the only state with overall increases in good / excellent quality categories of 5% or more for both corn and soybeans. Late in the period, the high pressure dominating the Midwest weakened, and storms started slipping around the Great Plains ridge into the northern Midwest. Heavy rains fell in western Wisconsin on July 17 associated with a mesoscale convective system, and much more widespread precipitation happened overnight at the end of the period, and will be included in next week's summary. Despite the usual association of dry conditions with heat in mid-summer, the dry areas of the Midwest were in fact dominated by a Canadian high pressure center most of the week. Temperatures were 2-6°F below normal over most of the Midwest (Figure 15). Towards the end of the period, though, a ridge built eastward from the Great Plains to the central Midwest, bringing very warm air at a time when crops were undergoing heat-sensitive fertilization processes.
The dry pattern of preceding weeks reversed during the July 18-24 period (Figure 16). Most of the Midwest received normal to above normal precipitation for the week. A persistent pattern of hot, humid conditions placed much of the Midwest under the threat of thunderstorms on each day of the period, with a few locations receiving well above normal amounts (Figure 17). At the beginning of the period, July 17 to 18, a group of training thunderstorms brought heavy rain to southeastern Indiana and southwestern Ohio. Amounts up to 6 inches fell in just a few hours, leading to the deaths of 3 people in Cincinnati suburbs (Figure 18, NWS). The convective precipitation was widespread over the next few days, but consistently returned to southeastern Missouri and west-central and southwestern Illinois from July 18-20. Some locations in a narrow north-south corridor received up to 8 inches of rain during the period (Figure 19, NWS). Finally, at the end of the week, heavy rains occurred in southern Minnesota and northern Iowa. Besides benefitting from the rain that did not runoff, these areas suffered spotty crop damage from wind and hail. A smaller but by no means less intense set of storms moved through central and eastern Illinois the evening of July 23rd, also bringing some crop damage due to 40-60 mph straight-line winds (Figure 20, Storm Prediction Center). Only parts of Michigan and eastern Ohio received far less than normal rain amounts this week, causing intensified concern for dryness affecting yields in these areas. Missouri was dry this week, too, but that was beneficial after massive rains in previous weeks. The rain, except in the three sub-areas mentioned, was not sufficient to lift the abnormally dry classification given to much of Iowa, Minnesota, Wisconsin, and Illinois last week, and dryness actually expanded in Michigan and Ohio (Figure 21, National Drought Mitigation Center). Finally, to compound matters, excessive heat in the Great Plains and much of the Midwest may have affected crop fertilization in some locations, and caused a number of heat-related deaths in the region (see below). Temperatures were 4-8°F above normal over most of the Midwest, except for the Ohio Valley and Kentucky, where clouds and fog held temperatures near normal (Figure 22).
The precipitation patterns during July 25-31 followed the outlines of established dry and wet areas in the Midwest (Figure 23, National Drought Mitigation Center). Most of Michigan, southern Wisconsin, central Minnesota, central Iowa, and northern Illinois received less than normal amounts of rain (Figure 24). On the other hand, already moist areas such as western and northern Minnesota, northern Iowa, Missouri and Kentucky received copious amounts of precipitation (Figure 25). Heavy rains in the previously dry parts of southern Illinois, southeastern Missouri, and western Kentucky helped to reduce the magnitude and spatial extent of drought in that region. Too much rain fell in the western and central two-thirds of Missouri from the 24th through the 28th, with some locations receiving more than 8 inches during the week (Figure 26). A stationary front extending from southwestern Missouri through northern Kentucky provided a focus for convergence and precipitation throughout all of Kentucky from the 26th to the 29th (Figure 27). At that time the boundary shifted northward, and hot and humid air returned to the central and eastern Midwest. This caused much of the region to register considerably above normal temperatures for the week (Figure 28).