Week 4: April 22-30, 2023

Return to Cold Temperatures

Despite prior spring-like temperatures, unseasonably cold temperatures were observed across the region as April came to a close. The entire Midwest was blanketed with below-normal temperatures; some pockets recorded average temperatures of up to 11°F below normal (Figure 1). At the state level, the largest mean temperature deviation was observed in Minnesota (8.8°F below normal) and the smallest temperature deviation was observed in Ohio (5.9°F below normal). Average maximum temperatures were well below normal throughout the entire Midwest and reached up to 16°F below normal in the western Upper Peninsula (Figure 2). Average minimum temperatures followed a similar, but not as drastic, pattern; the only differences were the slightly above-normal temperatures observed in the eastern U.P. (Figure 3). This abnormal cold caused 225 low minimum and 227 low maximum daily temperature records to be broken or tied (Figure 4). Minimum temperatures in the low-to-mid 20s were recorded throughout the week, even in the southernmost areas of the region, which led to the issuance of frost advisories and freeze warnings.

Scattered Precipitation Throughout the Week

The upper-level troughing pattern that was dominant throughout the week was the driver of the cool, wet, and gloomy weather that many observed. Figures 5a, 5b, 5c, and 5d show the below-normal 500mb heights at 1pm EDT on April 22nd, 25th, 28th, and 30th, respectively (Alicia Bentley). Everywhere in the Midwest, except for central Missouri, received some amount of precipitation this week, but only the north and the east recorded above normal totals. Eastern Ohio received up to 200 percent of normal precipitation and the U.P. received up to 300 percent of normal precipitation (Figure 6a). Most of the Midwest actually received precipitation that was between 0.5 and 1.5 inches below normal (Figure 6b). The main precipitation event came at the end of the week when a surface low set up over northern Michigan and the U.P. in the later hours on April 29th after a cold front had passed through (Figure 7, WPC). Precipitation transitioned to heavy, wet snow overnight, and continued throughout the day on April 30th. The highest snowfall totals remained localized to the western U.P. and far northern Wisconsin (Figure 8). Vilas County, Wisconsin, reported the highest one-day maximum snowfall total of 8.4 inches on April 30th. Final totals and records broken from this event will be discussed in next week’s climate summary as the event continued into the first few days of May.

While snow piled up in the north, multiple rounds of scattered showers and storms moved through the central Midwest on April 30th. Small hail and graupel were observed in some areas with this activity, as well as high wind speeds coming off of the surface low, primarily in the Duluth and Twin Cities areas.

Widespread Aurora Visibility

A severe geomagnetic storm impacted the Earth on April 23rd and resulted in rare Aurora Borealis visibility throughout much of the United States. Cloud cover reduced throughout the night, which gave viewers a better chance to see the lights. In the Midwest, the Aurora was visible in western Indiana, Missouri, northwestern Illinois, and northern Wisconsin, to name a few areas. For more information on the science behind the Aurora, please visit the Space Weather Prediction Center.

Drought and Streamflow Updates

The April 25th drought monitor update indicated some slight changes when compared to the April 18th drought monitor (Figure 9). The largest changes occurred in central Missouri where Moderate Drought (D1) expanded northeast to the Missouri and Illinois border. Some improvement was also noted in southern Minnesota as the D1 was downgraded to abnormally dry (D0). Slight improvement was also observed in central Iowa.

When compared to last week, streamflow values decreased to below normal in Missouri, central and northeastern Illinois, Indiana, and eastern Kentucky (Figure 10). Values in eastern Ohio and eastern Michigan increased slightly to near normal levels. While streamflows in Minnesota, Wisconsin, and the U.P. were still high this week from continual snowmelt, they returned to near-normal levels.