Week 3: April 15-21, 2023

Overview

The record warmth that was observed last week continued into this week as a warm air mass still remained over the area (Figure 1a, 4/15 1pm EDT, Alicia Bentley). A cold air mass quickly moved in as the warm air retreated, making for a drastic swing of temperatures within the first few days of the week (Figure 1b, 4/16 1pm EDT, Alicia Benley). Both warm and cold extremes were observed at some point during the week throughout the entire Midwest. As a whole, average temperatures reached up to 11°F below normal in the northwest, and up to 6°F above normal in the southeast (Figure 2). Maximum temperatures followed the same trend, but dipped to 17°F below normal in the northwest, and up to 9°F above normal in the southeast (Figure 3). Minimum temperatures stayed much closer to near normal levels (Figure 4). The Midwest saw some form of precipitation almost every day this week, including a winter storm in the north, which led to well-above-normal precipitation totals in Iowa, Minnesota, Wisconsin, northern Michigan, much of Illinois, and southeastern Missouri (Figure 5). An intense winter storm that lasted multiple days brought unseasonably high snowfall totals to the northern Midwest. Storms of this magnitude do not typically occur this late in the season, so as a result, almost all of Minnesota and Wisconsin, and parts of Michigan and Iowa, received upwards of 750 percent of normal snowfall (Figure 6).

Drastic Temperature Change

Towards the end of last week, record high temperatures were widespread throughout the Midwest. This warm air lingered for the first bit of this week, before being replaced by the complete opposite. The tight temperature gradient was visible on the temperature departure map from April 15th-16th (Figure 7a); northern Michigan observed temperatures of up to 20°F above normal, while parts of Minnesota and Iowa observed temperatures of up to 10°F below normal. As the cold front passed through the Midwest on April 16th, below normal temperatures blanketed much of the region and were especially prominent in the north during the second half of the week (Figure 7b). Some of the most notable temperature swings (some including data from last week) can be seen in Table 1.

It is not common for both high and low temperature records to be broken or tied in the same week, however, the Midwest saw both extremes this week: 125 high maximum, 43 high minimum, 101 low maximum and 6 low minimum temperature records were set or tied (Figure 8a, Figure 8b).

Cold Front Sparks Severe Weather

As indicated by the quick temperature drop, the cold front passing through the area was incredibly strong. The front remained stationary in the morning through early afternoon timeframe, before starting its journey east (Figure 9). Some scattered storms began popping up along the frontal boundary. Into the late afternoon, more severe storms began to develop along the boundary and the relative surface low, primarily in Missouri (Figure 10a: WPC, Figure 10b: NCEI, 4/16 4pm EDT). Over the next few hours, the system continued to intensify, bringing severe weather across the border to Illinois (Figure 11, 4/15 7pm EDT, NCEI). There were twelve preliminary reports of tornadoes in Missouri and Illinois from this system. One tornado in Maries County, MO was rated primarily EF-1 with some low-end EF-2 damage, and caused five injuries. Numerous hail reports were also received; the largest hailstones observed were of four inches in diameter in Dade County, MO. The main severe activity calmed down around midnight (EDT) on April 16th, but as the cold front continued to push through the Midwest during the day, some smaller storms inevitably popped up along the boundary, especially in Ohio.

Record Breaking Snowfall

Just days after record breaking warmth, northerners got a snap back to reality by an intense, winter storm system. The surface low remained on the backside of the front, and circulated over the Great Lakes from the evening of the 15th through the morning of the 17th. Minnesota and Wisconsin received rain late on the 15th and in the morning on the 16th, before transitioning to snow. Figure 12a (WPC) and Figure 12b (NCEI) show the surface analysis and associated radar imagery, respectively from April 16th at 7am EDT; Figure 13a (WPC) and Figure 13b (NCEI) show the same products from 12 hours later, 7pm EDT. The highest one-day maximum was 19.5 inches of snow on April 17th, recorded in Jackson County, Wisconsin. The areas surrounding Lake Superior received the most snowfall from this system (Figure 14). This snow tapered off in the early morning hours on April 18th.

Active End to the Week

A smaller system started to form over the Plains overnight on the 18th into the 19th, but largely diminished in intensity upon entering the Midwest. The system picked back up in intensity in far eastern Iowa and Wisconsin, and some lone cells developed in central and northern Illinois. Figures 15a and 15b show radar imagery from April 19th at 12pm EDT and 4pm EDT, respectively, and was indicative of the nature of this system (NCEI). Between these two times, consistent hail was reported. Most reports were for hail between one and two inches in diameter; the largest hailstone in this area was 2.5 inches and was reported in McLean County, IL. Numerous high wind speeds were reported as well. As this first wave exited the area overnight, more storms began to pop up and intensify in the western Midwest (Figure 16, 4/19 8pm EDT, NCEI). There were four preliminary reports of brief tornadoes in Pottawattamie, Crawford, and Fremont Counties in Iowa around this time, along with numerous hail reports in Iowa over the following few hours. More storms continued to pop up over much of the Midwest all day on the 20th. The severe storms tapered down around 10pm EDT on April 20th, and left rain to linger in the central Midwest throughout the early morning hours. View the storm reports from April 19 and 20 for more details about these storms.

The passage of a warm front during the day on April 20th gave much of the Midwest some relief from the unseasonably cold temperatures, but temperatures in the far north were still below freezing on April 21st. A pretty typical setup was present in the morning on April 21st: a cold front was out ahead of a surface low pressure center, with rain along the cold front and snow around the low. This activity started up around 4am EDT on April 21st (Figure 17a: WPC, Figure 17b: NCEI) and continued through around 7pm EDT (Figure 18a: WPC, Figure 18b: NCEI). Only a relatively small area received snow from this event, but totals were not insignificant; far northern Minnesota received up to 10 inches of snow (Figure 19).

The two late-season snow events caused the Midwest to break or tie 113 daily snowfall records (Figure 20). Additionally, Duluth, MN has now broken their previous record for highest seasonal snowfall, with a total of 137.1 inches so far!

Streamflow Update

When compared to last week, streamflow values decreased to below or much below normal in Ohio, Kentucky, Indiana, and near Chicago (Figure 21). Increases to much above normal or high were observed throughout Wisconsin and Minnesota. Consistent, heavy snow with large accumulations throughout the entire season led to rapid snowmelt as temperatures started to warm.