Torrential rains in Oklahoma; Summer in March continues for Midwest

A significant flood event is underway in Eastern Oklahoma, where widespread rainfall amounts of 3 to 5 inches have fallen since yesterday. Up to four more inches of rain is likely today, and the National Weather Service in Tulsa is warning of the potential for "widespread and potentially catastrophic areal flooding and river flooding" should some of the higher rainfall amounts being forecast materialize. Numerous main-stem rivers across eastern Oklahoma and northwest Arkansas are now approaching flood stage, and will likely experience moderate to major flooding into Wednesday. Rainfall has also been heavy over Eastern Texas, with widespread amounts of 2 - 4 inches. These heavy rains are causing some street flooding, but in general, will be a benefit, as moderate to severe drought conditions still cover most of the region.


Figure 1. Radar-estimated rainfall for Eastern Oklahoma since March 19, 2012, as estimated by the Tulsa, Oklahoma radar.

The storm system responsible is a massive, slow-moving trough of low pressure over the Western U.S. that is colliding with the warmest and moistest air mass ever recorded in March in the Central and Eastern U.S. According to the NWS in Minneapolis, Minnesota, moisture flowing northwards into Minnesota along the cold front early this week had the highest levels of moisture ever recorded so early in the year. At the boundary between the Western U.S. trough of low pressure and Central U.S. ridge of high pressure, a cold front is lifting huge quantities of moisture-laden air aloft, forcing torrential rains to fall. The cold front is also expected to trigger a Slight Risk of severe weather over East Texas, Western Louisiana, and Southern Arkansas today, says NOAA's Storm Prediction Center. Severe thunderstorm watches are already posted for much of East Texas, as seen on our Severe Weather Map. Three tornadoes were reported yesterday in Texas, and eleven touched down the previous day in Oklahoma, Nebraska, and South Dakota. The tornado that hit North Platte, Nebraska two days ago was rated a strong EF-3, and injured four people.


Figure 2. Predicted rainfall amounts for the 2-day period Tuesday morning through Thursday morning show an area of 3+ inches (orange colors) is expected over Eastern Oklahoma, Arkansas, Louisiana, and Southwest Missouri. Image credit: NOAA HPC.

Summer in March continues for the Midwest
The ongoing March heat wave in the Midwest will continue to set all-time heat records through Thursday, gradually shifting its peak intensity eastwards during the week. A few highlights from yesterday's records:

Pellston, Michigan in the Northern Lower Peninsula is called "Michigan's Icebox", since it frequently records the coldest temperatures in the state, and in the entire nation. But the past three days, Pellston has topped out at 80° - 82°F, the first 80°F March days in their history. Yesterday's 82° reading broke the previous record for the date (56° in 1976) by an amazing 26°, and was 44°F above average. Nearby Traverse City hit 83°F yesterday, the third consecutive day the city has experienced its hottest March temperature on record.

International Falls, Minnesota hit 78°F yesterday, 42° above average, and the 2nd hottest March temperature on record in the Nation's Icebox. The record of 79°F was set the previous day. Remarkably, the low temperature for International Falls bottomed out at 60°F yesterday, tying the previous record high for the date. I've never seen a station with a century-long data record have its low temperature for the date match the previous record high for the date. Yesterday was the seventh consecutive day that International Falls broke or tied a daily record. That is spectacularly hard to do for a station with a century-long weather record. The longest string of consecutive records being broken I'm aware of is nine days in a row, set June 2 - 10, 1911 in Tulsa, Oklahoma (with weather records going back to 1905.) International Falls has a good chance of surpassing nine consecutive records this week.

Record heat in Canada
Record-breaking heat has also penetrated into the Prairie provinces of Canada over the past week. Winnipeg, Manitoba has broken its record high for the past five days in a row, and hit 24°C (75.2°F) yesterday, its hottest March temperature on record. Forecast high temperatures for Wednesday and Thursday across Ontario are near 26°C, which will threaten the records for hottest March day in history for Windsor, London, Hamilton, and Toronto.


Figure 3. The jet stream pattern features a large, southwards dipping bulge over the Western U.S., creating a trough of low pressure with cold and snow, and a large, northwards looping bulge over the Central U.S., creating a record-strength ridge of high pressure.

Why the record early-season warmth?
The unusual warmth is due to a loop in the jet stream that has created a large upper-level ridge of high pressure that is stuck in place over the Eastern U.S.--a phenomenon known as a "blocking pattern." Since the jet stream acts as the boundary between cold air to the north and warm air to the south, and the large loop in the jet places its axis far to the north of the eastern U.S., summer-like warmth has developed over the eastern half of the U.S. Conversely, colder than average temperatures have developed over the western third of the U.S. behind the southwards-dipping loop of the jet stream. This jet stream pattern is too extreme to be stable, and the big loop over the Western U.S. will break off and form a giant eddy on Wednesday. The resulting area of low pressure will be known as a "cut-off low", because it will be cut off from the jet stream. The cut-off low will drift slowly eastwards during the week, gradually bringing an end to "Summer in March" over the Eastern half of the U.S.

Jeff Masters