Recent MSP South Flow Activity

Aircraft activity over some Twin Cities communities recently has neighbors in those areas asking,  “What’s going on?” During the past two months, many flights to and from Minneapolis-St. Paul International Airport (MSP) have been following flight paths and procedures in a south flow day after day, which is unusual, particularly during this time of year.

As a direct result of the prevailing wind patterns, the airport was in a south flow configuration for 35 out of 61 days in September and October 2015. At times, this configuration was used for many consecutive days as winds continued to blow from the south and east. When MSP operates in a south flow, aircraft depart from Runways 12L, 12R, and 17 (over Eagan, Mendota Heights and Bloomington); arrivals land on Runways 12L and 12R (over Minneapolis).  Persistent use of the south flow is rare, particularly during late fall, when north winds typically become more common.

How air traffic controllers use airport runways is highly dependent on weather. Air temperature, wind, and humidity contribute to the physics of flight and impact how aircraft perform. Wind direction is a particularly important consideration for runway selection because lift – the force that enables planes to become airborne -- is generated as wind blows over their wings. Therefore, the safest runway to use for aircraft takeoffs and landings is the runway that provides the strongest headwind.

At MSP in September and October 2015 the wind patterns have favored a south flow nearly every day because winds have blown steadily from the south and east, meaning planes have needed to take off and land facing toward the south and east. It is these southerly winds that have contributed to the unseasonably warm temperatures the region has experienced this fall.  In fact, September 2015 was the warmest September on record for Minnesota and Wisconsin since records were kept back to 1895. 

“The overall weather pattern for September was summer-like with wind patterns favoring warm moist air from the south and few Canadian cold fronts.” -- Minnesota Department of Natural Resources

Typically, fall and winter will have more Canadian cold fronts and wind out of the north or west, putting MSP into a north flow. When that happens, arriving traffic approaches the airport from the south and southeast and departing traffic takes off toward the north and west. 

The wind rose diagrams below depict the wind patterns from September 1-October 31, 2015 compared with the same period in 2014[1]: 

IEM Windrose Plot for MSP Sep-Oct 2015 and 2014When a south flow is used at MSP, aircraft approach the airport from the northwest as they are being queued for landing on Runways 12L and 12R at MSP. Due to strict spacing requirements, the number of aircraft in the arrival flow will determine how far out the queuing occurs. At some of the busiest hours at MSP the approach path may extend beyond 20 miles from MSP.

Departing aircraft use Runways 12L, 12R, and 17; after takeoff these aircraft will overfly the Minnesota River Valley and communities located south of MSP. When approved for takeoff by air traffic controllers, pilots are assigned a specific departure procedure that includes a heading and instructions for navigating the flight to its destination. Pilots departing from MSP are not following ground references except in rare instances, and only when the aircraft is following visual flight rules.

So if you’ve questioned why air traffic around MSP has seemed a little different this fall than most, the answer, as Minnesota’s famous son Bob Dylan sings, “is blowing in the wind. The answer is blowing in the wind.”

For more information about wind roses, please click the following link to view our response to this Frequently Asked Question (FAQ): What is a Wind Rose?

Note:
[1]The Iowa Environmental Mesonet (IEM) collects environmental data from cooperating members with observing networks. The data are stored and made available on this website: http://mesonet.agron.iastate.edu/