Memorial Day weekend forecast: Models trending drier overall?

Welcome to the unofficial start of summer in Minnesota.

Memorial Day weekend is the first of the big three summer holidays. Climatologically it is also the wettest and coolest of the three. The old weather wisdom says forecast rain and hope for better.

It’s interesting to note that forecast models are having a hard time locking onto the relatively small disturbances that may bring rain and thunderstorms this weekend. That’s not unusual in summer. There’s a good reason for this.

Long and short waves

Large-scale weather patterns are driven by large atmospheric waves called Rossby waves.

We could talk all day about this, but here’s the important idea from NOAA.

Rossby waves help to transfer heat from the tropics toward the poles and cold air toward the tropics trying to return the atmosphere to balance. They also help locate the jet stream and mark out the track of surface low-pressure systems. The number of longwaves at any one time varies from three to seven though it is typically four or five.

Their slow motion often results in fairly long persistent weather patterns. For example, locations between the trough and the downstream ridge can experience extended periods with rain or snow while at the same time 1,500 - 2,000 miles (3,000 - 4,000 km) upwind and/or downwind the weather is very dry.

Short waves

Short waves are, as the name implies, shorter than long waves. They ride embedded withing the overall long-wave flow pattern. They often trigger areas of precipitation out ahead of the wave.

Again, here’s more from NOAA.

Shortwaves, embedded within longwaves, are also the chief instigator of episodes of precipitation. Main precipitation bands will be typically localized near the short wave as it passes overhead.

Below is an example of a 500 mb chart. The height contours are in black. The brown arrows indicate direction of airflow. The large red dashed lines represent the location of the long wave troughs.

Memorial Day weekend forecast

So, how do long and short waves figure in our Memorial Day weekend forecast? Simply put, short waves are harder for forecast models to pinpoint several days in advance. The American and Canadian models suggest a short wave in southern Minnesota Saturday morning.

If this timing works out it would trigger a batch of rain Saturday. The Canadian model also picks up a second rain wave into Sunday below.

The European model is trending drier this weekend. It prints out less than half an inch of rain for most of Minnesota, with heavier rains across Iowa.

Stay tuned as the forecast models track the short waves across Minnesota this weekend. Their posting and timing will determine when it rains at your location.