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Weather blog: When should you plant your garden?

Posted at 12:10 PM, Apr 30, 2020
and last updated 2020-05-04 17:09:13-04

April is coming to a close, and we turn the calendar to May, many folks will be ready to plant the garden.

Let's take a look at when it's safe for gardeners in southeast Wisconsin to get seeds and sensitive plants in the ground without concern for frost or a hard freeze.

Let's start with a look at the Midwest. Here, we see a map outlining median dates of the last spring freeze. By median, we are looking at an average last date for a 32° freeze.

You can see by this graphic, for SE Wisconsin, depending on how close you are to the lake, these dates range from either April 21 to 30 (closer to the lake), or May 1 to 10 (inland).

The influence of Lake Michigan (currently at 40-42°) can help keep coastal areas from dropping below freezing. But, there is variability year to year. Let's take a look at some records.

Temperature records for Milwaukee can give us a glimpse at the past. If we look at record low temperatures for the month of May, we can see overnight lows CAN be cold, in the 20's and 30's.

We can see a record low of 29° occurred on May 27, 1961. So it is possible to dip below freezing, even into late May. BUT, most of these record lows were set in the 19th and 20th centuries. Only one record low on this list is from the 2000's.

Let's look at one more important element, the forecast for the first half of May. The Climate Prediction Center 6-10 day and 8-14 day outlooks show a high probability for below normal temperatures in Wisconsin, the Midwest, and much of the Northeast through mid May.

The Hazardous Temperature Outlook confirms this thinking. Note the slight risk (20%) of much below normal minimum temperatures from May 7 to May 10 from Minnesota to Maine.

Average minimum temperatures for SE Wisconsin during this time are in the mid 40's, so we would anticipate overnight lows cooler than mid 40s.

One more map, a look at the 500 mb weather map. This weather map can show us trough and ridge patterns, where troughs would indicate cooler air, ridges warmer air. Here we can see the pattern six days into May, a trough extending from the Great Lakes region to the Canadian Maritimes.

The TMJ4 Storm Team will keep a close eye on the potential for cooler than normal temperatures, and any potential for frost, through the first half of May. Stay with us on air and online for updates.

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