BATON ROUGE, LA (WAFB) -
Every year at about this time, especially after a run of warm winter days like those that have marked much of February, I get calls, letters, and social media posts asking, “Can I start my outdoor planting?”
The correct answer is “Probably, but not definitely.”
Everyone wants a “yes” or “no” answer, but weather doesn’t work that way. Sorry. It’s all about probabilities and statistics.
For metro Baton Rouge and areas south of the Red Stick, I put the chance of another light, brief freeze at about 10 to 15 percent or so, which is very low, but not zero. Chances for another freeze for WAFB’s coastal parishes is down in the single digits. I’d carry about a 30 percent chance for another light freeze for WAFB neighborhoods near and north of the LA/MS state line.
When I get one of these questions from viewers, I often ask a question right back: “Can you get your plants covered in the unlikely case of an unexpected late freeze?“ Be aware that Baton Rouge has had “freak” freezes as late as April twice in the past 100 or more years, so if you’re not ready (or able) to protect that vegetation, you may want to wait a couple of weeks at least. However, if you are ready to take the low-end chance and you can get that tender vegetation covered in the event of a rogue, late freeze, then this upcoming weekend may be time to get that garden underway.
Table 1 shows my assessment for the average range of dates for the last spring freeze for selected sites in the WAFB area. Keep in mind the average date means that there is still a 50 percent chance for another freeze later in the spring. The table also includes the dates for a 20 percent chance of a later freeze.
Many of my climate-science colleagues will post a single date for these categories rather than a range of dates. My main reason for using a range of dates is that any single date depends on the period of years used to generate the climatological statistics. For example, if we look at Baton Rouge, the average last spring freeze date is Feb. 21 if we use the period of 1981 to 2010. However, shift that 30-year period forward just five years (1986 to 2015) and the date becomes Feb. 23. Choose to look at the last 50 years of data (1969 to 2018) and the date moves again to Feb. 25. You get the idea.
Frankly, while these dates may differ in a statistics sense, they are telling us that a good date range for that 50 percent chance is generally around the last week of February for the Red Stick.
But wait! At a 50/50 chance (50 percent or average date), are you ready to risk your plants on a coin toss?
And there’s another threat many people forget. Air temperatures are measured approximately 4 feet above the ground. On a clear and calm night, the temperature difference between those measuring thermometers and the ground surface can be as much as 2° to 4°. That’s why you can sometimes see morning frost on the grass, rooftops, or windshield (it must have dropped to at least 32° for frost, right?) even when the reported low temperature only dips into the mid 30s.
In effect, that date for the 50 percent chance implies you may have better than a 50/50 chance of another ground frost. Just a little food for thought.
Table 1 also shows you the approximate dates for the 20 percent chance of a later freeze in the spring. These data are telling us on average, there is still a one-in-five chance for another freeze well into March for Baton Rouge.
Based on Table 1, you will note that my low-end chance for another freeze THIS spring for metro Baton Rouge (10 to 15 percent) is more like what the long-term climate statistics might suggest for a mid-March date.
Here are some of the keys I’ve looked at to propose a lower than typical chance for another freeze in the capital region:
- February temperatures have been running above normal and the Baton Rouge Metro Airport (BTR) hasn’t dipped below 36° this month. In fact, the last freeze was Jan. 25. What’s more, BTR has only had eight freeze days this season, about half the average expected through Feb. 20. Freezes have been in short supply this winter.
- If you have been following the WAFB Storm Team’s 10-day outlooks, you know we expect temperatures to remain well above freezing into March, so it will be a “freeze-free” February, which is not as uncommon as you might think. Historically, about 1-in-6 Februarys are freeze-free for Baton Rouge. But just by making a freeze unlikely until sometime into March means chances drop substantially.
- Although the NWS Climate Prediction Center (CPC) is currently suggesting a trend towards near average to below average temperatures across much of Louisiana for the first half of March, it’s also expecting early-to-mid March weather to trend wet for south Louisiana. Our local freeze threat tends to be a little lower (again, not zero) during rainy Marches.
- The CPC has also finally confirmed the onset of El Niño. Looking back at past springs (through 1950) when El Niño was active, only about one-third of those springs had freezes in Baton Rouge on or after Mar. 1.
- Of the ten freeze-free Februarys for Baton Rouge since 1950 (which includes February of 2017 and 2018), there were only two seasons (20 percent) with freezes after Mar. 1.
Given these five tidbits, it seems reasonable to assume the chance for another Baton Rouge freeze this spring is well below the long-term statistics. In fact, as was true in the past two seasons (2017 and 2018), it seems likely metro Baton Rouge and areas south of the Capital City are done with freezes for the year.