Post #G21-054: My first frost date will be late this year.

Posted on October 11, 2021


As fall progresses, it’s time to start looking out for the first frost.  For open-air gardeners, that’s when you either start hassling with some sort of overnight frost protection, or you call it quits for the year on any frost-sensitive plants.

I still have a lot of things growing in my garden that I would like to harvest before first frost.  These are plants that survived the summer (peppers, sweet potatoes) and vegetables planted specifically for fall harvest (lettuce, spinach, peas, green beans, eggplant).

For Vienna, VA, in Zone 7, first frost is expected on or about October 24 (Post #G21-052).  That’s the “30th percentile” first frost date.  Over the past three decades, first frost has occurred on or after that date 70 percent of the time.

As I noted in earlier posts, the “last spring frost” and “first fall frost” concepts are crude.  They are unconditional probabilities, that is, they simply summarize what occurred in the past.  They don’t account for the current weather this year, and they don’t account for the presence of long-term (e.g., ten-day) forecasts.

For the spring last frost date, the presence of good long-term (e.g., ten-day)  forecasts shifts the odds in your favor (Post #G21-005).  That happens because you won’t plant if frost is in the forecast.  That obvious observation converts the unconditional “30th percentile” spring date into a conditional “10th percentile” date.  Just by keeping an eye on the 10-day forecast in the spring, you can cut your odds of a post-planting frost from 30% to 10%.

The same should be true of the fall first-frost date, but without any significant real-world consequences.  As with the spring date, the current ten-day forecast should help you predict the first-frost date more accurately than the simple unconditional 30th percentile date.  But unlike spring, the plants are already in the ground.  This might give you a bit longer time to plan when to harvest the last of your garden, but that’s about it.

The statement above ignores the potential for significant predictive help from “seasonal forecasts”, which I take to mean forecasts of average weather conditions made months in advance.  There are a lot of issues there, the foremost of which being that these tend to be vague (e.g., the prediction will be whether or not a season will warmer or colder, wetter or dryer, than usual).

To put it plainly, even if the forecast is for a warmer-than-normal fall, nobody has done the analysis to translate that into a specific prediction for the fall first frost date.  It’s not even clear if it is feasible to do that.  And so, this year, the prediction is for a warm fall in this area (e.g., this reporting, or this reporting).  But I have no clue what that implies for first frost date.

You can access the official U.S. seasonal climate forecasts on-line.  As is the custom for the modern age, you can go play with them in an interactive map, courtesy of the National Oceanographic and Atmospheric Administration, below:


NOAA says there’s a good chance that temperatures will be warmer than normal in my area.  I’m sure that’s helpful to somebody, but surely not to me.  I’m guessing that’s about as good as they can do, and, given the inherently (mathematically) chaotic nature of weather, that may be about as well as they will ever be able to do.

As a result, I’m not holding out much hope for a super-accurate seasonal forecast.  Instead, I’m sticking with the idea that the only actionable information is the current ten-day forecast.

Source:  The Weather Channel, accessed 10/11/2021.

Based on today’s ten-day forecast for my area, I have little to worry about regarding the 10/24 first frost date.

I’d like to ask a couple of questions, given this forecast, but I don’t have the data, and I don’t think I can get my hands on the data.  First, I’d like to know the odds that it actually will freeze on October 24th, given that the forecast low is in the high 40’s.  I would also like to estimate what the actual first frost date is likely to be, given this forecast.  Both of those would require having historical data on the 10-day forecasts.  And, while I’m sure that somebody has stored that information, there’s no way for me to get my hands on it.

In any case, this has almost zero practical importance.  The only change this makes in my gardening is for two remaining pumpkin plants that I was about to pull out.  These were late to set fruit, to the point where neither of them was going to be able to produce an edible pumpkin by October 24.  I was about to clear that bed and set that up for over-wintering.

But now, given this forecast for warmth almost two weeks into the future, I think I’ll let them go.  You never know what another couple of weeks of growing season might bring.