The Trouble With 33 Degrees
October 30th, 2016
I noticed this month that the low temperature forecast can become a motivator for me to adopt some level of complacency. The trouble with a forecast low of 33 degrees (or a bit higher) comes from the sense that it isn’t going to be cold enough to cause trouble for the remaining garden plants. The National Weather Service does issue frost advisories late in the growing season when forecast lows are expected to come within 4 or 5 degrees of the 32 mark. However, on clear, calm nights many locations can be as much as 4 to 7 degrees colder than the official reporting sites. Surprises also occur later in locations which may have escaped the first freezes after frost / freeze advisories are discontinued for the season. At such times, the low temperature forecast may receive less attention. Next I will note what I observed in my area during October.
The Evidence – October History
Forecasting weather is difficult, and I have plenty of first-hand experience with that difficulty! So, the purpose in recounting this brief October history is not to find fault. I seek, rather, to inspire you to become a more attentive consumer of weather forecasts. At Marshall, MN during the first 24 days of October the morning low temperatures averaged three degrees colder than the forecast given 24 hours beforehand. In determining this average, I used just 19 of the 24 days, after tossing out five days when the daily low temperature occurred in the late evening, rather than in the morning. Among the remaining 19 days were five mornings when the NWS forecast low for Marshall, MN was above 32 degrees (a five-morning average forecast of 34.8 degrees F), but the actual low temperature at Marshall was at or below 32 degrees (with a five-morning average observed low of 28.8 degrees) resulting in an average error of six degrees! Since I track the low temperature forecast 24 hours in advance, it’s probable that some of these forecasts were adjusted downward in the late afternoon or evening — 10 to 15 hours before the actual occurrence of the freezing temperatures.
The Factors – Difficult to Predict
When October low temperatures get colder than forecast, it’s usually due to having less cloud cover than expected, less wind than expected at your location, or drier air (which cools more quickly) than expected. None of these factors are uncommon – especially in the mid to late fall and – by early winter when fresh snow cover can also complicate the temperature forecast. Low stratus clouds are more common in the fall and early winter and the timing of their coming and going is often difficult to predict. When you see skies clearing more quickly than expected, a colder night is likely! Wind speeds vary considerably from one location to the next based on elevation and land features such as windbreaks or slopes. If your winds are calmer than surrounding areas (or lighter than forecast) and skies are clear, expect a chilly surprise! Last Monday’s frost (October 24th) occurred when the dewpoint dropped a full ten degrees overnight – and I will admit to being surprised at that! A big drop in overnight dewpoint can happen when conditions are ideal for moisture (which prevents the air from cooling as easily) to condense out of the atmosphere as dew or frost.
The Remedy – A Grain of Salt
Maybe you haven’t been focused on risk of frost or freeze to the extent I was during October, but you may have a different temperature threshold of concern. Perhaps you’re applying ammonia to farm ground, monitoring hunting dogs in the field, or seeking out the ideal day for that last autumn hike or plucking your backyard tree’s remaining apples. Taking the forecast with just a grain of salt, and being watchful for developing surprises can help the forecast work better for you. The key idea here — if one aspect of the forecast (for example cloud cover) is not going as expected, other aspects (such as temperature or wind speed) often will change. To help my own vigilance, I like to create a small chart to track important forecast details. When I see conditions deviating from the forecast, I prepare accordingly. Send a message to firstname.lastname@example.org if you’d like me to prepare a weather tracking chart for your favorite late fall activity!