WHNT Weather

First Freeze of the Season Soon?

Posted on: 9:38 pm, October 15, 2012, by Jason Simpson, updated on: 08:40pm, October 15, 2012

The earliest “freeze” on record in Huntsville occurred on October 29, 2000 with a lowest temperature of 29 degrees, but in Muscle Shoals, it was all the way back in 1908 with a low of 31º on October 3rd.

Frosts usually happen earlier than “freezes.” A freeze is defined as “A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost. (Western Region Climate Center)

Alabama and Tennessee often see frost on the ground, rooftops, and car windshields with temperatures as much as 5 to 8 degrees above freezing, and since frost has already occurred for some around the Tennessee Valley, we are now looking toward what could be the end of the growing season with some air cold enough for a widespread freeze within the next two weeks – particularly around October 27th to October 31st.

Let’s say up front here that there are several global atmospheric signals working against a late October freeze for most of North Alabama, but since we are approaching the time of the normal first frost and freeze of the season, there is definitely some precedent to at least look at what some of the long-range model guidance is suggesting.

First off, here is the probability of a temperature below 35º F at Huntsville International Airport over the next 15 days based on the NCEP Global Ensemble Forecast System (GENS). I chose to interrogate the model to see what the probability of a temperature under 35º F would be for the entire model run; 35º is significant because once we get to that point, frost is likely as long as the the sky is clear and the wind is light, and a freeze is definitely possible for many of the typically colder areas surrounding Huntsville:

A probability of 33% on October 27th and 38% on October 29th definitely raises eyebrows that far out! If we drop the expectation down to a temperature under 32 degrees, the probability is still just a little higher than 10 percent:

The graphs above are from an American-run computer model ensemble system. If you want to learn more about what an ensemble is and what it does, check out this post from Brandon Chambers earlier this Fall: Ensemble Signal of Cooler Weather Ahead.

The European model (ECMWF) also shows some signs of being cooler than normal to end the month. The image below is the ECMWF ensemble spaghetti plot of 500 mb heights for 7 AM Saturday, October 27th. The orange line is the ensemble mean position of the 5780 height line. What’s important here is that the orange line (ensemble mean) is farther south than the green line (climatological average). When the 500 mb heights are abnormally low, that means the atmosphere is unusually cold. On the flip side of that, if the orange line were north of the green, then the height would be abnormally high suggesting warmer-than-normal conditions.

These are two independent forecast systems suggesting at least a chance of cold enough weather for a freeze; however, as I mentioned earlier, there are some things that don’t seem to be in place just yet. The North Atlantic Oscillation (NAO) and the Pacific North American Oscillation (PNA) do not give a look that is highly favorable for a surge of cold air late in the month. The NAO is trending toward neutral (with only a few ensemble members suggesting a continued negative look – negative is favorable for Eastern US cold). The PNA is trending toward a slightly negative or neutral number with only a two ensemble members pointing it into positive territory (positive PNA is favorable for Eastern US cold).

Here’s one of the old reliable maps to really give us clues to long-term weather forecasting. The Canadian GEMS shows a large area of near and slightly-below normal temperatures over the Eastern United States. Huntsville’s averages drop to near 70º for a high and 46º for a low by the end of the month; if we are a little below average in that time frame, seeing some lower and middle 30s definitely seems plausible between October 27th and October 31st!