First freeze arrives...
Posted November 8, 2010
For a number of days last week, forecasts pointed toward this past Sunday morning as a likely candidate for producing the first subfreezing air temperatures of the season across a sizable part of our area. The forecast worked out reasonably well in that respect, as many stations west of the I-95 corridor bottomed out in the upper 20s to low 30s, including a 30-degree minimum at the Raleigh-Durham airport. The National Weather Service had issued a freeze warning on Saturday covering most of central and eastern NC, and most of this warning verified reasonably well, with some exceptions across a good portion of the coastal plain, where persistent winds through the night mixed the lower atmosphere enough to prevent many areas from cooling quite to the freezing mark.
There was another chance for some of those coastal plain areas to fall to freezing or below this morning, and another warning was issued for areas that did not verify on Sunday. As I write this, stations like Ahoskie, Rocky Mt-Wilson, Fayetteville, Lumberton, Clinton and Pope AFB have fallen to freezing or below, while a few others just south and east have remained a touch warmer. Once a region has fallen below freezing for the first time, the growing season is considered to be over and typically the NWS will not issue another warning for the next occurrence.
You may recall that just last year, we set a record at the Raleigh-Durham airport for latest "first freeze" of the Fall, with November 28th being the morning that we finally got that cold. This year, we were still on the late side of normal, but not too far outside the typical range of variation. Our average first freeze sate at the airport is October 28th, as illustrated in the cumulative probability graph I included here, and has a standard deviation of about plus or minus 9 days. The graph here shows the likelihood of temperatures falling to or below a certain temperature (32 degrees in the case of the orange line second from left) by a given time of year. For example, the green line shows that we have about a 63% chance of having fallen to 24 degrees or lower by December 1st, and so on.
If you'd like to check out similar graphs for sites other than RDU (and some tabular data that also allows you to see when the earliest and latest first occurrences of those temperatures were), you can find them at the link I provided to the Southeast Regional Climate Center site. Just click on a dot for a station, then look down the left-hand side of the resulting page and select the "Fall Freeze" probabilities - there is also a "Spring Freeze" link that addresses the latest times in spring that the same group of temperatures are likely to be encountered.
For now, it appears that the rest of this week and the weekend ahead will likely be too mild for concern about any further freezes during that time frame.