December 1: Looking Back at Autumn, More Rain Ahead

WEATHER

Calm weather will prevail for the first couple days of December, with lots of sunshine and near-normal temperatures. We’ll reach the low 50s for highs today…
thursday-highs
…then drop to near freezing tonight:
friday-lows
Back up to the mid 50s Friday:
friday-highs

Increasing clouds Saturday, with a good chance of rain heading in from the southwest Saturday night and Sunday.  The RPM forecast model that we use most-frequently in Futurecast just looks 72 hours into the future, which means we run out of data in the pre-sunrise hours of Sunday morning.  But that’s enough to give you a look at the approaching rain, and to point out that the best chance of substantial rain looks like it will shape up in southern Middle Tennessee:
rpm-6p-sat rpm-9p-sat rpm-12a-sun rpm-3a-sun
Just rain with this system, no severe weather expected.

Another good chance of rain will move up from the southwest again by late Monday, followed by yet another rain chance midweek.  Still plenty of uncertainty surrounding all of these systems — the long-range forecast models have been maddeningly inconsistent, so while I’m confident in the overall “rainy pattern”…
rainy-pattern
…I’m not as confident in the details of the 7-day forecast:
wsmv-7-day-am
Stay plugged-into the forecast for changes if you’re trying to make plans from Saturday night onward.

Once the weather pattern calms down a bit, a big chunk of cold air will dive down into the middle of the country by late next week:
colder-pattern
We’ll be on the edge of that initially, but it looks pretty likely that we’ll see a short period of below-average temperatures late next week.

 

LOOKING BACK AT AUTUMN

Today is the first day of “meteorological winter”…meteorologists round off the seasons to coincide with the calendar — so, instead of using “astronomical autumn” of Sept 21 through Dec 21 (based on the Earth’s revolution around the Sun), we record autumn as Sept-Oct-Nov, winter as Dec-Jan-Feb, and so on.  It’s easier, and it just makes more sense.

Now that meteorological autumn is in the books, we can take a look back at exactly how exceptionally warm and dry it was.

  • The average temperature in Nashville in November was 55.3°, making it the 5th-warmest on record (temperature records in Nashville date back to 1875).
  • October was 4th-warmest, September was 15th-warmest.  When you add up all three months, the average temperature was 66.3°, the 2nd-warmest autumn on record!  Only 1931, with an average temperature of 66.8°, had a warmer autumn.
  • Just 4.17″ of rain fell in Nashville during the entirety of September, October and November.  That’s below the average rainfall for just November.
  • Autumn 2016 was the 6th-driest on record (precipitation records in Nashville date back to 1872).

When you look at Nashville’s temperatures in 2016 as a whole (through November 30), this is the warmest year on record with an average temperature of 64.5°.  (And before you ask, YES I’m just comparing temperatures through November of each year in the historical record, so it’s a strict apples-to-apples comparison.)  The next-warmest year was 1921, when the January-through-November average temperature was 64.0°.  1921 was the warmest year as a whole, ending up with an annual average temperature of 62.4°.

So here’s the natural question: since December is a colder month in general, what are the odds that 2016 maintains its first-place status?  We don’t have to do a whole lot of fancy math to find out the answer, thankfully.  The average temperature in December is 40.4° (adding up the highs and lows, dividing by 2).  If the month ahead brings us exactly normal temperatures, the average for 2016 overall would still be 62.5°, which means 2016 would be cemented as the warmest year on-record in Nashville.

The Climate Prediction Center’s outlook for the month of December shows “equal chances” of normal, above-normal, or below-normal temperatures.  That is, there isn’t a strong signal in the big global weather patterns (La Nina, North Atlantic Oscillation, etc.) that points toward either prolonged cold or warm weather over the next month.
off15_temp

Let’s get speculative — what would the December temperature have to be to drop 2016 into second place?  Try 38.6° — almost two full degrees below average!  When you’re trying to overcome 11 months of exceptional warmth, you need a pretty cold month to drag down the annual average temperature just a couple of tenths of a degree.  Put it another (and simpler) way: December would have to be among the coldest 25% in Nashville’s recorded history JUST for 2016 to not be the warmest year on record.

Now let’s get really weird: what if this December completely flips the script, and is the coldest on record — what would that do to the 2016-as-a-whole statistics?  The coldest December in Nashville occurred in 1989, when the average temperature was just 29.5° (that’s ridiculously cold for this part of the country).  Let’s say this month unleashes a series of cold snaps, and we end up with an average temperature of 29.4°.  (That’s not actually going to happen, but let’s stick with it as a hypothetical exercise.)  That kind of exceptionally cold weather throughout the month would drop 2016’s average temperature to 61.6°, which would still be tied for the sixth-warmest year on record.

Let me put some emphasis on that: 2016 has been SO WARM so far, even an all-time coldest December would still leave this as one of the six warmest years on record.  To be clear: this is local data…but it’s an almost-100% likelihood that 2016 will be the warmest year on record globally as well.

Adding in the expectation that 2016 will rank at least in the top six, that means 5 of the 12 warmest years on record in Nashville will have occurred since 2006.
thisisfine

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About paulheggen33

Morning meteorologist for WNCN-TV in Raleigh.
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One Response to December 1: Looking Back at Autumn, More Rain Ahead

  1. Fred says:

    Oops, didn’t see your new post;)

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