Saturday, November 24, 2012

MWN 2012-2013 Winter Outlook for the Mid-South

Winter outlooks are always a tricky endeavor here in the Mid-South.  In fact, I'll be honest, I don't like doing them, but they seem to be popular among our readers so I put them together each year with some hesitation.

In the interest of full disclosure, I'm not a long-range forecaster.  I specialize in nowcasting.  So while I can hold my own in the 0-6 hour time frame (and am not too shabby when it comes to 5-7 day forecasts), 3 month outlooks are a challenge.  What I can do is cull information from multiple sources, put it in the blender, add a dash of experience and knowledge in Mid-South weather patterns, and present it in a coherent manner.

Note that while this discussion is heavy on the effects of large-scale patterns that help determine winter weather regimes across the U.S., and specifically the Mid-South, I do offer my thoughts on what to expect locally, this winter, at the end of the post. Skip ahead to "The Bottom Line" if you wish.

Last year's MWN outlook

At this time last year, I predicted near normal temperatures and precipitation, a couple of chances at winter precipitation, and also a few shots at severe weather late in the winter.  As it turned out, temperatures were in the top 10 for warmth, precipitation was very close to normal, and most of the snow that we received was early in the season (late November-early December, before it got excessively warm) with one brief snow event around Valentine's Day.  Severe weather struck on January 22 and the Memphis area just missed large-scale events on February 29 and March 2 (both exploded just to our east).  Except for the miss on temperatures, I pretty much had the winter pegged!

Status of ENSO / Recent Mid-South Winters

The driving factor in most winter outlooks for the U.S. is the status of El Nino-Southern Oscillation (ENSO), which is somewhat predictable out at least a few months.  ENSO refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean and in air surface pressure in the tropical western Pacific.  These variations can bring somewhat predictable patterns to the weather in North America.  For instance, El Nino during the winter is associated with a more southerly track to the jet stream as it enters North America's western coast, resulting in stormy weather for California and much of the southern U.S.

The past few years have seen La Nina conditions the past two winters (2011-'12 and 2010-'11), El Nino in 2009-'10, and ENSO-neutral (or La Nada) in 2008-'09.  Here's how Mid-South weather has responded:
  • 2008-'09 (La Nada) - snowy (3.8" at the airport, 8.9" at MWN in Bartlett), temps/precip slightly above normal
  • 2009-'10 (El Nino) - 13th coldest winter on record, snowy (6.8"), precip slightly below normal
  • 2010-'11 (La Nina) - 8th driest winter on record, but 9.7" of snow; temps just above normal
  • 2011-'12 (La Nina) - 7th warmest winter on record, dry with little snow
Looking ahead to the winter of 2012-'13, one thing we know (or are fairly certain of) is that earlier predictions of an El Nino winter now appear to be fizzling and long-range models are nearly unanimous that an ENSO-neutral, or "La Nada" (neither El Nino nor La Nina), pattern will be present.  Looking back, the last ENSO-neutral pattern brought slightly above normal temperatures and precipitation and some snow of note (2008-09).

Typical effects of an ENSO-Neutral winter. Note the presence of a subtropical jet stream over the Mid-South.

The NOAA Winter Outlook

In fact, the "official" winter (December-February) outlook from the National Weather Service/NOAA for the Mid-South predicts a better chance of above normal precipitation (40%) than below normal (27%) (see second figure below).  From a temperature standpoint, equal chances of above and below normal temperatures are forecast, though above normal temperatures are likely across a large area just to the west of the Mid-South (first figure below).  NOTE: Not shown, higher chances of above normal temperatures are forecast heading into spring 2013, which could translate into late winter (Feb-Mar) as well.

Chances of above (A) and below (B) normal temperatures for Dec-Jan-Feb from NOAA/NWS.
Un-shaded areas have equal chances of above and below normal average temperatures.

Chances of above (A) and below (B) normal precipitation for Dec-Jan-Feb from NOAA/NWS.
Un-shaded areas have equal chances of above and below normal average precipitation.

Other factors, and their effects

Besides ENSO, there are several other factors that are much less predictable, especially beyond a week or two, that can have major influences on the weather pattern over North America.  To see the effects of one of the major players, look at the difference between the last two winters - both of which featured La Nina conditions.  2010-'11 had decent snowfall (9.7" officially), but was very dry when considering rainfall.  Last year was very warm with near normal precipitation and very little snow (all of it early in the winter).

The difference between these two winters was the strong influence of the NAO, or North Atlantic Oscillation.  The NAO changes based on the position and strength of the North Atlantic jet stream.  A positive NAO brings above normal pressure to the eastern U.S., which tends to result in above normal temperatures that can extend into the Mid-South. A negative NAO results in lower pressure along the Atlantic seaboard and below normal temperatures, as well as more general storminess, in the eastern U.S.

The winter of 2010-'11, which was a La Nina winter, resulted in several periods of cold and snowy conditions due to a persistent negative NAO.  The winter of 2011-'12 was influenced by NAO but was overcome in this region by one of the strongest La Ninas in recent memory, resulting in warm and dry conditions across the southern U.S. The negative NAO's effects were more pronounced along the eastern seaboard.  The NAO can vary several times over the course of a winter and is generally not predictable beyond a week or two in advance.  Thus it is a huge factor whose effects for a season cannot be accurately determined this far in advance.
Effects of the NAO in it's positive and negative phases (Image credit: Brad Panovich)

Another global factor is the Arctic Oscillation (AO), which produces similar results to the NAO - colder in the eastern U.S. when negative, warmer and drier when positive.  The AO is also unpredictable, though there are early signs that the AO could be in a more negative phase than positive at least early in the winter.  The AO was negative for much of the 2009-'10 (El Nino) and 2010-'11 (La Nina) winters, contributing to brutal winter storms over the eastern U.S. and near to above normal snowfall both winters in the Memphis area, despite an overall dry winter last year.

Effects of the AO in it's positive and negative phases (Image credit: Brad Panovich)

The bottom line

While we expect to see an absence of official El Nino or La Nina conditions, Pacific Ocean temperatures slightly favor El Nino warmth.  Couple this with the possibility of negative NAO and AO conditions over the North Atlantic and it's prudent to follow the NOAA experts in calling for a better chance of above normal precipitation than below normal here in the Mid-South.  These factors all indicate that, one way or the other, the region could be squarely in the path of a number of varying storm tracks.

However, temperatures (as everyone knows who has lived here for any length of time) are the biggest forecasting dilemma, even a day or two in advance of a winter low pressure system.  Frequently, the Mid-South sits right on the edge of temps that are cold enough (to the north) or too warm (to the south) to produce snow from passing winter weather systems (think "I-40 corridor").  I think that will continue to be the case this winter (which is not going out on a limb per se).

But, the chance of having more wet systems moving through the region gives us the potential for additional chances that cold air could be in place at just the right time to produce some snowfall.  I expect more snow than last year, but not necessarily enough to go down in the record books.

Lastly, given above normal precipitation and a more active storm track, as well as possible above normal temperatures, I also give the Mid-South a slightly elevated chance of experiencing severe weather, especially late in the winter.

MWN Winter Outlook for 2012-2013

Temperatures near to slightly above average
Precipitation - above average
Snowfall - near to just below average (average is 3.6")
Severe weather - slightly elevated chances

We'll see how it plays out and you can call me a liar when the official stats come in next spring!  Just be sure to re-read the disclaimer at the beginning of this article before calling me out...  However it pans out, WILL keep you updated on each system that affects the region throughout the winter, just as you have come to expect.

Erik Proseus, Meteorologist
Cirrus Weather Solutions/

El Nino -
NOAA/CPC seasonal outlooks -
NCAR/UCAR - "Hola La Nada!" -
NOAA/CPC - Teleconnections -
2012-'13 Winter Outlook by Brad Panovich (image credit) -

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