14 November, 2008
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WINTER OUTLOOK 2008-2009
For the purposes of this discussion, “winter” is defined Meteorological Winter, which begins December 1 and ends on February 28.
Well, I guess it’s time to play my winter weather cards and see if I hit the jackpot…or wind up having to fold! In any case, there are several considerations which factor into my winter outlook:
1. ENSO: El Nino/Southern Oscillation (ENSO) is an acronym used to describe the overall changes in the Pacific Basin, of which La Nina is the cold part and El Nino is the warm part. This runs on a 3-7 year repeating cycle, or oscillation.
According to the latest NOAA discussion, there is a lingering (weak to neutral) “La Nina” effect over the Pacific Basin. Sea temperature measurements show colder than average surface waters remaining in the eastern Pacific; this acts to suppress the moisture flux into the atmosphere and deflect the jet pattern northward. This leaves Texoma with a drier pattern, as we had last winter, for our part of North America. The average upper level wind flow (northwesterly with a weak to neutral ENSO) favors a greater number of cold outbreaks from Canada. These tend to be steered more to the east, but we would still be within the “outer limits” of each one.
Since La Nina weather is drier than normal, this allows for a wider daily range of temperatures; so we’d expect colder than normal nights and rather mild days, on average. A case study done by the Fort Worth NWS office shows that five of the eight coldest outbreaks in the past 58 years took place during weak or “neutral” La Nina winters…like we’ll be having this year.
2. PNA, NAO,AO,and MJO: Good grief: so many acronyms, so little time! Each of these describes a repeating pattern.
The Pacific North American (PNA) is a large cycle of variation within the Pacific Ocean currents and eastward over North America at mid levels in the atmosphere. While it can compliment ENSO, it changes much more often between positive and negative phases. The PNA has been in a positive phase for most of the past few months. This scenario favors below normal temperatures for the southern Plains (where we live). September 2008 was the 11th coldest on record for Oklahoma and 10th coldest for Texas. The fluctuations of the PNA into the “positive” mode lead to more cold outbreaks for a given winter east of the Rockies. Since we’re in a weak La Nina/neutral ENSO already, a positive PNA tends to skew the potential towards cooler weather.
The AO (Arctic Oscillation) and the NAO (North Atlantic Oscillation) change several times per winter. Both are measurements of the relationships between northern cold high pressure and warmer low pressure further south. The NAO is considered a regional version of the Arctic Oscillation. A “negative” mode of either one indicates strong high pressure up north; it favors winter storms and cold outbreaks in the eastern United States. Texoma tends to get cold dry air masses on the back side of these storms during a negative AO/NAO. Indications this winter point towards a neutral to negative AO/NAO, but it’s very hard to nail these down more than a few weeks out.
The MJO is the Madden-Julian Oscillation. This one resides in the tropics and was first identified in the 1970s. Research on the MJO has a long way to go; the basic theory centers on tropical rain systems and their movement and evolution from west to east. It can throw a “monkey wrench” in the overall seasonal outlook as it ebbs and flows on a fairly short cycle length of 30-60 days. A strong MJO can increase rainfall across the southwestern and southern U.S.; the NOAA forecast is for a weak MJO through the winter, once again reinforcing the case for a fairly dry winter in Texoma.
3. Volcanoes: There have been four major eruptions this year. The largest is the Alaskan volcano Kasatochi, which ejected 1.5 million tons of sulfur dioxide into the stratosphere in August 2008. This is sufficient to have some impact on global climate, although not a lot. In 1991, the much larger eruption of Mount Pinatubo in the Philippines shot 20 million tons upward. This one led to about two degrees of global cooling during the next two years. All four of the 2008 volcanic events together may have contributed a total of two million tons of SO2. This should be sufficient to provide a few tenths of a degree of global cooling during the next year or so. Not a lot…but still enough to be significant. It’s one more factor pointing to a cool (cold) winter.
4. Arctic Ice: Although very little has been said of it in the national media, the Northern Hemispheric ice caps have grown at a near-record rate this fall. The large expanse of ice cover offers a greater physical area for cold air development, as well as a path for Siberian (from Russia with love!) outbreaks where they will not “warm up” as when crossing an open ocean.
Bottom Line: my winter 2008/2009 forecast for Texoma is for one to two degrees below average temperature and below average precipitation.
While this may all read like a train wreck, please understand these phenomena all interact in very complicated and only partially understood ways. Given each of these influences, I am expecting somewhat below temperatures east of the Rockies (except warmer for the southeast) this winter. As for precipitation, I expect generally below normal rain/snowfall. This does not mean we won’t see some rain, snow, or ice this winter. I cannot forecast a specific event like that months ahead (and neither can anyone else except for a wild guess, which is luck not science!). But the pattern favors drier than normal overall.
While “one to two degrees” may not sound like much, averaged over 90 days it corresponds to the category of “moderately below normal”. The cold will be most noticeable on the many expected clear and dry nights. We will also see more sunshine than average if my forecast comes true. I do not expect a record cold winter, but certainly colder than average and drier than average. Feel free to post your comments…and stay warm!