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Kirk Mellish's Weather Commentary

Posted: 5:40 p.m. Wednesday, Dec. 29, 2010

Winter update, January thaws fleeting 

By Kirk Mellish

I am still on vacation but wanted to give you good folks a preliminary update. Next threat middle of January, but general time frame for more fun and games from winter anywhere from January 9th through the 22nd give or take.

We are near the end of just the first month of winter! Traces of snow are still on the ground in North Georgia. The first month of winter has been much colder than average in Georgia with the first White Christmas in Atlanta in 129 years.

December has been dominated by a trough in the Gulf of Alaska, a strong west-based -NAO, and a trough in the southeast US. This pattern has led to the axis of colder than normal temperatures form the Plains into the southeast US, with the strongest cold anomalies being located over the Ohio Valley to the southeast. The Arctic Oscillation (AO) and North Atlantic Oscillation are behind the cold winter of last year and the cold start to this one. They are tied to the Atlantic Ocean temperature pattern and to a lesser degree Pacific Ocean temperatures, both of which undergo multidecadal cycles (30-70 years).  The favored states of the AO and NAO are tied to the phases of these long term oscillations. The long term ocean cycles are natural changes and linked to variations in the meandering thermohaline circulation likely tied to solar influences. They behave predictably as an average condition over long-time frames (decade or more), but unpredictably over shorter time frames from months to years.

Clearly the wild card of this winter (the AO/NAO), which is only forecastable specifically on a daily to two-week in advance time frame, has been the dominant factor for the first month of winter. The negative trend of the Arctic and North Atlantic oscillations with the attendant Greenland blocking pattern of the jet stream has thus far overwhelmed any Pacific Ocean La Nina influence. This was anticipated in the long-range call for winter, BUT NOT TO THIS EXTREME. The fast, early start to winter and fade down the stretch forecast has started out well in terms of the pattern but not the numbers.  Obviously it remains to be seen if the fade for the core of winter comes or not.

Preliminary speculation is that Previous year's high latitude volcanic eruptions (Alaska and Siberia 1-3 years ago) combined with the historic low sun spot cycle are working together to aid the negative NAO causing the abnormal snow and cold in the South and East U.S. and Europe.

The tropical summer and fall weather patterns and ocean temperatures aided the historic hurricane season numbers, and likewise seem to have had more pull on the large-scale atmosphere than the La Nina in the Pacific. In addition the strength of the La Nina has been less than numerical and statistical equations predicted thanks to weakening from the Madden Julian Oscillation (MJO) in the equatorial Pacific.

This rare combination leading to the very cold December for much of the nation indicates there is no reason to believe there will be any lasting super warmth this winter. It also means that my forecast for milder weather to take over slowly but steadily in January and February over the Southeast states is in jeopardy. I think after about a period of 6-14 days of changeable weather with milder temperatures than what we have seen so far this winter, we will revert to a similar pattern to what we have seen in December. This means that mine as well as others forecasts of a warmer than normal mid to late winter may go by the wayside. While January gets its thaw, there is little to indicate widespread lasting warmth or any blowtorch warm-up beyond fleeting.

Cold will return to the East and South, but if La Nina has its way it will be closer to normal cold for January and February. So I am not totally abandoning the flip in the pattern previously expected yet, as it may simply be delayed but not denied. But pending new model output on the NAO and a review of analogs I may have to make that change.

The reason for not jumping the gun is two fold. One, the cold December itself helps to destroy part of its cause, by cooling the Atlantic Ocean and Caribbean Sea/Gulf of Mexico (positive AMO Atlantic Multidecadal Oscillation) which when warm in winter draws cold air down into it. Second, the hurricane analogs which suggested the cold December in the East and Southeast still show milder conditions for the January-February period in most of the East and South followed by Spring chill from DC northward. But at least the next 6-10 days or so will be much more moderate East of the MS River. After that there is a battle between the La Nina signal taking over and model forecasts of a SSW event at 10mb (sudden stratospheric warming above the North Pole) the first projects mild but the second argues for a return of polar air after the thaw.

The warming aloft this fall and early winter in northeast Canada and Hudson Bay is part of the very strong negative arctic oscillation pattern we have seen the last few years, in part related to the long solar minimum and high latitude volcanoes. The positive AO state is characterized by an anomalously strong polar vortex that traps cold air in high latitude and more zonal mid-latitude jet stream that allows maritime Pacific air to invade North America and Atlantic air to flood Europe often as far east as The Urals. In the negative mode, high pressure dominates the polar region and North Atlantic, shunting arctic air south in North America and Siberian air west to Western Europe. (see slide show)

A positive AO leads to increasing temperatures in the Northern Hemisphere continents with colder than normal air trapped in the arctic down to the northern parts of Canada and Hudson Bay.

As noted, the North Atlantic Oscillation, an important component of the overall arctic oscillation varies with the ocean temperature tripole in the Atlantic, known as the AMO or Atlantic Multidecadal Oscillation. When the Atlantic is in the warm positive AMO mode as is currently the case, the NAO and AO tend to be more negative.

When the AMO is positive with warm water in the North Atlantic and in the Tropical Atlantic, the NAO was mainly negative (1960s). When the North and Tropical Atlantic turned cold in the 1980s, the NAO was mainly positive. Note the AMO flipped positive (warm) in 1995 with a big dip in the NAO.

So far this late fall and early winter, the AO has been mainly negative and forecast by the model ensembles to remain so.

This unexpectedly strong -AO/NAO explains not only the brutal cold in Europe and Asia but also the amazing 11F negative anomalies in the southeastern United States. NOTE AMAZINGLY how even though we have replaced the strong El Nino with a strong La Nina, and the QBO (Quasi-biennial Osscilation) was EAST last winter but is WEST this winter yet the result so far has been virtually the same for the start of the Northern Hemisphere winter. It is even colder in the southeast US, in Alaska and India. In the southern Hemisphere note the cold December to date in Australia and much of South America (the start of their summer). Summer snow even fell in southeast Australia.

These run contrary to what has happened in the past and is verification of the concern expressed in the winter forecast release at the end of October that finding solid analog years was being made difficult by unprecedented ocean-atmosphere-sun alignments.

Analog years that have done a better job (but not a perfect match) so far this winter are 1917,1933, 1950, 1955, 1995, and 2005. And to a lesser extent 1969, 1985, 1989, and 2008. The winter of 1950-51 stands head and shoulders above the rest to date. The big unknown is if these analogs will work going forward or will the ANTI pattern continue to give unprecedented weather given the signals from past history? Slide show maps of update.


In case it has been forgotten, here is a snippet from the winter outlook write-up back issued way back in October:

"New Jersey and Far South Florida seem like the biggest toss up locations. NJ because it's more open to impact from a negative cold NAO, South tip of FL due to more clouds and rain rather than any arctic air. However, it is common for there to be conflicting indications in making forecasts, rare is when all or even an overwhelming majority point in the same direction. If it was easy, we would always be right.  At this point it looks like a "front-loaded" winter: November to early January the best odds of cold air from the Midwest-Great Lakes to East Coast, with the bulk of January a question mark, February most likely to be the warmest winter month, and March more of a toss up but leaning on the mild side.

While temperatures should average out near average in the winter as a whole for NJ and maybe NW TN, and the South third of FL, temperatures North of GA are more likely to be near or below normal much of November and December east of the Mississippi River. After that, a major thaw will follow in January bringing warmer-than-normal temperatures for the heart of winter. Then there could be a return of cold weather in March north of the Mason-Dixon Line. An axis from Denver to Chicago to Newark is likely to have several intense cold shots and larger than usual temperature swings during the next 4-5 months.

Making this years winter forecast more challenging or of lower confidence than normal are some unprecedented atmosphere-ocean-solar signals. The El Nino to La Nina change has occurred about as fast as it can, in fact it's the most rapid onset of a La Nina on record. It is also projected by modeling to be the strongest such reversal from a warm episode to cold episode on record. Also, we are in the longest solar minimum sunspot cycle in 100 or more years. This makes us concerned that this year's La Nina winter weather may not behave as would normally be anticipated. However, due to the historic nature of unfolding events, in what way cannot be quantified. In other words, it is harder to use the past to get a picture of the future when the past has never been quite like this before. What's more some of the past solar minimums and strong La Niña's predate reliable weather records so are not included in the climate composite data base, such as the La Nina of 1889 and 1892. Also, if we looked only at the top strength La Niña's then our sample size would be so small as to be problematic.

The main conflict in making this years winter forecast is the conflicting signals regarding jet stream blocking at high latitudes and the polar vortex. The strongest La Nina since 1955 suggests the AO/NAO (arctic oscillation and North American Oscillation) should be more positive which is a "warm" signal for the Eastern and SE USA. However, the solar cycle min and Atlantic Ocean temperature pattern and the decadal trend of the NAO favor a more negative or "cold" signal. Unfortunately there is a lack of skill in predicting the AO/NAO more than a couple weeks out. If this pattern ends up being strongly negative (not currently expected) then the La Nina above-normal temperature outlook will bust. Sometimes the atmosphere adjusts rapidly to the Pacific Ocean signals and sometimes it reacts more slowly. As in the daily and weekly weather forecast, timing can be everything. You can be right about the eventual evolution but get the timing wrong enough to throw the outlook off, which is why a 3 or 4 month average forecast can be more correct than a monthly break down, as mother nature does not pay attention to a flip of the calendar page. In TN, GA, FL an above-normal temperature winter does not mean there cannot be a significant cold snap or two within the pool of warmth, but mainly outside of FL. In fact, there is a nontrivial chance for the coldest low to be colder than last year's coldest of 13F in Atlanta! So at least one or two cold waves with lows in the teens or colder is still likely in the midst of a generally warmer winter. Another front-loaded winter, off to a fast (early) start but fades down the stretch...

The fine-tuned analog lists shows a December through February temperature pattern for Atlanta of between 0.38 F and 1.12 F degrees above-normal, with rainfall between one-half inch and 1.42 inches below normal. Snowfall at least 0.6 inches below normal or between half an inch and 1.4 inches.

IF there is snow it's more likely to be early or late. The highest snow amount in analog years was 4.8 inches in 1959 and 4.5 in 1933. There was at least a dusting of snow in 74% of the analog years in either winter or spring, ranging from November to March.

The known wild cards for this winter? If the NAO is more negative than I expect (blocking pattern) the winter will be much colder than I am forecasting. If the La Nina ends up being more west based than I am expecting, then the winter will be much warmer than I am forecasting with frequent golf weather."

Only time will tell what the winter was on average after we get into March, after one month we don't know yet. Click on the maps to go to the linked slide show to see graphically what I am writing about. I hope everyone had a Merry Christmas. Peace on Earth, Good will toward men. God bless us, everyone.

Click here to find out more!
White Christmas in Atlanta no longer a dream in a song

Photo: NWSFO Peachtree City, Georgia



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