Winter weather

Wow! I cannot believe we’re into November and looking ahead at another winter season. Where has 2012 gone!? We’re only a few weeks away from December, with the first day of that month marking the start of “meteorological winter” which runs through February 28th. With the season fast approaching, it’s time to gaze into the crystal ball and see what might be expected for South-Central KY during Winter 2012-2013.

Winter forecasting is NOT easy. That’s because there are numerous factors that come into play, including those that help drive the jet stream and steer systems across the country. I undershot the cold and snow two seasons ago - then horribly overshot last season on both accounts. Sometimes a winter storm tracking a few too many miles north or south of our region can be the difference between getting more rain vs. snow or missing out on winter weather altogether. The whopper of a winter storm that hit just days before Christmas 2004 comes to mind. That year, parts of our area saw snow and ice accumulate over a foot deep, while others saw mostly rain.

If you detest snowfall, well…you were probably in heaven last winter. Heck, we hardly had a winter! Bowling Green’s seasonal snowfall was a mere 1.4” with the area experiencing one of its warmest winters all-time (so did a lot of the nation, for that matter). Each season can be radically different, though. Take the winters of 2009-’10 and 2010-’11, for example. Both featured extended stretches of very cold air, with almost 16” of snowfall in the ‘09-’10 season followed by over 21” of snowfall in ‘10-’11. If you like snow, those were banner seasons by Bowling Green standards, especially when you consider that our average seasonal snowfall is around 9”. By the way, 2010-’11 was our snowiest winter since 1978-’79. As a former professor of mine so eloquently puts it, “no two winters are exactly alike in Kentucky.” That professor’s name: Glen Conner, and he should know. He served for decades as Kentucky State Climatologist. He’s right…and that‘s what makes comprising these forecasts so challenging.

That said, let’s first take a look at the various ocean patterns and upper-level pressure systems that play key roles in how a North American winter plays out. For the sake of keeping this from becoming a novella, I won’t go through every single item, but I’ll hit the “main players”. The terminology may get a little technical at times so bear with me as we pick them apart.

1.) EL NINO/LA NINA (EL NINO-SOUTHERN OSCILLATION)

Chances are you’ve probably heard about El Nino and La Nina since these two often steal the headlines. Basically, El Nino refers to an abnormal warming of the sea surface water in the Central Pacific ocean off the South American coast. La Nina, on the other hand, is a cooling of those same waters. If the water temperature at the sea surface is right at or very near normal, neither El Nino nor La Nina is present. That’s what we call an “ENSO Neutral” condition.

elnino_lanina

Each of the past two winters featured a La Nina pattern. However, La Nina broke down early this year. There had been a trend toward at least weak El Nino conditions heading into Fall. But the latest observations suggest that warming of the Central Pacific waters is happening slower than was previously forecast. You might ask, “what does this mean for us?” Well, it may not mean a whole lot, especially given all the other atmospheric players. But it is clear we are not looking at a third straight La Nina winter. Based on my research, this will have a significant bearing on my forecast…especially since La Nina winters tend to have wilder temperature swings and more precipitation in our region than El Nino seasons. While it may be marginal, I will lean toward weak El Nino conditions being present for most of the upcoming season.

2.) PDO/AMO (Pacific Decadal Oscillation/Atlantic Multi-decadal Oscillation)

The PDO refers to water temperatures in the North Pacific south of Alaska and off the coasts of western Canada and the U.S. Pacific Northwest. The general belief is that the PDO will be in a “cold phase” this season due to chilly waters there. The AMO refers to the overall water temperature in the Atlantic. The AMO typically runs in 30 year cycles. Throughout much of the late 1950s, 60s, and 70s, the AMO was in a “cold phase”. Since the 1980s, however, the AMO has been in a “warm phase”. This is something that could help fuel more “nor’easter” type storms this year - the kind that pummel the U.S. East Coast with gale force winds, heavy rains and even big snows.
ao

3.) NAO/AO (North Atlantic Oscillation/Arctic Oscillation

These pressure systems are the “wild cards” when it comes to winter forecasting. They gave many a forecaster a hard time last year (yours truly included). That’s because unlike the indices mentioned above that deal more directly with sea surface temps, these refer to the movement (a.k.a. oscillations) of semi-permanent pressure systems in the North Atlantic and Arctic, respectively. Those movements can be erratic, and it’s difficult -- even for a skilled meteorologist -- to forecast them beyond two to three weeks out.
nao
There are two phases of both the AO and NAO. When each is in a negative phase, the polar jet tends to dip down into the eastern U.S., sometimes sending cold, arctic air into our region. By the same token, a positive phase of each of those indices generally means the eastern U.S. (including our region) is mild to warm. The AO/NAO stayed positive most of last winter, which was a major reason why the season was so mild. However, there are signs that point to more fluctuations in these indices this year. This will be key to our forecast, especially since these fluctuations could become the dominant player in what looks to be a very weak El Nino.

The jury is still out on solar cycles and how big a role they play in winter. Some speculate that more solar flares/activity leads to milder conditions. Another factor to consider is snow cover across polar regions and Canada leading into November.
Here’s a look at where we stand on that right now:
snow_cover

By now, you’ve probably gathered there is much to consider when coming up with a winter forecast. The variability of the AO and the NAO makes it tougher. It helps to look back at previous winters to find those that saw atmospheric conditions and ocean temperatures similar to what is expected this go round. Those seasons are referred to as “analog” seasons. Here are the ones I found that seem to most closely resemble the kind of pattern I think shapes up this winter:

1957-’58
1976-’77
1987-’88
2002-’03
2009-’10

If you like snow, you’ll be happy to know that many of the winters mentioned above were pretty generous in bringing the white stuff. The season that may raise a few eyebrows might be 1976-’77. That El Nino season featured some brutally cold weather (esp. in January ‘77) and over 20” of snow! More recently, 2002-’03 brought nearly a foot of snow to BG with a few inches more than that in ‘09-’10. I will say ‘76-’77 is the extreme/worse-case scenario in terms of cold - a scenario that does not look (at this writing) to be repeated this season.

But don’t get ahead of me…none of this is a guarantee!

With all that said, it’s time to get into what I’m sure you came here for and that’s the Winter Outlook. I will point out that stranger things have happened in these parts… including freak snows in November as well as March and April (BG’s snowiest month of all-time was March 1960). However, my focus will be on the three months that make up “meteorological winter” December, January, and February.

DECEMBER: I look for this December to be seasonably cold overall with some mild spells. December 2009 (our most recent El Nino winter) behaved that way. Bowling Green’s average temperature that month was 36.6 degrees, which was one degree below normal. We also had a few light snow events that month (including flurries on Christmas Day - the year before our first official White Christmas since 1993). Total rainfall that month was a little over an inch below normal, typical of an El Nino winter here. I believe most systems next month will bring us rain, but a couple could bring us rain to light snow situations. December 2002 brought us a sizable snow/ice storm early that month. While I won’t rule something like that out, I believe the bulk of our wintry precipitation this season comes later.

DECEMBER TEMPERATURES: (1-2 degrees below normal)
DECEMBER PRECIP: Below normal
DECEMBER SNOWFALL: 1” or less (near normal)

JANUARY: This is normally the coldest of the winter months, and I look for this one to follow suit. We should see the polar jet take more plunges southward this month with at least a couple of extended periods of cold. I don’t look for the entire month to be bitterly cold, though. Past history of years with patterns similar to this one indicate we will see some mild periods (January thaws) from time to time. January 2003 and January 2010 both had their fair share of very cold temps but we did see the cold relax for roughly a 10 day stretch in those months. What about snow? Amounts will depend on how the polar jet interacts with the subtropical jet. In previous weak El Nino winters, South-Central KY usually came close to, or was right in the middle of a major winter storm. A storm in late January 2010 brought 5.5” of snow to Bowling Green…the most from one system since 1996. However, in January 2003, a similar system took a jog further south and nailed Nashville with 8” of the white stuff while Bowling Green wound up with only three inches. Another winter storm took a track almost like that one in January 1988 (another El Nino season) with parts of Tennessee getting more snow than South-Central KY. History could repeat itself if we see the southern-branch jet stick closer to the Gulf coast this season, but I think our area gets in on at least a couple of decent snows in January 2013.

JANUARY TEMPERATURES: (1-2 degrees below normal)
JANUARY PRECIP: Slightly below normal
JANUARY SNOWFALL: 4-6” (near to slightly above normal)

FEBRUARY: This is often the toughest month to make a prediction as there is more uncertainty about how the patterns will behave this far out. So again, we must look at past history with our analog seasons. In most of those seasons, we had at least one significant snow event during February with one or two other minor events. Something else we may have to watch for this season is an enhanced risk for an icing episode. February 2003 brought a major ice storm to northern portions of the WBKO viewing area as well as Northern/North-Central KY. Of course, the Ice Storm of January 2009 is fresher in the memories of many. That ice storm happened during a “neutral” winter season, but we may see a weak El Nino revert to neutral late in the year. Ice storms happen when moisture and warmth run up and over colder air at the surface. Do I think we’ll see a repeat of January ‘09? No, as that was a once in a 40 or 50 year kind of event. However, I would not be surprised if we encounter at least one winter storm that could bring us at least a tenth to a quarter-inch of ice before the season is out. I believe February starts cold before some late-month warming takes place. This will help balance the month since I am banking on a weak El Nino fading by then.

FEBRUARY TEMPS: Near normal
FEBRUARY PRECIP: Near normal
FEBRUARY SNOWFALL: 4-6” (near to slightly above normal)

When it all shakes out, here is what I expect for the season as a whole:

TEMPERATURES: Slightly below normal
PRECIPITATION: Below normal
TOTAL SNOWFALL: (9-12” - near to slightly above normal)
CHANCES OF SEEING AT LEAST ONE ICING EVENT OF A TENTH OF AN INCH OR GREATER: 60%
CHANCES OF SEEING AT LEAST ONE SIGNIFICANT SEVERE WEATHER EVENT BETWEEN DEC. 1ST AND FEB. 28TH: 30% (can never rule that out completely but chance lower this year with more cold air and less active subtropical jet expected)
SEASON’S COLDEST TEMP: 3 degrees (should happen in January)

There you have it. Basically, when it’s all said and done, I think this season is colder and snowier overall than last but not as cold and snowy as Winter 2010-‘11. I suppose if you don’t like snow, you can take comfort in knowing this should not go down as one of our snowiest winters. On the other hand, I think there will be enough wintry weather this season to bring smiles back to those who were bummed out from how little snow we saw last year.

Could this forecast bust? Admittedly, the potential is always there. If the subtropical jet is more active than expected and interacts with the polar jet more often, then the snowfall forecast could wind up being too conservative. On the other hand, if the subtropical jet stays suppressed to the south and/or the polar jet stays locked up into Canada more often than expected, than we could wind up with less snow than forecast.

I’d like to know what YOU think. Do you agree or disagree with my outlook? Let me know either via my professional face book page, my twitter feed HYPERLINK "mailto:(@Main_Event_Wx"(@Main_Event_Wx) or via e-mail at HYPERLINK "mailto:shane.holinde@wbko.com"shane.holinde@wbko.com. Just keep the comments clean, please.

Overall, be prepared for no matter what Mother Nature throws our way this winter season. Have a plan of safety should you ever need to use it. Whether an ice storm or a tornado, you should always be prepared. As we like to say around here “Know the weather before it knows you!”

Let the fun begin.

Shane


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