Alright...December is almost here, which means the official start of "meteorological winter" is almost here, too. It's time to take a look into the old crystal ball to see what lies ahead for the next three months. If the recent shift to a more active weather pattern is any indication, this winter will keep Chris, Stephanie, and me VERY busy!! In fact, Thanksgiving Day--a day in which we experienced near record warmth and snow within a 12 hour span--could just serve as a microcosm of what's to come. Speaking of snow, yes I think we will see some more in the weeks and months ahead. However, I will be shocked if we match last year's tally of over 15", Bowling Green's biggest in 25 years (just what you snowbirds wanted to hear, I am sure!). I'll explain why as we go.
Let's get to it, shall we?
First, the main players:
1. La Nina: Simply put, this refers to significant cooling of the surface waters of the Pacific Ocean near the equator. This is the opposite of "El Nino", where waters in the central Pacific warm. Last winter was an "El Nino" winter for North America, but this time out we will be influenced by La Nina conditions through the season.
There is some indication that La Nina may break down as we head into February and March, something that could lead to a cold finish to this winter. However, past moderate to strong La Ninas have brought generally mild to warm conditions for KY. This one will likely fall into that category. Why? Well, in La Nina years, the Pacific jet tends to jog farther north, coming in from northern California before streaking across the Rockies and central Plains into the Midwest and Ohio Valley region. The polar jet (our northern branch) often keeps a ridge over the northern Pacific while occasionally dipping south from Canada into the upper Midwest. That pattern doesn't always hold in place, however. There will be times this season when the polar jet plunges southward into the eastern U.S., bringing cold, arctic air into KY. But I think those cold shots will often be short-lived, especially during what is normally the dead of winter (January). This is where the other players (aka "teleconnections") get involved.
. North-Atlantic Oscillation: This refers to the east and west movement of upper high pressure in the North Atlantic region (generally between Newfoundland/Greenland over to Iceland and the Azores). Basically, when the NAO is in the "negative" phase, we often see blocking over/near Greenland, where a big ridge forms in the polar jet. This often forces the polar jet southward into the eastern U.S. In a "positive" phase, ridging usually dominates over the eastern U.S., which for us typically means mild to warm temperatures. While there is not much skill in determining NAO phases beyond a couple of weeks out, there is indication that the NAO is likely to be "negative" to start December before trending "positive" later in the month and into January.
3. Arctic Oscillation: This often works in tandem with the NAO. This basically refers to the change in barometric pressure over the North Pole. A negative phase of the AO means a good chance for arctic air to come south into the eastern U.S. (especially when the NAO is negative), while a positive phase generally means the really cold stuff stays bottled up to the north.
4. Pacific Decadal Oscillation: Much like the NAO, the PDO refers to the movement of semi-permanent pressure systems over the North Pacific Ocean, just south of Alaska. This has a huge impact on how both the northern and southern branches of the jet streams position themselves.
5. Other Players/Considerations: I like to check out snow cover over North America heading into the winter season to see how it's trending. I will say there is actually more snow cover showing up over the arctic regions and in Canada now vs. this time a year ago. That is something to consider, as more snow/ice cover tends to have a refrigeration effect of sorts on the surroundings. Sunspots are another "player" of sorts in my winter outlook. After several years of decreased activity, sunspots seem to be on the uptick. With more sunspots typically comes increased solar radiation onto the earth's surface.
Those are the players. Now, about those analog years I mentioned earlier. Those are winter seasons with patterns and teleconnections similar (though not EXACTLY the same) to those expected for this season. Here are the ones I came up with:
There are many common denominators that show up with each of these seasons listed. For one, each winter was mild overall with occasional shots of cold. Also, each winter had below-average snowfall for Bowling Green. Oh, and more thing, all these winters featured major severe weather events with strong tornadoes either during the dead of winter (a la 1989, 1999 and 2008) or early spring (a la 1965 and 1974) either in our near South-Central KY. With an active Pacific jet, I think there is a better-than-average chance of seeing at least a couple of significant, broad-scale severe weather episodes between now and March. February 2008 was proof that we can never let our guard down when it comes to severe weather, no matter what time of day or year!
So, here's a month-by-month breakdown of what I expect:
DECEMBER: This may actually wind up being the coldest of the three "meteorological" winter months. One trend I have noticed in looking at the data from those analog years is that some of those seasons started rather cold. Not mentioned above was the winter of 2005-06, which featured a weak La Nina. That season started cold before a flip to warm late December into January. This season could go along those lines. Look for a very progressive weather pattern throughout the month, with at least average if not slightly above average precip. Here's where some of you may ask, "Hey, what about snow?!" Well, typically we don't see much in December (only 0.9" on average, to be exact). In fact, you have to go back to December 2002 to find the last time we had a snow of more than 1" in Dec for Bowling Green (and that was in an El Nino season). While we haven't seen measurable snow in December in the last 5 years, I am going out on a bit of limb and will say that we pick up some significant snowfall at some point this month. That's partly based on my call for a wet month, and partly based on the NAO forecasts mentioned here earlier. In all likelihood, I think that happens sometime during the first half of the month...not necessarily good news for our "White Christmas" prospects.
Average December Temp: 36-38 degrees (near to slightly below average)
December Precip: Near to slightly above average (5-6")
December Snow: 1.5" (.6" above average)
JANUARY: This is "normally" the coldest of the winter months. Last year, it lived up to that billing with over 8" of snow (our snowiest Jan since 1985). I highly doubt we'll see a repeat of that this time, though. The reason? I look for a large upper high to plant itself over the southeast U.S. this month, keeping the polar jet farther north. That will likely result in many days with south to southwest winds that keep us mild if not warm. I won't be at all surprised if we see some 70 degree days during this month; that's how warm I think it could be at times. Now that's not to say we won't have some occasional cold shots--we will--but when they do happen, look out! I can see a situation like what occurred back on January 2, 2006 possibly happening again this January, where we go from 70s one day to 30s and 40s the next. That's when trouble usually comes. As for snow, don't look for much this month. In many of the analog years listed above, there was little more than a trace of snow for the entire month of January. That fact weighed heavily in my forecast here. The pattern should stay quite active, though.
Average January Temp: 38-40 degrees (4-6 degrees above normal)
January Rainfall: 4 1/2 to 5 1/2" (about 1/2" to 1 1/2" above normal)
January Snow: 1/2" (3 1/2" below normal)
FEBRUARY: This month's usually the toughest to call simply because it's so far out. There is much more uncertainty inherent to the computer model forecasts when stretched out over a longer period of time. However, I believe the month starts warm before finishing out cold. This goes back to something I mentioned earlier about the expected breakdown of La Nina conditions late in the season. This happened back in 2008, when we didn't see any measurable snowfall until near the end of February. We also saw a big winter storm early that March, one that dumped anywhere from 3-12" of snow over South-Central KY. That's why I say if you are a fan of the white stuff, don't think that just because this will be a mild winter overall doesn't mean we'll be shut out altogether. Now I do think this will be the only month of the next three with below average rainfall, in part because of a less active Pacific jet with a weakening La Nina late in the going. However, if we can get a system to track across the Dixie states and send overrunning moisture into a cold air mass, anything can happen!
February Temp: 37-39 degrees (near normal)
February Precip: 3-4" (up to 1" below normal)
February Snow: 3" (1" below normal)
Just a footnote about March (even though it's technically not a part of meteorological winter): I think we could see a little more snow early that month before the pattern likely flips to one that's more zonal with the jet stream over North America.
So there you have it. In summary, I am going for above-average precip overall for the season (though Feb could be a little drier than normal) with below average snowfall (thinking 5-8" for a seasonal snow total across South-Central KY, which for BG is 2-5" below normal). Look for a cold start and a cold finish sandwiching what should be a mainly mild mid-winter (though with cold shots from time to time). As for any major ice storm threat, that appears to be low for this season. However, don't be surprised if we see severe weather in the dead of January, especially given our history with moderate to strong La Nina setups. Oh, one more forecast of sorts: I look for only one night of single-digit temps this season, with no sub-zero (haven't had that here since Jan 2003).
Could I be completely off base with this forecast? Admittedly, that's possible! Who knows...we could have one blockbuster winter storm that could bust the numbers I posted earlier. For that matter, we could see even less than the forecast amount (1949-50 only had a trace of snow for the ENTIRE winter season). I am just calling it as I see it! :)
Thanks for reading. Have a fun and safe winter!