Thursday, November 6, 2014

Flash Weather: Get Ready for Frozen Turkeys

When it comes to long-term weather forecasting,  as I've said before to quote the great James Spann: "There's simply no skill in pinning down details past seven days."

In other words, while it takes specialized training to know how to dicpher real-time computer model data, when the conversation changes to, "Will we see snow on Christmas Day this year?", it's really anyone's guess.

Thus, I completely understand if one considers long-term forecast specialists to be professionals in "crap shooting". After all, the weather can seem to have a mind of its own.

However, as the clock has turned into November, there's been considerable buzz surrounding the weather pattern for Thanksgiving week...which is only fair as it's one of the most traveled weeks of the entire year.

And though I remain fairly conservative in posting any resemblance of a forecast 2-3 weeks out, I have to admit, I'm pretty impressed with model consistency regarding weather happenings in the 18-25 day time frame.

Seriously, I'm getting goosebumps just thinking about it.

'Cause bottom line: I can't remember the last time there was so much model agreement on an large-impact event over 14 days out. Regardless if this event is currently overhyped (or even downplayed), it's pretty incredible what's going on in the weather world right now.

With that said, let me just cut straight to the point: If you live in the eastern two-thirds of the U.S., we're headed for a pretty cold least by November's standards. No, this isn't going to be like January, when the polar vortex trended almost the entire month; however, we could see several days with January-like temperatures.

Thus, in terms of negative temperature anomalies, I wouldn't be surprised if we a) end November around five degrees below-average and b) experience the coldest November since...(*gulp*)...1976.

Now, I won't go into any detail on the epic winter of 1976-77; however, I have heard several credible sources remark on how that winter is showing up as a top analog year for the upcoming winter. For those who are unfamiliar, when I say 'analog year', this simply means a featured year from the past that best resembles the current year, taking into consideration the state of current teleconnections (the ENSO, PDO, QBO, PNA, AO, NAO, etc.). So when a meteorologist says the atmospheric stage for winter 2014-15 is most comparable to 1976-77, this should be music to any snow lover's ears.

But enough bambling about.

For the remainder of this blog, I'm going to harp on three reasons why this month is going to be cold and why we should keep a close eye on Thanksgiving week.

1) The Arctic Oscillation

Check out the AO forecast chart to the right...and note how the AO starts to tank in just a couple of days. Pretty remarkable how amplified the AO outlook looks right now. For all you weather geeks reading this, you may notice how a similar drop occurred one month ago in early October...and how it helps ignite a stretch of below-normal temperatures and our first frost of the season. Being we're one month closer to the start of meteorological winter, it's fair to wonder how low the temps will go next week...and if they will be tolerable for wintry precip.

Now, some of you may be wondering: What does a falling AO mean? Fair question. Basically, a falling AO, especially once it drops into negative territory, means higher pressures set up in the polar regions, in turn, promoting weaker zonal winds and polar, continental air to dive southeastward from Canada into the eastern third of the conus. This is a) why a negative AO is virtually synonymous with colder-than-normal temperatures for this part of the country and b) why the GFS's depiction of the AO's behavior for the next week has caught many meteorologists' attention.

What's triggering such a drastic plunge? We'll get to that in just a moment.

But first, I want to provide a little visual refresher concerning the difference between a +AO and a -AO. Check out the graphics below. When it comes to comparisons, the key is where the ridging and troughing axises set up (which is influenced by lower pressures setting up at high latitudes and higher pressures setting up at mid latitudes).

With a +AO, the dominant players are the Aleutian and North Atlantic polar vortexes, which combine to maintain a strong, fast moving jet stream around the Canadian/US border. In essence, this becomes the dividing line between polar air locked up to the north and the mild, Pacific air caught up in a zonal flow to the south. Note how the jet stream is oriented west to east, with little amplification. This is a trademark of a +AO. See how all the cold air is confined to the arctic circle? This is why the majority of the U.S. experiences warmer-than-normal temperature whenever a +AO is established.

However, with a -AO, the pattern is practically reversed, where the polar jet is much weaker and more susceptible to blocking patterns, such as the Greenland Block and the East Pacific Ridge. With lower pressures at mid-latitudes and higher pressures at high-latitudes, the Aleutian polar vortex is displaced further west, while the North Atlantic polar vortex becomes a non-factor. This allows the Hudson Bay polar vortex to take center stage and deliver much colder air further south. In extreme cases, a cross polar flow can establish itself through the 'Siberian Express' and deliver record-breaking or record-threatening arctic outbreaks. This is essentially what happened back in January.

2) Typhoon Nuri, The Bering Sea Rule & The Lezak Recurring Cycle Nuri - According to NOAA officials, forecasters are watching the development of what could be the strongest Bering Sea storm on record. Despite recent weakening, not only does the storm maintain strength equivalent to a category 4 hurricane, but will likely "bomb out" and encounter a period of rapid intensification. In fact, this past Monday night, the European model was forecasting the extratropical cyclone's barometric pressure to plummet down to a record-shattering low of 916 mb on Saturday morning, while other models, like the GFS-parallel, show an even stronger storm setting up at 914 mb (Note: These graphics below denote two things: 1) the geopotential height values in color and the mean sea level pressure (MSLP) values with the black pressure contours).

Folks, if this forecast verifies, this will easily break the all-time Bering Sea record of 927 mb set at Dutch Harbor in 1977. But while the ECMWF ensembles (a collected average of 52 members) forecasts the storm to be a little less strong at 938 mb, the main takeway for those thousands of miles away is that the most accruate forecast models in the world all show this very powerful cyclone merging with other atmospheric teleconnections to produce a series of arctic outbreaks across the central and eastern U.S. the next 2-3 weeks.
And just for perspective, check out the ECMWF MSLP Anomaly graphic to the right. See that giant black spot west of Alaska? That's the monster storm we're talking about. Note how much deeper the pressure is compared to other low pressure systems in the vincinity. There's really no comparison.

For those who read my 2014-15 Winter Weather Prediction piece, you'll remember how inferences on long-term weather outcomes can be determined by assessing the state of Pacific Ocean temperature anomalies at various locations. When we talk about low pressure events, this translates into colder, stormier conditions, while the opposite can be said about high pressure events. So even though we're talking about a more immediate scenario, the atmospheric principles still apply.

The Bering Sea Rule The Bering Sea Rule, a term that became popular when Joe Bastardi was still working at AccuWeather, takes the occurrence of high and low pressure systems in the Bering Sea and extrapolates their re-occurence over the US 2-3 weeks later. Thus, if a typhoon recurves north and east as it approaches Japan, means of a strong cold front, the weather in the eastern US is often teleconnected to the event a couple weeks out.

This is why it makes sense to monitor what happens with Typhoon Nuri. If we take the storm in the image above (valid on 11/8) and apply 2-3 weeks to it, we can see how there could a significant storm in the November 23-29 time frame. Talk about crazy timing, IF the BSR happens to verify in this case.

The Lezak Reocurring Cycle - According to the theory developed by Gary Lezak in the late 1980's, the Lezak's Reocurring Cycle refers to the unique weather pattern that sets up between October 1st and November 10th every autumn. During this time, "long term" longwave troughs and ridges become established over the northern hemisphere, marking the weather pattern in a way in which it repeats itself every 45-60 days until it slowly weakens and falls apart in the summer.

3) Sudden Stratospheric Warming - When we talk about a SSW event, it means we're talking about temperature anomaly behavior at the 10 millibar level (almost 20,000 meters up). According to this animation from CPC, last month featured a very early stratospheric warming event expanding across Eurasia and the North Pacific. Like the Bering Sea Rule, there's a rough time period between pattern emergence and encountered effects. In this case, when we find evidence of a SSW, especially on the other side of the globe, it takes roughly 2-4 weeks for this to translate into a colder pattern in our neck of the woods. Essentially, what a SSW does is weaken the polar vortexes enough to the point they become more mobile and displace further south towards to the U.S., thus, reinforcing lows setting up at the low-lattitudes. As mentioned above, this correlates to a -AO, which is necessary if the mid-south/southeastern quadrant of the U.S. are to have a decent shot at seeing any snow. So overall, we can see how the duo of Typhoon Nuri and the ongoing SSW is going to radically shape the AO (and NAO) pattern for the rest of the month.


Perhaps you've scrolled down straight to this section just to get all the juicy details without the mumbo-jumbo. Well, when we combine all three of the forementioned ingredients together (Typhoon Nuri, the current SSW and the state of the AO), it translates into the increased likelihood of a blocky, cold pattern establishing itself pretty much everywhere east of the Continental Divide for the next few weeks. Thus, it's no wonder why the Climate Prediction Center is so confident with how the weather will fare in the medium and long-range. How about a 80% chance of middle Tennessee seeing colder temperatures in the 6-10 day time frame for starters? If you check your weather apps, you'll see how highs in the low 40's/upper 30's start to pop up next Wednesday. But this is only the beginning! What happens next week will, for the most part, stick around for the majority of the month. Could middle Tennessee see some early snow like we did last year? It's becoming more of a possibility, though climatology says: Don't get your hopes up too soon.

While the map above assesses the probabilities of temperature anomalie beheavior, the ECMWF map (valid through 11/13) below accounts for the actual temperature anomalies at 850 mb (or ~5,000 feet) in ºC. So if this map is correct, mean temperature values for the next week should be ~10-11º F below normal.

Believe it or not, the CFS ensembles suggest the colder air will likely continue into early December! By the look of the map below, you can see how temperatures way out in the 25-35 day time frame run 5-7 degrees below normal. So essentially, this cold pattern will not be a transient one. Such a notion lines up well with the -AO and the SSW/Bering Sea storm combo platter. 

So keep the winter coats handy...and stay tuned for a wild ride of weather as we approach the holiday season. As always, I will keep you posted & ahead of the storm...

~ Cameron

  • The Weather Centre
  • The Weather Prediction
  • The Washington Post
  • Kentucky Weather Center
  • Climate Prediction Center
  • Tropical Tidbits

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