Tuesday, December 3, 2013

December 8-10 Potential Major Winter Storm

Models are hinting at the potential for a major winter storm in the Midwest and Ohio Valley between December 8th and 10th.

Model guidance indicates a storm system will push down into the Southwest and eventually shift eastward to begin its trek across the Plains and East US. Pattern will likely hold ridging in the Southeast for this potential storm, meaning the East Coast (except the Northeast) is essentially out of the race for this potential winter storm. It is expected that the storm system will move northeast after it ejects into the Southern Plains, and current expectations are for the storm to traverse an area from Texas to western New York state.

Models are having a bit of trouble with this system, as there is the chance of phasing prior, during or just after this storm moves through the Midwest and Lower Great Lakes. My take is that we should see snowfall amounts strengthen from the meager totals they currently are, and I wouldn't be surprised to see a northward shift occur as well, depending on future model solutions. The reason I say I wouldn't be surprised with a northward shift is because that Southeast ridge should most likely prevent any major southward shift. Again, models are iffy to trust, and should be iffy to trust up until the event actually happens. Here are the solutions from the GEM and ECMWF, respectively:

You can see the spread in the GFS ensembles, showing the discrepancies that still have to be resolved.


Risk for Severe Cold, Significant Winter Storms Rising for Mid-Late December

There could (keyword is COULD!) be an atmospheric pattern evolving that may be setting up for some severe cold weather and potentially significant winter storms for the latter half of December.

Long range hovmoller projections for the Outgoing Longwave Radiation (OLR) suggest that an MJO wave will begin moving towards the 180 degree longitude region, which will begin the process for this atmospheric pattern. The OLR works with blues and purples signifying negative OLR anomalies and enhanced convection, as well as yellows and reds portraying positive OLR anomalies and suppressed convection. The MJO wave has actually started its progression early from roughly the 90E longitude line, as the blues and purples indicate just above the black line towards the bottom of the image (that black line illustrates the line between observed OLR anomalies and forecast OLR anomalies, which appear above and below that line, respectively). It is expected that the wave will push from the Phase 2-ish region eastward, before weakening yet again, as the dashed black oval illustrates in between the two blue swaths in the bottom left of the graphic.

This graphic from the Australian Bureau of Meteorology (BOM) illustrates each MJO phase and its respective OLR anomaly. Just like the hovmoller above, blues show enhanced convection (active MJO phase) and yellows show suppressed convection. Taking a look at the graphic at the top of this post, we see the blue oval near the bottom of the image starting out at the 120E longitude line. A glance at the composite graphic above tells us this puts the MJO in Phase 5. The enhanced convection, outlined by that blue oval, is then predicted to continue travelling towards the 180 longitude line, which places the Madden-Julian Oscillation at Phases 6 and 7. We're going to switch topics, but will revisit this MJO topic a little later on in this post.

Take a look at this forecast from the ECMWF ensembles. To many of you, this is just a big ridge of high pressure, but it is actually much more. This ridge, as well as the ridging that has hit the Gulf of Alaska recently and will continue to do in the near future, is part of a Rossby Wave train that has set up over the Pacific. The Rossby Wave involves either a large high or low pressure system pushing up into the upper latitudes (in the case of an anticyclonic Rossby Wave) or pushing down towards the mid-latitudes (in the case of a cyclonic Rossby Wave).

The graphic above gives an example of a cyclonic Rossby Wave, and you can see that pronounced low pressure system dropping south, resulting in adjacent ridging in the jet stream. A similar situation happens with the anticyclonic jet stream- the high pressure system pushes up into the upper latitudes, and adjacent dips in the jet stream develop as the wave penetrates further into the upper latitudes.

Atmospheric Angular Momentum (AAM) tendencies confirm the emergence of the anticyclonic Rossby Wave in the mid-latitudes of the Pacific with a sharp increase (shown in oranges and reds) in the AAM tendency in the top right of the image, which is below the 60N parallel.
AAM tendencies
So now that we are pretty much expecting this anticyclonic Rossby Wave to develop in the Pacific, like it's been doing recently, the question goes to 'why will it shift west into Russia?' The answer - you guessed it- goes back to the MJO.

Take a look at this composite above, developed by Allan Huffman. This shows the mid-level geopotential height anomalies for the Northern Hemisphere during a December Phase 3 MJO. As I mentioned above, in the MJO discussion at the start of this post, we are expecting an emergence of the MJO into Phase 3 before the weakening occurs, and before the enhanced convection (think blue oval in the hovmoller) returns in Phase 5 later on. You'll notice deep negative height anomalies over the Bering Sea and Gulf of Alaska, and then ridging extending from the Pacific into far northeast Russia in this composite image. That ridging is expected to be the anticyclonic Rossby Wave train location when the MJO emerges into Phase 3. If you recall, in yesterday's post, I discussed how troughing in the Gulf of Alaska will lead to a positive East Pacific Oscillation (EPO) to develop. The Phase 3 MJO composite confirms this prospect, as the anticyclonic Rossby Wave being shoved west takes our current cold weather away and pushes it into the Gulf of Alaska.

When the MJO weakens again, the Rossby Waves should gradually shift east before they really get pushed east, and that's where the real fun begins.

Remember how we saw the blue oval enhanced convection returning in the hovmoller, roughly in mid-December, and we saw how that meant a Phase 5 MJO would develop? Well, here's the composite image for mid-level height anomalies for a December Phase 5 MJO wave. You can see the ridging in the Pacific, but now it's displaced in the Gulf of Alaska. Think about what that means. That means, if it can sustain itself, the anticyclonic Rossby Wave train will come back into the Gulf of Alaska, and give North America a real freeze, as those dips in the jet stream adjacent to each side of the ridge come back into play, like we discussed with the example of the cyclonic Rossby Wave image above.

The long range OLR forecast then tells us the MJO will progress into Phase 6, and look at what the December mid-level geopotential height composite for the Phase 6 MJO shows us. The ridging is now steadily fixated in the Gulf of Alaska, now knocking on the door of the West Coast of the United States. This would mean that the anticyclonic Rossby Wave train would also push east, resulting in the frigid weather produced by this ridging to shift east and impact not only the Plains, which up until this point would be receiving the main brunt of this icebox-like air mass, but also the Midwest, Great Lakes, Ohio Valley, all the way down to the Gulf Coast.

For what it's worth, the Phase 5 and 6 temperature composites for the MJO are very warm for the nation, but I can tell you now that such a warm US wouldn't verify if the Rossby Wave train did progress into Phases 5 and 6, and if the LRC works out well, as I'll discuss later on in this post.

As an added bonus, a 6-variable MJO projection created by Michael Ventrice (right panel below) confirms this whole evolution of the MJO, at least out until the 10th of December. It doesn't show the weakening of the MJO like other global MJO forecasts show (you can see them by going to our Weather Models page and scrolling down to the Teleconnections region), but still confirms the idea of the MJO progressing through Phase 3, and eventually making its way to the other phases as we get into the long range. I personally feel that the ECMWF MJO forecast (left panel) goes better with this current situation, especially when you look at the weakening of the OLR in the hovmoller at the top of this page and see the ECMWF weakening its forecast as well when it goes into Phase 4. However, both forecasts have their benefits, and at the end of the day, I've got a good feeling the MJO will progress through the phases we have covered thus far.

ECMWF MJO forecast (left) and Michael Ventrice's 6-variable MJO forecast (right).
So you've heard my reasoning behind why the second half of December could get extremely cold, but not the significant winter storms. What's the reasoning behind that?

Well, for one, as long as the Rossby Wave train sticks around in the Pacific, it will produce anomalously amplified waves of low pressure in those jet stream dips we've been discussing that appear on each side of the anticyclone. Thus, when those strong storm systems push out into the United States (or anywhere around the world), you've got the likelihood that the Rossby Wave will enhance the tendency for those storm systems to be significant in terms of potential effects, especially in the winter department.

Another impoartant factor will be the projected evolution of the MJO into Phase 5 and 6.

Check out the precipitation composites for a November-December-January MJO wave in Phases 5 and 6. There is a clear trend towards above normal precipitation in the Ohio Valley and Midwest, as well as the Great Lakes and a portion of the Northeast. That dryness you see in the Southeast is most likely a byproduct of the storminess around Greenland during Phase 5 and 6 MJO events in December, which produces a positive North Atlantic Oscillation (NAO). You weather junkies know that a +NAO then produces ridging in the Southeast and directs storm systems north into the Midwest, Ohio Valley and a part of New England. With the MJO projected to head into Phases 5 and 6 for the latter half of December, it does appear likely that the stormy trend we have been seeing recently for the Plains and Midwest will continue, should these MJO projections verify.

The Lezak Recurring Cycle also appears to be throwing its support behind a mid-late December colder trend. The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a 40-60 day interval for the next ~10 months before it dissipates over the following summer. This season, we will have a cycle length of 57 days, meaning the weather pattern on October 19th should correlate similarly to the weather pattern on December 15th, 57 days later. Likewise, the October 22 weather pattern should come back around on December 18th. This time period has a deep depression in the jet stream, along with ridging in the West. If we combine the MJO projections with our knowledge of the MJO composites and potential location of the anticyclonic Rossby Wave train during Phases 5 and 6, and put that together with the LRC, it would seem that very cold period of weather is on the way after around December 15th, give or take a couple of days. This doesn't mean it will 100% verify, but the LRC is one factor I place high confidence in, and the MJO willing to transfer this potential anticyclonic Rossby Wave train into the Gulf of Alaska would mean that the cold wouldn't just be "cold"- it would be potentially brutal. In addition to the LRC seeing cold around the mid-late December timeframe, it also points out the presence of stormy conditions in the Plains, Midwest and Ohio Valley during this timeframe, adding credibility to the MJO Phase 5 and 6 precipitation composite.

In case you didn't get any of that, here's the summary.
-The Madden-Julian Oscillation is predicted to move into favorable phases for pushing a large, repetitive anticyclone east towards the US.
-The weather pattern then favors potentially brutal cold in the nation for the mid-late December period.
-Stormy conditions in the Midwest, Plains, Great Lakes, Ohio Valley, and Northeast are then possible.

The usual long range forecasting caveats apply in this case, including the chance that this may not happen at all. My confidence is decent that this should happen at least to some extent, but there's always something that could go wrong, thus the caveats of long range forecasting.

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