Stratospheric Warming Events May Be Suppressed This Winter

Latest data suggests the probability for stratospheric warming events is relatively low for much of the winter. While they are not completely ruled out of possibility, the likelihood of them happening is reduced.

One culprit of this suppression of sudden stratospheric warming events (SSW events) is the below-normal state of the 100 millibar poleward eddy heat flux. To summarize, the poleward eddy heat flux, when above normal, helps initiate SSW events and helps to weaken the polar vortex by enhancing these attempts to push warm air into the stratosphere to kick off those SSWs. Below normal heat flux values suggest a stronger polar vortex, as the willingness of the stratosphere to kickstart SSW events is lower than normal. We have seen a rise in the lower stratosphere eddy heat flux values, but I doubt we will be seeing significant warming events in the stratosphere until this factor, among others, complies.

Keeping with the observed conditions in the stratosphere, temperature data across the entirety of the stratosphere indicates temperatures are well below normal. While this chart for 50 millibars (commonly identified as the middle of the stratosphere) tells the story of a colder than normal stratosphere, you could look at any level of the stratosphere from the 1 millibar level (the top of the stratosphere) to the 70 millibar level (one of the lower regions of the stratosphere) and find the same result, which says the stratosphere is colder than normal. This really isn't good news for those wanting some stratospheric activity. It's one thing for one region of the stratosphere to be colder than normal, but it's a whole different matter when the entire stratosphere is colder than normal. If a specific region is colder than normal, it isn't a gigantic block to SSW chances. But with the entirety of the stratosphere colder than normal, the chances of warming events are significantly diminished. Add to that the below-normal eddy heat flux values and the final factor we will discuss below, and the stratosphere really isn't looking so hot, both literally and figuratively.

To add to all the troubles the stratosphere is having, the Quasi-Biennial Oscillation (QBO), a wind-related oscillation entrenched in the stratosphere, is doing nothing to help matters. The QBO switches between positive/westerly and negative/easterly phases, where the positive QBO strengthens the polar vortex and the negative QBO weakens it. The Quasi-Biennial Oscillation had previously been in a deep negative trend, as shown by the solid negative phases and significantly weakened positive phases over the past few years. Now, the new positive QBO is breaking through that barrier and is at levels not seen for several years. To summarize, this positive QBO is bad for stratospheric warming prospects, especially now that the +QBO continues to strengthen.

Michael Ventrice has been able to divide the QBO into eight phases, not unlike the Madden-Julian Oscillation. Each phase has its own mid-level (500 millibar) height anomaly distinction, as Mr. Ventrice's composite chart for December-January-February shows below.

As you can see, Phase 6 of the QBO (which we are currently in) promotes significantly below-normal height anomalies across the Arctic Circle, leading to ridging over the Southeast and East Coast, in addition to ridging over the Gulf of Alaska. This has been displayed well in the recent weather pattern, and medium-range forecasts show no end to the Phase 6 QBO. We are all looking forward to when (or if) the QBO enters Phase 7, which is projected to happen later on this winter. When the QBO shifts to phase 7, the composite chart tells us that high pressure will dominate the upper latitudes, leading to a favorable winter pattern over the Central and Eastern US. Now, this does not mean that the weather pattern will always be high pressure over the Arctic and cold in the Lower 48. But it does mean that those conditions become more possible/probable with the emergence of that Phase 7 QBO. However, until that Phase 7 evolves, we're really out of luck in the stratosphere, as the unfavorable QBO, anomalously weak eddy flux values and colder than normal stratosphere temperatures combine to create a trifecta of unfavorable SSW conditions.

Andrew

ALSO: ICYMI:
-December 2-4 Potential Blizzard
-December 5-7 Potential Winter Storm
-December 8-9 Potential Southeast Snowstorm/Ice Storm

Data in this post was contributed by Eric Webb.

December 8-9 Potential Southeast Snowstorm/Ice Storm

There is a chance that a snow and ice event may evolve across the Southeast over the December 8-9 period.

The GFS model predicts that areas of ice pellets, freezing rain and snow will begin to form across Louisiana, Mississippi, Alabama, Georgia, and North Carolina for the morning hours of December 8th, in conjunction with liquid precipitation falling across the rest of the aforementioned states. It appears that snowfall would be favored the further north you go, meaning North Carolina, far northern Georgia and upper Alabama would be under the gun for some snow.

The real show begins just 12 hours later, in the evening of December 8th, when the GFS model indicates the precipitation switches over to rather heavy frozen precipitation, particularly across the Carolinas and Virginia. This image, provided by Levi Cowan, does not differentiate between different frozen precipitation types like the WeatherBell image did above, so we are only left to fear that this will either be a potent ice storm, or a decent snow event. The whole event itself is still in question, so there is a lot to still be figured out in the next several model runs. As always, updates will be provided when needed here at The Weather Centre.

Andrew

December 5-7 Potential Winter Storm

There is an increasing potential for a winter storm in the Midwest, Plains and Great Lakes over the December 5-7 period.

A look at geopotential height anomalies over the middle of the atmosphere from November 28th into the 29th suggest that stormy weather was over East Asia, as evidenced by deep blue negative height anomalies. Using the correlation that indicates weather over East Asia is reciprocated throughout the United States just 6-10 days later, it is expected that stormy weather will be brought upon the US between the December 5-9 period. Model guidance suggests that this potential storm will occur between the December 5-7 period.

Model guidance systems from the GFS (shown above) to the Canadian GEM to the European ECMWF agree that this will not be a huge snowstorm for those that get snow. There is general agreement that a region from Missouri and Kansas to Michigan will get in on some snow for this storm system, with the states of Iowa, Illinois and Wisconsin also getting in on the action.

The GFS model's snowfall forecast for this system (shown above) is actually a bit south of other forecasts from the GEM and ECMWF, as shown below:

GEM Model total snowfall forecast (storm system's snowfall seen over MO/IL/IN/MI)

The European model is obviously much more aggressive with this storm system, and I don't blame it. It will take a little while longer for models to nail down exactly how much precipitation will be in the below freezing air mass, and how much will be in the above freezing air mass. The European solution keeps more precipitation below freezing, while the GFS/GEM agree on more liquid precipitation rather than snow. Regardless of how much precipitation actually ends up in the above and below freezing air masses, it is apparent that someone in the MO/IL/MI region will end up with some accumulating snowfall. I'll stick with the GEM/GFS for now out of caution and a wish to refrain from going all-in on the highest snowfall forecast, but depending on the consistency of each model in the future, my preference may change.

ICYMI: December 2-4 Potential Blizzard

Andrew

December 2-4 Potential Blizzard

There is growing potential for a substantial winter storm to fall upon the Northern Plains and upper Midwest, possibly to blizzard proportions. 

Reanalysis of the weather around East Asia reveals a storm system passed over the region on November 26th, resulting in the apprehension for the chance of a storm system for the nation between December 2nd and December 6th. This storm would appear to be related to that East Asian connection, where weather anomalies in East Asia can be reciprocated in the United States approximately 6-10 days later. Model guidance is now catching on to what may be a blizzard in the December 2-4 timeframe.

Precipitation type and MSLP projections for December 3rd, when this storm is at its peak, show very heavy snow impacting the Northern Plains, including the Dakotas and Minnesota, as well as the Upper Midwest (with Wisconsin and the Michigan peninsula included). The storm's minimum pressure looks to be in the 980 millibar range, which classifies this as a very strong storm system. This is confirmed by the tight pressure contour arrangement around the storm system. With the tightening of those pressure contours comes increased winds, and this is why my concern is shifting towards a blizzard threat. It remains to be seen if the criteria for a blizzard will be met, but based on current model projections, I wouldn't take that idea off the table.

Snowfall amounts from this storm range from half a foot in far North Dakota to over a foot in southeast ND and a portion of South Dakota. Totals above the 12 inch mark are widely projected across Minnesota, extending into Wisconsin before a drop-off to the 6-12 inch range is seen across the Badger State. I'm generally anticipating amounts close to this forecast, as a system as strong as this one will be capable of producing some rather large snows.

Andrew

December 4-8 Major Winter Storm

It is looking increasingly likely that a winter storm will affect the Plains over the December 4-8 timeframe.

Height anomalies over East Asia in the last several hours suggest the passage of a bout of stormy weather, centered over Japan. This is signified by the deep blues and indicate the presence of such stormy weather. There is a known correlation that indicates storm systems or ridges of high pressure over East Asia can be reciprocated in the United States 6-10 days after the event's occurrence in East Asia. As we see a stormy period begin in East Asia over November 28th, I would expect December 4-8 to be the timeframe for a winter storm in the US as a result of this East Asian correlation. Now that we have been tracking the models for the past several days, and have taken the Lezak Recurring Cycle into account, we are getting some returns on possible tracks for this storm system.

The Lezak Recurring Cycle would indicate that the storm system that passed through the Plains on October 11th should come back to haunt the region, roughly around the December 7th timeframe. As the mid-level chart from October 11th shows above, the system brought its strongest impacts to the Dakotas and Northern Plains, though I suspect that won't be completely the case this time around. For those unfamiliar with the concept, 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's cycle is roughly 57 days, which is why we have highlighted December 7th as the connection date for this October 11 system.

The GFS model has this storm system actually becoming two storm systems, though it is the northern-most one that drops snow across the Plains. This model puts down snow from Montana to Wisconsin, also allotting some snow to the Dakotas, Minnesota and Canada. With the presence of an Arctic front pushing south, I support this idea of the system taking a more zonal approach to its track when compared to the October 11th track. While model forecasts will continue to change until this storm system, I do think that more of the upper Plains and upper Midwest will experience the wintry side of this storm when compared to the portion of those affected in early/mid October.

The GFS model puts down totals in excess of one foot across North Dakota, much of Minnesota and a slight portion of Wisconsin. Lower amounts greet the other half of the Dakotas, the rest of Minnesota and the upper half of Wisconsin. This does look to be a rather substantial event, should current forecasts verify, with plowable snow extending across a wide majority of the far northern states.

Andrew