THE DREADED POLAR VORTEX
A MESSAGE FROM TSWAILS
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ONE DAY AT A TIME...
Looking out the window today, it won't appear much different than any other February day. However, globally, changes are taking place in the atmosphere that are going to make life in the Midwest certainly more challenging and interesting in the coming 2 weeks.
Recently, I've been highlighting some of the changes by way of teleconnections, which involves the use of analogs and correlations from similar patterns to visualize potential trends. Now, models are beginning to latch onto what the teleconections were indicating, and the details are gradually becoming more apparent. That said, we are still out 6–7 days before the more impactful aspect of the wintry change occurs, meaning there are still significant details to work out in terms of timing and intensity of both cold and snow.
With regard to the cold, I pointed out recently that the Arctic Oscillation (AO), is an indicator of the potential Arctic air has to be discharged into the mid-latitudes of the Midwest. The teleconnection is shown strongly negative through the 18th, which analogs to increased chances of Arctic outbreaks.
The reason why is that in the negative phase, higher than normal pressure over the Arctic regions leads to weaker westerlies. The westerlies which are typically quite strong, slow considerably allowing the dam to break and cold air to be released much further south than typically witnessed.
Another factor that is getting some play is occurring in the stratosphere, the highest level of the atmosphere. That's where the Polar Vortex resides, harboring its bitterly cold air. Data indicates that the Polar Vortex is weakening and shown to be splitting into 2 pieces in February. These 2 circulations are what's known as "daughter vortices" and one of these is likely to be displaced and repositioned further south over North America.
The Polar Vortex, as it relates to us, is a term that describes the broad but intense winter circulation over the Northern Hemisphere. If you were to see it in 3D, the Polar Vortex from top down would have a vertical structure that looks like this, connecting to the lower levels and the weather patterns below. It's rotation is similar to a giant atmospheric tornado.
What's really fascinating is that the PV is found at the highest levels of the atmosphere within the stratosphere, where a deep dry layer of air is found. We on the other hand reside in the troposphere, the layer of air near the ground where all of our weather occurs.
For this reason, we separate the entire Polar Vortex into an upper (stratospheric) and a lower (tropospheric) part. They both play their role differently, so we monitor them separately. But both parts together form the winter circulation across the Northern Hemisphere and the resulting weather patterns. Below you can get an idea of what the jet stream in the troposphere would look like when the Polar Vortex is weakened (or in the coming case, splits into daughter vortices).
Currently, the PV looks a bit stretched but fairly typical in both size and intensity on the EURO at 10mb.
By Friday the 14th, the split is shown to occur.
It's even more evident lower at 50mb with its two distinct cores of cold air.
Finally, around February 15-16th, the jet stream at 500mb in the troposphere (where our weather takes place) is in this configuration. One of the cold vortices is approaching the Great Lakes.
Look what that does to our temperatures. Departures in some parts of Iowa are shown to be 40 degrees below normal.
The GFS
The EURO, showing departures even more expansive than the GFS.
One last thing. Look at the departures in the Polar regions and Canada. Many are well above normal while here in the Midwest, we are seeing the greatest anomalies in all of North America relative to normal. The higher pressure at the Arctic has exerted enough force to send the bitter cold that is usually over the Pole, southward in the Midwest. Lucky us. In a nutshell, that is likely what is driving teleconnections such as the Arctic Oscillation into phases that correlate to cold. It's also why I think we are in for a pretty good stretch of frosty temperatures starting next week.
The final thing I will say about stratospheric warming events is that forecasting them with specifics is really tough. It's a very complex part of the atmosphere where data is limited far above the surface. It's a known contributing factor to Arctic outbreaks, but our understanding of the process remains limited. Thus, even though parameters are bullish on paper, it doesn't mean the cold will fully translate to reality.
Forecasting snow at this distance is a bit of a crap shoot. The EURO and GFS don't show much until the night of Tuesday the 11th, roughly a week away. If nothing else, both do show snow accumulations. There could also be some light snow in the north Saturday night, (maybe an inch or snow) but most of it remains further to the north.
With days before we get into any decent snow potential, all we can attempt to do now is determine trends where snow is most likely. Model ensembles, give us an average of multiple solutions based on different inputs. Some are high, others not so much. Here's the average amounts indicated by the ensembles of the EURO and GFS through February 17th. Most of this from the 11th to the 17th.
The EURO ENSEMBLE
The GFS ENSEMBLE
Operational models are based on a single solution and give more detail. However, if things are not properly initialized, they can be quite off at 6–7 days. They are fine for determining or confirming trends and ideas, but not for specific amounts. These will bounce around the next few days, but tonight there is exceptional consistency in my local area. Again, the majority of this is shown falling on or after February the 11th.
The EURO OPERATIONAL
The GFS OPERATIONAL
Last but not least, after a day Monday where highs hit 60 or above from I-80 south, much cooler readings will prevail Tuesday. Highs are expected to range from a crisp 20 far north to perhaps 32-33 far south. Wind chills will be a factor in the range of 5-20 later in the day, but even worse to start, especially across the north. These are 8:00am wind chills.
Wednesday afternoon in the south, (evening in the north), the low level cold air at the surface will be overrun by warmer air aloft. Models, to varying degrees, suggest the potential for light freezing drizzle or rain. Some mixed snow (or rain south) is even possible, with temperatures generally running close to freezing or slightly below. Amounts are not expected to amount to much, but a little glaze goes a long way. The recent warmth should help to keep the threat marginal in the south, where ground, surface, and pavement temperatures will be warmer. The EURO remains the most bullish on the threat showing this for freezing rain.
Personally, I am leaning more to what the HRRR shows for amounts. Not much, but a little dash is all it takes to cause problems.
That's where I will leave it for now. I'll have more on the freezing rain threat tomorrow and no doubt more on the return of winter in my next post. There are plenty of things going on, and I will be digging through fresh data to determine new trends in coming posts. That's all for now. Roll weather and if you can spare a donation, I will see to it that it's money well spent....TS
