First of all the question is not appropriately titled. The polar vortex ALWAYS exists as explained in this URL - What is the Polar Vortex ?. The real question is whether a polar vortex split up at the poles and the smaller vortices moved equatorward or not. In February of 2018 the polar vortex did split high up in the atmosphere due to a SSW event. If we take a look at it from GFS forecast data from the period - February 2nd 2018 10 hPa wind data and then a few days later - February 12th 2018 10 hPa wind data.
Content owned by The National Observatory of Athens, Greece confirms it in this youtube video - Polar Vortex split. An associate of mine who creates popular science videos for the larger public came up with a layperson's view of the polar vortex in this youtube video -SSW explanation plus polar vortex split
The key point in Simon Clark's video of the sudden warming of the stratosphere can be seen in GFS forecast data as well - Air temperature before polar vortex split at 10 hPa and this Air temperature after the polar vortex split
There is a 40 degree centigrade warming of the stratospheric air at 10 hPa over some locations. Over the temporal scale (3-4 days) that is fairly dramatic and that effect is eventually "felt" at the surface level. While the mechanism behind this is still a matter of deep research the impact is synoptic to planetary scale to cause a cold wave through Europe as was asked in this question - Where did the daily temperature variation go?.
Finally I am not sure of the impact on US stations but I believe it affected Western Canada and Europe.
With NOAA partially restoring their database and their website the images are available now. I am displaying the 10 hPa air temperature for two different dates. One is before the SSW event (9th February 2018) and another one after the SSW event (13th February 2018). The stereographic projection is over the northern hemisphere
As one can make out over large areas of the North Pole the air temperature at the 10 hPa has increased by around 40-50 degree Kelvin and the colder air has been displaced equatorwards. Unlike the GFS forecast images shown earlier these temperature values are the mean values averaged over a single day and are therefore more representative than a single time associated with the GFS data.
Finally the vector wind at the 10 hPa levels on both those two days and you can see clearly the split vortices in this one
Looking at the literature for the polar vortex splits it appears that they also use the geopotential height increase over the poles to indicate the impact of a SSW.
The first image is prior to a SSW which indicates the presence of a stable polar vortex(lowered geopotential height)
The second image is after a SSW and one that indicates the instability of the polar vortex(it's weakened state) and split vortices over Western Canada and Northern Europe. (Geopotential height increase over the poles).
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