5:00 PM CDT | December 14, 2017
Christmas is just around the corner. A lot of people are getting ready to hit the road next week. Will the travel weather be good? Well, it is impossible to say right now as it is still many days away. You are probably thinking, "Then, why am I reading this?" While I cannot say exactly what will happen around Christmas, I want to discuss what certain indicators are suggesting. Plus, why is there all this talk of snow on social media? This update is to try to decipher the weather "chaos" around Christmas.
The part of the pattern we are coming up on is what we saw in early November. If you remember from past discussions, the cycle has repeated roughly every 51 days. On the morning of November 3rd, we had a system moving into the Pacific Northwest. This system did not dig southward into the Rockies but progressed eastward. Flow strengthened across the US with the jet stream pushing a bit southward. This allowed for a cold but shallow airmass to move into the state on November 6th. Embedded disturbances were able to lift moisture northward and set up an overrunning event by November 8th. The Texas Panhandle saw some snowfall during this period with a chilly rain farther south.
DFW Airport's high temperatures from the 3rd to the 9th were 72*F, 90*F, 94*F, 75*F, 64*F, 51,*F, and 57*F. Quite the roller coaster ride! On November 8th, the coolest day, DFW recorded .81" of rain. This was the most rainfall observed in November with only a trace recorded on the 12th and 15th. How does this translate to what is on the way? Take a look at the latest GFS (American) and CMC (Canadian) weather models. These are valid on December 20th and show the 500mb pattern over the United States. What do you see? For one thing, they depict another system moving into the Pacific Northwest 47 days from the first cycle.
There is a consensus right now that we will see another system move into that part of the country by mid next week. This is in agreement with the recurring cycle theory, so confidence in this occurring is high. Watch what happens as we go further in time.
What just happened? Well, what happened between the 20th and the 23rd was that the GFS model dug the system into the Southwest US and cut it off from the main flow as ridging offshore amplified. The CMC model takes the system eastward through the Plains and into the Eastern US. The latter scenario was closer to what we saw in early November, but the other scenario cannot be discounted. The more western GFS scenario would be capable of producing greater amounts of moisture for the region, though. Again, the recurring cycle theory lets us know when certain features will come into play again, but it does not guarantee that they will unfold the same. Other factors such as Arctic Oscillation, seasonal differences in the jet stream, and so forth have an impact on how the upper-level energy behaves. What both of these models and the European model agree on right now is an intrusion of a cold but shallow airmass. This is what happened in early November as well, so the models may certainly be onto something. I would recommend taking a look at the weather update from November 6, 2017. It foreshadowed the pattern that we could see around Christmas. This is why I am monitoring the trends so closely.
One final thing I would like to mention is that there has been much talk of cold and snow around Christmas on social media. This is coming from fellow weather enthusiasts and meteorologists alike. The large majority of these posts simply show model graphics with no explanation or reasoning. Let's say we do see a wintry situation develop in the Plains (which is not a guarantee at this point), snow may not be the biggest concern in regards to wintry precip. It could come in the form of freezing rain/sleet if the airmass is shallow in nature as it was in the first part of November. We will hopefully know more about the weather on and around Christmas in a few days. I hope this discussion helped to clarify the upcoming weather and put it into perspective.
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