Due to the timing of this post, it will be a shorter, but sweet, if you like cold air, blog. This will also be a text based blog, pictures come later when we get closer to the event. First things first, when does it look like this outbreak may happen? Next week, models have suggested that an arctic outbreak occurs over the eastern half of the United States. This is still being watched being several days in advance of course, but at this time, the second week of December is being watched for the super cold air potential. Not going to nail out any exact temps, but will just say, COLD. This blast is in response of a very strong arctic cold front. Now, since we are warm right now, and will continue to warm up, when you have two clashes between airmasses, if you have the right set up, severe weather can be involved. That looks like it may be the case. THIS IS STILL GOING TO CHANGE, WE ARE A WEEK AWAY FROM ANY POTENTIAL ACTION. That goes for this ENTIRE blog as well. Will only touch on the severe potential. The newest GFS run actually has a very, very significant cold outbreak in the very long range, but as we know, this will change too. It could trend either way, intense cold or a warm up. Just for the sake of a little model fun, I will say that if you are north of Pine Bluff, you do not make it out of the 20s that entire day. Of course this will CERTAINLY change, but we are keeping an eye on it for you guys. There will be a new blog in the coming days regarding the cold air/possible severe weather potential. Thanks for reading and have a good night.
Good evening Arkansas! As most of you know, we have had a very up and down fall thus far. The upcoming week will be discussed in your nightly outlook. This article however will look deeper into the following week, where one of the biggest travel days and shopping days of the year occur. You have heard a lot about the overall pattern and it being stuck in this back and forth mode. Lately data has been very consistent on bringing some really nice cold air to the southeastern U.S. during the week of Thanksgiving. If you remember the last few ones have been rather warm. This year though, things are looking a tad different. Leading up to today, data had shown a deep cool down that looked very impressive. Even I raised an eyebrow thinking without the proper snow cover in the plains that would be hard to do. That being said, data has come a bit more to reality about temp’s thanksgiving week. Let’s dig in.
Below is data from the 12z GFS and GEFS. I personally like the GEFS as it is a bit newer and also takes a deeper comparison into the many ensemble thrown into it. I have compiled some images into a GIF to show the pattern over the course of the week. GFS is slightly warmer than the GEFS but both show the transition from warmer to cooler. Earlier in the week, this model had the cold front coming in before thanksgiving with colder air behind it. As the week has gone on, it has moderated those temps some as well as delayed the front to after thanksgiving or even on Thanksgiving Day. Given this far out, timing of a front will obviously change somewhat. Both models have a cold front at some point during the week so confidence in a cool down is high. However, based on the trends of the data a roller coaster ride looks to continue into the holiday week with temps near to slightly below average but nothing we haven’t already seen. We warm up ahead of a cold front, cool down for a few days then warm right back up. If you notice, the deep cold air gets shifted more towards New England vs across the south, another classic weak la Nina fall pattern.
As you can see, while different days, both the GFS and the GEFS have similar setups as to when cool air intrusions make their way through the region.
Now let’s compared the “king”, the European model. What I typically look for on this data as we approach winter is blocking in Greenland and western Canada. This would typically bring the cold air shooting south. Looking at the Euro, it does show the blocking we look for when you look for cold air to surge south. However, that blocking is displaced in the wrong spots. Therefore, that sends the colder air to more of a glancing blow vs the deep cold air. Without a snowpack to keep cold air from moderating, it tends to be warmer, even with a cold air mass in place. Looking at thanksgiving week, the euro is just slightly off from the GFS, one again keeping us right at or just slightly below average for this time of year with the colder air displaced to the east. Again, typical for the pattern.
Here you can see the euro showing the warm ridges (red) vs the cooler airmasses (blue) The blocking we look for would be further east than where it is to bring colder air south. So with this type of setup, we get a glancing blow while the northeast gets the deep cold air.
Another thing to look at when we look for a pattern change, is the AO/NAO/PNA schemes. These tell us what could transpire up stream and how that will affect the atmospheric pattern. Generally, on when we look for cold air intrusions, we look for negative readings on the AO/NAO, as well as look for a negative reading on the PNA to determine the strength of the ridging out west. Well, earlier on in the forecast period, things were looking decent for a good cold shot. Now both schemes after heading fast towards a cold pattern, quickly reverse to show that any cold air intrusion will be brief or in our case a glancing blow. To get sustained cold air intrusions, you especially want to see the AO stay negative for a long period of time not a quick turnaround. Another reason as to why I am skeptical on the cold forecast.
Comparing the NAO/AO schemes, you can see why the deep cold air is short lived. To get it consistent it needs to stay negative. I do think the EURO is on to something though and that needs watched. The PNA, which measures the strength of the ridge out west, needs to be negative to get cold air shots. The weaker the western ridge, the less of a cold shot the lower 48 gets.
One extra tidbit here for you. Will it rain or even snow during my Thanksgiving week? The answer? Almost neither. GFS show rain with a cold front around Thanksgiving but dry for the rest of the week. As you can see below, a rather dry week before the holiday. Take this with a grain of salt because this will change a lot between now and then. No snow though for those of you wanting it.
As you can see, that while the overall pattern has been roller coaster as of late, there is no reasoning as to why that won’t continue. Thanksgiving week, the pattern favors a cooler but not really cold week. All the signs and everything we look at to make a forecast show us that a rather average to slightly below looks most reasonable at this time. If I were to forecast this tonight I would go with high’s in the 50s and 60s. That is NOT my forecast however, given it is too far out for exact details. La Nina patterns are very hard to predict and can turn on a dime overnight so we must monitor and watch for trends. The trend has been slightly warmer each run leveling off around average. So, do expect a change before this month is over, just will it be before or after Thanksgiving Day. That is a question that will be answered hopefully late next week. Remember this is just to give you an idea of what to expect during your holiday week coming up. This can and will change some. I will have a new article as we get closer to the week, revisiting and re-updating for an official outlook.
~Chief Forecaster Derek
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Images by Pivotal weather and Weather bell.