~Cameron (Severe) Derek (Winter)
As we kick off the first weekend of December, we may be having severe weather on our hands here in the natural state.
A powerful and potent storm system will approach The Natural State as we move towards the Friday/Saturday timeframe. This system will pull up a good bit of gulf of mexico moisture and will also pull up some instability towards the state. At the same time, as drawn on the maps, this system will take on a negatively tilted position over our area resulting in the possibility of strong/severe thunderstorms over our area. Shear will be sufficient over our area for severe weather but instability & timing will be the huge question in regards to how significant this threat could be. If it's overnight as current guidance has it, the threat will be marginal and main primary threat will only be damaging winds along the front and possibly heavy rainfall. But if this system trends towards the afternoon hours then the tornado threat may increase as instability will be greater during the afternoon hours.
This is the GFS and EURO's idea on the instability and Precip as they both have the system moving through in the overnight hours. Marginal instability + High Shear = Damaging Wind Threat and possibly a spinup or two along the line as its moving through our area.
While the thunderstorm and warm air threat to start the month is becoming more certain another thing is the cold side to the start of December. Also along with the colder air, there are storm systems showing up as well. Below I have added the GFS loop through the middle of December, the GFS FV3, which will be replacing the GFS in January, Images for the 2 systems on the 4th and the 10th, as well as the the CMC and the EURO for comparison. As you can see, the system shows up on some but not all. This is very common this far out to see systems show up in the long range but not come to fruition. Comparing the differnt guidance members, clearly there is a system to watch, but the details are far from certain. One thing that is agreed upon is the colder air coming in. To get snow in Arkansas you have to have the cold air in place and the system coming in with the cold air already entrenched and this is the main thing to watch for and also much easier to predict. Lets take a look.
Below are the temperature anaomlaies into the first week of December. As you can see, there is remarkable consensus that colder air will invade the the eastern half of the U.S. Temepetaures below to well below nornal are seen on all of the latest opertaional members. This would support the cold start to December that we have talked about and this is the start of Meterological winter so therefore, the start of winter being cold. This would obviously be after the Saturday system. Again if we can get an active subtropical jet into this cold air, we will see the potenital for winter weather and one of those systems mentioned above could be our snow maker with the setup we are heading into.
Now, you can't simply look at temperature outputs alone to get a forecast or potential forecast. You also have to look at the upper atmosphere to see if the models hold any weight. To do that we look at the three main Oscillations that affect our weather here, the AO, NAO, and PNA. To get cold weather and the potentail for any type of wintry weather in the state, you want to see a negative AO and NAO and a netural to slightly Positve PNA. The more negative the AO and NAO are, the greater chance we have to see colder air. The more neutral the PNA or positve, the stronger the ridging is the west. You need this ridging in the west to bring cold air south. Below are the latest outputs for what is mentioned above and as you can see, it fits into the idea of a colder pattern setting up.
Below I have included the CPC outlooks for the upcoming 6-10 and 8-14 day periods. As you can see the Climate Prediction Center has a colder pattern moving across which also favors a colder pattern setting up and goes along with what we are seeing in terms of colder weather. On the storm system side, they as well have above average precip. So with that, as stated above means that we will need to watch the systems being advertised by some of the guidance as even the CPC is hinting at the possibility of an acitve pattern as we head into December.
So as you can see, there is definitely something to watch other than quiet and sunny. Will we have warmer days? Yes. But in the midst of that, there will be a lot more colder days as we head into December. With the pattern starting to really get going, we could see one of those systems meet up with the colder air and bring us a round of winter weather, but that remains to be seen. Unfortunately, we will have to get through what looks like a round of strong to severe thunderstorms to get to the colder air first. Stay tuned as we continue to update as things get wild and crazy as we head into December.
~Cameron (Severe) Derek (Winter)