Winter Has Finally Arrived!

I realize this title is a little misleading and would seem uneducated, so allow me to explain: Winter has finally arrived in Raleigh, NC. Finally the 10-day, then 7-day, and now 3-day model runs of the European Model are in line with the American models with enough confidence to create a forecast preparing the general public for the Triangle's first winter storm of the season.

This season has been a wetter, and colder winter than average, yet we have not seen ANY wintry precipitation in the triangle aside from a couple of flurries. Why? Well, conditions have to be so perfect for a winter storm event, especially a snow event, that we just don't typically see them around central NC. What are those conditions? I have explained them directly below. If you just want to know the "What" and "When", then you can scroll down to the "Outlook" section. The "How" is the most important part to me so I threw it in here!

Cold pool at the surface: Typically, cold air at the surface in Central NC is established by a cold air damming event. As high pressure sets up over the NE United States and a Low centers over the Gulf states, warm air backs up on the western side of the Appalachian Mountains and cold Canadian air is funneled down the East side. This very cold pool of air, however, is not very deep, only typically about 1-1.5 km thick from the surface, but depending on where the high and low pressure centers are located, the pool can extend from the mountains all the way out to the Greenville and Fayetteville. When this happens, warm air and moisture are advected above the cold pool by a southeasterly or easterly wind (from the Atlantic Ocean). This moisture then falls into the cold layer, freezes, and falls to the surface.

Pressure Center Locations, Pressure Gradient: The best case scenario for that strong cold pool at the surface is a high pressure center located over the Great Lakes/NY/PA and a Low over the Gulf/Gulf states. When the pressure centers orientate as such, a strong East-West pressure gradient establishes itself over the mountains. Winds generally flow along the pressure gradient with low pressure to their left, but when this setup occurs cold Canadian air is forced rapidly in a clockwise direction around the high and shot downward into the Carolina's and Virginia's. Another atmospheric force, which is proportional to the magnitude of the wind speed, is a force vector which pulls the wind to the right in the Northern Hemisphere (direction is based on the planetary vorticity - to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere). As the wind is shot southward, the Coriolis Force pulls the advected cold air into the Appalachian Mountains, and it begins to get blocked. After a couple days of blocking, the cold pool (mentioned above) finishes developing and is henceforth maintained.

Presence of Moisture: The timing of moisture advection into the cold pool is crucial to the development of wintry precipitation. If the moisture is advected after the cold pool has been eroded or before it is established, it will only rain.

Atmospheric Thickness: The thickness of the atmosphere is also an important parameter to look at when forecasting winter weather. Meteorologists have what is called the 540-line. 540 is the thickness of the 1000 millibar to 500 millibar pressure levels and is measured in decameters. Atmospheric Thickness is determined by the temperature of the air in that layer, and generally at 540 decameters the atmosphere is cold enough for the production of wintry precipitation.

The main problem with NC during the winter is that behind a cold frontal passage, the cold air makes it into the Piedmont, but the moisture typically gets blocked by the mountains as it moves from the Northwest to the Southeast.

Outlook: We do not have the pressure center locations currently that I specified above, but we have moisture being advected into cold air. The main event will not occur until Monday night, but currently we have snow report all across the state from Durham to accumulation in Boone.

The front is moving through tonight. Temperatures are going to get bitterly cold, dropping from the current 50 degrees to a low of 16-17 within the next 10 hours. It will be windy through the night with sustained winds in the 20's and gusts possibly reaching and topping 50 mph!

There are good chances for blowing light snow tonight along with this front because we have biting cold air aloft and decent moisture present. After timing analysis, it looks like the real wintry precipitation will begin early in the afternoon (about 1-2 pm) on Monday, but I will have another post tomorrow evening with more details about the event (this post is primarily for background). Precipitation is going to consist of snow in the afternoon and overnight, snow/ice mix as temperatures reach freezing, then back to snow Tuesday night after the temperature drops below freezing again.

Class Forecast: Based on what I am seeing right now, there will likely not be class on Monday night (after 6 pm), Tuesday, Wednesday, or Thursday morning (before noon). There are a lot of factors going into that bold #Statement (I had to throw that in there, especially after our men's basketball team had such an awesome game against No. 9 Louisville earlier today), which is why I will write another post tomorrow which is more of a forecast (again, this post is analysis). It's going to be another "Snowpocalypse" in Central NC!

Things to Stay Aware of:

1. Be careful of wind. Powerlines could be taken down by trees or by the wind itself tonight and driving will be dangerous! If you can stay home overnight, stay home. It will also be very windy tomorrow, so if you will be driving on the interstate as I will be in the morning, keep two hands on the steering wheel and do NOT have any distractions while driving. Your car can be pushed into the shoulder very easily by the wind.

2. Be aware of temperatures. The low tonight is going to be in the mid teens and cold temperatures with strong winds make for a very quick loss of body heat. If you have to be outside for an extended period of time, BUNDLE UP!

3. Watch out for black ice. If you are in higher elevations or areas where precipitation is currently falling, and there is water on the road, it WILL FREEZE overnight. Temperatures have been consistently cold enough for ground temperatures not to hinder the freezing of precipitation as the air temperatures drop to 17 tonight.

That's all for now! Have an awesome and safe night! I will touch base again tomorrow!