2015 "Ice Storm" Forecast

It's the end of the weekend and the weather is moving in. This is what we've all been waiting for! So, like I was saying in my post yesterday, in order to get snow, we need to be on the northern side of the 540-line. Unfortunately for the system tomorrow, it looks as though we will have temperatures aloft that are too warm for the development of snow. It looks like around the 1-2 km level in the atmosphere will be above freezing, meaning frozen precipitation will melt before it refreezes in the surface layer. The primary precipitation for Raleigh, NC will be sleet/freezing rain, and much less snow than we were initially expecting. Though, it looks as though we will still get between 2-4 inches of accumulation over the next 48 hours.

I have noticed a lot of brine solution on the road today as I have been driving around Wake Forest, Raleigh, Cary, and Apex. I am glad to see that NC is preparing, but I fear that it may not do much good with the precipitation coming in.

Here is a rundown for each day:

Monday

Temperatures will be cold and winds will still be whipping on Monday morning and afternoon, and it looks like winter precipitation will start falling as a snow/sleet mix around 5:00-6:00 Monday night. It will be light at first, but after midnight precipitation will really begin falling. Accumulation looks to be around 1" of sleet/snow/freezing rain on Monday overnight. 

WRAL winter potential

Tuesday

Wind will be breezy and temperatures more mild. The temperature on Tuesday morning will be around 25 and will increase to around 33, though that extra degree above freezing most likely won't have much of an effect on melting the precipitation. Freezing rain and sleet will likely be the scenario before lunch, but after lunch it will be primarily snow again for a few hours before clearing out. All in all, we should see between 2-4 inches of precipitation in Raleigh. 

Wednesday

We will not likely have any precipitation on Tuesday night or Wednesday, but the temperature is going to stay below freezing, and as a matter of fact the low on Wednesday night is possibly going to be in the single digits depending on the how the wind calms down and how the clouds clear out. Any precipitation, frozen or liquid, on the roads and grass will refreeze on Wednesday night. 

Now for what everyone is waiting for. 

Class Cancellation Forecast

Nothing is definite yet, but I would presume based on prior years of experience with University closings that...

Class Monday night - gym closes early, classes after dinner cancelled

Class Tuesday - Cancelled all day

Class Wednesday - Morning cancelled, classes after 12 noon may still occur

Class Thursday - This is a stretch 4 days out, but IF there is any precipitation still left on the ground Wednesday night, it will refreeze and classes before noon on Thursday will be cancelled. 

Word to the wise, I would still prepare for afternoon classes Wednesday and all classes Thursday in case they are still held. It is more likely that we have them than not. 

What To Watch Out For

1. Wintry precipitation. Monday night I would put your windshield wipers up so that they do not get frozen to the windshield. If your business is still open on Tuesday, be aware that you will need to start your car earlier because it will be frozen shut

2. Black ice. If you HAVE to go out on Tuesday, drive slowly and carefully. Bridges WILL be frozen, even with the brine solution on the road.

3. Power Outages. Yes, Power Outages. With freezing rain in the amounts the models are saying, this is a very real concern for especially Southern and Eastern North Carolina.

Hope this helps! Everyone please stay safe over the next few days! Remember, if forecasts are off from any forecasting site, including my amateur forecast here, forecasting winter weather anywhere, but especially in NC, is VERY difficult because of the number of factors which affect both amount and type and the slight changes between each. This is NOT going to be a massive snow event like we had in 2000, or a crazy crippling ice event like we had in '04. This next 48 hours is going to be a messy winter weather period. Stay tuned to your local stations!

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!