Winter is Coming... But How Much? Where? When?

Happy New Year Everyone!

It is only fitting that the first post of the new year would be a snow storm, as these are arguably the most exciting for students and the most challenging for forecasters. This, in my humble opinion, is starting off 2017 correctly!

Now getting to the meat; this post is primarily going to be for North Carolina, however I will mention accumulation forecasts for other parts of the southeastern United States. In addition, there is still uncertainty among computer model members and meteorologists alike as to where the areas of heaviest precipitation will be and what types of precipitation will be experienced. I will try to sort some of that out for you all here. 

The sections below in order include a description of the main meteorological factors contributing to this situation, my forecast for precipitation accumulation and location, and the hazards and impact associated with the outcome (as well as the class cancellation forecast should it be necessary). While I understand many are only concerned with the forecast, I encourage all of you to read the why associated with this system. It is a very complicated system with many uncertainties and ultimately we as meteorologists will not know exactly what will happen until it happens with this storm. As Nate Johnson, meteorologist and Executive Producer at WRAL-TV put it today on the radio, "there's a lot of 'bust' potential with this storm". It's not easy to predict the future, but it's possible to prepare for what may happen.

Setup / Meteorological Factors

Model Forecast Temperatures Overnight Friday Night

Model Forecast Low Temperatures Sunday Morning

Cold Air: Cold air is being forced into the south by the Canadian high pressure system in conjunction with an upper-atmospheric ridge (area of high pressure higher up in the atmosphere), and is banking up against the eastern side of the Appalachian Mountains as a result of the Coriolis Force, or the apparent right turning of objects and fluids in the northern hemisphere (and left turning in the Southern Hemisphere) as a result of the earth's spinning on it's axis. This cold air has another role: aiding in accumulation. The 4" soil temperatures in the Piedmont and Coastal Plains are much too warm at this point to be conducive to accumulating frozen precipitation, however cooling temperatures over the next few days will bring these temperatures to a more manageable temperature. On Monday, soil temperatures in the Triangle were around 50 degrees Fahrenheit. However, by late Friday night into early Saturday morning, cold air and some already fallen precipitation will assist the ground temperature to reach that desired (or dreaded) freezing point. 

Model Forecast Low Temperatures Monday Morning

Moisture: As this ridge has been building, an area of low pressure is being forced around it. This low pressure last weekend had not yet crossed onto the Continent and was still out over the Pacific Ocean. As this low pressure center progresses, it is expected to travel over the Gulf of Mexico, cross over Florida, and proceed northwest along the coastline while remaining at sea. The importance here is that winds flow counterclockwise around the low's center, meaning that as the center is located over the warmest water in the Gulf and the Gulf Stream, winds to the north will be flowing toward the continent carrying moisture-rich air parcels. 

Atmospheric Ascent / Lifting Mechanism: In order to receive precipitation from that moisture, some sort of lifting mechanism is needed to force those air parcels upward in the sky where they can cool and condense into liquid precipitation from water vapor. The setup we have for this weekend includes at least two lifting mechanisms: orographic, and atmospheric. The orographic lifting mechanism resides in the mountain range. As the wind blows toward the mountains, it has nowhere to go but up, and rises over the range allowing the moisture in the air to cool. Atmospheric lifting will occur as surface air is forced toward and over the "cold dome". That dome of cold air is created and maintained by continental surface winds originating from the north/northeast (in VA, NC, SC, and GA) continuing southward and turning into the Appalachian mountains.

Precipitation-Type / Timing / Accumulation

P-Type and Timing:

Precipitation Type Model Guidance through 7:00 a.m. EST Saturday

Precipitation Type Model Guidance through 1:00 p.m. EST Saturday

At this point it looks like the main swath of precipitation will run through the eastern Piedmont and western Coastal Plains of NC angled northeastward from Greenville, SC up through Norfolk, VA... essentially following the I-85 corridor. A gradient of accumulation will likely develop on either side of this corridor. Due to warmer temperatures at the onset of the precipitation, p-types will likely begin as rain in the afternoon across the board with the exception of the higher elevations to the west. After a couple hours, the precipitation will likely shift over to a mix of snow, sleet, and freezing rain. Temperatures will be cooling off very rapidly, and less than a couple hours later around dinner time or slightly after we will begin seeing the transition to snow. Snow will be the main precipitation type for the Coastal Plains and Piedmont for Virginia, North Carolina, South Carolina, and Georgia. Those living on the coast will likely experience a mix of sleet and snow due to slightly warmer temperatures through the duration of precipitation.


Wintry Accumulations:

Accumulations will likely not be as high as many of the shared pictures have shown, and to be completely honest there is still a decent chance of some areas to the northwest in NC and southeast in SC who will not see precipitation at all. That being said:

Deep South: Places including Northern Alabama and Mississippi could see a trace to some measurable accumulation around 0.5". There are also chances for a wintry mix in southern Mississippi and Central Alabama.

Southeast: Northeast Georgia could receive up to between 1-2", and areas in Northwest South Carolina could receive between 2-3".

East: The range I am forecasting now for Southwest to Northeast North Carolina is a main swath of between 3-5" with some areas possibly seeing 5-6" with a gradient up to the Northwestern NC foothills of 0.5-1", increasing again to into the higher elevations with between 2-3" in places like Boone, Grandfather Mountain, and Asheville. It is looking like the Virginia Beach area and Norfolk area will receive part of that main swath so probably 2-3" in areas up there.

Coastline: I want to do a special accumulation forecast for the coastline because it is likely that those living between Charleston and the Outer Banks (at this point at least) will receive some accumulating freezing rain. I am not sure how much, however some model representations approximate up to 0.5" of freezing rain accumulation in places of extended freezing precipitation. This will provide grounds for power outages (as I will mention in "Hazards").

Hazards

1. Slick Roads: Whenever we get wintry precipitation, the roads fail to remain clear of drivers, and in the case of snow drifts, pedestrians as well. At first, rain will aid in the road surfaces cooling, however after the temperature drops to below freezing, that layer of water will quickly become a layer of ice. Be mindful of how much time you spend on the road as it is unsafe for yourself and others.

2. Wind: While there should not be too many instances where the wind will blow rapidly, gusty conditions at times cannot be ruled out. In the cold and potentially freezing precipitation, this could spell power outages. Power outages in the cold could cause many health issues.

3. Biting Cold: Not only are we going to be below freezing for the majority of the precipitation event, but after the precipitation comes to a halt the temperature will drop even more quickly. The sky will clear on Saturday afternoon into Saturday evening, and cold air advection (the movement of cold air from location to another, in this case from Canada down to NC) and radiative cooling, or the process of longwave electromagnetic radiation being emitted from the earth's crust and released into the atmosphere, will work in tandem to cool the temperature into the single digits. There is a possibility that we will wind up setting some new low temperature records, and possibly even a few lowest highs. The American weather model even has us getting down below 0 degrees F without including wind chill. It is going to be very, very cold.

4. Hypothermia/Frost Bite: When it gets this cold, it does not take long before your body begins to slow down and show signs of frost bite or hypothermia if you have been exposed to cold water. I have posted here the National Weather Service Windchill Chart. After some model analysis, we can expect wind chill values throughout Sunday into Monday to drop into the single digits with possible values below 0. When it gets this cold, a time of only 30 minutes is necessary before frost bite affects exposed pieces of the body. If your power goes out, be sure to have a supply of candles and blankets ready to keep your living spaces lit and warm.

5. Bursting Pipes: This is usually not a hazard for us in the Southeastern United States, however with an air mass as cold as this one is expected to be, make sure you have your faucets dripping at night.

Class Cancellation Forecast

Unfortunately, church will likely be cancelled on Sunday, however I would recommend having your quiet time even so.

I have a hard time believing that classes will resume for the Spring semester on Monday, January 9. Temperatures will be so cold that we will be experiencing a hard freeze during the middle of it, and anything that has a special opportunity to melt will freeze again on Saturday and Sunday Nights.

I believe that because of warming weather Tuesday will not be a complete snow day, but a half day.


That is all I have for you right now! Get your bread, milk, and be sure to fill up your cars with gas! I hope everyone has  a great couple of snow days and stays safe!