12/28/12 Forecast Update

Type of precipitation (green = rain, red = freezing rain, purple = sleet, blue = snow)

UPDATE*** Please, please be careful driving tonight and tomorrow. This snow coming in will be partially melted when it hits the ground, forming a higher density (8:1 ratio instead of 10:1) slushy accumulation. Coupled with sleet, freezing rain, and eventually all snow, untreated surfaces will be very slick.
The amount of dry air in the mid levels this afternoon and evening may be the difference maker tonight in terms of what type of precipitation we see. My thoughts are this: the High Country will see a mix of rain/fat melted snowflakes to start this event (possibly starting as just rain for the foothills north of HWY 18). The degree of evaporative cooling and the precipitation rate will play a large role in determining what kind of precipitation falls. Watch the radar tonight: when a "darker" signature (heavier precipitation) shows up over you, watch the precipitation change to wet snowflakes. Evaporative cooling will cool down the surface enough to keep this as a mostly frozen mix through early Saturday morning however the precipitation rate is not high enough to keep it at all snow. Prevailing warm air aloft will create a mixed precipitation even through the night. Saturday morning does transition to all snow showers. Snow accumulations through Saturday are at just 1-3" max (dusting to 1" for foothills), with a 0.15" ice accretion total.  I am concerned about the higher ice accretions for the interior valley regions and for the eastern slopes.

   

Percent of frozen precipitation

Saturday evening a cold front pushes through with a short but concentrated burst of northwest flow snow showers. Another 1-2" max (up to 3" for high peaks along the NC/TN line) is expected. We are expecting a 2-4" grand total of snowfall.

An Introduction to the Stratosphere

About every third post on the weather page I write something about a stratospheric circulation pattern but have yet to go into full detail about what it is that I am taking about. I have mentioned the ramifications but not the underlying processes. Well, we're about to dive into "stratosphere 101".

The Polar Vortex
The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered around the polar regions. It strengthens when the temperature gradient when the stratosphere over the mid-latitude is warm and the stratosphere over the polar regions is cold. The explanation from the Atmospheric and Environmental Research Center (AER) is the best one I can find.

"Strong Polar Vortex
 
Strong is the more common state of the polar vortex. When the polar vortex is strong, this creates strong low pressure in the Arctic region.  Because of the pressure difference between the Arctic and mid-latitudes, air flows into low pressure and this confines the cold air to high latitudes closer to the Arctic. Therefore it is often mild across the Eastern US, Europe and East Asia during winters when the polar vortex is strong.

Weak Polar Vortex

When the polar vortex is weak or “perturbed”, the flow of air is weaker and meanders north and south (rather than west to east). This allows a redistribution of air masses where cold air from the Arctic spills into the mid-latitudes and warm air from the subtropics is carried into the Arctic. This mixing of air masses also favors more storms and snow in the mid-latitudes. Air flows away from the high pressure Arctic. The north to south direction of the polar vortex carries cold Arctic air into the mid-latitudes of Eastern US, Europe and East Asia. Therefore it is cold across the Eastern US, Europe and East Asia during winters when the polar vortex is weak."

Where's Our Snow?

Temperature anomalies (in Celsius). Very cold less than -3*C, Cold is between -1*C and -3*C, near normal between -1*C and +1*C, Warm is between +1*C and +3*C, very warm is greater than +3*C

 Thankfully once we get out of the first 2/3 of December, we do not see predominant warm anomalies showing up until the end of February. Is this a repeat of last winter, which brought unusual warmth and a lack of snow? The short answer is no. A weak El Nino in the Pacific exists where there was not one last year. The Stratosphere above the Arctic is warming rapidly whereas last year there was no sign of this. A colder stratosphere above the Arctic means a warmer winter for us across the Southeast. The recent spike and the pattern that will take place over the next two weeks will keep this trend into effect. It will be a slow start into our winter, but once it is here, our cooler weather will be here to stay.

Black line = temperature this year, blue line = average

If anyone paid attention to my call of 41" of snowfall for Boone, NC, it's actually the same 36" I predicted back in October, except I added the Sandy snowfall event to the winter total. Why? No winter forecast already accounted for a once-in-a lifetime storm that took place in October. 36" of snowfall for the rest of the winter is actually 3" below Boone's average of 39".  You include Sandy's snow accumulation and you get 41" instead, which is right around average for Boone.

Temperature deviations from the norm (Celsius)

Ouch. The first half of this month is looking painful for ski resorts across the Southeast to get much of a base down. Crazy enough is that these most similar winters 1)have a trough moving through around the 12th that is conducive for snow making 2)Greenland blocking starts setting in the last 1/3 of December, helping to drive colder weather farther south.

If someone did account for a storm like Sandy contributing to a heavy snowfall for a winter forecast, please put me in contact with them immediately!