The Challenge of Forecasting Winter Precipitation Type in the Appalachian Region

By Steve Keighton

Science and Operations Officer

Can you remember a time when the forecast called for several inches of snow and you got nothing but a cold rain?  Or maybe the forecast was for significant ice accretion and what you got was a light glaze but with several inches of sleet accumulation?  Many of you can probably recall several such scenarios.  Forecasters at the National Weather Service office in Blacksburg typically have to deal with several tricky winter precipitation forecasts per season, so we certainly remember when things didnt go exactly as planned.  The following explanation of what creates these challenging forecast situations, and why they are relatively common in the Appalachian Region, will hopefully help you appreciate the tough job we sometimes have in the Blacksburg NWS forecast office. Additionally, we hope it will help you understand why your timely reports of snow, sleet, and ice accumulation, or when it is changing from one type to another, are so important to helping is provide the best forecast we can.

The region along and to the east of the Appalachian mountains of the eastern U.S. is actually one of the few places in the country, and even the world, where mixed precipitation type, especially freezing rain and sleet, is actually a rather common wintertime occurrence. The phenomenon largely responsible for this, known as cold air damming, is common only to a couple of locations in the country as seen in the figure below (although it has been occasionally observed in some other parts as well).

Common locations for cold air damming in the U.S.

A closer view, in the figure below, actually shows that cold air damming and its effects can occur as far south as northern Georgia, and as far west as the higher valleys of the Appalachians such as the New River Valley in Virginia and the Greenbrier Valley in West Virginia.

Locations where cold air damming can influence precipitation type several times a season.

What exactly is cold air damming?  Well, imagine what happens when cold, dense air settles down over New England, and because of intense sinking motion under the strong high pressure, it wants to spread out.  One of the directions it tends to spread is toward the southwest, and it would keep spreading west if it not for the mountains, which cause it to dam up against the east side, and usually spilling over into some of the mountain valleys as well.  That process is shown in the figure below.

Cold high pressure and surface wind flow typical of cold air damming situations.

Next, out of the southwest comes a developing storm system (low pressure), pumping warm, moist air toward the low-level cold air locked in place along and east of the Appalachians.  As moisture rides up over the cold air dome, it squeezes it out in the form of precipitation, at the same time warming the air above the cold dome, and trying to erode it from the top, as well as the southern, western, and eastern fringes at the ground.  Often this is a tough battle, and typically the cold air dome holds tight in many areas, especially in the mountain valleys and immediately east of the Blue Ridge in southern VA and northwest NC.  One reason the low-level cold air has a tough time going away is because as precipitation falls into this relatively dry air mass, evaporation causes it to cool even more, at least at first, until it becomes saturated. As long as there is a low-level feed of drier air from the northeast, it will help to reinforce the cooling as precipitation falls into this air mass from above.

Eventually, the warm air aloft will begin to work its way to the surface on the outer fringes of this cold air dome, typically first in the Mountain Empire region of southwest VA and the higher ridges of northwest NC. Sometimes next it will reach places like Bluefield, WV, and also the Piedmont regions of north-central NC and Southside VA (eroding from the southeast on this side of the dome).  Once in a while the warm air will mix down to the surface into the New River Valley and along the Blue Ridge toward Roanoke, but more typically these areas cant rid themselves of the stubborn cold air dome near the ground until the surface cold front eventually sweeps through from the west.  Ironically, the air mass behind the cold front actually may result in warmer temperatures at the surface than existed ahead of the front!

So why doesnt only snow fall under the cold air dome (assuming the surface temperature is below 0 C or 32 F) and only rain fall outside the cold air dome? Because the atmosphere is three dimensional, and the type of precipitation that reaches the ground depends on the temperature profile of the atmosphere in a column above the ground.  As the warm air aloft battles the low-level cold air just above the surface, this temperature profile is constantly changing, and so the precipitation type at the ground is usually changing too! 

To visualize how the temperature profile of the atmosphere effects the precipitation at the ground, the two figures below may be helpful.  The storm system moving up from the southwest usually has deep moisture associated with it. Deep enough so that the precipitation first develops high enough in the atmosphere where it is forming as ice crystals, or snow.  The warm layer brought in by the storm system is actually relatively low in the atmosphere (between roughly 3,000 and 7,000 feet is common), but still could be below freezing (meaning the snow would remain as snow as it fell through the relative warm layer). However, in many cases, since these storms are pumping up low level air from the Gulf of Mexico or Atlantic Ocean, the air is actually above freezing.  In these cases, the snow begins to melt as it falls through the warm layer. 

What happens next becomes more complicated. Depending on the depth and warmth of this warm layer, the snow may or may not completely melt into rain before then reaching the cold air dome just above the surface. If the snow flakes only partially melt, and then fall back through the cold air dome, they will reach the ground as tiny ice pellets, more commonly known as sleet.  Sleet can accumulate on the ground, eventually turning the ground white much like snow, however it takes a lot more liquid equivalent amounts of the ice pellets to accumulate inch by inch than it would take for fluffy snow flakes.  The diagram below shows the temperature profile and process of forming sleet at the surface.

How sleet is formed from partially melted snow flakes falling through a shallow warm layer, then through the surface based cold layer.

Now, lets say that the warm layer is becoming warmer and deeper with time, which is usually does. Soon, it will be warm and deep enough to completely melt the snow flakes into rain droplets, which can only re-freeze into solid ice pellets if the cold air near the surface is very cold and deep.  More typically, the rain reaches the ground still as liquid rain, but if the ground itself is below freezing, the water will then freeze on contact, forming a glaze of ice accretion. This can eventually weigh down trees and power lines enough to begin to cause problems (not to mention creating extremely treacherous roadway and sidewalk surfaces).  Of course, if the temperature at the surface is above freezing in this scenario, then only a cold rain will fall with no ice accretion. 

The animating figure below shows how different temperature layers can result in either snow, sleet, freezing rain, or plain rain.

How different combinations of warm and cold layers aloft can result in a variety of precipitation types at the ground.

Finally, if the temperature profile stays at or below freezing through all these layers, then just snow will reach the ground (the example on the left in the image above).  Most storm systems pump some degree of warm air into lower layers of the atmosphere, but several effects can help to balance that push of warm air, such as partial melting of snow flakes (which actually results in slight cooling of the air), and strong upward motion.  When the surface low pressure center tracks to the southeast of your location, you tend to be in a more favorable location for strong  upward motion, generally colder temperatures, and precipitation will more likely to remain all snow (if conditions are cold enough to begin with). If the surface low tracks to your northwest, you will tend to either get rain, or if the cold air damming is strong enough freezing rain or sleet is possible.  Most scenarios are more complicated than this, with a primary low center tracking to the northwest of the Appalachians, then a secondary low forming over eastern NC or VA, resulting in a widespread area of mixed precipitation types, and complicating the evolution of the cold air dome as well as precipitation amounts.

It turns out, very subtle temperature differences, or changes in depth of the warm and cold layers, can result in changes to precipitation type.  Sometimes, it is pretty obvious that a location will see all snow, or all rain, but when it appears that freezing rain or sleet might be dominant, very small differences in the temperature profile aloft can result in more of one type than another.  Commonly, there is a complete variety of precipitation types, sometimes two or three types falling all at the same time.  You can imagine how difficult this makes it for forecasters to predict the total amount of snow accumulation, sleet accumulation, or ice accretion, even if we know how much liquid equivalent is expected (not always an easy task in and of itself).

Understanding how cold air damming develops and erodes under different types of weather patterns, and how the behaviors of different types of storm systems can interact with and influence the dome of cold air, is one key to better forecasting winter precipitation type.  Another key is getting the best observations that we can, whether it be from automated surface observing systems, upper air observations from instruments launched by balloons, Doppler radar, or from our volunteer spotters letting us know what is actually occurring on the ground.  Your timely and accurate measurements of snow or sleet accumulations, ice accretion on surfaces (including on the ground or raised surfaces such as branches or railings) from freezing rain, and when precipitation changes from one type to another, are absolutely critical to our ability to keep the forecast as up to date and accurate as we possibly can.  Its great to get a storm total report at the end of the event, but even more helpful, if it is possible, are your frequent reports throughout the event. This is especially the case when amounts are approaching or crossing key thresholds (such as the start of ice accretion, and when it reaches on the ground or on tree limbs, or your first inch of snow), and of course when the type changes.

We hope to hear from you at some point during the upcoming winter season, and we thank you in advance for your helpful reports!  We commit to you that well use them to better understand how a given weather system is evolving, and use that knowledge to update and enhance the forecast during the event as necessary.