NWS Little Rock, AR - Forecasting the Winter Storm of December 25-26, 2012

Big winter storms in Arkansas are often not a surprise. If heavy accumulations of snow and ice are coming, forecasters can usually figure this out several days in advance. Even the graphics to left were put together a couple of days before the event on December 25th and 26th.

The question is not if a storm is coming, it is who will get what and how much. In Arkansas, several precipitation types are often in play, with snow here, ice there and rain somewhere else. Model data that arrives every few hours helps in the decision process, but each data package has a different answer. Available moisture, which determines how much precipitation will fall, can go from a lot to not so much. Throw in fluctuating temperatures (warming and cooling), and you have a very complex situation on your hands.

How about snow to water ratios; that is, will this be a wet or dry snow? The standard is a 10:1 ratio, or 1 inch of water yielding 10 inches of snow. But if you expected this to happen, and there was a 20:1 ratio instead (a dry powdery snow), your accumulation forecast would be way underdone. If a bunch of sleet unexpectedly mixed with the snow, now you are looking at overdone snow totals. All of this has to be taken into consideration.

The Christmas, 2012 winter storm was no different than any other forecast puzzle. Rain was expected initially, with a changeover to snow during the afternoon of the 25th in northern and western sections of the state. The changeover would take place farther south/east after dark. This was pretty much on the money.

This was identified as the forecast was prepared during the wee hours of the 25th, with freezing rain/ice added from the Ouachita Mountains to Little Rock (Pulaski County) and along Highway 67/167 toward Searcy (White County).

The big dilemma involved the heavy snow band, and where it would set up. There was some question if the band would be over the northern counties or more toward central Arkansas.

Counter-clockwise flow around system was intense on the back (cold) side of the system. There was a tight pressure gradient, which works somewhat like a nozzle on a hose; that is, the tighter it is...the faster the flow. Strong and gusty northerly winds were anticipated during the evening of the 25th, especially in the eastern half of the state. It was felt that gusts over 40 mph would create whiteout conditions with blowing snow and low visibility. That is why a Blizzard Warning was posted in the northeast (the first such warning issued by this office).

Putting everything in forecast form gets complicated when dealing with a winter case. In the old days, forecasts for groups of counties were issued to the world via teletype. These days, hourly grids with a resolution of a few kilometers are populated with elements such as temperature, wind, sky, and expected weather. This is done in the Graphical Forecast Editor (GFE).  Model data can be loaded into the grids directly, but many of the grids are edited by hand. If an incoming storm flips and flops like the Christmas system, the grids are revamped and elements are adjusted to fit the pattern.

A significant wrinkle in this event was thundersnow. There was enough instability to create a few thunderstorms in the heavy snow band, and this amped up snow amounts. While the 6 to 10 inch forecast was not far off, there were some places that got accumulations over a foot.

For more of what was in the actual forecast, check out the media briefing below (that was posted early on the 25th)