Climate Data
Yearly Reports
Interested in what kind of weather occurred in a recent year? Check out the most memorable events below.
 
Forecasting the Winter Storm of December 25-26, 2012
 
 A storm system ("L") brought rain and some freezing rain to begin the event on 12/25/2012, but precipitation changed to snow during the afternoon and evening. 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.
In the picture: A storm system ("L") brought rain and some freezing rain to begin the event on 12/25/2012, but precipitation changed to snow during the afternoon and evening. This happened as the system tracked from Texas toward the Tennessee Valley, with cold air wrapping around the back side of the system as it departed.

 

Link of Interest
Special Weather Statement (Issued at 532 am CST on 12/23/2012)

 

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.

 

What was not foreseen right away was the extent of the cold air as precipitation began. Early forecasts had temperatures in the mid and upper 30s in areas such as Little Rock (Pulaski County). in reality, subfreezing air swept into central Arkansas early on the 25th as rain built into the region from the south. The surface map showed below freezing temperatures spreading from northern into central Arkansas at 600 am CST on 12/25/2012.
In the picture: The surface map showed below freezing temperatures spreading from northern into central Arkansas at 600 am CST on 12/25/2012.

 

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.

 

At one point (as late as 12/24/2012), the axis of heavy snow (at least 4 inches in 12 hours or 6 inches in 24 hours) was expected to be over northern Arkansas. The track of the incoming storm system would determine which scenario would verify. In general, heavy snow falls to the left (north) of the track, and the expected track wobbled leading up to the event.
In the picture: At one point (as late as 12/24/2012), the axis of heavy snow (at least 4 inches in 12 hours or 6 inches in 24 hours) was expected to be over northern Arkansas. This was based on a storm ("L") track from around Texarkana (Miller County) to Helena-West Helena (Phillips County). The track was actually a little south of this, and the axis shifted into central sections of the state.

 

Late on the 24th/early on the 25th, it was apparent where snow would pile up. A swath of 6 to 10 inch amounts was painted from west central into central and northeast Arkansas, with lesser amounts over the northwest and also in the southeast. Expected conditions and accumulations posted on this website early on 12/25/2012.
In the picture: Expected conditions and accumulations posted on this website early on 12/25/2012.

 

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).

 

Link of Interest
Hazardous Weather Outlook (issued at 500 am CST on 12/25/2012)

 

A quick look at the Graphical Forecast Editor (GFE) used by the National Weather Service.
In the picture: A quick look at the Graphical Forecast Editor (GFE) used by the National Weather Service. On the left: (1) hourly temperature grids, (2) maximum (high) temperature grids, (3) minimum (low) temperature grids, (4) hourly wind grids, (5) hourly sky grids and (6) hourly weather grids. On the right is how a weather grid might appear.
 

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.

 

The sounding (temperature and dewpoint profile with height) at North Little Rock (Pulaski County) around 600 pm CST on 12/25/2012.
In the picture: The sounding (temperature and dewpoint profile with height) at North Little Rock (Pulaski County) around 600 pm CST on 12/25/2012. The sounding was saturated in a deep layer, with temperatures nearly identical to dewpoints (good for making precipitation). Temperatures were pretty much below freezing, with snow likely. There was a stable layer or inversion (rising temperatures with height) toward the ground, with an unstable layer aloft. It was unstable because saturated air parcels were warmer than the surrounding environment (to the right of the red line), and were allowed to ascend (at the moist adiabatic lapse rate). Apparently, there was enough vertical motion to create thunderstorms, and thundersnow was reported.
 

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)

 

 

Link of Interest
Audio Only Briefing (MP3)

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