Post-Christmas Storm Analysis and Overview.

I promised a few of my fellow Eastern weather forecasters that I would write a detailed overview and analysis of the storm.

There will be two parts to this article; Storm summary (extent, stats, damage, etc. longevity), analysis (dynamics, movement, development, etc.), and included within the analysis is what went wrong for the areas that were "shafted".

Storm Summary

The latest snow cover analysis from NOAA shows that yesterday's blizzard produced snow liquid equivalent precipitation of up to two inches over a widespread area from the lower Ohio Valley into the Northeast. During this storm, the average snow ratio ranged from 12:1 over Michigan and Indiana, to 9:1 in Central Ohio through Maryland, and Pennsylvania. Using these ratios, as well as real-time reports, suggests that parts of Michigan, Ohio, and Indiana had snowfall in the range of 3-10 inches, and areas from southern Indiana to Northeastern Ohio had over a foot or more. Recent reports from today show that many areas in the Northeast also saw more than a foot, and its likely that any areas downwind of the Great Lakes will see additional accumulations of 6 or more inches.

Not only did this system produce heavy snow, there was also very high winds in the regions of heavy snow. In fact, the NWS had issued blizzard warnings from Arkansas into Central Indiana and Ohio. Winds gusted to 55 mph at times, producing whiteout conditions in the heaviest bands. Some of the heavier bands of snow were producing snowfall rates of nearly 2.5 inches per hour at times in Southern Indiana! This was truly a massive blizzard for the Ohio Valley region.

(Note: For any Easterners reading this article, please notify me or provide snowfall accumulation maps for the Northeastern states as well. At the time that I wrote this article, I couldn't find many. Thank you.).


This system was not just a snow and wind "producer" but it also had a significant impact in regards to severe weather and heavy rainfall. On Christmas morning, the makings of a significant severe weather outbreak were beginning over Eastern Texas, with the development of widespread heavy showers and thunderstorms on the edge of a major surge of moisture. Eventually, a major severe weather outbreak developed farther east along the Gulf Coast, with many reports of damaging tornadoes along and ahead of the cold front. One notable tornado was a wide, wedge tornado which ripped through Mobile, Alabama late Tuesday evening.

Here's the storm reports map from the SPC. As you can see, the most significant reports were of strong tornadoes.  

(This section is not completed yet as the storm total damage and effects are still being assessed so this will be updated.).

Storm Analysis: Dynamics and Development

This part is extremely technical; Skip down to the part that says, "STOP HERE" if you do not wish to read the technical details.

This storm system was not only notable in its widespread heavy snowfall, high winds, heavy rainfall, icing, and severe weather, but also in its dynamics, and development. 

The initial shortwave that eventually developed into the storm made landfall on December 23rd, at around 7 p.m. EST. Even though it made landfall on the 23rd, it had been forecasted to develop by numerical weather guidance several days beforehand. At the time of landfall, there was still significant differences in regards to how the storm would evolve, or where the primary low would track. 

By the afternoon of Christmas Eve, a strong jet streak was entering the backside of this shortwave as it traversed across the Rocky Mountains. At this point, it was rapidly deepening, and spread in numerical guidance was still rather widespread. At this point, the NWS, as well as many other forecasters, were gaining confidence in a major snowstorm/blizzard setting up from the Southern Plains into the Ohio Valley and Northeast. Winter storm watches, warnings, and advisories were beginning to be issued. Storm Prediction Center outlooks were also issued for the potential of significant severe weather on Christmas Day..

Also notice the weak shortwave "scooting" along the Ohio Valley regions. This shortwave produced snow, ice, and rain in the Mid-Atlantic and Northeastern regions. Not only this, but it brought a new wave of colder air into the Ohio Valley, paving the way for a significant snowstorm.

The weak Arctic front caused 850 millibar temperatures to drop from around 0 degrees Celsius to -8 to -10 degrees Celsius throughout the Northern Ohio Valley.

By the morning of the 25th, a significant surface wave had developed in Texas, as the shortwave deepened rapidly. The jet streak entering the shortwave was only helping matters, and was quickly deepening the surface low. A strong Arctic high was descending into the United States, and keeping the warm front further south. It was also one of multiple reasons as to why the low pressure system tracked into the Tennessee Valley. Strong vertical motion was occurring over Texas at this point, producing widespread showers and thunderstorms.

This radar image is from the early morning hours Christmas Day, 2012. Showers and thunderstorms as well as moderate to heavy snow developed rapidly over Oklahoma, New Mexico, and Texas over the next few hours.

By 8 or 9 a.m. (EST) Christmas Day, the low was beginning to consolidate over Eastern Texas, and was beginning to move along a stalled frontal boundary. To the north of the storm, a very deep Arctic air mass was enroaching upon the storm, turning heavy rain and thunderstorms into heavy snowfall.

By 8 or 9 a.m. (EST), showers and thunderstorms were expanding, as well as the heavy snowfall regions.

By early afternoon, the strengthening shortwave was rapidly developing, and becoming increasingly negatively tilted. At this point, the surface wave was rapidly developing as well, with increasing regions of showers and thunderstorms. Strong upper level forcing was forming ahead of the shortwave, and leading to the development of a shortwave ridge ahead of the storm. Thus, began the Sutcliffe self-development process.

Strong isentropic ascent had developed ahead of the warm front by late Christmas Day, with widespread cloud cover as far north as the Indiana/Michigan border. The jet streak located over Northeast Michigan, was beginning to "couple" with the jet streak over Texas, enhancing vertical motion over the Ohio and Tennessee Valleys.

Precipitation was becoming more widespread as well with large regions of heavy snowfall over Arkansas and parts of Texas. At this current time, severe weather was developing rapidly around the Gulf coast, with numerous reports of large tornadoes.

Before I move on to the day of the event, I would also like to point out that numerical models had been slightly too far west with the placement of the surface low pressure system. But I would also like to point out that numerical guidance had placed the upper level system much farther east than it actually was, with most of the dynamics occurring west of the surface low.

Due to strong divergence aloft in the left exit and right entrance region of two potent jet streaks, in tandem with isentropic ascent, and 850 millibar frontogenesis, snow bands rapidly developed over Central Indiana, as noted by the lone banding. This in itself defied the mesoscale models, which were underdeveloping the snow shield for this storm system. Also, note the strong region of reflectivities over far SW Indiana. That region had snowfall rates approaching 2.5 inches per hour at times, combined with high winds, produced conditions of zero visibility for nearly 4 hours straight.

*STOP HERE

Here's another look at the radar imagery a few hours in the future. Note the expansion and redevelopment of heavy snow bands in the northern section of the precipitation shield.

While the heaviest bands over Central and Southern Indiana did weaken, heavier bands continued to persist over West-central Ohio, and Northeastern Indiana.

The map above is a surface map from approximately 9 a.m. (EST) on December 26, 2012. At 9 a.m., snow was beginning to fall across most of the area. By 10 a.m. (EST), heavy snowfall intensified. At this point, banding became extremely intense due to mesoscale processes, unforeseen by regional models. Strong northeasterly flow intensified, and along with the heavy snowfall, produced near whiteout conditions, especially along Route 24 and east.

As for the entire storm system, it was moving into South Central Kentucky at the time, not quite becoming a secondary low just yet, but it was getting ready to transfer, as noted on the 15z RAP PV analysis.

Heavy snow was rapidly expanding, and east as well. Cities such as Washington, D.C. and regions in the Mid-Atlantic states actually had snow that wasn't even forecast to develop! This was a major blunder for the numerical weather models.

As for the Northern Ohio/Northern Indiana region, heavy snow bands were beginning to move off to the NW and persisting, producing snowfall rates that approached 1.5 inches per hour, producing near whiteout conditions at times, and terrible road conditions.

There were three main bands as indicated on radar: One was sitting of to the west of here in Western Indiana, another was sitting right over the Route 24 corridor, and the other was in Van Wert, Adams, and Wells Counties.

Snow bands began moving off to the north. Snowfall rates were just under an inch per hour over most of the area, while areas within the main bands were seeing snowfall rates of over an inch per hour.

THIS SECTION IS HIGHLY TECHNICAL

Meanwhile, the larger storm system was beginning to occlude, focusing dynamics over Northern Indiana and Ohio, while the warm conveyer belt was being "cut off" at mid levels over the lower Great Lakes region. The second image is of the average Potential Vorticity values between the 400-200 millibar levels. It indicates how strong a storm system is. At this point, the shape of the upper level wave also indicated that secondary development was commencing over the Mid-Atlantic states (note the higher PV advection values over South and North Carolina).

Here is where things started to go wrong. The large black line indicates the position of the dry slot. The dry conveyer belt was beginning to interact with the upper level wave, and was being forced to the surface as upper level "foldogenesis" was commencing in the rapidly evolving upper level wave. This basically caused subsidence in the region that was should've been in a region of strong quasi-geostrophic ascent (showing some of the problems of the QG framework). The dry slot worked its way into the area, cutting off any decent snow banding, leaving only light snow for areas NW of the Route 24 corridor. Snow banding persisted over North Central Indiana, but weakened very rapidly as the main dynamics became focused on the secondary low.

Not only this, but the strength of the upper level wave eventually cut off the warm, moist conveyer belt as well, which reduced the supply of moisture for the snow bands over Northern Indiana and Michigan. Even though snow banding was decent over Michigan after snow began cutting off over NW Ohio, it was much weaker due to the lack of moisture.

As for areas farther east, strong warm air advection, as well as stronger CVA, and potential vorticity advection, continued to produce heavy precipitation, at times heavy snow and icing. The primary low continued to rapidly weaken as the secondary low moved northeast into the Mid-Atlantic states and offshore.

By 20z on December 26th, a full and closed secondary low had developed over North Carolina, and was beginning to move northeast. The wave became increasingly negatively tilted at all levels of the atmosphere, focusing the strongest dynamics over the Central Appalachians, Pennsylvania, and Maryland.

Strong upper level CVA, in combination with the propagating PV anomaly induced even stronger bands of precipitation over Central Pennsylvania, and eastern Ohio. Most of this convective precipitation was heavy snow, north of South Central Pennsylvania, and rain south of that line.

Strong warm air advection caused snow to change over to rain in some areas of the Mid-Atlantic, while only enhancing the snow shield over the Northeast. 850-700 millibar frontogenesis caused convective snowfall over central and Northern Pennsylvania, even causing thundersnow.

 (Note: This part is not yet complete, and will not be so for a few days. Sorry for the inconvenience.)

Conclusion

Throughout the storm's developmental and mature phase, strong upper level vorticity advection, as well as the propagation of the PV anomaly allowed for strong upward vertical motion over large areas. This allowed for the development of a strong and widespread precipitation shield as the low tracked from Eastern Texas and into the Tennessee Valley. Strong winds, heavy snow, and near whiteout conditions resulted from the Ohio Valley into the Northeast, with widespread snow accumulations over one foot in many areas.

THIS ARTICLE WILL BE UPDATED AS NEW INFORMATION COMES IN. THANK YOU FOR READING THIS.