Fredericksburg snow and ice

Snow and ice covering the ground.

Now that we are rounding the corner and expecting a decent warming trend in the coming days, many are taking time to reflect on exactly what happened the past week or two.

 

WHAT HAPPENED?

A few weeks ago, you may recall the posting of a weather blog that hit on the idea that we would be dealing with very cold temperatures Feb. 10-Feb.16. This was based on extremely high pressure values associated with a building dome of cold air across Alaska.

Cold air is heavy and dense. In essence, cold air sinks.

Pressure levels were higher than previously seen in Alaska's weather history, and it became obvious that it would have to spill somewhere. In reality, the leading edge of this system was delayed several days, making it appear that we might miss it altogether, but air like this is never handled well by computer models. Weather balloon data in Alaska is sparsely populated, and this can lead to modeling errors.

Despite this fact, ominous signs of cold air dislodging across the country was becoming widely accepted and more likely with each model run.

 

WHY SO COLD?

Once the air mass was too heavy to remain stationary, it had to move somewhere. When you spill a glass of milk or water, it usually follows the path of least resistance. Arctic air works the same way: Once it spills out of the polar regions, it follows the path of least resistance.

It followed a path mainly east of the Rockies, because it could easily spill southward without large mountain ranges bottling up the cold air.

An old saying argues that once the glass of water has spilled, it will continue spilling until the glass is empty. The arctic air escaped Alaska and allowed the state to warm up, as we entered the deep freeze left by a spilled glass of arctic air.

It also followed the path of the river of air known as the "jetstream," and it was easily transported southward across most of the country.

 

THE PRECIPITATION

We have more than one jetstream in continental weather patterns: polar jetstreams and Pacific subtropical jetstreams. They both came together to bring us storm system after storm system from the Pacific Basin.

The storm systems would override the super cold air at the surface to produce periods of ice, snow, sleet and even thunder.

 

ICE, SNOW, ICE, SNOW

In the beginning, the air was shallow and temperatures were warmer only a few thousand feet above the surface. Despite being in the 20s on the ground, warmer air was located just above the ground and allowed the precipitation to fall as rain. This would freeze once it hit the ground, where it was below 32 degrees.

The next system would produce snow because the air was deeper and less shallow, and it was below freezing at all levels of the atmosphere, creating a recipe for heavy snow.

It is very hard to get the ingredients just right for such a combination.

It tried to warm up before our next cold front, and this allowed another round of ice and freezing rain to develop. The deeper air mass Thursday produced snow again, thanks to deeper cold air in the atmosphere.

 

HISTORICAL WEATHER?

No storm system is ever the same. Ingredients for a recipe will not always produce the same taste. Likewise, ingredients in the atmosphere will not always produce the same effect.

When the word "historical" is loosely applied, all storms fall into that category. It's just a matter of minutes here and snowfall totals there, but the ingredients are basically the same nonetheless.

It takes cold air, moisture and a trigger such as a low pressure system to produce snow, sleet or freezing rain.

 

DECEMBER 1983

The power outages make this February's storm system different from most of our previous cold weather events.

Long-time residents of Kerrville may recall the historic cold spell that happened in December 1983. This polar outbreak affected the Hill Country for nearly two weeks with a brief respite of a day or two here and there.

Lows at night bottomed out at 10 degrees on Dec. 22 and 23. It then dropped to 8 degrees on Dec. 24 and 7 degrees on Dec. 25 and 26. It remains the coldest Christmas ever with ice on the Guadalupe River.

A two-day break from single digits occurred afterward, with lows of 16 and then 25 degrees on Dec. 27 and 28. The next arctic front dropped us to 10 degrees again on Dec. 29 and then 4 degrees on Dec. 30.

We ended that month with a low of 6 degrees on Dec. 31, 1983.

Statistically, this was much colder than what we just experienced, and it lasted Dec. 16-31, with barely a day above freezing.

We dropped to 9 degrees again the following month with additional polar outbreaks during the first month of 1984. Ice was observed with this snap, but it only produced 0.03 inches of measurable precipitation, falling short of our most recent system in that regard.

 

FEBRUARY 2011

In recent history, it has been 10 years since we experienced three consecutive days below freezing.

After a balmy high of 73 on Jan. 31, 2011, arctic air dropped us to 23 degrees on the morning of Feb. 1, and we remained below freezing until the midday hours on Feb. 4.

Highs remained in the teens and lower 20s for the bulk of the three-day period, with lows between 10 and 15 degrees.

On paper, it was not as cold as what we saw this year, although daytime highs were similar. It was also a dry air mass, with little or no ice and snow accumulation with it.

 

1989 POLAR OUTBREAK

A similar polar setup was observed Dec. 11-24, 1989.

During that stretch, 10 morning low temperatures in the teens and single digits were observed. It was the last time Kerrville officially touched zero, which happened on the morning of Dec. 23, 1989.

Likewise, precipitation was not as heavy and amounted to trace amounts of snow and ice.

 

HISTORICAL SNOW

Monday's snow system was similar to what happened on Jan. 11-12, 1985. That event would go on to produce a record 13 inches of snow at the USDA in Kerrville. Medina reported 23 inches of snow from this system.

Even this storm could not compete with that snow machine. It was not as cold, with a low of 19 and a high of 22 degrees. However, we had numerous polar fronts throughout the month setting the stage for that storm to happen.

We had a low of 9 degrees on Jan. 21, 1985, and several days with highs only in the 20s and 30s.

 

COLDEST EVER

This system does not hold a candle to our all-time low temperature of -7 degrees, recorded on Jan. 31, 1949.

It started on Jan. 28, when temperatures dropped to 12 degrees, followed by a low of 12 again on Jan. 29.

A snow event that dumped 6 inches of snow on Jan. 30 may have helped out with clearing skies when the low dipped to -7 on Jan. 31.

Regardless, arctic outbreaks frequented the month of January and lingered into February 1949.

 

THE PERFECT STORM

In reality, there were ingredients in all of those events that were in close communion with what we just observed.

While no storm system is ever the same, we had polar air in place for all of them.

 

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