Gustavo M. Riggio

12/17/2013

ATMS 613

2004 Christmas Snow Event in South Texas

On the 24th and 25th of December of 2004, parts of South and Southeast Texas experienced a

rare and record breaking snowfall event. This storm, which took place during Christmas Eve

brought for the first time snow to some areas managed by the Corpus Christi weather

forecasting office. Measurable amounts of snow were experienced by several counties,

including Brownsville, Corpus Christi, San Antonio/Austin, and Houston/Galveston. A total of 4.4

inches was observed in a 24 hour period by the Corpus Christi weather forecasting office,

breaking the previous record of 4.3 inches set in 1895. In addition, this event also produced the

heaviest snowfall in recorded history for Christmas day in Corpus Christi, breaking the 0.1 inch

record observed in 1918. Although snowfall to the northern counties of the Corpus Christi

weather forecasting office is not unusual, as observed in 1895 for example, this event was the

first to bring 12 or more inches to the Victoria area since 1895.1,2,3,4

The February 1895 snow event in southern and southeast Texas, like the 2004 event, produced

a narrow snow band that produced elevated levels of snow.3 Different from 2004 however, the

observed snow amount in 1985 was almost double of what was observed in 2004.3 The amount

of snow observed in 1895 was mostly likely due to a much longer period of snow fall, three

days, compared to the 8 to 12 hours period of snowfall in 2004.4

Although uncommon in the south, analysis of the weather maps and dynamics of this event

shows similar characteristics to northern snow events. In this event, a narrow band of heavy

snowfall fell about 30 to 40 miles west-northwest of Corpus Christi and extended for more than

200 miles.3 Figure 1 shows the area affected by the heavy snow band in 2004. Figure 2 shows

the 2004 Corpus Christi (CRP) county warning area.

Starting on December 23rd 2004 at 0000 UTC, most of the central portions of the United States

started to develop a system dominated by a low level and upper level trough that extended

from Canada to central Texas, and eventually extended westward to Baja California. Associated

with this elongated trough, an arctic airmass feature dominated the dynamics of this event and

helped produce a synoptic scale lift that shaped the conditions favorable for this snow event.

The impressive extent to which the arctic airmass propagated down in the continental United

Figure 1. Snowfall analysis showing the heavy snow band and the affected counties in the 2004 event.3,4

Figure 2. Corpus Christi county warning area (bordered by blue lines), as well as counties (bordered by black lines) and observation sites (orange) in south and southeast Texas.4

States can be observed on the 300 and 800mb maps (Figure 3). It is not until 1200 UTC

December 23rd however that the role the arctic airmass played in producing such snow event in

southern Texas started to become clear on how it would affect the weather in southern Texas.

Because of such elongated arctic airmass, the extent to which the upper trough was able to dig

southward was very important to maintain a deep freezing layer over South Texas. At 300mb a

strongly positive tilted axis started to push eastward into an area where a strong jet was

located (Figure 4 top right image), indicating unfavorable dynamics for any special weather

even to occur as positive tilts generally indicate weak differential advection and increased

stability, poor vertical wind shear, and weak jet streak near the base of the trough.

Therefore, at this time it was not yet clear if any precipitation would be observed in the Corpus

Christi area since the relative humidity of the region was very low and other factors were not

favorable for cyclonic development. In addition, radiosonde data suggests a very shallow depth

Figure 3. ETA analysis valid for 0000 UTC December 23 2004. Left side figure shows the 850mb heights, temperature, and winds. Right side figure shows the 300mb heights and winds. Note the extent to which the upper and lower level trough dug southward within the continental United States bringing the arctic airmass down to Texas, and eventually Baja California.

of polar airmass. Figure 4 bottom right quadrant shows an area of extremely low humidity over

Texas.

Moving forward in time, the 24th of December started to show a more interesting scenario

developing for northern and southern regions of Texas. By 0000 UTC 24 December, the

positively tilted trough advanced eastward maintaining its tilt, but also propagated south as a

result of a strong jet oriented almost in a north-south manner originated from western Canada

(Figure 5). At this same time, as a result of a cutoff moisture pool existent in northeast New

Figure 4. ETA analysis valid for 1200 UTC December 23 2004. Top left quadrant shows the 850mb heights, temperature, and winds.Ttop right quadrant shows the 300mb heights and winds. Bottom left quadrant shows the surface pressure and thickness. Bottom right quadrant shows the relative humidity of the 850-500mb layer. The extent the colder artic airmass expanded to include parts of Southern Texas and Baja California in the 850 and 300mb maps are important to maintain a freezing level over the southern U.S.

Mexico, radar and visible satellite image showed snow and clouds moving into Texas from New

Mexico (Figure 6).

Figure 5. ETA analysis valid for 0000 UTC December 24 2004. Quadrants represent the same as noted for figure 4. Compared to figure 4, note the eastward propagation of the tilted trough. Humidity now shows northwest of Texas, improving the chances of an ice of snow event to happen over northern Texas, and even South Texas as the colder airmass extents south with the air of the strong jet from western Canada.

Figure 6. Radar image at 0000 UTC on December 24 2004 showing snow falling on North and Northeastern parts of Texas.

Correspondingly, the 500mb map (figure 7) shows an area of strong positive vorticity over

northern Mexico that propagated in an elongated fashion up to the Great Lakes, matching the

location of the jet streak seen in the 300mb map. Such elongated feature will later be seen to

aid in the propagation of the trough southward into southern Texas and even northern Mexico

as the upper arctic jet mixes with the tropical jet positioned in northern Mexico.

As the trough moved east and southward (figure 8), on 1200 UTC December 24th, Corpus Christi

radiosonde also indicated freezing temperatures only a few hundred meters above the surface,

showing a very deep cold air layer over the region. Surface temperatures were close to 2 oC.

Radiosonde data also showed an area of convective instability at mid to high elevations (Figure

9), improving any chance of freezing precipitation to develop given that enough moisture was

Figure 7. 500mb and absolute vorticity ETA analysis valid for 0000 UTC December 24 2004. Note the elongated vorticity area that propagates from northern Mexico to the great lakes. Its eastward propagation will later aid in the development of snow showers over southern Texas.

available in that area. At this point, the moisture pocket propagated, following the progression

of the tilted axis. Moisture prominent from the Gulf of Mexico also starts to show and will later

play an important part in this snowfall event.

Figure 9. Radiosonde data valid for 1200 UTC December 24 2004. The depth of the cold air at this time is well defined by this sounding. Starting at only a few hundred feet above the surface, conditions are favorable for snow or ice given that enough moisture is present in the atmosphere at this time.

Figure 8. ETA analysis valid for 1200 UTC December 24 2004. Quadrants represent the same as noted for figure 4. Note the eastward propagation of the tilted trough and how the humidity pocket has also moved south-southeastward.

Over the next 12 hours, on December 25 at 0000 UTC, changes began to take place and snow

began to fall in Southern Texas. The upper trough was now located over Northern Mexico and

Southern Texas (Figure 10 top right figure), while cold air at the surface was almost entirely

over Texas (Figure 10 top and bottom left figure). Low level moisture is seen southeast of the

state (Figure 10 bottom left figure). Advection of vorticity over the area (Figure 11) was clear at

this time as an occluded low pressure system appears to take shape but does not develop into a

cyclonic circulation on the southeastern part of Texas. Not only that but a deep arctic airmass

(Figure 12) also present in the area coincided with the area of strong vorticity advection over

south Texas.

However, as it is observed, the areas in which the most significant weather features are present

at 0000 UTC on December 25th 2004 do not truly match the area of snowfall. In order to better

understand what caused snowfall, the formation of snow bands between 0000 UTC and 1200

UTC on December 25 2004 as the system quickly moved eastward must be investigated.

Confirming snowfall on southeast and south Texas, radar (figure 13 and 14) shows a strong

signal of a snow band southeast of Texas and into the Gulf of Mexico as soon as 0600 UTC.

Figure 10. ETA analysis valid for 0000 UTC December 25 2004. Quadrants represent the same as noted for figure 4. Note the eastward propagation of the tilted trough and the fact that surface temperatures have substantially decrease in all of the state of Texas.

Figure 11. Absolute vorticity for 0000 UTC December 25 2004. An area of positive absolute vorticity is present over northern Mexico and quickly moving northeasterly to south Texas.

Finally, the time of heavier snow occurred between 0000 UTC February 25 and 1200 UTC

February 25. During this time of progression, between 0000 UTC and 1200 UTC, temperatures

at the surface were still cold enough to maintain a freezing layer, and the humidity was still

present to provide the system with enough moisture to generate precipitation. Compared to

the previous time period, warmer temperatures are observed at the surface but even colder

temperatures are observed at 300mb as the system has moved easterly. Moisture is still

present but only at the very eastern part of Texas (Figure 14).

Supporting the development of a wintry system, the 500 mb layer vorticity coincides with the

area that has received most of the snow (Figure 15). This positive vorticity advection will hold

the snow band in the area for a while. At 0000 UTC February 26 the system has completely

moved out of Texas and temperatures well above freezing were again observed.

Figure 12. Radiosonde data valid for 0000 UTC December 25 2004. The depth of the cold air at this time has dramatically increased and surface temperatures are now very close to 0 oC.

Figure 13. Radar image at 0600 UTC on December 25 2004 showing snow fall on the south and southeastern parts of Texas.

Figure 14. ETA analysis valid for 1200 UTC December 25 2004. Quadrants represent the same as noted for figure 4. Surface temperatures over Texas have increased while temperatures aloft over the eastern portion of the state have decreased. The moisture is now confined to the very south- eastern part of the state.

Figure 15. Absolute vorticity for 1200 UTC December 25 2004. An area of positive absolute vorticity is now present over south Texas.

The sounding for this time period shows moderate backing of the winds close to 700mb,

representative of cold air advection, and reinforcing the cold spell the region is receiving from

the frigid airmass (Figure 16).

Radar for this time period shows that the snow band has started to move out of the area, with

heavier precipitation over the ocean and parts of southern Louisiana, indicating that the snow

event was close to being finalized (Figure 17).

At 0000 UTC February 26 the system has completely passed the area and temperatures

returned to above freezing. This was the first time Texas had seen snow measurable amounts of

snow in a long time, especially in the southern part of the state. The combinations of strong

cold air advection from the arctic combined with strong relative humidity were the main

contributing factors for this system to develop.

Figure 16. Radiosonde data valid for 1200 UTC December 25 2004. The depth of the cold air at this time has dramatically increased. Strong cold air advection is observed with the backing of the winds.

Figure 13. Radar image at 1200 UTC on December 25 2004 showing that the wintry system is moving out of the area.

1- Glenn E. Lader. STORM SUMMARY NUMBER 5 FOR MAJOR WINTER STORM. Retrieved on Dec. 8, 2013.

2- Griffths, J. F., G. Ainsworth, 1981: One hundred years of Texas weather 1880-1979, Monograph Series No. 1, Office of the State Climatologist Department of Meteorology, College of Geosciences, Texas A & M University.

3- Wilk, G. E., R. F. Morales, J. Runyen, R. Burton 2007: The Historic Christmas 2004 South Texas Snow Event: Model Performance, Forecaster Response, and Public Perception. NOAA/NWS Southern Region Technical Memorandum.

4- Morales, R.F 2007: The Historic Christmas 2004 South Texas Snow Event: Diagnosis of the Heavy Snow Band. NOAA/NWS