THE RIO GRANDE VALLEY CHRISTMAS SNOWSTORM: A ONCE IN LIFETIME EVENT

Kurt M. Van Speybroeck\*, Jesus Haro, Andrew Patrick, and Jason Straub

National Weather Service Forecast Office, Brownsville, TX

Abstract

The snowstorm that developed over Deep South Texas on 24 December 2004
and persisted through the early morning hours of 25 December 2004 was
truly an historic weather event. A review of climatological records
reveals that snowfall is possible during the heart of winter in the
northern hemisphere at the lower latitudes, with extremely rare snowfall
events occurring in the latitudes ranging from 25 to 27 degrees north.
The meteorological developments that lead to the snowfall in Deep South
Texas, while not unique or individually significant, all contributed to
a larger scale system that became favorable for a "once in a lifetime
event."

An analysis of the operational aspects of this event will be completed
to evaluate and highlight the opportunities and best practices of
operational meteorologists when forecasting a climatologically rare
event. The analysis will begin with a review of the meteorological
events preceding the snowfall, an analysis of the quality and
consistency of the numerical models and guidance will also be performed.
A thorough review of the synoptic, mesoscale, and microscale features
will lead to the post event storm summary. Finally, an examination of
the operational products, practices, and situational awareness aspects
will be completed. The results will show that "significant impact
weather events" can be predicted with a higher degree of confidence
through situational awareness training, sound meteorological analysis,
and an effort to collaborate the forecast internally and with
neighboring National Weather Service Offices these

**1. INTRODUCTION**

The likelihood of receiving measurable snowfall in the eight counties of
Deep South Texas is remote, at best. Snowfall has occurred in Deep South
Texas less than 5 times since weather records began around 1870. These
records have been compiled by the National Weather Service (NWS) in
Brownsville, TX and archived at the National Climatic Data Center in
Asheville, NC (2005). An examination of the geographic elements and
topography across the region will demonstrate the rare nature of
\_\_**\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_**

\*Corresponding author address: K. Van Speybroeck, 20 S. Vermillion,
Brownsville, TX 78520; email: <kurt.vanspeybroeck@noaa.gov>

snowfall during any given winter season. A review of the climatological
records will reinforce the fact that snowfall on Christmas Day is
extremely rare. Next, the analysis of Numerical Weather Prediction (NWP)
model guidance highlighting the consistency, timing, and accuracy of the
model data will be explored. Using the NWS Weather Event Simulator, a
reconstruction and analysis of the synoptic, mesoscale, and microscale
aspects of the storm will illuminate the delicate balance and timing
necessary to generate frozen precipitation in this temperate climate. A
post event investigation of the operational aspects, including
situational awareness, will identify several areas that meteorologists
can focus on to correctly identify and properly forecast historic
events.

**2. CLIMATE INFORMATION**

The geographical area of responsibility for NWS Brownsville (BRO)
consists of the eight counties of Deep South Texas (Lower Rio Grande
Valley) and the adjacent Gulf of Mexico from the lower Texas coast out
to 60 nm. The topographic features of northeast Mexico also have a
significant impact on the climate and weather of Deep South Texas. The
Sierra Madre Oriental mountain range in northeast Mexico, with several
ridges and peaks above 3500 m (11,480 ft) and a median elevation of 2200
m (7218ft) (Mexico: A Country Study) contributes heavily to historic
weather events, from excessive heat, severe weather, and this most
recent snowfall event. (Figure 1)

![](media/image1.png){width="3.247916666666667in"
height="2.6493055555555554in"}

Figure 1. *Low Resolution Topographic map of northeast Mexico and Deep
South Texas.*

**a. December Climate Data**

Brownsville's average daily temperature during December is 16.2 degrees
C (61.1degrees F), the average daily maximum temperature is 21.2 degrees
C (70.2 degrees F) and the average daily minimum temperature is 11.1
degrees C (52.0 degrees F). The temperature extremes for Brownsville in
December range from a minimum of -1.11 degrees C (30 degrees F), to a
maximum of 34.4 degrees C (94 degrees F). (NWS Brownsville Station
Records). Snowfall in this climatological regime is quite rare with
measurable snowfall (accumulations greater than trace amounts) occurring
within the city of Brownsville only twice since 1878. The most recent
measurable snowfall was recorded in February 1895(Travis, NCDC station
records). Frozen precipitation (i.e. snow flurries, ice pellets, or
sleet) has been recorded 27 times from 1870-2005. Prior to the Christmas
Eve snowfall event, the most recent occurrence of snow flurries occurred
in December, 1997. Snow flurries have been recorded as early as November
and as late in the season as March. As expected, these events are
characterized as flurries or mixed precipitation, with no measurable
accumulations. About 40 percent (11 of 27), of the frozen precipitation
events occurred during the month of December. The average occurrence of
snowfall in Brownsville, TX during any winter season is 3.7 percent. The
average occurrence of snowfall on December 25th is 0.74 percent.
(Percentages include the snowfall from December 25, 2004).

3.  **Analyses**

The meteorological events during the week of 19 -- 25 December, 2004
were not singularly significant. However, the numerical model guidance
suite available to NWS meteorologists in Deep South Texas began to
suggest a possible, significant pattern change. The emerging pattern
shift began to resemble an arctic outbreak signature, known as the
McFarland pattern (1988) (Figure 2). This pattern is generally
characterized by a high amplitude 500 hPa ridge over western North
America, coupled with a significant 500 hPa cyclone centered over Hudson
Bay or Greenland. A major short wave in the flow, generating lee side
cyclogenesis is necessary for major freeze potential in the lower Rio
Grande Valley. The recognition of this developing pattern began with the
0000 UTC Mid Range Forecast and Global Forecast System

![](media/image2.png){width="3.2444444444444445in"
height="2.8270833333333334in"}

Figure 2. *12 UTC Dec 20* *MRF/GFS 500hPa Heights and MSL Pressure valid
00 UTC Dec 25.*

(MRF/GFS) model output from December 20. However, the presence of these
freezing or sub freezing free air temperatures only set the stage for
the possibility of snowfall.

The McFarland pattern guidance, in conjunction with reliable numerical
weather prediction model guidance, is useful for anticipating a pattern
shift and significant change of temperature regime. The predictability
of snowfall however, is more unreliable, since the occurrence is
extremely rare in the NWS Brownsville county warning area (CWA).

The NWP model guidance remained consistent, depicting a synoptic scale
arctic outbreak pattern for the Lower Rio Grande Valley. As this
scenario advanced in the model guidance, an additional 500 hPa low
pressure system was forecast to move across northern Mexico and Deep
South Texas. This combination of cold surface air and a relatively warm
core trough, suggested that winter type precipitation was possible. This
"TRough Of Warm air ALoft" system, known as a TROWAL, has been studied
extensively in the central and northern plains region of the United
States by Crocker et al. (1947). Significant snowfall totals have been
associated with the TROWAL and their associated thermodynamic structure.

**a. Model Interpretations**

Meteorologists at NWS Brownsville began to recognize and describe the
consistent pattern developing in the National Center for Environmental
Prediction (NCEP) GFS model guidance. The depiction of a large scale
pattern shift to a meridional trough over the eastern United States with
a high amplitude ridge over the western United States was consistently
advertised by consecutive cycles of the GFS and ETA (renamed North
American Model NAM) models. The preliminary indications were evident
during the 1200 UTC cycle on December 20, 2004. Each successive model
cycle added definition and forecaster confidence, as the identifiable
pattern shift was carried forward in time by each model suite member.

By midweek (*Wednesday December 22, 2004*), the GFS, NAM, and ensemble
model suite were consistently projecting a sharp, cold outbreak as far
south as northeast Mexico (Figures 3-4).

![](media/image3.png){width="3.248611111111111in"
height="3.091666666666667in"}

Figure 3. *18 UTC December 22 MRF/GFS 500 hPa Heights and MSL Pressure
valid 00 UTC December 25.*

![](media/image4.png){width="3.2430555555555554in"
height="2.9868055555555557in"}

Figure 4. *18 UTC December 22 ETA/NAM 500 hPa Heights/MSL Pressure valid
00 UTC December 25.*

Forecast model soundings were depicting a significant cold surface air
mass from Corpus Christi through Brownsville, into the foothills of the
Sierra Madre Oriental. The model guidance continued to forecast a 500
hPa closed low pressure system moving through the southwestern United
States into northeast Mexico and Deep South Texas. This upper level
system was expected to draw moisture and provide sufficient upward
vertical motion to generate liquid precipitation above the deepening
sub-freezing surface layer. By 1200 UTC Friday (*December 24, 2004*)
both the observed and forecast sounding data were pointing to a
significant (measurable) snowfall event for the area of Deep South Texas
along the Rio Grande and Gulf Coast. (Figure 5 though 7)

![](media/image5.png){width="3.2465277777777777in"
height="2.5972222222222223in"}

Figure 5. *Observed Radiosonde data from Brownsville, TX 12 UTC 24 Dec
04.*

![](media/image6.png){width="3.2465277777777777in"
height="2.5972222222222223in"}

Figure 6. *Observed Radiosonde data from Brownsville, TX 00 UTC 25 Dec
04.*

![](media/image7.png){width="3.2465277777777777in"
height="2.5972222222222223in"}

Figure 7. *Observed Radiosonde data from Brownsville, TX 12 UTC 25 Dec
04.*

Given the rarity of occurrences of frozen precipitation in the region,
snow accumulation forecasts were developed with a consensus forecast
effort to highlight the most significant temporal and areal impacts.

**4. Synoptic/Mesoscale Observations**

National Weather Service network observations show the development of
the McFarland pattern, the progression of the surface cold air through
Deep South Texas, and the storm track of the 500 hPa low pressure system
across northeast Mexico through the Gulf Coast region of Texas. These
components merged along the Rio Grande and in Deep South Texas
generating a swath of snowfall that extended west to east from Laredo,
TX to Brownsville and South Padre Island, TX. The area extended from
south to north from Matamoros, Tamaulipas, Mexico to the
Houston/Galveston metropolitan area.

The heaviest snowfall was recorded along the coastal counties, where
greater amounts of available atmospheric moisture were tapped from the
adjacent Gulf of Mexico. A swath of one to three inches of snow
blanketed the Rio Grande Valley from Rio Grande City to South Padre
Island. An interesting feature developed over northern Cameron and
southern Willacy counties. NWS Doppler radar images from Brownsville
(KBRO) show a slight increase in reflectivity between 06 and 09 UTC
December 25, (Figure 8), which corresponded to

![](media/image8.png){width="3.2291666666666665in"
height="3.2291666666666665in"}

Figure 8. *KBRO WSR-88D Reflectivity image showing slightly higher
return energy north of Brownsville. Observers reported seven inches of
snow in this area*.

isolated higher snowfall totals. A National Park Service Ranger and part
time weather observer recorded nearly seven inches of snow at the Palo
Alto National Park and at another location near Rio Hondo, TX.

Reports of four to six inches of snow were received from weather service
cooperative spotters and the general public from Raymondville to
Kingsville, TX. As the storm moved northeast along the Gulf Coast,
snowfall amounts increased, with up to 14 inches of snow being recorded
northeast of Corpus Christi, TX near the town of Victoria (Table 1).

The storm accelerated northeast into southwest Louisiana by 1300 UTC on
December 25. Conditions in the wake of the snowfall event transitioned
quickly. Rapidly clearing skies and nearly full sun on December

25 produced a maximum temperature of 11.1 degrees C (52 degrees F) in
Brownsville, causing a rapid melting of the first "White Christmas" or
"Blanca Navidad" in recorded history (Figure 9).

  ------------------------------------- ---------------------------------
  Location                              Total Snowfall (in inches)

  Brownsville                           1.5

  Harlingen                             1.5

  San Benito                            1.5

  La Joya                               1.5

  South Padre Island                    2.5

  Port Isabel                           3

  Rio Grande City                       3

  Sarita                                3

  McAllen                               3.5

  Raymondville                          3.5

  Hebbronville                          5

  Corpus Christi                        4

  Victoria, TX                          11

  Brazoria, TX                          13
  ------------------------------------- ---------------------------------

Table 1. *Observed/Reported storm total snow accumulations along the
Texas Gulf Coast.*

**5. Operational Products**

The number of NWS products issued for this event, show that the
operational and situational awareness of the forecast team at NWS
Brownsville were both elevated during the week preceding the snowfall.

The product suite began with a series of Special Weather Statements
(SWS) and Hazardous Weather Outlook (HWO) statements during the
afternoon forecast suite. The first SWS was issued at 510 PM CST Monday
December 20, 2004. This initial weather product highlighted the
significant weather changes including the drastic temperature change,
potential freeze, and *"In addition to freezing temperatures...there
will be the potential for a mixture of wintry precipitation late
Thursday night and early Friday."* Although the forecast was nearly four
to five days (eight to ten forecast periods) away, the idea that a
potentially significant impact weather

![](media/image9.png){width="3.2465277777777777in"
height="2.5972222222222223in"}

Figure 9. *Observed Radiosonde data from Brownsville, TX 00 UTC 26 Dec
04.*

event was likely is the message that our partners received.

A total of 10 Special Weather Statements were issued between December 20
and 25 highlighting the potential for a high impact winter weather event
in Deep South

Texas. In addition, 12 Urgent Weather Messages, (identified as Non
Precipitation Warnings or NPW), were issued highlighting the timing of
the event, the onset of freezing or below freezing temperatures, frost
formation, and the increased potential for snowfall and freezing
precipitation. As the event became imminent, two Winter Weather Messages
were issued approximately 12 hours prior to the onset of accumulating
snowfall.

There were no NWS equipment failures noted that were directly attributed
to the freezing temperatures or snow accumulation.

**6. Conclusion**

The increasing accuracy of numerical weather prediction model output and
guidance has increased the overall productivity of the line office
operational meteorologist throughout the NWS. However, NWP model
forecast continue to generate many opportunities for operational
meteorologists to add value by improving accuracy and highlighting
significant weather impacts.

The "Christmas Blizzard of 2004", as it is known in the Rio Grande
Valley is an excellent example of these opportunities. The combination
of machine generated forecasts and the subjective analyses added by the
human element can produce truly impressive results.

By utilizing the developed local expertise or knowledge base, in concert
with the object NWP model guidance, operational meteorologists were able
to forecast a "once in a lifetime" event with confidence and accuracy.
By maximizing local knowledge, the proper application of pattern
recognition, awareness of NWP model biases and performance the
meteorological team was able to issue a series of products and forecasts
that increased awareness and allowed for several days of preparations
before the impact weather changes occurred.

In addition, the collaborative efforts of the NWS Brownsville team
facilitated the concept of consensus forecasting both internally and
regionally. Using sound meteorological reasoning, diplomatic discussion,
and brainstorming sessions, the individual meteorologists all
contributed to a more consistent and eventually more accurate depiction
of the upcoming weather events.

Finally, the continuous training efforts of operational meteorologists
played a key role in predicting this major weather event. The NWS
Weather Event Simulator (WES) has been instrumental for individual case
review, and for "practicing" operational meteorology during seasonal
weather minima. The review of previous case studies and the availability
of reference materials was also a useful element during the initial
forecasting of this one in a lifetime weather event with confidence.

**7. References**

Crocker, A. M., W. L. Godson, and C. M.

Penner, 1947: Frontal Contour Charts. J.

Meteor., Vol. 4, 95-99.

McFarland, M. J., 1988: Useful Relationships

Between 500 mb Features and Major Freeze

Events in the Lower Rio Grande Valley.

NWS SR Technical Memorandum.

Mexico: A Country Study, 1998: Federal

Research Division of the Library of

Congress.

NCDC, 2005: National Climatic Data Center,

Asheville, NC.

NWS Brownsville, TX 2005: Local Station

Climate Records Database.

Travis, R., History of Freezes in the Lower Rio Grande Valley Parts
I-III. Palm Society of South Texas Bulletin.