THE HISTORIC CHRISTMAS 2004 SOUTH TEXAS SNOW EVENT: DIAGNOSIS OF THE HEAVY SNOW BAND Ronald F. Morales Jr. NOAA/NationalCorpus Christi, Weather Texas Service Weather Forecast Office Abstract On 24 and 25 December 2004, a rare, banded heavy snow event occurred over portions of south Texas. Maximum snow amounts of 20-33 cm (8-13 inches) were reported in the band region. Such amounts had not been observed in south Texas since the late 1800s. Moisture, lift, stability, and -1 across thermal profiles for the event were examined. On the synoptic scale, the event was associated with a strong low-latitude upper- level trough (26-28˚N) and jet stream maximum of 72-77 m s northern Mexico and southern Texas. This trough and jet maximum combined to produce large scale lift over an already sufficiently moist south Texas region. At lower levels, a deep freezing/sub-freezing advectionair mass was at lower present levels. as far south as northern Mexico. Confluent upper-level flow located northeast of the It snow was surmised region, helped that the to rarity maintain of the a deepevent coldwas dueair mass to the by combination producing northerlyof the very flow low- and latitude cold hPa heights and 850 hPa temperatures within the trough over north central Mexico were around four standardupper-level deviations trough, andbelow the normal deep cold during air themass. event. Anomaly calculations confirmed that both the 500 To diagnose the snow band region, cross sections of saturated equivalent potential vorticity (EPV*), Petterssen frontogenesis, saturated equivalent potential temperature (θes), and relative humidity (RH) were constructed perpendicular to the snow band. During the entire heavy snow period, the cross However, the θes surfaces in the cross sections showed a transition from the release of conditional instabilitysections indicated (CI) and the upright presence ascent of at negative the beginning EPV* located of the event,just above to the an release axis of of mid-level conditional frontogenesis. symmetric instability (CSI) and enhanced slantwise ascent about half way through the event, as the θes surfaces became more horizontal. Corresponding author address: Ronald F. Morales Jr. NOAA/NationalE-mail: [email protected] Weather Service 5777 South Aviation Ave., Charleston, South Carolina 29406 Morales 1. Introduction On 24 and 25 December 2004, much of south Texas stations were chosen because they were the closest official received a record snowfall. Measurable snow was reported National Climatic Data Center (NCDC) climatological sites within/near the snow band region. Table 1 shows that 72% of Corpus Christi’s snow events had daily totals of from the Galveston/Houston region, southward to Corpus one inch or less. Similarly, Table 2 reveals that 83% of heavierChristi, Brownsvillesnow band. and This into bandNortheastern produced Mexico maximum (Fig. brokeVictoria’s their snow records events for 24 had hour daily snowfall totals totals,of three 4.4 inches snow1). Embedded depths of within 33 cm the (13 larger inches), snow a half field width was a(distance narrow, andor less. 12.5 Forinches this respectively. event, both CorpusIn addition, Christi many and locations Victoria between maximum snowfall and half that amount) of approximately 48-64 km (30-40 miles), and a total length Rio Grande Plains and northeast Mexico, recorded their from the Houston/Galveston area, southward to the lower only one heavy snow band, a composite of the radar data Climatic Data Center 2004). of more than 320 km (200 miles). Although Fig. 1 reveals first white Christmas since local records began (National 88D) sites across south Texas indicated that there were Year Date Amount atfrom least Weather two distinct Surveillance bands that Radar-1988 formed during Doppler this (WSR-event. 1888 1.0 The entire snow event occurred mainly between the hours 1888 0.2 of 0000 and 1200 UTC 25 December 2004. The heaviest January 15 snowfall of 5 to 10 cm h-1 (2 to 4 inches h-1) occurred from 4.3 January 16 0200 UTC to 0800 UTC within the band region. 1.0 1895 February 14 To put this Christmas 2004 snow event into historical 2.0 perspective, the last time Corpus Christi or Victoria, TX, 1897 January 25 3.0 1897 January 28 0.1 1897 January 29 bandreceived of heaviersimilar 24 snow hour across snowfall south totals and was southeast February Texas, 1895. 0.1 butThe theFebruary snow totals 1895 weresnow nearly event doublealso produced (25-50 cm,a narrow 10-20 1906 February 7 0.5 1912 January 12 0.4 1914 February 25 storminches) since those it ofhad the a currentlonger duration case (Griffiths (three anddays) Ainsworth than the December 25 0.1 20041981). event, Snowfall which amounts persisted were for likely approximately higher with the10 to1895 12 1918 January 11 December 20 0.1 hours. Tables 1 and 2 show all of the recorded snow events 1918 for Corpus Christi and Victoria, TX respectively. These two December 21 0.2 1924 1.5 1924 March 10 0.2 1926 January 23 1940 1.6 1932 0.1 January 19 0.2 1940 January 21 0.5 1949 January 29 0.1 1958 February 12 0.2 1967 January 9 1.0 1973 January 11 0.1 1973 February 8 2004 December 24 2.3 1973 February 9 2004 December 25 2.1 Table 1. Snow events for Corpus Christi, TX from 1888 to Fig. 1. 2004. Snowfall amounts are daily totals (inches), listed in based on measurements obtained at 1200 UTC 25 December chronological order. The December 2004 event was a record 2004. SnowfallAnalysis ishighlighting contoured snowfallevery 2 inches, totals overwith southa maximum Texas, 24 hour snowfall amount (4.4 inches), 2.3 inches on December contour of 12 inches within the primary band region. 24, and 2.1 inches on December 25. 136 National Weather Digest The Historic Christmas 2004 South Texas Snow Event: Diagnosis of the Heavy Snow Band The primary motivation for this study was to diagnose to diagnose the features associated with the snow band the potential cause(s) for the narrow snow band, which will be discussed in section 3. Section 4 includes a brief was not predicted by operational numerical weather synoptic and mesoscale overview of the event (including satellite and radar trends), a discussion on the origin and of a seemingly rare, low- latitude heavy snow event. The maintenance of the deep cold air mass, and an attempt keyprediction meteorological (NWP) models, features and present to document during thethis occurrence case were: to quantify the rarity of the low-latitude upper low and cold air mass across northern Mexico and southern Texas. Section 5 focuses on diagnosing the cause(s) of the a strong, low-latitude upper-level trough (26-28˚N), and heavy snow band, and section 6 provides a summary and investigationa deep, saturated, into sufficientlyits origin andcold maintenanceair mass. Given will thebe conclusions. provided.significance In ofaddition, a deep itcold will air be massshown for that this it wascase, possibly a brief the simultaneous occurrence of a vigorous low-latitude 2. Assessing the Stability and Forcing Mechanisms for a Heavy Snow Band upper- level trough and a deep sub-freezing air mass that will reveal that, despite the rare snow totals and deep cold a. Diagnosing stability in the banded region airsignified mass theover true low-latitudes, rarity of this the event. associated Overall, synoptic this study and mesoscale dynamics were not necessarily unique when Many studies in the refereed literature have focused on compared to other more northern snow band events. the relationship between instability (upright or slantwise) Section 2 will provide a general background on some methods used to diagnose the stability within heavy snow bands, as well as a possible mesoscale mechanism to and the existence of heavy banded snow (e.g., Seltzer release the instability. The data and methodology used et al. 1985; Moore and Lambert 1993; Martin 1998; Wiesmueller and Zubrick1998; Nicosia and Grumm1999; Year Date Amount thatMarket any and instability, Cissell 2002; including Jurewicz conditionaland Evans 2004; symmetric Moore 12.0 instaet al. 2005). Schultz and Schumacher (1999) emphasized 0.2 within a saturated region where the instability exists. 1895 February 14 Crossbility sections(CSI), will of not equivalent be realized potentialuntil ascent vorticity occurs 1.0 (EPV) or saturated equivalent potential vorticity 1906 February 6 3.0 (EPV*) and equivalent potential temperature (θe) or 1918 January 11 saturated equivalent potential temperature (θ ), taken 2.6 es 1923 February 4 perpendicular to the heavy snow band, can be used to help March 10 1926 January 23 March 11 to as moist potential vorticity (MPV) (e.g., Bennetts and 1932 0.7 diagnose the stability. EPV/EPV* has also been referred 2.0 1932 1.7 3.0 Hoskins 1979; Shields et al. 1991; Martin et al.1992; 1940 January 20 1.0 MooreTo better and describeLambert both1993; the McCann mathematical 1995; andNicosia physical and 1940 January 22 3.4 Grumm 1999). 1949 January 30 4.0 1958 February 12 inrelationship (1). Similarly, between EPV* EPV can andbe calculated stability, Moore in (1) andby replacing Lambert 1.2 θ(1993)with derived θ a two dimensional form of EPV as shown 1960 February 12 e es 1.0 in the form for EPV, where Mg = vg + fx is the geostrophic 1973 January 11 absolute momentum in (1). For (v simplicity,is the geostrophic the equation wind wasnormal left 0.2 g 1973 February 9 to the cross section, f is the Coriolis parameter, with the 2.1 x-direction perpendicular to the thermal wind vector 1976 November 28 2004 December 24 10.0 and increasing toward the warmer air), and g is the 1985 January 12 gravitational acceleration.