Old and New Perspectives on Forecasting Rare Snowfall Events in the Lower Rio Grande Valley of Texas (141)

Joshua Schroeder, NOAA/National Weather Service, Brownsville/Rio Grande Valley, TX

Christopher Birchfield, NOAA/National Weather Service

Geoffrey Bogorad, NOAA/National Weather Service

Barry Goldsmith, NOAA/National Weather Service



Measurable snowfall is very rare in Brownsville, TX, owing to its subtropical latitude (25&deg55' N) and proximity to the Gulf Coast. However, on 08 Dec 2017, widespread snowfall occurred across the Lower Rio Grande Valley (RGV) of Texas. For Brownsville, this represented the first measurable snowfall since 25 Dec 2004, and only the second recorded since 1895! Arctic-air outbreaks are a necessary, but not sufficient, condition for such events.

McFarland's (1976) study noted several correlations between certain 500-hPa features and the occurrence of major freezes in the RGV. The popularity of the pattern recognition technique has made the term &ldquoMcFarland Signature&rdquo a common phrase in the Texas meteorological community, including the Texas media, today. The current study re-examines his six cases, using modern reanalysis techniques, and incorporates several other freeze or snow events which have occurred since the original 1976 publication. The composites and anomalies are then analyzed to assess the hypothesis that recognition of the McFarland Signature continues to be a useful forecast tool.

The best forecasts are usually made through a combination of pattern recognition and interpretation of numerical model guidance. Leading up to the event, a few forecasters monitored the snowfall probability and accumulation forecasts from the National Blend of Models (NBM Tew et al. 2016). Qualitatively, the NBM was found to have captured well the amounts of frozen precipitation which were observed, as well as correctly placing the axis of highest snowfall accumulation (a band of 3 to 6 inches which fell on ranches along the Rio Grande south of Laredo through Corpus Christi). Close collaboration with NCEP's Weather Prediction Center was also critical in formulating this forecast. When placed within the context of similar synoptic patterns from previous cases, incorporating the latest blended guidance can add skill and confidence to forecasts, even for very rare events.