20th Conference on Weather Analysis and Forecasting/16th Conference on Numerical Weather Prediction


The role of antecedent precipitation in the development of the 24–25 January 2000 U.S. east coast snowstorm

Michael J. Brennan, North Carolina State University, Raleigh, NC; and G. M. Lackmann

The role of latent heat release (LHR) in extratropical cyclone dynamics has been well documented in previous research, with LHR playing a critical role in numerous cases of cyclone genesis and evolution. This relationship can be conveniently viewed through the framework of potential vorticity (PV). LHR can impact cyclone development by reducing the effective static stability of the atmosphere, which enhances the interaction of the upper-level and surface waves. The LHR also builds the downstream upper ridge ahead of the cyclone as well as generating a lower-tropospheric PV maximum. Several studies have found that this lower PV maximum can favorably enhance the coupling of upper and lower boundary disturbances and can contribute a significant fraction of the cyclone’s intensity from both a vorticity and wind field perspective. Furthermore, this anomaly can contribute significantly to the moisture transport into a cyclone, impacting precipitation distribution.

The cyclone of 24-25 January 2000 produced heavy snowfall over the southeastern and mid-Atlantic portions of the United States and was poorly forecasted by operational numerical weather prediction models, which underestimated the cyclone strength and transport of deep moisture into the region from Georgia to Virginia. Observations and model analyses show that an area of precipitation developed over the Deep South around 06 UTC on 24 January along a lower-tropospheric front, prior to coastal cyclogenesis. This precipitation expanded and moved into the cold, stable air north of the front, producing between 0.5-1 in. of precipitation across portions of Alabama and Georgia by 12 UTC 24 January. The precipitation was poorly represented by the 00 UTC 24 January operational models, therefore the resultant diabatic PV alterations were also poorly represented. We hypothesize that this area of precipitation and its resultant impact on the PV distribution were critically important to the cyclone strength and track, and the inability of the operational models to resolve this antecedent precipitation is a major reason that their forecasts of this cyclone and its precipitation distribution were inaccurate.

A PV budget using model-derived temperature tendencies from an MM5 simulation has been computed, unambiguously showing a large lower-tropospheric positive PV anomaly developing over the southeastern U.S. by 12 UTC 24 January. This anomaly is coupled to large values of LHR in mid-troposphere produced by the model’s explicit precipitation scheme. Forecasts from the Eta model failed to resolve this lower tropospheric PV maximum and produced a cyclone that was too weak and tracked too far offshore when compared to observations. Work is currently underway to invert this PV anomaly and quantify its contribution to the cyclone development, particularly to wind field around the cyclone as it relates to the transport of Atlantic moisture into Georgia and the Carolinas.

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Session 8, Case Studies: Part II (ROOM 605/606)
Monday, 12 January 2004, 4:00 PM-5:30 PM, Room 605/606

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