Event Summary      National Weather Service, Raleigh NC

January 25, 2000 Winter Storm Updated 2007/1/25 (Click on the image to enlarge.) Event Headlines ...A powerful winter storm dropped record amounts of snow across central North Carolina... ...The all time single storm snow accumulation record for Raleigh-Durham was set with this storm when the airport received 20.3 inches of snow... ...Over 100,000 customers were without power and many schools and business closed for several days... ...The storm was not well forecast by numerical weather prediction models... ...Following the event, research at NC State indicated that the inability of the operational models to resolve antecedent precipitation across the Deep South was a major reason for their forecast failure of the developing cyclone... ...This event reinforces the need for forecasters to maintain situational awareness of upstream conditions and to compare model forecasts with observations... Objectives ...Summarize the January 25, 2000 snowstorm... ...Discussion of the some of the model errors with this storm... Event Summary This storm was the third of four storm systems to impact central North Carolina during the last two weeks of January, 2000. This two week period featured an active winter storm pattern that featured a deep trough over the eastern U.S.. Winter storms impacted North Carolina on January 18, 2000; January 23, 2000; January 25, 2000 and January 30, 2000. The January 25, 2000 storm was preceded by a rather weak storm that dropped up to an inch of snow and some freezing rain across the North Carolina Piedmont on January 23. As this storm moved off the Mid-Atlantic coast, a second low pressure system developed over Florida on the morning of the 24th. This storms system rapidly strengthened late in the day. Snow began falling across portions of North Carolina during the afternoon and evening on January 24, as the storm moved north along the North Carolina coast, dropping well over 10 inches of snow throughout central North Carolina in 24 hours. The heaviest snowfall amounts of 15 or more inches occurred from Anson County northeast through Wake County and continuing into Virginia. Areas both east and west of this line received 5 to 10 inches of snow. Even some locations near the coast, such as Wilmington, received up to 5 inches of snow. This storm received a great deal of media attention including a news story on CNN. Snow Accumulation Map

Storm Summary Synoptic Overview As the January 23 storm system and upper-level jet began to lift northeast away from North Carolina, a second short-wave trough was digging southeastward into the Deep South. At 00Z on the 24th, a large area of precipitation associated with the departing storm stretched from the North Carolina coast through northern Florida. At around 06Z, an area of convection developed along the Gulf Coast over Louisiana and Southern Mississippi. By 12Z, the upper-level short-wave had begun to amplify and become negatively tilted, and a low pressure system formed over northern Florida. As the deepening upper-level wave drew cold air southward, precipitation began falling into the relatively dry air airmass over the state, and surface temperatures fell to below freezing across much of the area. The upper-level support, coupled with a pre-existing baroclinic zone draped across the southeast, allowed the surface low to rapidly strengthen off the South Carolina coast. As the area of new convection moved across the Southeast, it rapidly merged with the developing low pressure, creating an "instant occlusion" of the storm, which leads to rapid cyclogenesis. Heavy snow began to fall just after midnight in the Raleigh-Durham area on the 25th (see surface observations below). The low pressure system would eventually deepen to around 986mb and move up the Atlantic seaboard, spreading snow across much of the Mid-Atlantic and New England by 06Z on the 26th. Surface Observations at Raleigh-Durham (KRDU) The intensity of the snowfall, combined with the accumulation and strong winds, caused thousands of business and residents to be without power for several days. Observations from the Raleigh Durham International show six out of seven consecutive hours reporting heavy snow (*Note: 11Z Observation unavailable): KRDU 250551Z 01016G27KT 1/4SM +SN FZFG SCT002 BKN006 OVC011 M02/M02 A2954 KRDU 250651Z 36012G19KT 310V030 1/4SM +SN FZFG OVC002 M02/M02 A2955 KRDU 250751Z 01013G25KT 1/4SM +SN FZFG OVC002 M02/M03 A2953 KRDU 250851Z 36009G19KT 330V040 M1/4SM +SN FZFG SCT002 OVC008 M03/M03 A2951 KRDU 250951Z 36013G29KT 310V030 M1/4SM +SN FZFG SCT002 OVC008 M02/M03 A2950 KRDU 251151Z 35010G21KT 310V050 1SM -SN BR SCT002 OVC009 M02/M03 A2952 KRDU 251251Z 34012G20KT 300V360 1/4SM +SN FZFG SCT002 BKN009 OVC014 M02/M03 A2952 Radar Observations Heavy precipitation can be seen in the regional radar reflectivity image from 06Z (100 AM) on January 25, 2000 below. A band of high radar reflectivity indicative of heavy precipitation is shown stretching across North Carolina. As the storm system intensified and moved up the coast the band evolved and rotated but it generally remained over central North Carolina resulting in the significant snow accumulations. A radar loop from 00Z January 24 (700 PM January 23) through 11Z January 26 (600 AM January 26) shows the evolution of the storm system including the initial storm on January 23, the development of precipitation across the Gulf Coast states resulting from an intense upper level jet, to the rapid deepening of the primary storm on January 24 and 25, 2000.

Model Error In general, forecast models preformed very poorly during this event. An example of this is shown in the image below which compares the 48 hour, observed liquid equivalent precipitation with the Eta model 48 hour forecast valid at the same time (click on the image to enlarge, image provided by Michael Brennan, NC State University). The model failed to capture the significant precipitation over the Carolinas and Virginia; in fact the Eta forecast had no precipitation over Raleigh and Richmond where over an inch of liquid equivalent was observed. As forecasters scrambled to adjust forecasts to reflect the unfolding snowstorm, there was little time to analyze the full reason why the forecast had gone bad. In the wake of the storm, many were left wondering how nearly two feet of snow could have fallen in less than 24 hours with very little warning. Was there something that could have been analyzed ahead of time that would have lead to questioning of model precipitation forecasts? Or was there a physical mechanism that was misunderstood, or missed by the models? Below is a brief summary of some of the forecast errors and research topics that arose from this case. Incipient Precipitation and Latent Heat An area of precipitation developed along a frontal system over southern Alabama and southern Mississippi at around 06Z on January 24, 2000. The area of precipitation expanded and intensified as it moved into the cold airmass north of the front in Georgia by 12Z on January 24. Analysis of surface observations and radar imagery show that this area of precipitation was poorly forecast by the Eta model. Research into this storm indicates that this under forecasted area of precipitation that fell into a cold and somewhat dry airmass across the Deep South had a significant impact in how the models handled the developing storm system. Based on quasi-geostrophic (QG) theory, height rises (falls) occur above (below) a mid-level latent heating maximum, due to changes in the density of the air above and below. This response also creates a low-level maximum in cyclonic potential vorticity (PV), as low-level static stability is increased. The effects of the induced cyclonic PV max are manifest in wave amplification and enhanced rotation around the PV center. The precipitation across the Deep South induced a PV anomaly that enhanced the easterly flow downstream, creating stronger westward moisture advection over the Carolinas from off the Atlantic (Brennan and Lackmann 2005), thereby extending the heavy snowfall further inland. The inability of the models to accurately predict the precipitation across the Deep South was a major reason that the model forecasts of this storm were so inaccurate. The combination of increased moisture advection, enhanced dynamics, and a deep subfreezing column of air (see GSO soundings below), created an environment that was primed for intense winter weather.

Conclusion The "Blizzard of 2000" was one of the most powerful winter storms on record in parts of North Carolina. It was also one of the most poorly forecast. Many lessons have been learned from this event, including the importance of human analysis of the impending weather situation and awareness of upstream conditions. It is vital to compare model forecasts with observations, not only over the regional area, but also well upstream, in order to validate the model forecast for use. Another useful outcome of the storm has been the research on the role of latent heat release on the surrounding environment. The enhanced circulation around an area of positive PV where latent heating is strong was found to be instrumental in supplying high amounts of moisture to the forecast area. This knowledge can help forecasters in the future to recognize this feedback, thus improving precipitation forecasts and situational awareness. Case Study Team Barrett Smith Jonathan Blaes References Brennan, M. J., and G. M. Lackmann, 2005: The influence of incipient latent heat release on the precipitation distribution of the 24-25 January 2000 U.S. East Coast cyclone. Mon. Wea. Rev., 133, 1913-1937. Michael Brennan, 2004: PV - The Use of PV thinking in Operational Forecasting. Power Point Format. Available online at http://www2.ncsu.edu/eos/service/pams/meas/sco/research/nws/visit/pv_20041216.ppt. News story from the CNN.com archive. For questions regarding the web site, please contact Jonathan Blaes. NWS Disclaimer.