Super Tuesday and awful Wednesday: the

5-6 February 2008 Severe Weather Outbreak By

Richard H. Grumm National Weather Service State College, PA

And Seth Binau

National Weather Service, Lacrosse, WI

1. INTRODUCTION A winter storm brought one of the largest and most significant severe weather events to the central United States on 5-6 February 2008. This may have been one of the most significant mid-Winter severe outbreaks in recent history. The event produced 453 severe events, to include 91 tornadoes on the 5th and 59 severe weather reports and 2 tornadoes on the 6th of February (Fig. 1). Preliminary reports suggest that 59 people lost their lives in this deadly tornado outbreak making it one of the most deadly tornado outbreaks in 23 years. The last deadly event of this magnitude was in May 1985. Severe weather in association with winter storms is actually quite common. Galway and Pearson (1981) examined winter tornado outbreaks. Most of these events were associated with winter storms which produced snow in the cold air. They found that these events accounted for about 9% of all tornadoes over the course of any given year. Of the 23 outbreaks they examined, 19 were associated with storm which produced snow and ice in the colder air to the north. The noted that these events produced a large number of tornado related deaths. The “super storm” of March 1993 produced severe weather and tornadoes as it’s trailing cold front pushed through Florida (Kocin eta 1l

Figure 1Storm Prediction Center (SPC) storm report by type for the 24 hour periods ending 0800 CDT on 6 and 7 February 2008. Reports are color coded by severe weather event type. http://www.spc.noaa.gov/climo/reports/

Courtesy of the SPC.

Figure 2 NAM 00-hour analysis of 850 hPa winds (kts) and u- and v-wind anomalies valid at (left) 1800 UTC 5 February 2008, (center) 0000 UTC 06 February 2008, and (right) 0600 UTC 6 February 2008. Upper panels show 850 hPa winds and u-wind anomalies. Lower panels show 850 hPa winds and v-wind anomalies.

1995 and Uccellini et. al 1995).The storm also produced a squall line and severe weather over western Cuba (Alfonso and Naranjo 1996). These examples demonstrate the potential of winter storms to produce severe weather. Other examples include the 24 January 1997 event which produced 13 tornadoes in Tennessee and the 49 tornadoes on 7-8 January 2008. The increased in tornadic activity over the central United States, slightly north of the climatologically preferred position is likely related to the current La Nina. During La Nina, the storm track over the United States shifts to the west. This diminishes the threat of East Coast storms, favoring a track up the Ohio Valley. This in turn allows Gulf moisture to surge northward into the Mississippi, Tennessee, and Ohio Valley’s contributing to the increased tornadic activity. Marzban and Schaefer (2000; hereafter MS) showed an increase in strong and violent tornadic activity in association with cooler

Pacific sea surface temperatures (La Nina). It should be noted that the found that the activity should increase over the northeastern United States (area 2 in MS Fig. 1) which include Kentucky on the southern limb and Illinois on the western limb. Hart and Grumm (2001) showed how climatic anomalies can be used to determine if a system anomalous. That is, whether a system is departing significantly from normal in terms height, pressure, winds, and moisture fields. They developed a method to rank such system. Grumm and Hart (2001) applied these methods to cold season events demonstrating how anomalies can be used to discriminate between a minor and a significant winter storm. Stuart and Grumm (2007) showed how this can be used as a forecast technique to identify significant East Coast Winter storms. The purpose of this paper is to document the severe weather event of 5-6 February 2008. Climate anomalies will be used to show that

Figure 3 As in Figure 2 except showing mean sea level pressure (hPa) and anomalies in the upper panels and precipitable water (mm) in the lower panels.

this storm contained signals, such as anomalously strong low-level winds and anomalous moisture which may have provided clues to the potential severity of this outbreak. Forecast products and analysis products will be presented. 2. METHODS Data for this study include re-analysis climatological data from the NCEP/NCAR global re-analysis project (GR: Kalnay et al 1996). The means and standard deviations were used to compute standardized anomalies, displayed in standard deviations from normal (SDs). Forecasts from the NCEP GFS and NAM are used to show the verifying patterns. Ensemble data shown here were primarily limited to the NCEP SREF and a mix of SREF, GFS, and SREF data were examined to produce a pseudo super ensemble. The climatological data used to compute anomalies was restricted to those produced by the NCEP/NCAR GR data set (Hart and

Grumm 2001). They will be presented in relation to both NAM and GEFS output. All data was displayed using GrADS. Anomalies were computed as described Hart and Grumm (2001) and Grumm and Hart (2001). Shaded values show the standardized anomalies computed as:

SD = (F – M)/�)

Where F is the value from the reanalysis data at each grid point, M is the mean for the specified date and time at each grid point, and σ is the value of 1 standard deviation at each grid point. For brevity times are presented in the format of 06/1200 UTC which signifies 06 February 2008 at 1200 UTC. Both model and EPS initial and valid times are presented in this format. 3. RESULTS i. NAM 00-hour analyses

Figure 4 As in Figure 2 except NAM 00-hour forecasts of 850 hPa temperatures (C ) and heights (m) from 1800 UTC 5 February through 0600 UTC 6 February 2008.

Figure 2 shows the NAM 00-hour forecasts of 850 hPa winds and u- and v-wind anomalies from 05/1800 UTC through 06/0600 UTC. The key parameter associated with severe weather is the strong low-level 850 hPa jet. In these instances, the 850 hPa jet was 3 to 4 SDs above normal at 05/1800 and 06/0000 UTC as it propagated northward from Louisiana to western Kentucky. By 06/0600 UTC the winds had increased and were in the 4 to 5SD above normal range over Tennessee, Kentucky, and southern Ohio. Clearly, a strong southerly jet was impacted the affected region. Figure 3 shows the NAM 00-hour forecasts of mean sea-level pressure and precipitiable water (PW) for the same time periods as shown in Figure 2. These data show the surface cyclone moving from Oklahoma into Ohio during the 18 hour period covered by these images. The surface cyclone had pressure anomalies on the order of -2 to -3 SDs below normal. More significant were the PW values and the PW anomalies. The PW anomalies were 4 to 5 SDs above normal in the warm air by 06/0000 UTC. The high PW air was in the warm air near the nose of the

strong and anomalous southerly jet shown in Figure 2. The high PW air is often associated with relatively high CAPE air and in the winter months, high PW air facilitates keeping temperatures high further impacting the low-level stability. Figure 3 shows the 850 hPa temperatures and the 850 hPa heights over the over the eastern United States from 05/1800 through 06/0600 UTC. Above normal 850 hPa temperatures dominated the entire region with +1 to +2 SD above normal 850 hPa temperatures. Some areas had 850 hPa temperature on the order of 2 to 3 SDs above normal. The lower panels of Figure 2 and the upper panels of Figure 3 clearly indicated that an unseasonably warm moist air mass was in place over much of the eastern United States. A strong baroclinic zone (Fig. 3) and a strong moisture boundary separated the unseasonably warm air from the colder air to the north and west. Though not shown, with time the cold front in the Mississippi Valley (Fig. 3 right) at 06/0600 UTC began to accelerate as it moved eastward.

The easterly jet and negative u-wind anomalies in Figure 2, on the north side of the surface cyclone (Fig. 3), shows the area which was affected by the snow. As indicated by Galway and Pearson (1981) many winter tornado outbreaks are associated with cyclones which produce snow. The 850 hPa cyclone track appeared to delineate the boundary between the warm air and colder air and the snow fell to the north and west of the 850 hPa low track. Figure 5 shows the evolution of the frontal system and the position of the key features at 06/1800 UTC. The 850 hPa cyclone wsa -3SDs below normal and the surface cyclone central pressure was near -3SDs below normal. The 850 hPa jet was about +2 to +3 SDs above normal over the Ohio Valley. The anomalous easterly jet was over Illinois and Wisconsin. The 850 hPa isotherms and the PW field showed the cold front about halfway across Kentucky. The PW anomalies in the warm air were near +3.5 SDs above normal ahead of the frontal system.

ii. Precipitation analyses.

Figure 5 As in Figure 2 except NAM 00-hour forecasts valid at 1800 UTC 6 Febuary showing (left) a) 850 hPa temperatures and anomalies and b) 850 hPa heights, (center) 850 hPa wind and a) u-wind anomalies and b) v-wind anomalies and (right) mean sea level pressure and b) precipitable water.

Figure 6 shows the accumulated rainfall from the Climate Prediciton Center (CPC) unified precipitation data set for the 24 hour periods ending at 06/1200 and 07/1200 UTC.. These data show the heavy rainfall fell on the warm side of the frontal boundary, in and near the high PW air and in the area of the above normal PW values. The 48 to 64 mm contours was present from Missouri into southern Indiana for the 24 hour period ending at 06/1200 UTC (Fig.6a). A few areas had 65 to 96 mm of precipitation as indicated by the purple areas in southern Illinois, Indiana, and Kentucky. Precipitation was lighter on the 6th (Fig. 6b). The area of snow over Wisconsin in Michigan was evident with areas of 16-25mm of precipitation and few areas of 25-32 mm. Other areas of heavy rain included the southeast and the northeast. iii. Long range forecasts

Figure 6 Unified precipitation data (UPD) showing accumulated precipitation (mm) for the 24 hour periods ending at a) 1200 UTC 6 February and b) 1200 UTC 7 February 2008. Values in mm as indicated on the right.

The number of forecast permutations available is exhaustive based on 4 cycles per day and the fact that this event was well predicted 5-7 days in advance. Thus, for brevity focus will be on a few fields from the NCEP GEFS (also known as the MREF) furthermore, displays will be limited to those produced by the cycles initialized at 0000 UTC. The GEFS forecasts initialized at 31/0000 UTC of PW and 850 hPa winds are shown in

Figure 7. These data show the strong southelry jet over the Ohio Valley and the surge of high PW air into the eastern United States. These forecasts were too fast with the frontal evolutions. Though not shown, the pattern at 05/1200 UTC was more similar to the observed pattern in Figures 2 & 3 at 06/0000 UTC than the forecast valid at this time. Thus, these images show a timing error and the cyclone center was too far north as indicated by the easterly wind anomalies displaced into

Figure 7. NCEP GEFS forecasts initialized at 0000 UTC 31 January 2008 valid at 0000 UTC 06 February 2008. Fields shown include (left) 850 hPa winds and a) u-wind anomalies and b) 850 hPa winds and v-wind anomalies and (right) precipitable water (mm) with a) spaghetti plots and spread about the mean and b) the ensemble mean and the departure of this field in standard deviations form normal.

Canada and position of the 850 hPa circulation center in Figure 7. The timing and position errors began to decrease in significance by the 02/0000 UTC GEFS forecast cycle. As shown in Figure 8, the frontal system as indicated by the PW anomalies was within a 100-200km of that shown in Figure 2 at 06/0000 UTC. The GEFS anomalies were on the order of 3 to 3.5SDs above normal, quite large for a 4 day forecasts and reasonable based on the verifying NAM analysis. Such large anomalies at this time range require significant convergence of the ensemble members toward a similar solution, a relatively high confidence forecast. The 850 hPa winds showed the strong southerly jet with +2 to +3 SD anomalies. The u-wind anomalies were not as well forecast. The GEFS was having some difficulty with the cyclone along the frontal boundary.

As the forecast range decreased, the details and the anomalies steadily converged toward those observed in the 00-hour NAM forecasts. The GEFS forecasts initialized at 03/0000 UTC (Fig. 9) showed the frontal system over the Mississippi Valley with the anomalous PW values to the east. The PW field also showed the strong baroclinic and moisture zone from New England to Wisconsin then southward into Louisiana. By this cycle there were clear indications of the potential fore severe weather. A few GEFS members forecast CAPE as high as 1200 JKG-1. The forecasts of the probability of CAPE > 600JKg-1 and shear is shown in Figure 10. The GEFS showed over a 90% chance of CAPE over 600 JKG-1 at 05/1800 UTC, with strong shear over the region. By 06/0000 UTC the CAPE dropped measurably likely the impact of the solar cycle in the model. Shorter range GEFS forecasts continued to

Figure 8 As in Figure 7 except forecasts initialized at 0000 UTC 2 February 2008.

focus on the event and timing better and the CAPE values steadily rose. For brevity short range forecasts are presented in the next section. iv. Short range forecasts Emphasis here is on the Short-range ensemble forecast system (SREF). Individual models such as the NAM and GFS could be used as there was good agreement between the models and the ensemble members. For brevity only the 05/0900 UTC SREF is shown. Earlier cycles performed well and the pattern, converging closer to the NAM analyses was visible in these forecast similar to those presented in the GEFS. From a predictability stand points, the pattern in the warm sector was relatively highly predictable. Figure 11-13 show the SREF forecasts initialized at 05/0900 UTC valid at 05/2100 and 06/0000 UTC. The products shown are

similar to those presented earlier from the GEFS and the NAM 00-hour analyses. These data show the frontal zone as defined by the PW field with anomalous PW in the warm sector. (Fig. 11). The spread was relatively small suggesting a high degree of agreement in these short-term forecasts. The anomalies increased to 4 to 5SDs above normal over Illinois and Ohio by 06/0000 UTC. These values compared well with those shown in Figure 3. The cold frontal system was well placed in the SREF. The heavy rainfall axis in Figure 6 lined up well with the SREF PW anomalies. The SREF 850 hPa winds are shown in Figure 12. The SREF showed the anomalous southerly winds ahead of the cold front and the anomalous easterly winds on the cold side of the PW boundary. The v-wind anomalies increased to 3 to 4SDs above normal by 06/0000 UTC as forecast by the SREF. By 06/0600 UTC the SREF showed strong 850

hPa southerlies from Tennessee into Indiana. Clearly, the low-level winds were forecast to increase in intensity from 06/0000 through 06/0600 UTC. This increase was evident in both the southerly winds associated with the convection and the northeasterly winds associated with the snow on the cold side of the storm. Figure 14 shows the quantitative precipitation forecasts (QPF) from the SREF valid for the 24 hour period ending at 06/1200 UTC showing the probability of 0.1 and 1.00 inches of QPF. The 1.00 inches in the 24-hour period can be compared to the analysis in Figure 6. The area of heavy rainfall was relatively well forecast. The SREF missed some of the 1 inch amounts observed farther south in Kentucky, Tennessee and Alabama. Clearly, the convective line and the enhanced convective rainfall it produced was under forecast by the SREF. The 0.10 inch image showed the convective line and the potential precipitation associated with this feature, however the

SREF produced 0.2 to 0.60 inches of QPF with this mesoscale feature. The SREF also failed to capture some of the higher rain amounts in Illinois, Indiana, and Kentucky. These higher amounts due to convection are not likely to be well captured by an ensemble of 32 to 48 km resolution and using a convective parameterization scheme. 4. CONCLUSIONS On 5-6 February 2008, a major severe weather and tornado outbreak affected the central United States from Texas to Ohio. The event produced over 91 tornadoes, several of them were deadly tornadoes. Despite how well forecast this event was and the issuance of tornado warnings providing advance notice, 59 people lost their lives in this deadly event. It was one of the largest mid-winter tornadic events on record and the deadliest tornado outbreak in the United States since May 1985.

Figure 9. As in Figure 7 except forecasts initialized at 0000 UTC 3 February 2008.

Figure 60. NCEP GEFS forecasts initialized at 0000 UTC 3 February 2008 showing the probability of CAPE greater than or equal to 600JKg-1 (upper panels) and the bulk shear in the 0 to 1.5km level and the probability of the shear greater than 6x10-55s-1. Data are valid at 1800 UTC 5 February and 0000 UTC 6 February 2008.

The NAM 00-hour forecasts showed that a modestly strong winter storm produced heavy snow in the cold air and an exceptionally strong and deadly severe weather outbreak in the central United States on 5-6 February 2008. The NAM analyses clearly showed anomalous southerly winds and anomalously high PW values suggesting the potential for severe weather. The set up was similar to previous events indicated by Galway and Pearson (1981) and with other documented winter severe weather events, such as the event associated with the March 1993 “Super storm” (Uccellini et al 1995). Though not shown, there was a strong upper-level jet with anomalous 250 hPa u and v-wind anomalies. However, the strong baroclinic zone and the strong 850 hPa jet allowed this feature to be inferred by the reader. Overall, the NCEP ensemble prediction systems forecast the larger scale pattern quite well. Furthermore, the pattern was forecast quite well providing several days lead-time for

this potentially devastating event. Both the NCEP GEFS and SREF performed quite well. Clearly, the advanced notice of the high risk of severe weather from the Storm Prediction Center (SPC) suggests that pattern recognition and experimental ensemble products are having a positive impact on weather forecasting. Though not shown, the SPC issued a rare high risk for severe weather product well in advance of the event. This event, with 59 fatalities ranks as the 11th deadliest tornado outbreak since 1950. There were two 58 fatality events including 3 March 1966 and 21 April 1967. The 10th event had 71 fatalities on 15 May 1968 and there were 76 fatalities on 31 May 1985. These previous ranked events were based on 1 day outbreaks and the 5-6 February event spans two days1.

1 Data provided and summary provided by Greg Carbin, of the Storm Prediction Center.

Figure 71. As in Figure except SREF PW forecasts initialized at 0900 UTC 5 February 2008 valid at (left) 2100 UTC 5 February and (right) 0000 UTC 6 February 2008.

Clearly, ensembles can aid in defining patterns conducive for severe weather, heavy snowfall, and heavy rain. When used with climatic anomalies, the EPS data can provide useful information with which to gage the intensity of a severe weather outbreak. Despite how well this event was forecast and how timely the warnings on the numerous strong and violent tornadoes were, the loss of life was high. Clearly, we need to find better means to communicate the risks to further reduce the loss of life in these deadly events. The total precipitation produced by the event is shown in Figure 16. These data are a simple arithmetic sum of the data presented in Figure 6. An axis of heavy rainfall, 32 to 96 mm extends from Missouri into southern New England. There are hints of the mesoscale snow band from northwest Missouri across Wisconsin and Michigan. There are also bands

of heavier rainfall amounts extending to the south indicating the contribution of the severe convection to increased rainfall from eastern Okalahoma into Georgia. One final point, the La Nina pattern that has been part of the winter of 2008, with a storm track up the Mississippi and Ohio Valleys, appears to have produced a significant increase in severe weather over the central United States (Fig. 15). The severe weather event of 7 January 2008 produced 332 severe reports and 75 tornadoes and 5 fatalities. This event include a rare tornado in the State of Wisconsin (SPC Severe weather events website). The event of 8 January produced an additional 178 severe reports to include 16 tornadoes and 1 fatality. The event of 10 January 2008 produced 182 reports with 36 tornadoes, and the event of 29 January produced 305 severe reports to include 3 tornadoes to include 2 fatalities. Clearly, the

first 37 days of 2008 have been exceptional active in terms of severe weather. 5. Acknowledgements The Storm Prediction Center for their webpages and severe weather database. We would specifically like to thank Greg Carbin, John Hart, and David Bright of the SPC for severe weather information and data. 6. REFERENCES Alfonso, A.P and L.R. Naranjo 1996: The 13

March 1993 Severe Squall line over western Cuba. Wea. Forecasting,11,89-102.

Doty, B. E., and J. L. Kinter III, 1995:

Geophysical data and visualization using GrADS. Visualization TechniquesSpace and Atmospheric Sciences, E. P. Szuszczewicz and Bredekamp, Eds., NASA, 209–219.

Galway,J.G. and A. Pearson, 1981: Winter Tornado Outbreaks:, Mon. Wea. Rev,109,1072-1080.

Grumm, R.H. and R. Hart. 2001:Standardized

AnomaliesApplied to Significant Cold Season Weather Events:Preliminary Findings. Wea. and Fore.,16,736–754.

Hart, R.E, and R.H. Grumm 2001: Using

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and L W. Uccellini, 1995: Overview of the 12–14 March 1993 Superstorm. BAMS,76,165-182.

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correlation between US tornadoes and Pacific sea surface temperatures.Mon. Wea. Fore.,129,184-895.

Figure 12. As in Figure 11 except showing SREF 850 hPa winds and u-wind and v-wind anomalies valid at (left) 2100 UTC 5 February, (center) 0000 UTC 5 February and (right) 0600 UTC 6 February 2008.

Stuart, N.A and R.H Grumm 2006: Using wind anomalies to forecast East Coast Winter Storms.Wea.Forecasting,21,952-968.

Uccellini, L.W, P. J. Kocin, R S. Schneider,

P. M. Stokols, and R A. Dorr, 1995: Forecasting the 12-14 March 1993 Superstorm, BAMS,76,183-199.

Figure 13. As in Figure 10 except showing SREF forecasts initialized at 0900 UTC 5 February 2008 valid at 2100 UTC 5 February and 0000 UTC 6 February 2008.

Figure 14. SREF forecasts inialized at 0900 UTC 5 February 2008 showing the probability of precipitation and the ensemble mean precipitation for ethe 24 hour period ending 1200 UTC 6 February 2008. Left side showsthe potential of 1.00 inches aright panels show the probability of 0.10 inches. Upper panels show the probability (shaded) and the ensemble mean (blaccontour) 1 or 0.1 inch contour respectively. Lower panels show the ensemble mean (shaded) and each members 1 or 0.10 inccontours.

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Figure 15 Storm Prediction center summaries of severe weather showing (upper) all severe reports by event type from 01 January through 06 February 2008 and (lower) tornado trends including the 10 year average and the tornadoes over the past 4 years.

Figure 86 As in Figure 6 except UPD data showing the 2 day accumulated precipitation (mm) for the 48 hour period ending at 1200 UTC 7 February 2008.