Draft: The value of standardized Anomalies in Understanding the Historic

Tornadic Event of 27-28 April 2011: by

Richard H. Grumm

National Weather Service State College, PA Abstract:

An historic and tragically deadly tornado event struck the eastern United States on 27-28

April 2011. This was comparable to the deadly 3-4 April 1974 and 18 March 1925 events which caused over 300 and 740 deaths respectively. The expansive event of April 1974 affected 11 States and was associated with large low-level wind anomalies and an unseasonably strong upper-level jet stream.

The 850 hPa low-level jet peaked at over +4.5s above normal on 27 April 2011, accompanied by precipitable water anomalies in excess of +2s above normal. The 250 hPa jet core was over 3s above normal, over the Great Lakes with a broad implied jet entrance region of the southern United States.

This anomalous pattern is shown using the GFS 00-hour forecasts. Forecasts from the NCEP models are used to show how predictable this pattern was. Clearly, National Weather Service’s forecasters used these data and this and additional information to anticipate this historic event. Undoubtedly, anticipating this event and communicating this event to the public by the broader weather community reduced the loss of life. Despite the successful forecasts and communication of this information, this event will go down as one of the deadliest tornado events in United States history

1. INTRODUCTION

A devastating severe weather event affected the eastern United States from 26 to 28 April (Table 1). On

27 April there were 806 reports of severe weather to include 211 tornadoes. Tornadoes were observed

from the Gulf of Mexico to New York with strong tornadoes impacting Mississippi, Alabama and

Georgia. Initial indications are that over 300 people were killed by the severe weather on 28 April 2011,

making this one of the deadliest tornado events in the United States (Table 2). Comparable cases include

the 3 April 1974 event where an estimated 300 people lost their lives in a widespread 11 State tornado

episode and the 18 March 1925 event which was estimated to have taken 747 lives.

.

Figure 1. Storm Prediction Center reports of severe weather, by type for the 24 hour period ending at 1200 UTC 28 April 2011.

This historic event shard many of the common characteristics often associated with tornado outbreaks.

These conditions and the climate anomalies of several fields will be shown here. Hart and Grumm (2001)

and Graham and Grumm (2010) have demonstrated the value of standardized anomalies in identifying

meteorologically and climatologically significant events The range of events spans identified by this

method spans heat waves, snow storms (Stuart and Grumm 2006), historic floods (Bodner et al.

2011;Junker et al.. 2008), and severe weather events.

The ingredients associated with severe events often include high convective available potential energy

(CAPE), shear, and lift (Doswell et al. 1996;Doswell 1982; Davies and Johns 1982) . High values of low-

level moisture, often gauged by precipitable water (PW) are often found in close proximity to high values

of CAPE. The importance of mid-level dry air over this region, often associated with elevated mixed

layers (EMLs: Banacos et al. 2010;Carlson and Ludlam 1968; Carlson et al. 1983) is often good

indicators for the potential for severe thunderstorms. Some of the more notable supercell storms and

severe outbreaks have been associated with the presence of EML’s to include the widespread tornado

event of 31 May 1985 (Banacos and Ekster 2010).

This paper will document the historic tornado event of 28 April 2011. The focus is on the synoptic scale

pattern and the anomalies associated with this meteorologically and climatologically significant event.

The NCEP GFS 00-hour forecasts are used to show the pattern. Forecasts from the NCEP ensemble

forecast systems (EFS) and GFS forecasts are presented to show the value of anomalies in anticipating

potential high impact severe weather events.

2. METHODS AND DATA

The 500 hPa heights, 850 hPa temperatures and winds, other standard level fields were derived from the

NCEP Global Forecast System (GFS), North American Mesoscale Model (NAM), Global Ensemble

Forecast System (GEFS), Short Range Ensemble Forecast (SREF) and the NCEP/NCAR (Kalnay et al.

1996) reanalysis data. The means and standard deviations used to compute the standardized anomalies

were from the NCEP/NCAR data as described by Hart and Grumm (2001). Anomalies were displayed in

standard deviations from normal, as standardized anomalies and are used to show the anomalies,

probability of heavy rainfall and Quantitative Precipitation Forecasts (QPF) associated with this event.

All data were displayed using GrADS (Doty and Kinter 1995).

The standardized anomalies computed as:

SD = (F – M)/σ ()

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.

Model and ensemble data shown here were primarily limited to the GFS and GEFS. The NAM and SREF

data were also available for use in this study. Displays will focus on the observed pattern and some

forecast issues associated with the pattern.

Both the unified precipitation dataset (UPD: Seo 1998) and the stage-IV precipitation data (Lin and

Mitchell 2005; Nelson et. al. 2010) were used to evaluate the rainfall associated with this event. The

stage-IV data are presented in this document.

In addition to computing standardized anomalies for distinct times associated with individual

meteorological rainfall events, anomalies were computed by averaging the fields and each instantaneous

standardized anomaly value over 1 to 2 day periods. This was facilitated identifying the signal and the

persistence in the long-term signal of the pattern. This concept was then used to obtain multi-day pattern

forecasts from the NCEP GFS and GEFS.

For brevity, times will be displayed in day and hour format such a 28/0000 UTC signifies 28 April 2011

at 0000 UTC.

3. METEOROLOGICAL OVERVIEW

i. Synoptic scale pattern

The 500 hPa pattern in 6-hour increments from 27/1200 UTC through 28/1800 UTC (Fig. 2) shows the

deep 500 hPa trough moving into the southern and eastern United States along with the strong ridge to the

east. This ridge played a critical role in the evolution of the weather over the eastern United States. The

strong southerly flow between these two systems produced an elongated south-to-north 250 hPa jet (Fig.

3). This clearly allowed the transport of deep moisture from the Gulf of Mexico into the eastern United

States.

The low-level response is clearly visible in the PW field (Fig. 4) with generally high PW which showed a

surge to the north and east (Fig. 4b-e) as the 500 hPa wave moved eastward. PW anomalies of 2 to 4s

were present over the southeast between 28/0000 and 28/0600. This surge of high PW air was associated

with a strong 850 hPa low-level jet (LLJ:Fig. 5) with 850 hPa wind anomalies at 4-5s above normal over

western Mississippi at 28/1800 UTC (Fig. 5a) peaking over 5σ above normal in Alabama at 28/0000

UTC near the time of the tornado event in the southern States. There was a brief period of 6s 850 hPa

anomalies in these 6-hourly data at 28/1200 UTC over Maryland (Fig. 5d). The pressure field (Fig. 6)

shows the deep cyclone which tracked to the north and west of the region of severe weather.

The persistence of the pattern (Fig. 6) over the 48 hour period is shown in the composite. The red square

is Birmingham, Alabama. The deep trough and sharp ridge are evident in these data along with the strong

southerly flow and surge of high PW air. The composited PW field implies a flow of moisture from both

the Gulf and southwestern Atlantic. The strong jet between the trough and ridge is evident in these data.

The composite pattern from 27/1200 through 28/0000 UTC shows the strong 250 hPa jet, strong 850 hPa

jet, which was in excess of 24ms-1 over Alabama for the period and was +5s above normal over the

region. The surge of high PW air is also evident in these data.

ii. Forecasts-GFS

iii. Forecasts-GEFS

iv. Comparative events (Pakistan?)

.

4. CONCLUSIONS

An historic tornado outbreak affected the eastern United States on 28 April 2011. With at least 211

reported tornadoes, this was one of the largest tornado outbreaks during the month of April. There are on

average 160 tornadoes observed during the month of April. Until 2011, April 1974, with 267 tornadoes

held the monthly record for April. As of 29 April 2011 there have been approximately 600 tornadoes for

the month. With over 310 fatalities this event surpassed the old record of 310 fatalities set in April 1974

(Table 2) and ranks among the deadlier tornado outbreaks in US history. This event was relatively well

predicted and the large scale pattern in which the event occurred was nearly a textbook example of the

conditions associated with tornadoes. The standardized anomalies associated with many of the key fields

indicated an environment conducive to a significant severe event.

It should be noted that the number of tornadoes (211) is larger than the number of tornadoes observed in a

typical April. This may reflect both the observation bias due to the convergence of technologies and

public awareness. Cell phones, Twitter, and other forms of social media allow for quick access to and

sharing of data related to a wide range of weather. Other contributing factor to this event would have to

include changes in population over the southern United States, an improved warning and communications

network and a pattern which brought together the key ingredients and all of these ingredients departed

significantly from climatological norms. The warning and communications technologies likely saved

untold number of lives. The tragic 18 March 1925 (747) and 21 March 1932 events occurred well before

the advent of weather radar and prior to our ability to warn on such storms.

The pattern during this event included an abnormally deep 500 hPa trough with a strong ridge over the

western Atlantic. This produced strong southerly flow over the Mississippi Valley with a strong 250 hPa

jet and a strong 850 hPa LLJ. A plume of deep moisture moved into this region, providing the fuel for the

evolving storm. At times the PW was 2 to 3σ above normal and the 850 hPa winds were 5σ above

normal. The lift was provided by an advancing frontal system, which ended a prolonged period of heavy

rainfall which caused significant flooding in the Mid-Mississippi Valley.

Composites of the pattern showed that the anomalous pattern and strong flow persisted over several days

and reached its apogee on 27 April 2011. A comparison to the historic event of 3 April 1974 implies a

similar pattern was associated with both events. It should be noted that the data used to analyze the 1974

event is on a 2.5x.2.5 degree grid. However, these coarse data due show a pattern and key anomalies

similar to those indicated on 28 April 2011.

5. Acknowledgements

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Date Tornado Wind Hail Totals 26-Apr 77 382 280 739 27-Apr 211 402 193 806 28-Apr 13 75 25 113 Totals 301 859 498 1658

Table 1. Severe weather from the Storm Prediction Center by day during the period of 26-28 April 2011. Data is displayed by type and summer for each day and type. Return to text.

Event Date Fatalities 18-Mar-25 747 21-Mar-32 332

27-Apr E318 17 May 1840 317

3-Apr-74 310 11-Apr-65 260 21-Mar-52 205

Table 2. Partial list of record tornado events provided by the National Weather Service. Data is from Grazulis (1993). The 27 April event numbers are not final. Return to text.

Figure 2. GFS 00‐hour forecasts of 500 hPa heights and height anomalies in 6‐hourly increments from a) 1200 UTC 27 April through f) 1800 UTC 28 April 2011.

Figure 3. As in Figure 2 except for 250 hPa winds.

Figure 4. As in Figure 2 except for precipitable water and PW anomalies. Return to text.

Figure 5. As in Figure 2 except for 850 hPa winds and wind anomalies.

Figure 6. As in Figure 2 except for mean sea level pressure and pressure anomalies. Return to text.

Figure 7. GFS composites of 6‐hourly data from 1200 UTC 26 through 1200 UTC 28 April 2011 showing a) 500 hPa heights and anomalies, b) 250 hPa winds and anomalies, c) 850 hPa wind and anomalies and d) precipitable water and anomalies. Return to text.

Figure 9. Figure 9. As in Figure 7 except showing mean sea‐level pressure.