Impacts of the April 2013 Mean trough over central North America

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1 Impacts of the April 2013 Mean trough over central North America By Richard H. Grumm National Weather Service State College, PA Abstract: The mean 500 hpa flow over North America featured a trough over the continent and ridges along the East and West Coasts. The mean trough was associated with several surges of cold dry air which penetrated into the Gulf of Mexico. This produced several cold episodes and a 10 day period where over 100 record low high-temperatures were set or tied. The relatively cool air led to 3 significant late season snow falls from the Rockies to the Great Lakes. There were two surges of deep moisture and high PW air. The first surge produced 3 of the 6 severe events which had 100 more severe reports between 9-11 April The second surge around the 18 th of April was associated with a second round of severe weather and a heavy rainfall event in the Mid-West which produced river flooding in Illinois and Michigan. The mean trough limited the surges of high PW which in turn limited the severe weather and tornado activity in April 2013.

2 1. Overview April 2013 saw several surges of cold air into the central United State which produced several periods where record low high-temperatures were set or tied (Fig. 1). During the mid-april cold snap, a deep trough developed over northeastern North America which extended into the northern Plains (Fig. 2a). A sharp ridge was present over the western Atlantic during this period. The flow about the ridge kept the plume of high precipitable water (PW) air focused over Florida and off the East Coast (Fig. 2c). Most of North America experienced a period of near to below normal PW for this period of relatively cool weather. Beneath the trough, 850 hpa temperatures were below normal over most of northern North America (Fig. 2d). Florida and western Alaska were relatively warm. Two massive surface anticyclones were present during this period of time, one over the eastern Pacific and northwestern North America and the other beneath the ridge over the western Atlantic (Fig. 2d). This cold period impacted the monthly mean pattern significantly (Fig. 3) which showed the mean trough and the ridge over the western Atlantic, though these features were considerably weaker. The persistent ridge over the northwestern Pacific was a dominant feature. The PW patter showed most of North America was near to slightly drier than normal (Fig. 3c). The pattern and low PW value likely contributed to the relative dearth of severe weather and tornadoes in the United States during April There were only 6 severe weather outbreaks which contained 100 more severe reports (Table 1). The event of 11 April 2013 produced 180 severe reports including 26 tornadoes in the Gulf States (Fig. 4) and it was part of a 3-day event which began the central Plains on 9 April 2013 and produced 491 severe reports on April This paper will examine the pattern over the United States during April 2013 and summarized some of the key impacts. The focus is on the pattern and anomalies associated with the pattern. 2. Data and Methods The large scale pattern was reconstructed using the Climate Forecast System (CFS:Saha 2010) 6- hourly data and CFS based 30-year climatic means and standard deviations. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). Storm reports were obtained from the Storm Prediction Center. Processed SPC data was used to overlay on the anomalies and the base SPC images were used. The Stage-IV data was used to produce the monthly precipitation data. 3. The region

3 The pattern for the month of April (2013) showed the mean trough over central North America with a strong ridge over the northeaster Pacific and a weaker ridge over the western Atlantic. At the surface, ridges were present beneath these features. The flow about the eastern ridge kept the plume of deep moisture off the East Coast most of the month. The PW data implied most of North America was relatively normal to slightly below normal during most of the month. The number of daily high temperatures which were record low values (Fig. 1) implied a surge of cold air near the middle of the month. Figures 5 & 6 shows the evolution of the pattern over 9 days around this time and represent a portion of the data used to compute Figure 2. These data show start out showing the last vestiges of the blocking high which had been over northeastern Canada in late March and early April 2013 (Fig. 5a). A strong 500 hpa trough over the northern plains developed during most of this time period, eventually carving out a deep trough in the central United States by 19 April (Fig. 5e). During most of this period heights were above normal along the West Coast and a large ridge developed along the East Coast and western Atlantic as the trough developed (Figs. 5c-g). During this 9-day period, the region of above normal heights over northeastern North America was replaced by a region of well below normal heights. At 850 hpa the evolution of the trough was accompanied by a surge of -1 to -4σ below normal 850 hpa temperatures (Fig 6). The surge of the cold air was into the central United States, through the Plains, Mid-Mississippi Valley (MMV) and Gulf States. These regions showed up repeatedly in the daily records of low high-temperatures (Fig. 1 & Table 2). 4. Impacts The impacts of the pattern on the United States were significant including several late-season snow events along the eastern slopes of the Rockies and into the Plains. Several significant snow events impacted the western Great Lakes region. The total precipitation for the month (Fig. 7) suggested an active storm track from the southern Plains (Oklahoma) northeastward into northern Michigan. Many of the large precipitation events during this time fell as snow on the northern edge of this storm track. In the eastern United States, a strong ridge and warm surge around 10 April produced severe weather and locally heavy rainfall. Despite the early date, several mesoscale convective systems developed on April 2013 (Grumm and Ross 2013). The large ridge over the eastern United States at this time (not shown) was associated with a deep trough in the western United States which produced a 3-day winter storm from Colorado, across the plains and into the Great Lakes on April 2013 (Fig. 8). The severe weather and snow were associated with the development of strong, but short-lived ridge in the east (Fig. 9d-f) and a deep 500 hpa trough in the western United States (Fig. 9c-g). Most of the snow was in close proximity to the closed 500 hpa cyclone. The deep trough and strong ridge allowed for surge of high PW into the MMV,

4 across the Mid-West and into the East Coast from 9-12 April 2013 (Figs. 10d-g). This surge of high PW air was associated with 3 of the 6 larger severe weather events of April 2013 (Table 1). Ahead of a strong frontal system, heavy rainfall was observed on April Flooding was observed in Chicago on 18 April when 100 to 150 mm of rainfall impacted the region (Grumm 2013). The rain fell in strong southwesterly flow ahead of the deep trough shown in Figure 5. This was one of the few periods where deep subtropical moisture penetrated into the eastern United States. Widespread rains in excess of 100 to 200 mm were observed from Missouri to Michigan resulting in flooding along many rivers in these States. Some of the more significant river flooding was observed in Illinois and Michigan along the Illinois and Grand Rivers respectively. Along the northern edge of the precipitation shield of this system heavy snow fell from western Kansas and Colorado into Minnesota (Fig 9 upper panel). A third snow event of note occurred April (Fig. 10 lower). This event produced snow from the Plains to the western Great Lakes. The snowfall on the 24 th is shown in Figure 9. The deep trough during the middle part of the month (Fig. 2 & Fig 5) contributed to the two significant snowstorms during April The severe weather activity during the month of April was limited to 6 events which produced 100 or more severe reports (Table. 1) and only 1 significant multi-day severe event (Fig. 4) which occurred during a period when a strong ridge was present along the East Coast and a deep trough was present over the western United States (Fig. 8). The mean trough in the center of the United States during long periods of time limited the intrusion of warm moist air into the central United States suppressing convective activity and limiting the number of observed tornadoes in April Summary A mean trough over central North America and ridges along each coast dominated the pattern during much of April The trough limited the surges of deep moist air from the Gulf and western Atlantic which in return limited the number of large severe weather and tornado outbreaks across the United States during April The mean trough was the result of several surges of energy and intrusions of cold air which led to several late season snow events from the Rockies into the Great Lakes. The middle of month was period of sustained cold (Fig. 1 & Table 2) producing a 10 day period where over 100 daily low high-temperature records were tied or broken. As the pattern transitioned in early April a ridge built over the eastern United States on 9-11 April 2013 (Fig. 9). This allowed for a surge of high PW into the eastern United States which produced a rain and severe event in the warm air. Farther west, a trough developed and along with an intrusion of cold air produced a late season snowfall in the plains (Fig. 8).

5 The eastern ridge moved over the western Atlantic and a series of troughs and cold surges led to a prolonged period of cool weather in the central United States (Figs. 1 &2). The mean trough limited the surges of warm moist air into the United States limiting the instability and low-level southerly flow often associated with spring-time severe weather. During a cold surge around April a surge of high PW produced a heavy rainfall event from Missouri into Michigan. On the cold side of the boundary another late season snow event occurred. The 100 to 200 mm of precipitation in this region (Grumm and Ross 2013) contributed a significant portion to the total rainfall over the same region for the month (Fig. 7) as only 150 to 300 mm (6-12 inches) of precipitation fell over most of this region for the entire month of April. The higher amounts of precipitation for the month in this region fell over northern Michigan. The early April high-latitude ridge over northeastern North America was replaced by a deep trough with significant height anomalies (Fig. 9d-e). The strong block which had kept most of the eastern United States cool in March and April broke down. However, the evolution of a positive PNA pattern, with sharp ridge along the West Coast did not lead to sharp warm up. The pattern eventually transitioned into another one which allowed the intrusion of cold air into the United States and suppressed much of the severe weather activity. The 6 severe events which produced 100 or more reports were each associated 2 distinct frontal systems. The event of 9-11 April was associated with a strong ridge and cold front. The strong ridge produced a series of summer-like MCS s in the eastern United States. The second systems were associated with a strong cold front. 6. Acknowledgements 7. References Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C., Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Saha, Suranjana, et. al., 2010: The NCEP Climate Forecast System Reanalysis. Bull. Amer. Meteor. Soc., In Press (DOI: /2010BAMS3001.1).

6 Record Low high-temperatures Number of records broken Records tied/broken Records Records Tied Apr 8-Apr 15-Apr 22-Apr 29-Apr 6-May 13-May Day of Month Figure 1. Daily plots of record low high-temperatures including total number of record tied or broken, the number of records set and the number of records tied. Return to text.

7 Figure 2. The mean pattern over North America from the Climate Forecast System for the period of 0000 UTC April Data include a) mean 500 hpa heights (m) and height anomalies, b) 850 hpa temperatures and anomalies, c) precipitable water (mm) and anomalies, and d) mean sea-level pressure (hpa) and anomalies. Anomalies in standard deviation from normal as shown in the color bar. Return to text.

8 Figure 3. As in Figure 2 except for the pattern from 0000 UTC 1 April to 0000 UTC 1 May Return to text.

9 Figure 4. Storm Prediction center storm reports for the 2-day severe event of 10 and 11 April Return to text.

10 Column1 Date Severe Tornadoes 9-Apr Apr Apr Apr Apr Apr Table 1. Storm Prediction Center list of severe weather events with 100 more reports. Data include date, total severe reports and number of the reports which were tornado reports. Return to text.

11 Figure 5. As in Figure 2 except for single time plots of 500 hpa heights and anomalies in 24-hour increments from 1200 UTC April Return to text.

12 Date Records Records Tied Records tied/broken Locations 1-Apr Northern Lakes 2-Apr S. Plains and NE 3-Apr Southern and NE 4-Apr Southern US 5-Apr Gulf States 6-Apr FL 7-Apr AK NW 8-Apr NW 9-Apr AK N. Plains 10-Apr Plains and Lakes 11-Apr Plains and Lakes 12-Apr Mississippi Valley 13-Apr Great Lakes 14-Apr Northern Tier US 15-Apr Rockies N Plains 16-Apr N-C Plains 17-Apr Plains 18-Apr Central US to Gulf 19-Apr Central US to Gulf 20-Apr Central US to Gulf 21-Apr Great Lakes/Southeast 22-Apr SE and Northern Plains 23-Apr Central US to Gulf 24-Apr Central US to Gulf 25-Apr Central US to Gulf 26-Apr South 27-Apr Mid-Mississippi Valley 28-Apr Mid-Mississippi Valley 29-Apr AK SE 30-Apr AK NW Table 1. List of daily record low high-temperature set, tied and the total. The regions in the far right column describe the geographic area where many of the records were set on a given day. Return to text.

13 Figure 6. As in Figure 5 except for 850 hpa temperatures. Return to text.

14 Figure 7. Stage-IV monthly precipitation (inches) for the period of 0000 UTC 1 April to 0000 UTC 1 May Values in inches starting at 1 inch and contours 2,4,6,8,12,16 and 24 inches. Return to text.

15 Figure 8. Snow precipitation for the 3 days from April Return to text.

16 Figure 9. As in Figure 5 except for the period of 1200 UTC 6-14 April Return to text.

17 Figure 10. As in Figure 9 except for precipitable water and anomalies. Return to text.

18 Figure 11. As in Figure 10 except for snowfall on 19 and 24 April Return to text.

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