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Eastern North American Subtropical Ridge and heat of May 2010 By Richard H. Grumm National Weather Service State College, PA 16803 1. Introduction A large subtropical ridge developed over the eastern United States during the week of 24 May 2010 (Fig. 1). This feature was associated with some interesting, if not unique weather over much of the eastern United States and Canada. Warm moist air transported poleward of the ridge produced convection and mesoscale convective complexes over Canada which moved over northern New England. Dry conditions and lightning produced fires in Quebec. Under the ridge, the warm air produced record high temperatures over southern Canada and the eastern United States (Table 1). At the height of the event, daytime highs exceeded 90F from Chicago to Hartford. Many high temperature records were set in central New York and southern New England on 26 May 2010. Locations in Ontario and Quebec saw high temperatures in the 80s and 90s from 25-27 May 2010. This large subtropical ridge brought atypically warm weather to eastern North America. The 850 hPa temperatures were forecast to exceed 20C over portions of New York and New England on 26 May 2010. These forecasts proved to be quite accurate and rare readings over 20C at 850 hPa were observed at several locations including Albany, New York. Not surprisingly many locations in eastern New York and New England set or tied record high temperatures and several sites recorded the earliest record high so early in the season. This paper will document the conditions associated with this impressive subtropical ridge. The focus is on the pattern and the anomalies. Additionally, forecasts are shown to demonstrate that this event was relatively predictable. 2. Methods The 500 hPa heights, 850 hPa temperatures and winds, other standard level fields were derived from the NCEP GFS, GEFS, 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. 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. For brevity, times will be displayed in day and hour format such at 26/0000 UTC signifies 26 May 2010 at 0000 UTC. 3. Results i. Pattern Figure 1 shows the 500 hPa pattern at 0000 UTC from 20-28 May 2010. By 24/0000 UTC the 500 hPa anomalies were over 2SDs above normal from Illinois into Ontario and Quebec. A close 5880m contour was present in these data. This feature shifted northward over southern Canada by 25/0000 and begin to weaken by 27/0000 UTC. The ridge appeared to redevelop or retrogress to a position over the northern plains by 28/0000 UTC (Fig. 1i). The 850 hPa temperatures (Fig. 2) showed above normal 850 hPa temperatures associated with the tropical ridge. A closed 20C contour was evident over Illiniois at 24/0000 UTC and closed 22C contour was evident at 25/0000 UTC. This pocket of warm air followed the ridge eastward and was over western New England by 27/0000 UTC. Thermal anomalies ranged from 2 to 3SDs above normal at 850 hPa with this pocket of warm air. The air clearly modified by 28/0000 UTC (Fig. 2i). The precipitable water analysis at 0000 UTC 20-28 May is shown in Figure 3. These data show the surge of high PW air north and west of the subtropical ridge. This is a common feature associated with many heat episodes. PW anomalies over 3SDs above normal were present in Canada south of James Bay 24-25 May 2010. This area shifted eastward on 26 May (Fig. 3g). Not surprisingly during this period MCS activity, was observed over Quebec and into northern New England. The traditional “ring of fire” with a subtropical ridge had been displaced well north and east of its traditional location. From a sensible weather impact, Figure 4 shows the high temperatures on 24-26 May 2010. Temperatures above 90F extended from the Gulf States into the upper Midwest and into Canada with readings over 90F just south of James Bay. Figure 5 shows a GOES-IR image valid at 25/1831 UTC. Generally clear skies were present under the massive subtropical ridge. The surge of high PW air over along the western edge of the ridge was producing convection over Illinois and Wisconsin. A ridge-runner MCS was visible over Quebec and northernmost Maine. The surge of high PW air, wrapping over the ridge produced this atypically northward are of MCS activity. Figure 6 shows an MCS moving out of Ontario into northern New York at 27/0015 UTC. To the east of this MCS a smaller developing MCS was visible. This eastern MCS would produce severe weather from Vermont to Long Island as it moved southward. The lower panel in Figure 6 shows this feature over Connecticut and Long Island at 27/05045 UTC. ii. Forecasts Thus the GEFS may pay a penalty with only 6-hourly data resolution. The overall pattern for the subtropical ridge was well predicted by then NCEP ensemble forecasts systems (EFS) to include the Global Ensemble Forecasts System (GEFS) and the short-range ensemble forecast system (SREF). Figures 7-10 shows select forecasts. 4. Conclusions The 500 hPa pattern and 850 hPa temperatures were well predicted by the GEFS with high confidence (Fig. 7) from the 23/0000 UTC GEFS. Figure 8 shows 9 selective forecasts of 850 hPa temperatures and the standardized anomalies. These data show the consistent forecasts of as a strong subtropical ridge and 2-3SD 850 hPa temperatures over New York and New England. Many aspects of this event were well predicted, to include the retrogression but the focus here is on the warmest day. The SREF forecasts from showed both high confidence (Fig. 9) and a closed 18C contour at 26/1800 UTC. The 9 select SREF forecasts all showed this warm air. The data shown are at 2100 UTC because that was the time the SREF had the maximum 850 hPa temperature forecasts. Temperatures were slightly lower at 27/0000 and 26/1800 UTC. Figures 11-12 show SREF threats of variables associated with warm episodes. These data show that the strong 500 hPa ridge, warm 850 hPa temperatures, warm 2m temperatures and high PW air with the subtropical ridge were relatively well predicted. A large subtropical ridge developed on 24 May 2010 and persisted through 27 May 2010. This feature produced a closed 5880m contour over southern Canada and 2SD above normal height anomalies. At the surface, high temperatures over 90F were common beneath this feature and the pocket of 20 to 22C air at 850 hPa. This large subtropical ridge produced or tied many high temperature records over eastern North America. Additionally, this feature produced convection over northeastern Canada and MCS activity over an area where such activity is quite rare, even in summer, let alone in late May. Clearly, this strong tropical ridge pushed high PW air and conditions favoring deep convection well north of the climatologically favored areas. The “ring’ of fire associated with subtropical ridges was pushed well northward with this unusual system. As this event ended, the large scale ridge retrogressed. As it moved or redeveloped to the west, the track of MCS activity moved westward too. The record heat in New England ended with as strong MCS that moved nearly north-to-south trough the region. This ridge-roller (Galarneau and Bosart 2006) or ridge-runner produced a cluster of many wind and hail reports in Vermont and Massachusetts across Connecticut and Long Island. This event was well predicted by the NCEP models and EFS. Only EFS data were shown here. The key to this forecast was the subtropical ridge. Subtropical ridges, (Galarneau et al. 2008) play critical roles in warm episodes and modulating the distribution of convection. 5. Acknowledgements Walt Drag for records in New England. Lance Bosart for conference paper access, and the Albany map for discussions on many aspects of this event. 6. References Galarneau, T.J and L.F Bosart 2006: Ridge Rollers: Mesoscale Disturbances on the periphery of cut-off anticyclones. AMS Severe Local Storms symposium, Atlanta, GA. Galarneau, T. J., Jr., L. F. Bosart, and A. R. Aiyyer, 2008: Closed anticyclones of the subtropics and middle latitudes: A 54-yr climatology (1950-2003) and three case studies. Synoptic–Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, Meteor. Monogr., No. 55, Amer. Meteor. Soc., 349–392. Location Bradley Fields (KBDL) State CT Max Comments and notes 99 Tied 20 May 1996 record for month. Broke 1965 reading of 94. Warmest since 2 August 2006 when it was 100F. Worester MA 94 Warmest May reading earliest 94F or greater reading. Providence Burlington RI VT 95 91 Broke record set 26 May 1965 tied 1946 record Montpelier Islip LaGuardia VT NY NY 87 92 94 Broke record from 26 May 1978 86 in 2007 Ties 1965 record Kennedy International NY 91 Ties 1965 record Bridgeport CT 94 breaks 86 set in 2007 Table 1. Selected records by site and State set on 26 May 2010. Figure 1. JMA 500 hPa heights and height anomalies valid at 0000 UTC from (a-i) 20 to 28 May 2010. Heights in meters every 60m. Anomalies in standard deviations from normal in 1 standard deviation units. Return to introduction. Figure 2. As in Figure 1 except for 850 hPa temperatures ( C) and temperature anomalies. Temperatures every 2C. Return to pattern. Figure 3. As in Figure 1 except for precipitable water (mm)n and precipitable water anomalies. Return to pattern. Figure 4. High temperatures for 24, 25 and 26 May 2010. Figure 5. GOES IR image valid at 1831 UTC 25 May 2010. Local archive was limited to the eastern United States. . Return to satellite Figure 6. As in Figure 5 except valid at 27 May at 0015 and 0545 UTC. Return to satellite. Figure 7. GEFS forecasts initialized at 0000 UTC 23 May 2010. Left side shows 850 hPa temperatures with each members 0,8,and 16C contour and the spread. Lower panel shows the ensemble mean 850 hPa forecasts and the standardized anomalies. The right side shows 500 hPa heights. Upper right shows each member’s 5460 and 5760m contours and the spread about the mean. Lower panel is the ensemble mean field and the standardized anomaly. Return to text. . Figure 8. GEFS forecasts of the mean 850 hPa temperatures from all 21 members and the standardized anomaly of this field. Temperatures every 2 C valid at 0000 UTC 27 May 2010. Initialization times are a) 0000 UTC 23 May, b) 1200 UTC 23 May, c) 0000 UTC 24 May, d) 0600 UTC 24 May, e) 1200 UTC 24 May, f) 1800 UTC 24 May, g) 0000 UTC 25 May, h) 0600 UTC 25 May and i) 1200 UTC 25 May 2010. Return to text. . Figure 9. As in Figure 7 left side except for SREF 850 hPa forecasts of 850 hPa temperatures and anomalies valid at 2100 UTC 26 May from forecasts intialzied at (left) 0900 UTC 25 May and (right) 2100 UTC 25 May 2010. Return to text. . Figure 10. As in Figure 8 except for SREF forecasts valid at 2100 UTC 26 May 2010 from forecasts initialized at a) 0900 UTC 24 May, b) 1500 UTC 24 May, c) 2100 UTC 24 May, d) 0300 UTC 25 May, e) 0900 UTC 25 May, f) 1500 UTC 25 May, g) 2100 UTC 25 May, f) 0300 UTC 26 May and i) 0900 UTC 26 May 2010. Return to text. . Figure 11. SREF forecasts initialized at 0300 UTC 24 May 2010 showing the probability of a) 1.5SD or greater 500 hPa height anomalies., b) 2.0 SD 850 hPa temperature anomalies, c) 2.0SD or greater precipitable water anomalies. and d) 2.5SD or greater 2m temperature anomalies. , Figure 12. As in Figure 11 except showing a) probability of 500 hPa heights over 5840m, b) 850 hPa temperatures over 18C, c) precipitable water greater than 20mm, and d) 2m temperatures over 22C.