Mesoscale Convective System and heat episode July 2005 by Richard H. Grumm and Mathew Steinbugl

Transcription

1 Mesoscale Convective System and heat episode July 2005 by Richard H. Grumm and Mathew Steinbugl 1. INTRODUCTION A strong upper-level ridge brought unseasonably high temperatures to the Midwest and eastern United States from 23 through 28 July In most locations, this event did not qualify as a heat wave as record and near record high temperatures did not remain persist for 3 or more days. Thus it qualified as a warm episode. Though it was not a heat wave, it shared many of the common characteristics of a heat wave. An anomalous upper-level ridge, with 500 hpa heights in excess of 594 dm moved over the affected region; an area of anomalous precipitiable water moved over the ridge and was mainly poleward of the hottest temperatures; both 850 and 700 hpa temperatures were above normal; and finally the thermal anomalies were vertically stacked. This paper will examine the conditions associated with this heat wave with an emphasis of the affects over Pennsylvania. 2. METHODS Data was collected from AWIPS and from archived National Centers for Environmental Predictions (NCEP) model and ensemble model data. Climatic anomalies were produced using a mixture of operational ensemble data and the NCEP-NCAR global reanalysis (GR) data. All departures are show in standard deviations (SDs) from normal referred to as standardized anomalies. 3. RESULTS a) MCS of 25 July 2005 Figure 1 MREF mean 500 hpa heights (dm) valid at 1200 UTC 23 July Anomalies are shown by shading as indicated by the color key to the left. Heights are contoured every 6 dm. A strong upper-level ridge was present over the plains at 1200 UTC 23 July The heights were 2.5SDs above normal over Nebraska at this time. As this upper-level ridge moved eastward over Pennsylvania, a surge of above normal precipitiable water moved into the region late on the 24 th. Strong southwesterly winds associated with this warm moist air produced an MCS over Pennsylvania. Shortly after 25/0000 UTC an area of convection developed over southern Ontario, Canada. This area showed some organized convection by 25/0100 UTC (Figure 2). This convection developed in

2 Figure 2 GOES IR image and surface data centered over southern Ontario at 0100 UTC 25 July Figure 4 As in Figure 3 except valid at 0500 UTC. Figure 3 As in Figure 2 except valid at 0300 UTC 25 July the strong warm advection associated with the advancing upper-level ridge and unseasonably warm air. Surface temperatures and dew points were in the 80s and 70s south and west of the area of convection. To the east, temperatures were mainly in the 70s with dew points mainly in the 60s. By 0300 UTC (Figure 3) a well developed mesoscale convective system (MCS) was visible in the IR imagery. The system was moving to the southeast and moved into western Pennsylvania by Figure 5 As in Figure 2 except valid at 0900 UTC 25 July UTC. Thunderstorms were present over much of central and northwestern coldest cloud tops were over western Pennsylvania, New York, and Lake Erie. The system rapidly moved southeastward and was over southeastern Pennsylvania by 25/0900 UTC (Figure 5). By 25/1200 UTC the initial MCS was over the Mid-Atlantic region with scattered convection over Ohio, eastern

3 observations. This feature could be seen in the radar data valid at 0600 and 0700 UTC. Figure 7 shows the 0700 UTC 0.5 degree base reflectivity and velocity data. The stratiform rain area was present to the northwest with stronger convection to the south. At the radar, northwesterly winds were present. An area of convergence can be seen south of the radar, this area lines up well with the more intense reflectivity data. b) heat Figure 6 As in Figure 2 except valid at 1200 UTC 25 July Pennsylvania and New York. Overnight temperatures in western Pennsylvania had remained nearly steady as the warm and moist air moved into the region. Table 1 shows the high temperatures associated with the upper-level height (Figure 1) and thermal anomalies over the plains and Midwest on 23 July Most reporting sites in Nebraska reached or exceeded 100F. As shown in the table, upper 90s and lower 100 degree readings were typical on this day under the large ridge. Though not shown, north of the area which experienced the heat, a large MCS developed. As the MCS moved through the region, a meso-high was present in the surface Figure 7 KCCX base reflectivity and velocity data from the 0.5 degree scan valid at 0700 UTC 25 July Reflectivity and velocities as indicated by the color code at the top of each image.

4 Figure 9 Maximum observed temperatures on 26 July 2005 from the local cooperative observation network. On the 24 th the heat moved eastward. Midway airport hit 104F during the afternoon, about 1F lower than the alltime high recorded at that site. Officially, the high in Chicago was 102F. Temperatures reached 100F from Iowa and Missouri eastward to Ohio. St Louis reached 102 and it was 101 in Iowa City. The heat moved eastward on the 26 th with temperatures reaching 100F in West Virginia. Temperatures of 100F and greater affected Virginia and North Carolina on the 26 th and 27 th of July. Though hot humid weather affected Pennsylvania, the intense heat and 100F readings remained to the south of the area. Temperatures were mainly in the 80s and 90s after the MCS passed through on the 25 th. The warmest day state wide was on the 26 th. Temperature approached 100F in eastern parts of the state (Figure 8). Thunderstorms late on the 26 th brought cooler air to western sections of the State on the 27 th. A line of severe thunderstorms moved across the State on the 27 th bringing an end to this event. The hottest day of the summer to date affected New Jersey on the 27 th where Teterboro and Newark hit 103F and 101F respectively 1. In the Washington, DC and Baltimore areas, temperatures were in the 90s on the th. No sites exceeded 99F. However, the Baltimore sun reported that While not a weather record, power use set a new record for Baltimore... and "For Baltimore Gas and Electric 1 Courtesy Alan Cope, National Weather Service Mount Holly, New Jersey.

5 Company, the record was 7,070 megawatts of peak electrical usage at 5 p.m. Tuesday,... and topped Monday's record-breaking peak usage of 6,807 megawatts.". 2 Thus the heat wave and demand on power was significant. Farther south in Virginia temperatures also were mainly in the 90s. Clouds and convection precluded record high temperatures in this region. Figure 9 shows the temperatures across Pennsylvania on 26 July This was the hottest day over the state during this warm episode. Examining Figures shows that the surge of warm moist air moved over the region on the 25 th but lower precipitiable water values quickly moved back in. The warmest anomalies at both 850 and 700 hpa passed to the south along with the strong upper-level ridge and attendant subsidence. Figure 10. GR data valid at 0000 UTC 25 July 2005 showing a) 500 hpa heights, b) 850 hpa temperatures, c) 700 hpa temperatures and d) precipitiable water anomalies and departures from normal in standard deviations from normal (shading). 4. CONCLUSIONS An MCS moved over Pennsylvania on the 25 th of July This MCS ushered in warm humid air into the regions. Most locations of the state had 1-2 days where daily high temperatures exceeded 90F. The hottest air remained well south of the region. The thermal anomalies and height anomalies (Figs 10-12) show that the warmest air and strongest subsidence passed to the south. Thus, this heat episode was not as significant over Pennsylvania as it was over the Midwest and cities like Chicago and St Louis. Many heat waves are associated with strong ridges aloft and the presence of a 2 Courtesy Steven Zubrick, National Weather Service Office, Sterling Virginia. Figure 11 As in Figure 10 except valid at 0000 UTC 26 July moisture anomaly to the north. Areas which experience the warmest temperatures are often equatorward of the moisture anomaly. The close proximity of Pennsylvania to the moisture anomaly, and thus the potential for convection, likely was a signal to suggest that the heat episode would not be as severe as points farther south. The

6 NWS Offices in State College, Mount Holly, New Jersey, Sterling Virginia and Blacksburg, Virginia for information on the heat and temperatures in their regions. Also Paul Knight and Fred Gadomski of the Pennsylvania State University for data and plots of maximum temperatures. Figure 12 As in Figure 10 except valid 0000 UTC 27 July passage of the ridge and largest thermal anomalies to the south also preclude triple digit temperatures over most of the region. By true definition, this event did not qualify as a heat wave over most locations. Few locations were able to sustain 3 or more consecutive days of near record heat. Note in Figure 12 the strong frontal system poised to bring cooler and drier air into the eastern United States, thus ending this warm episode. An interesting aspect of this event was the demands for power. Despite this event not being a record, the increased population in the DC and Baltimore area in recent years suggest a major and prolonged heat wave could but incredible demand on the power grid. The lack of prolonged record heat in the past 10 years may be masking a lurking problem. Temperature values for the region are shown in the appendix. 5. ACKNOWLEDGEMENTS:

7 State Location Max temperature Iowa Des Moines 98 Iowa Ottumwa 99 Iowa Burlington 97 Iowa Spencer 97 Iowa Fort 97 Madision Iowa Keokuk 99 Iowa Clarinda 97 Iowa Council 97 Bluffs Iowa Red Oak 97 Iowa Shenandoah 100 Kansas Topeka 99 Kansas Concordia 101 Kansas Lawrence 103 Kansas Manhatten 103 Kansas Emporia 99 Minnesota Redwood 96 Falls Minnesota New Ulm 95 Missouri Kansas City 99 Missouri Sedalia 103 Missouri Chillicothe 101 South Chamberlain 110 Dakota South Huron 109 Dakota Nebraska Lincoln 105 Nebraska Offutt 102 Nebraska Norfolk 101 Nebraska Omaha 105 Table 1. High temperatures at select sites on 23 July Return to text. Map.jsp

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