Low-end derecho of 19 August 2017

Transcription

1 Low-end derecho of 19 August 2017 By Richard H. Grumm and Charles Ross National Weather Service State College, PA 1. Overview A cluster of thunderstorms developed in eastern Ohio around 1800 UTC on 19 August A point near New Philadelphia, OH is a reasonable starting point of the convection. The cluster organized into a quasi-linear convective system as it raced across eastern Ohio and Pennsylvania (Fig. 1). In Pennsylvania nearly all the reported damage was from strong convective winds. One of the farthest east reports was in Berks County, PA at 0100 UTC 20 August 2017 (SPC storm reports) The distance between the Ohio location and the Berks county location was about 290 miles using the Google Maps distance tool thus this event qualifies as a derecho. The Storm Prediction center definition of a derecho (SPC Derecho Page) states if the swath of wind damage extends for more than 250 miles (about 400 kilometers), includes wind gusts of at least 58 mph (93 km/h) along most of its length, and also includes several, wellseparated 75 mph (121 km/h) or greater gusts, then the event Figure 1. Storm Prediction Center (SPC) plots of storm reports by weather type. Courtesy of the SPC. may be classified as a derecho. This events wind damage was based almost entirely on wind damage along the path. So though it met the length requirements at the surface wind measurements were not available to fully qualify this event as a derecho. However, 0.5 degree velocity data on radar did indicate at lower elevations in the damaging wind swath in central Pennsylvania Doppler winds were on the order of 50 to 70 kts (57 to 80 mph). Thus we qualify this event as a low-end derecho for the purists who want to see surface observed winds of 58 to 75 mph. This short paper will document the low-end derecho of 19 August This event was confined mainly to Pennsylvania similar to the event of 29 June 2012 which was overshadowed by the

2 super derecho event of 30 June 2012 (Guastini and Bosart 2016 see Fig. 1; SPC review) which affected the Washington, DC metropolitan area and produced in excess of 2.9 billion in damages 2. Methods The pattern was reconstructed using the Climate Forecast System reanalysis data and the GFS 00-hour analysis. These data were displayed in GrADS. Radar data were displayed using GR2Analyst. Forecasts from the NCAR ensemble were retrieved from the NCAR 3km website. 3. Pattern The large scale pattern at 1800 UTC 19 August (Fig. 2) showed strong short-wave moving across the Great Lakes associated with a secondary cold front moving into the eastern United States. The precipitable water values were in the 30 to 35 mm range head of the front in western and central Pennsylvania. A broader frontal system had moved Figure 2. CFS analysis of the pattern at 1800 UTC 19 August 2017 showing the a) 500 hpa heights and anomalies, b) the 850 hpa temperatures, c) the precipitable water, and the d) 850 hpa winds and V-wind anomalies. Anomalies are in standard deviations from normal. through the region and moved the deep moisture well to the east. That frontal system was still present just off the East Coast (Fig. 2b-c). The trough and front were over eastern Pennsylvania by 0000 UTC 20 August (Fig. 3).

3 The fast moving line of storms produced heavy rainfall in eastern Ohio (Fig. 4) and stripe of 4 to 12 mm of rainfall across central Pennsylvania. Rainfall amounts to the east were lower as the derecho moved relatively fast and did not have deep moisture to tap into. It is clear that severe weather was focused relatively close to the axis of heavier rainfall in central Pennsylvania. 4. Forecasts Figure 3. As in Figure 2 except valid at 0000 UTC 20 August The NCEP HRRR and the NCAR 3km EFS each forecast a line of convection to move across Pennsylvania. The NCEP HRRR indicated a cold pool in the 2m temperature (Fig. 5) as the area of convection and attendant rain and enhanced radar moved across Pennsylvania (Fig. 5). The NCAR 3km EFS 40 dbz probabilities are shown in Figure 6. The two images show the early organization in the model of the cluster around 1800 UTC and the forecast peak intensity at 2200 UTC over central Pennsylvania. Relative to radar (not shown) these were quite successful forecasts. A sounding from the NCAR EFS at 2000 and 2100 UTC (Fig. 7) at a point near State College showed a very steep low to mid-level lapse rate and dry air below about 850 hpa. The latter likely favored the development of a cold pool while the former favored convective development. It is interesting to note that the 2100 UTC sounding was shortly after most members generated convection which clearly caused the sounding profiles to diverge showing the growing uncertainty as the convection developed.

4 The hodograph on both sounds was very linear which favored a QLCS and a relatively fast moving system. 5. Radar The radar imagery is from the central Pennsylvania radar (KCCX). Similar features were evident to the west the Pittsburg radar (KPBZ). The radar image at 2100 UTC shows (Fig. 8) the multi-cellular line over west-central Pennsylvania at 2101 UTC. Note there are several reports of wind damage which were reported along and just behind the position of the Figure 4. Stage-IV estimated QPE (mm) for the period of 1200 UTC 19 to 0600 UTC 20 August Figure 5. HRRR 2m temperature forecasts and base reflectivity forecasts valid at 2200 UTC 22 August Note the zoomed in view allows the cold pool to be visible but makes it appear to be more frontal in nature. However a zoomed out view showed a self-contained area of cold air associated with an area of 2-12 mm of QPF.

5 line at this time. The inbound winds of 50 to 65kt appear to show the structure and organization better than the cells along the line. By 2151 UTC (Fig. 9) the line was just west of State College. The strongest inbound winds were to the west of the radar and nearly all the reports of wind damage were near the more north-south oriented cells along the line which were oriented nearly orthogonal to the low-level winds. Note farther south the cells were oriented more from southwest to northeast. By 2233 UTC (Fig 10) the line was east of the radar and the strong winds were all outbounds with peak values in the 60 kts range. The plotted wind reports showed the focus of wind damage along the track of the line as it moved to the east. The final image is from 2357 (Fig. 11) showing the line east-central Pennsylvania, just before the leading elements entered Lebanon, PA. Shortly after this time the author took picture of the outflow and shelf cloud as it moved into Lancaster, PA (not shown). 6. Summary A fast moving low-end derecho developed over Ohio and moved rapidly across eastern Ohio and Pennsylvania. With a track over 250 miles in length and wind damage all along the path this convective system likely met the minimal qualifications for a low-end derecho. However there were no reports of wind values at the surface normally associated with a derecho. The damage and the radar derived winds likely suggest this was a low end derecho. This system and its evolution were relatively well forecast by the NCEP 3km HRRR and the NCAR 3km EFS. The NCAR EFS and HRRR both were able to forecast the convection with significant lead-time. The HRRR produced a cold pool and a modest rain event (note only 1 time period was shown). The NCAR EFS showed the derecho in the 40 dbz forecasts and the forecast sounding showed the low to mid-level instability, the straight hodograph, and the ability for cooling below favoring the cold pool evolution which the HRRR and NCAR EFS (not shown) forecasts. The main impact was wind damage along the path of the system. Rainfall amounts were generally on the low side. This event is a good example of the radar signatures associated with straight line wind damage and how with the correct thermodynamics and dynamics mesoscale models can simulate and thus forecast these types of events.

6 Figure 6. NCAR 3km ensemble initialized at 0000 UTC 19 August 2017 showing the probability of 40 dbz or greater cores. Times shown are near formation time of the cluster in the forecasts in Ohio at 1800 UTC and the forecast peak organization in Pennsylvania at 2200 UTC.

7 Figure 7. NCAR 3km EFS showing the sounding near State College around 2000 and 2100 UTC 19 August The map shows the location and the hodograph is beneath the map. Return to text.

8 Figure 8. KCCX radar valid 2101 UTC data include (clockwise from upper left) the 0.5 degree reflectivity, velocity, SRM (storm motion 270 at 35), and the correlation coefficient. Return to text.

9 Figure 9. As in Figure 8 except valid at 2153 UTC. Return to text. NWS State College Case Examples

10 NWS State College Case Examples Figure 9. As in Figure 8 except valid at 2233 UTC. Return to text.

11 NWS State College Case Examples Figure 10. As in Figure 8 except valid at 2357 UTC. Return to text.