Hurricane Harvey the Name says it all. by Richard H. Grumm and Charles Ross National Weather Service office State College, PA

Transcription

1 Hurricane Harvey the Name says it all by Richard H. Grumm and Charles Ross National Weather Service office State College, PA Overview Hurricane Harvey crossed the Texas coast (Fig. 1) as a category 4 storm between Port Aransas and Port O Conner around 0300 UTC 26 August 2017 near Rockport, TX (Fig. 2) were winds of 130 mph were recorded (TPC). Harvey was the first land falling category 4 storm to impact the United States since Hurricane Charley in The storm caused extensive wind damage and some damage with the storm surge. However, the enduring heavy rainfall (Fig. 3) associated with the storm will likely have the largest impact on east Texas. The storms origins likely began with an easterly wave which was tracked as early as 13 August Around 17 August the storm began to show indications in numerical guidance that the system could develop and deepen as it moved across the Gulf of Mexico. Tracking of the storm resumed on 17 August Figure 1. Track of Hurricane Harvey from the time the TPC began fully tracking the storm after it had weakened in the Caribbean. Green icons are Tropical depression, yellow are tropical storm, and red with H are hurricane points. Data through 1200 UTC 29 August 2017.

2 Figure 2. KCRP radar showing the approximate time of landfall of Hurricane Harvey near Rockport, Texas. Image time is 0259 UTC 26 August (Fig. 1). The storm was not easily recognized in the Climate Forecast System reanalysis (CFSR) until 0000 UTC 24 August (Fig. 4) when the storm was deep enough be analyzed by the CFSR. The precipitable water (PW) field and satellite data (not shown) showed a deep plume of moisture with the wave prior to 24 August. Forecasts from the NCEP GEFS (Fig. 5) and other models indicated a landfalling storm near Rockport, TX from forecast cycles initialized at 0000 UTC 22 August through 25 August. The storm intensity varied from each forecast cycle. The black dot in Figure 4 is Rockport which suggests relatively accurate forecasts of the ensemble mean position of the surface cyclone. The earliest closed cyclone on the Texas coast was produced the by the 0000 UTC 21 August 2017 GEFS (not shown). The track, timing and intensity of this storm were relatively well predicted. But the aspect of this storm which was also relatively well predicted was the heavy rainfall. This storm was forecast in deterministic models, which make the core of the ensemble forecasts systems (EFS) to produce rainfall totals in excess of 30 inches and these numbers went up as the forecast length decreased. This paper will present the pattern and anomalies associated with this historic event. The focus will be on the landfall of Harvey and the rainfall during the first 72 hours of the storm. The NCEP GEFS is the primary forecast tool used here. Other higher resolution and convective allowing guidance provided incredible guidance which will likely be discussed in subsequent research and likely at the AMS 2018 meeting.

3 Figure 3. Total estimated rainfall from 0000 UTC 26 to 1200 UTC 29 August Values in mm as in the color bar. Rainfall had not ended by the time of this image. Return to text. 2. Methods and data Radar data was retrieved from the Amazon real-time NEXRAD data archive server and displayed in GR2Analyst. The 0.5 degree velocity (V) and reflectivity data (Z) were examined. Fifteen minute loops were produced to examine the system evolution. Data from KCRP and KHGX were used to make the 15 minute 3 day loops. An example image is shown in Figure 1 near the time of landfall. The Climate Forecast Center reanalysis data was used to show the pattern and evolution of the system. These data are far to course to draw conclusions related to tropical storms and hurricanes but they do serve as a dataset of record. These data were displayed using GrADS. The Stage-IV rainfall data were used to show 1-hourly, 6-hourly, 24-hourly, and storm total estimated rainfall amounts. These data were displayed in GrADS and Python. GEFS forecast data were from the NCEP website and were plotted using GrADS. The predictability horizon diagrams were produced using the same data but were processed with Python. 3. Pattern and anomalies The large scale pattern over the United States and Gulf of Mexico from 1200 UTC 22 to 1200 UTC 27 August 2017 is shown in Figure 6. These data initially showed above normal 500 hpa heights over the

4 Figure 4. CFSR mean sea level pressure and anomalies every 24 hours from 0000 UTC 22 to 0000 UTC 27 August Isobars every 4 hpa. Anomalies as in the color bar. Return to text. Gulf of Mexico. The CFSR detected a 500 hpa low by 1200 UTC 24 August (Fig. 5c) which moved into eastern Texas. At the surface (Figure 4) high pressure over the Gulf of Mexico weakened from the 22 nd to the 23 and by the 24 th low pressure dominated much of the Gulf of Mexico and the circulation associated with Harvey was clearly identified by the CFSR. As the storm moved into Texas (Fig. 7) the CFSR analyzed a deep surface cyclone with -5σ pressure anomalies at 0000 UTC 26 August. The CFSR showed the surface circulation moving inland and then wobbling while maintaining a deep cyclone center. Though not shown, higher resolution guidance from the NAM and HRRR 00-hour forecasts showed a stronger and deeper cyclone center. The deep surface cyclone was associated with a broad plume of high PW air (Fig. 8) with PW anomalies on the order of +2 to +3σ above normal. The 850 hpa winds were also above normal with the strong surface cyclone. The 850 hpa u-wind anomalies were on the order -6 to -7 σ below normal north of the cyclone and equally anomalous south of the nearly symmetric cyclone center (Fig. 9). The deep cyclone, plume of high PW air, and strong southeasterly flow north of the cyclone center produced heavy rainfall along and north of the cyclone track on 26 August 2017 (Fig. 10a). The heavy

5 rain near Rockport was focused close to the eyewall rainbands from 0000 to 0600 UTC 1. Note the broad area of 100 mm of rainfall along and north of the storm center and the enhanced area of 150 and 200 mm or more QPF associated with bands which developed in the strong southeast flow. This area would expand over the next 48 hours. The extreme rainfall on 27 August (Fig 10b) suggested the feeder band did not move far and the inflow produced rainfall in the 500 to 600 mm rain in portions of eastern Texas south and east of Houston. This area of extreme rainfall was embedded in an impressive area of 150 to 200 mm of rainfall which extended well inland contributing to the heavy rainfall for the event as shown in Figure Forecasts and Predictability The scope of hurricane Harvey makes it difficult to find a forecast aspect to converge upon. The focus here will thus be on the landfall of Harvey and the threat for heavy rainfall and the potential resulting flooding. The forecasts from 6 GEFS cycles showing a tropical storm crossing the Texas coast are shown in Figure 5. These data show that the GEFS was quite consistent with strong cyclone coming onshore around 0000 UTC 26 August near Corpus Christi, TX. The anomalies were larger at longer range due to differences in the intensity and track of each individual GEFS forecast. At shorter ranges these differences were smaller and hence the ensemble mean produced a deeper and thus more anomalous cyclone. Though not shown, the 0000 UTC 21 August GEFS also had a land falling cyclone in the same region around the same time. From a surface cyclone perspective the GEFS provided at least 5 days of lead-time. The probability of 150 mm or more QPF from the same GEFS cycles and the ensemble mean QPF with each member 200 mm contour are shown in Figures 11 & 12. Note that all 6 forecasts produced over 150 mm or QPF over a large swath of East Texas. The heaviest rainfall was focused near and south of Houston (black dot). There was a subtle convergence of solutions for over 150 mm of QPF by 0000 UTC 23 August (Fig 11b) and the high confidence area increased in size as the forecast length decreased. The PH diagram for El Campo, TX shows (Fig. 13) the extreme rainfall forecast by the GEFS. It also shows the convergence toward an extreme rainfall event from forecasts initialized at 0000 UTC 22 June. These forecasts slowly ramped up the total QPF, with some wobbles (48 hour forecasts) as the mean QPF climbed to around 200 mm. This PH diagram suggested a high end QPF event was possible with 102 h of lead time. Confidence increased in a 100 mm event by 90 h and for over 150 mm by forecasts issued with about 60 h of lead time. These are extremely high numbers for a global ensemble. The GEFS QPF relative to the GEFS QPF climate showed that for days and nearly ALL forecast cycles, the GEFS was producing a record QPF event. The NCEP GEFS was forecasting an historic QPF within its internal model QPF. In reality, the GEFS under forecast the QPF as real atmosphere produced far more QPE than the model could generate. 5. Summary Hurricane Harvey came onshore near Rockport, TX around 0300 UTC 26 August 2017 as a category 4 storm. Winds in Rockport were around 130 mph. The winds produced extensive damage and there were numerous reports of tornadoes in the bands about the storm. Rockport 1 When examining 1 and 6 hour data this area of heavy rain stands out.

6 was the epicenter for heavy rainfall as the storm approached and then moved into Texas. As the storm moved inland rain and resulting flooding became the primary issue. The impact of the wind was significant locally but in the end the widespread damage and high impact was clearly produced by the extremely heavy rainfall and the resulting flooding. Harvey came onshore with strong winds and intense rainbands. The combination of the strong easterly flow and the deep moisture produced extreme rainfall rates and rainfall accumulations. The weak steering flow allowed the surface circulation to meander maintaining the deep moist flow into eastern Texas, northeast of the storm center. The resulting rainfall amounts were in the 200 mm (8 inches) over an expansive area of eastern Texas with embedded areas of over 700 mm (28 inches) in a 48 hour period as observed by radar. Multi-day rainfall totals will likely exceed 1250 mm (50 inches). Some aspects of this storm were well forecast to include the relative location and timing of landfall and the potential for incredible rainfall amounts. As shown here, the relatively coarse GEFS forecast landfall along the Texas coast within approximately hundred kilometers of the landfall location with at least 4 days of lead-time. The GEFS also forecast several 24 hour periods of over 200 mm of QPF (8 inches: Fig 14) of precipitation. And the M-Climate data from the GEFS indicated that for several days the GEFS was forecasting the largest amount of QPF that the forecast system had ever produced for nearly every forecast length. The GEFS predicted record rainfall with at least 4 days lead-time and forecast record rainfall to persist for several consecutive days. Given the bias in course models, these M-Climate data provided important guidance as the potentially historic nature of the event. There was considerable uncertainty with this event. The area of heavy rainfall shifted a hundred kilometers with different cycles and members and the focus of the heavy rainfall shifted as shown in the PH diagrams. But overall the GEFS clearly predicted the general region and general population centers to be affected by the heavy and the sustained heavy rainfall. Clearly, the GEFS is not the system of choice to forecast record rainfall amounts and exact locations. This is the role of convective allowing models (CAMs) and CAM based on ensembles. Surely the performance of these systems will be examined in coming conferences, workshops, with flood the AMS journals over the next 1-2 years. Here we offer an example from the NCAR 3km EFS initialized 24 hours prior to the landfall of hurricane Harvey. The total accumulated QPF and the probability of 10 inches or greater QPF 2 are shown (Fig. 15) and the probability matched mean QPF is shown in Figure 16. The probability matched means (Fig. 16) showed over 12 inches of rain in eastern Texas. The maximum values were likely over 12 inches however, the scale of the data was operationally limited to 12 inches. The ensemble maximum QPF did product (not shown) showed bands of 12 inches (350 mm) and greater QPF. The salient point here is that CAM ensembles were able to predict extreme rainfall amounts in close proximity to where extreme rainfall was observed. This case demonstrates the efficacy of CAM models and ensembles and the need for an operational high resolution CAM EFS to effectively predicted extreme weather events. 2 During the event 10 inches was the highest category available. This may change in the future as this event blew by that value.

7 6. Acknowledgments Lance Bosart for editorial comments and suggestions not all which could be reproduced here. The Albany MAP for insights and data links.

8 Figure 5. NCEP GEFS ensemble mean, mean sea-level pressure and pressure anomalies valid at 0000 UTC 26 August from forecasts initialized at a) 0000 UTC 22 August, b) 0000 UTC 23 August, c) 0000 UTC 24 August, d) 1200 UTC 24 August, e) 0000 UTC 25 August, and f) 1200 UTC 25 August Return to text. Return to forecast section.

9 Figure 6. CFSR 500 hpa pattern and 500 hpa anomalies in 24 hour increments from 1200 UTC 22 to 1200 UTC 27 August Return to text.

10 Figure 7. As in Figure 3 except CFSR MSLP every 12 hours. Return to text.

11 Figure 8. As in Figure 7 except for PW and PW anomalies every 6 hours. Return to text.

12 Figure 9. As in Figure 8 except for 850 hpa winds and 850 hpa u-wind anomalies. Return to text.

13 Figure hour accumulated rainfall from 6-hour Stage IV data for the 24 hour periods ending at a) 0000 UTC 27 and b) 0000 UTC 28 June Values in mm as in the color bar. Return to text.

14 Figure 11. GEFS QPF forecasts showing the probability of 150 mm or more QPF for the 48 hour period ending at 0000 UTC 28 August 2017 from forecasts produced at a) 0000 UTC 22 August, b) 0000 UTC 23 August, c) 0000 UTC 24 August, d) 1200 UTC 24 August, e) 0000 UTC 25 August, and f) 1200 UTC 25 August Return to text.

15 Figure 12. As in Figure 10 except the GEFS mean QPF (mm) and each members 200 mm contour if present. Return to text.

16

17 Figure 13. PH diagram for a point near El Campo, TX showing the forecast 36 hour QPF (mm) for the period ending at 1200 UTC 27 August Return to text.

18 Figure 14. GEFS QPF forecast (inches) for the 48 hour period ending at 0000 UTC 27 August Data show the mean QPF and the percentile of the QPF verse the QPF climatology. The table to the right shows the percentile for each QPF time verse the climatology by forecast length. Return to text.

19 Figure 15. NCAR 3km EFS showing the probability of 10 inches or more of QPF and the ensemble mean rainfall. Return to text.

20 Figure 16. As in Figure 14 except the probability matched mean 48-hour accumulated rainfall in inches. Return to text.