Heavy Rainfall Event of June 2013

Transcription

1 Heavy Rainfall Event of June 2013 By Richard H. Grumm National Weather Service State College, PA 1. Overview A 500 hpa short-wave moved over the eastern United States (Fig. 1) brought a surge of moisture (Fig. 2) into the Mid-Atlantic region and areas of locally heavy rain to both the Mid-Atlantic region and coastal regions of the Northeast on June 2013 (Fig. 3). The 850 hpa cyclone (Fig. 4) tracked from northern Illinois (Fig. 4a) across the Great Lakes and New York (Fig. 4a-f) with strong easterly flow in the relatively cold air north of the surface cyclone (Fig. 5). The surge of rainfall in the Mid-Atlantic region (Fig. 6) occurred in the warm sector east of the Figure 1. CFSV2 data showing 500 hpa heights (m) and height anomalies in 12 hour increments from a) 0000 UTC 10 June 2013 through f) 1200 UTC 12 June Return to text.

2 advancing surface cyclone (Fig. 5) and 850 hpa cyclone (Fig. 4). Most of the rain in fell between 0600 UTC 10 June through 0600 UTC 11 June (Fig. 6a-d) and was well aligned with the strong southwesterly flow (Fig. 7). Despite the time of year, the system lacked significant convective available potential energy (CAPE: Fig. 8). The rainfall in central Pennsylvania, which fell just a few hours either side off 0000 UTC (Fig. 6d) produced flooding in the State College NWS office county warning area. The rain fell well to the west of the broader heavy rainfall area along a north-south oriented line of showers and thunderstorms which developed around 2100 UTC June 2013(see inset to right in : Fig. 9). and moves slowly eastward. Hourly radar and radar rainfall data implied the heaviest rainfall fell between 2300 UTC 10 June and 0200 UTC 11 June, with 3-hourly rainfall estimated of over 75mm (3inches) in that period of time (Fig. 9). Local experience and flash flood guidance typically show that 75mm of precipitation is a good first guess for flooding over much of central Pennsylvania. Antecedent conditions can of course lower this this threshold. Figure 2. As in Figure 1 except for precipitable water (mm) from a) 1200 UTC 10 June through f) 1800 UTC 11 June Return to text.

3 This note examines the locally heavy rainfall event of June The focus is on the pattern and distribution of the rainfall. Forecasts are presented of the event to show how predictability of the event. 2. Data and Methods The large scale pattern was reconstructed using the 00- hour forecast of the Climate Forecast System version II data as first guess at the verifying pattern. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). Model data was obtained from NCEP and also analyzed with GrADS. Figure 3. Stage-IV accumulated precipitation form 1200 UTC 10 to 1200 UTC 11 June Units in mm as the color key. Return to text. Rainfall data was obtained from the Stage-IV data in 1 and 6-hour increments. The shorter duration rainfall was obtained from the National Mosaic and Multi-sensor QPE (NMQ) site. Composite reflectivity too was obtained from the NMQ site. 3. Forecasts The 16km SREF showed little accumulating QPF during the period when the heaviest rain fell in eastern Pennsylvania (Fig. 10). The probability of 25m or more QPF during the critical time covering the 9 hours (Fig. 11) within which the heavy rain fell also shows no strong signal. No members of the SREF produced 50 mm or more QPF so the 50mm was not shown. The NCEP deterministic GFS (Fig. 12) and NAM (Fig. 13) offered few strong clues to the locally heavy rainfall. The 15 hour period from 10/2100 UTC to 11/1200 UTC showed an early

4 wet GFS run with a closed 50 mm contours from forecasts issued at 09/1200 UTC (Fig. 12a) but the timing of the system and thus the QPF changed pushing the precipitation axis to the east over time. Similar to the GFS, 12km NAM (Fig. 13) initially had some residual QPF over Pennsylvania but as the timing of the overall system changed the QPF shield for values in excess of 12.5mm shifted to the north and east. Thus they offered little clue to the heavy rainfall threat over eastern Pennsylvania. The 13km RAP is updated hourly with real-time radar data. Hourly forecasts from the RAP made attempts at times to show areas of 25 to 50mm of QPF over eastern Pennsylvania (Fig. 14). These data varied markedly from run-to-run showing how sensitive the forecasts are to initial conditions. Shorter range forecasts at times offered clues and perhaps the potential for 25 to 50 mm of QPF in eastern Pennsylvania (Fig. 14a & e).. The RAP reflectivity near the time the intense rainfall was commencing is shown in Figure 15. These data showed little in common with the pattern of observed rainfall (Figs. 2 & 9) though they may have provided an indication for precipitation. 4. Summary A fast moving short-wave (Fig. 1) produced rainfall and areas of over 48mm of heavy rainfall from Maryland, across Pennsylvania and southern New England (Fig. 2). The heaviest rain, over 64mm, fell over portions of New Jersey and southern New York. A significant portion of the rainfall and the areas of heavier rainfall over Pennsylvania fell late in the event (Fig. 6) mainly between about 2300 UTC 10 June and 0300 UTC 11 June This led to some local flooding in central Pennsylvania where over a small area rainfall exceeded 75 mm (3 inches). The pattern was a pattern associated with precipitation and most of the higher precipitation amounts fell in the region affected by the 1-2.5σ above normal precipitable water plume (Fig. 2c). The PW field also showed a more north-south boundary from south-central Pennsylvania into the Carolinas which lined up well with the more north-south band of convection which affected northern Maryland and Pennsylvania between about 10/2200 and 11/0300 UTC. This sharper line with limited instability in Pennsylvania but 1200 to 2400 JKg-1 to the south produced severe weather and tornadoes from Delaware southward into the Carolinas (Fig. 16). The area affected by the severe weather was also experienced the passage of a strong low-level jet with +3σ 850 hpa v-wind anomalies (Fig. 7). The rainfall, after the main area of rain moved to the east (not shown) over central Pennsylvania, locally exceed 75mm. The SREF, GFS, and NAM clearly showed that after 2100 UTC 10 June the area where this rain fell was not an area the model atmosphere s expected heavy rainfall. Not

5 a single SREF member had predicted much more and 25mm in the affected region. The shorter range RAP showed some clues for increased rainfall potential. This system was clearly convectively driven. The relatively dry NAM and SREF suggest forcing was on a small scale as these systems tend to over predicted larger scale rainfall during the warm season. The RAP offered some clues as the rainfall potential, though the timing and location remained elusive. The RAP radar showed variable areas of potential precipitation. At best these data alerted forecasters to the potential for locally heavy rain in Maryland and eastern Pennsylvania. The poor quality of these extremely short-range forecasts in their efforts to predict precipitation raises issues about effectively using mesoscale models. At times the radar output from models is used to express where heavy rain and severe weather will be at a specific times. But this case and many others show that these forecasts are extremely uncertain and thus unreliable and provide but a glimpse of what things could or may look like. They should be used at best to heighten awareness and increase vigilance, realizing how poorly they may actually verify. 5. Acknowledgements Thanks to the Pennsylvania State University for data access, system software upgrades, and storage space. 6. 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., Figure 4. As in Figure 2 except for 850 hpa winds and u-wind anomalies. Return to text.

6 Figure 5. As in Figure 2 except for 850 hpa winds and u-wind anomalies. Return to text.

7 Figure 6. As in Figure 2 except 6-hour increments for the 6-hour periods ending at a) 1200 UTC 10 June, b) 1800 UTC 10 June, c) 0000 UTC 11 June, and d) 0600 UTC 11 June Return to text.

8 Figure 7. As in Figure 2 except for 850 hpa winds and v-wind anomalies. Return to text.

9 Figure 8. As in Figure 2 except for convective available potential energy. Return to text.

10 Figure 9. NMQ site 3-hourly estimate precipitation estimates from 0000 through 0300 UTC 11 June 2013 from gage correct radar and 1-hour composite reflectivi to right at 2300 UTC 10 June, 0000 UTC 11 June and UTC 11 June Return to text.

11 Figure 10. Return to text. NWS State College Case Examples

12 Figure 10. Return to text. NWS State College Case Examples

13 Figure 12. GFS QPF (mm) showing values of 12.5mm or greater QPF for the period of 2100 UTC 10 June through 1200 UTC 11 June 2013 from NCEP GEFS initialized at a) 1200 UTC 09 June, b) 1800 UTC 10 June, c) 0000 UTC 10 June, d) 0600 UTC 11 June, e) 0600 UTC 11 June, f) 1200 UTC 11 June, f) 1800 UTC 11 June Contours and shading as indicated in color bar and values below 12.5mm not shown. Return to text.

14 Figure 13. As in Figure 12 except for the 12km NAM. Return to text. NWS State College Case Examples

15 Figure 14. As in Figure 13 except 13km RAP valid for the 4 hour period of 2100 UTC 10 June through 0300 UTC 11 June Forecast initialized at a) 1600, b) 1700, c) 1800, d) 1900, e) 2000 and f) 2100 UTC 10 June Return to text.

16 Figure 15. As in Figure 14 except for RAP reflectivity (dbz) valid at 0000 UTC 11 June Return to text.

17 Figure 16. Storm Prediction Storm reports by type for 10 June Return to text.