Eastern United States Wild Weather April 2014-Draft

Transcription

1 1. Overview Eastern United States Wild Weather April 2014-Draft Significant quantitative precipitation bust By Richard H. Grumm National Weather Service State College, PA and Joel Maruschak Over a 3-day period extreme weather was experienced across the United States including extremely heavy rainfall (Fig. 1) and flooding over western Florida (AP 2014) which produced over 20 inches of rainfall in portions of the Florida panhandle. The weather during the period was the result of a strong short-wave which at 500 hpa moved over the Pacific northwest on 26 April (Fig. 2a) and rapidly moved eastward. As the trough moved eastward a ridge developed over the western Atlantic allowing moisture from the Gulf of moisture to move into the central United States (Fig. 3). The high heights over eastern North America produced a cold air damming situation over the northeastern United States. The deep moisture (Fig. 3). and strong low-level winds (Fig. 4) produced a severe weather event over the eastern Plains and Mississippi Valleys on April (Fig. 5). The severe weather diminished as the system moved eastward and encountered relatively cold low-level air. The boundary between the cold air and the warm moist air produced heavy rainfall and flooding from northern Virginia into southern New York (Fig. 6). The surge of high PW air and strong winds over the frontal boundary in the northeast had the appearance of a Maddox synoptic type event (Fig. 7 & 8). This paper will focus on the two areas of rainfall which produced flooding over the eastern United States. The focus is on the rainfall and how well it was predicted with the NCEP operational forecast systems around Pensacola and Philadelphia. The Stage-IV rainfall data is used to compare the rainfall timing and amounts relative to the forecasts systems. 2. Data and Methods The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global Forecast System 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). The focus on the larger scale pattern was to show the standardized anomalies and how they may help identify potential high impact weather events (HIWE).

2 The NCEP GFS, GEFS, and SREF were used to examine the forecasts of this event. As the pattern was generally well predicted the focus is on the quantitative precipitation forecast (QPF) produced by these systems. The stage-iv data were used to get a first guess at where, when, and how much precipitation was observed. These data were analyzed in planview with GrADS and in Python to make point diagrams showing the timing of the rainfall. Python was used to plot all-time series and plume diagrams. 3. Forecasts i. Western Florida Figure 9 shows the 6-hour gridded rainfall accumulations over Florida with 454 mm of rainfall which accumulated over about 30 hours. The rainfall was associated with a slow moving mesoscale convective complex (not shown). The rain being convective in nature is often difficult for larger scale models to forecast. There is some evidence (not shown) that convective allowing models provided better forecasts than the coarser guidance. These data were not available and were not used here. The SREF plume diagrams from 0900 and 1500 UTC 28 April (Fig. 10) shortly before the heavy rains began show that the SREF grossly under predicted the rainfall over western Florida. These data suggest that the SREF did relatively well with the timing but missed the magnitude of the rainfall. The SREF at 16km likely had some issues and the 6.09 inches of rainfall may have approached the maximum rainfall in the SREF implying the need to know the systems internal climatology. Forecasts from the GFS, SREF, and GEFS (Fig. 11 &12) are presented in Plainview which all show that these NCEP systems had great difficulty predicting the heavy rainfall and in-effect were incapable of forecasting the rain as observed. The point data in the SREF suggests there was some utility in using the SREF but this would have required knowing the extreme of the models internal M-Climate. Though not shown here, the NCEP models correctly predicted the surge of high PW air, the strong low-level jet and the potential for MCS development. The focus here was not on the pattern but the QPF issues related to the extreme rainfall. ii. Mid-Atlantic region. The rapid rainfall accumulations in Baltimore, MD and Philadelphia, PA (Fig. 13) show that these two locations had heavy rainfall. To a first guess, 3 inches of rain typically will cause flooding in much of the Mid-Atlantic region though this can vary based on antecedent conditions. The plume diagrams from two SREF runs show that the predicted heavy rainfall for

3 Philadelphia, PA (Fig. 14) and Baltimore, MD (Fig. 15). The QPF error was smaller than that in Pensacola, FL likely due to the stronger forcing during this phase of the event. Comparisons between the 27 km GFS, 16km SREF and 55km GEFS (Fig 16) show that all three systems had heavy rainfall predicted in the Mid-Atlantic region. The area affected varied with forecast length but in general these systems alerted users to the potential for heavy rainfall and the errors relative the Florida case were small. Figures 17 and 18 show 6 GEFS and 6 SREF forecasts for the event in the Mid-Atlantic region. There was considerable variability with regards to the axis of heavy rainfall but the systems did show the general regions affected. As implied in the plume diagrams and in ensemble mean plots (not shown) these systems predicted local maximum which were close to the observed maximum. 4. Summary A strong trough moving across the United States during the period of 26 April through 1 May 2014 (Fig. 2) opened the Gulf of Mexico allowing a surge of high PW air into the eastern United States. This resulted in a multi-day severe weather event, an MCS which produced over 20 inches of rain over the Florida Panhandle, and heavy rain in the Mid-Atlantic region. The heavy rains produced flooding in Florida and in the Mid-Atlantic region. The overall QPF errors in the event over Florida are somewhat troubling. All forecast systems grossly under predicted the heavy rainfall. Perhaps internal model climatologies (M-Climate) data would be of value. The 6 inches produced in several SREF members may have indicated a record rainfall in the forecast system, information which could be of use to forecasters. Additionally, as this rainfall was convectively driven, intense events such as this may require convective following models. There were some indications that 3-4km models did produce as much as 15 inches of rainfall over or offshore of the Florida panhandle. The rain and flood event over the Mid-Atlantic was well predicted relative to the event over Florida. This was likely due to the stronger meso-α and synoptic forcing, conditions where the operational non-convective allowing models perform well. The plume diagrams show that the QPF error was smaller for Philadelphia (Fig. 14) and Baltimore (Fig. 15) relative to Pensacola, FL (Fig. 9) likely due to the stronger synoptic scale forcing which the models were better able to resolve. The mesoscale nature of the Florida flood event limited the utility of the NCEP operational non-convective following models. In convective situations, ensemble outliers, convective allowing models, and monitoring observations are likely the best means to predict significant convective precipitation systems.

4 5. Acknowledgements The Pennsylvania State University for data access and providing students for research and case study projects. 6. References Bodner, M. J., N. W. Junker, R. H. Grumm, and R. S. Schumacher, 2011: Comparison of atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods. Natl. Wea. Dig., 35, Doswell,C.A.,III, H.E Brooks and R.A. Maddox, 1996: Flash flood forecasting: ingredients based approach. Wea. Forecasting, 11, 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., Durkee, JD., L Campbell, K Berry, D Jordan, G Goodrich, R Mahmood, S Foster, 2012: A Synoptic Perspective of the Record 1-2 May 2010 Mid-South Heavy Precipitation Event. Bull. Amer. Meteor. Soc., 93, doi: Junker, N. W., R. S. Schneider and S. L. Fauver, 1999: Study of heavy rainfall events during the Great Midwest Flood of Wea. Forecasting, 14, Lorenz, E. N., 1963: Deterministic nonperiodic flow. J. Atmos. Sci., 20, Lorenz, E.N, 1969: The predictability of a flow that possesses many scales of motion. Tellus, 21, Lorenz,E.N, 1993: The Essence of Chaos. University of Washington Press, 227pp. Palmer, T. N., 2005: Quantum Reality, Complex Numbers, and the Meteorological Butterfly Effect. Bull. Amer. Meteor. Soc., 86, doi: Stuart, N., Grumm, R. (2009) The Use of Ensemble and Anomaly Data to Anticipate Extreme Flood Events in the Northeastern United States. Nat. Wea. Digest 33: Toth, Z, Y.Zhu,and T Marchok, 2001: On the ability of ensembles to distinguish between forecasts with small and large uncertainty. Weather and Forecasting, 16,

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6 Figure 1. Stage-IV rain estimated for the 24 hour periods ending at a) 1200 UTC 30 April and b) 1200 UTC 30 April Over Over the southern United States and zoomed over Florida. Return to text.

7 Figure 2. GFS 00-hour forecasts of 500 hpa heights (m) and height anomalies in 24 hour increments from a) 1200 UTC 26 April through f) 1200 UTC 1 Mayl Return to text.

8 Figure3. As in Figure 2 except for precipitable water (mm) and precipitable water anomalies in 6-hour increments from a) 0000 UTC 27 April through f) 1200 UTC 8 April, the time of the heaviest rainfall. Return to text.

9 Figure 4. As in Figure 3 except for 850 hpa winds (kts) and v-wind anomalies. Return to text.

10 Figure 5. Storm reports from the Storm Prediction Center from April Return to text.

11 Figure 6 As in Figure 1 except for Stag-IV data over the Mid-Atlantic region form 0000 UTC 30 April through 0000 UTC 1 May Return to text.

12 Figure 7. As in Figure 4 except for 1200 UTC April through 1800 UTC 30 April in 6-hour periods. Return to text.

13 Figure 8. As in Figure 3 except for for 1200 UTC April through 1800 UTC 30 April in 6-hour periods. Return to text.

14 Figure 9. Stage-IV point data from a point near Pensacola, FL. from 0000 UTC 28 to 1800 UTC 30 April values in mm. Return to text.

15 Figure 10. SREF accumulated precipitation from a point near Pensacola, FL from the 0900 UTC SREF and the 1500 UTC SREF. Return to text.

16 Figure 11. GFS QPF (mm), SREF and GEFS probability of 50 mm or more QPF valid for the 24 hour period ending at 1200 UTC 30 April 2014 from a) GFS initialized at 0000 UTC 27 April, b) SREF initialized at 0300 UTC 27 April, c) GEFS initialized at 0000 UTC 27 April, d) GFS initialized at 0000 UTC 28 April, e) SREF initialized at 0300 UTC 28 April, f) GEFS initialized at 0000 UTC 28 April Return to text.

17 Figure 12. As in Figure 11 except for forecast on 0600 UTC 27 and 28 April and the SREF offset to 0900 on both days. Return to text.

18 Figure 13. As in Figure 9 except for Baltimore and Philadelphia total estimated Stage-IV QPE. Return to text..

19 Figure 14. As in Figure 10 except for point near Philadelphia, PA. Return to text.

20 Figure 15. As in Figure 14 except for Baltimore, MD. Return to text.

21 Figure 16. As in Figure 12 except GFS and GEFS initialized at 0000 UTC 28 and 0000 UTC 29 April and SREF initialized at 0300 and APirl Return to text.

22 Figure 17. NECP GEFS forecast of 50 mm or more QPF for the 24 period ending at 0000 UTC May 2014 for GEFS initialized at a UTC 26 April 2014, b) 1200 UTC 27 April, c) 1200 UTC 28 April, d) UTC 29 April, e) 1200 UTC 29 April, and f) 0000 UTC 30 April Return to text.

23 Figure 18. As in Figure 19 except for SREFs from a) 0900 UTC 27 and b) 28 April, c) 2100 UTC 28 April, d) 0900 UTC 29 April, e) 1500 UTC 29 April, and f) 2100 UTC 29 April Return to text.

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25 Figure 19. Not used MCS image