This paper will document the pattern which produced the record rainfall of 30 September The goal is to show the pattern the

Transcription

1 The Historic Synoptic-Hybrid rainfall event 30 September 2010 By Richard Grumm And Michael Kozar National Weather Service State College, PA INTRODUCTION A slow moving north-south oriented frontal boundary and surges of tropical moisture along this boundary produced heavy rainfall in the eastern United States. Rainfall amounts over 4 inches were common from Virginia into eastern Pennsylvania. A broad area of 4 to 5 inches extended from North Carolina into Pennsylvania with embedded areas of over 10 inches of rainfall (Fig. 1). Maximum estimated rainfall values over 20 inches were reported in North Carolina. Based on 24-hour rainfall amounts from the Unified Precipitation data set (Table 1) this event will likely be one of the top 24-hour rainfall events since North-south frontal systems with surges of high precipitable water air in the warm sector typify the Maddox Synoptic heavy rainfall events (Maddox et al. 1979). This event met this criteria. The interaction of this event type has historically produced some of the most significant heavy rainfall events in the Mid-Atlantic region. This record events, the interaction of Maddox- Synoptic Pattern with a tropical system is termed a Synoptic-Tropical event. This is simply a hybrid of the more generalized Maddox-Synoptic Event type. This event was associated with a plume of deep moisture of tropical origins. This atmospheric river (AR: Ralph et al. 2006) was associated with significant precipitable water standardized anomalies (Hart and Grumm 2001). Precipitable water anomalies on the order of 2 to 4σ above normal have been shown to be good predictors of heavy rainfall events. Combination of high PW anomalies and anomalous southerly winds is a key ingredient in many synoptic scale heavy rainfall events (Grumm and Hart 2001; Graham and Grumm 2010 and Junker et al 2009). It will be shown that during this event 850 hpa wind and v-wind anomalies were on the order of 6σ above normal during this event. It will be shown that the large scale pattern along the East Coast was relatively well predicted for this event. Additionally, severe NCEP model and ensemble runs predicted PW values to exceed 60 mm with the potential for 850 hpa winds to 6σ reach above normal. With the large scale pattern relatively well predicted, the models and ensembles predicted heavy rainfall. Many SREF and GEFS cycles should a high potential for over 100 mm (4 inches) of rainfall from North Carolina into New England. High probabilities of 100 mm or more QPF are not common occurrences in ensemble forecasts systems (EFS). An aspect of this rainfall event of interest is the interaction the remnant circulation of short-lived tropical storm Nicole. The surge of tropical moisture up and ahead of the general north-south frontal boundary were critical in the heavy rainfall. This paper will document the pattern which produced the record rainfall of 30 September The goal is to show the pattern the

2 forecasts of this pattern. The forecasts are used to relate back to the high probability outcome which was for widespread heavy rainfall. A connection is made showing the values of the pattern and its rarity and the confidence in the outcome, in this instance the high probability of heavy rainfall. 2. METHOD The large and regional scale patterns were produced from the 500 hpa heights, 850 hpa winds, precipitable water and other fields level fields were derived from the National Centers for Environmental Prediction Global Forecast System (NCEP GFS) 00-hour forecasts. The means and standard deviations used to compute the standardized anomalies were derived from the NCEP/NCAR Reanalysis data as described by Hart and Grumm (2001). Anomalies were displayed in standard deviations from normal, as standardized anomalies. All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies are computed as: SD = (F M)/σ (1) Where F is the value from the reanalysis data at each grid point, M is the 21-day running mean for the specified date and time at each grid point and σ is the value of 1 standard deviation at each grid point. A similar methodology was applied to produce anomalies of key forecast fields from the NCEP models and the NCEP EFSs. Additionally, the anomalies were binned to Figure 1. Q2 precipitation analysis over the Mid-Atlantic region. Upper panel is a 2-day total ending at 1100 UTC 1 October 2010 and the lower panels of the last 24 hours ending 2300 UTC 30 September Temporary image until event ends. compute probabilities of exceedance of these fields to show potentially high probability outcomes of significant anomalies. The ensemble data, mainly the quantitative precipitation fields, was used to extract probabilities of exceedance of key

3 thresholds. In this unique event, 100 mm (4 inch) probabilities were exceeded by most EFS members and led to a rare event with high probabilities of over 100 mm of QPF. Rainfall data taken from the National Mosaic and Multi-sensor QPE site 1. These provided a first guess of QPF. The Stage-IV data were used to produce regionalized maps of the event. The datasets are derived differently and thus produce slightly different results. No attempt is made herein to quantify which is a better analysis. Perhaps an ensemble of the two might provide the best set of values. The ranking of the top 20 rainfall events in Table 1 was based on the UPD data from 1 January September The maximum rainfall in these data over the 24 hour rainfall ending at 1200 UTC daily were used to find these data. These data are heavily biased toward COOP data, but they represent the longest running continuous record from which these records can be easily estimated. This event will clearly be in the top 10. The classification of events in Table 1 was conducted using the re-analysis data and determining the frontal orientation and winds. The comments were removed, but most of the tropical storms were named storms. The classic Maddox classification was used. However Synoptic events which interacted with tropical systems were termed hybrids. Where applicable the tropical storm name was identified. Since of the top 56 heavy rainfall were associated with named tropical storms. 1 No formal references yet. The link is 3. RESULTS i. Large scale pattern Figures 2-4 show the large scale pattern over the United States from 29/1200 UTC through 01/1200 UTC (not all in yet). The 500 hpa height shows (Fig. 2) an amplified trough over the central United States with large subtropical ridges to the east and west of this feature. The subtropical ridge over the Atlantic combined with the deep trough produced deep southerly flow (Fig. 3) from the eastern Caribbean to New England. Height anomalies in the ridges were on the order of 1σ with -4 to -5 σ values in the trough. The largest anomalies were near the developing TD#16 which became tropical storm Nicole (Fig. 4). The mean sea level pressure field in Figure 4 shows the anomalous surface pressures with tropical storm Nicole. Most of the rain on 30 September was observed well north of this circulation along the inverted trough which extends down the East Coast. The 250 hpa winds showed strong southerly flow with total wind anomalies peaking in the 3 to 4 σ range. The strong low-level flow will be shown in the next section along with the plume of high PW air that was advected northward with this system. ii. Regional pattern The plume of high PW air or atmospheric river associated with the heavy rainfall is shown in Figure 5. These data show a northward surge of high PW air into the Mid-Atlantic region around 30/0000 UTC (Fig. 5c) which increased rapidly on 30 September (Fig. 5c-e). The raw PW values in the GFS were on the order of 60 mm and anomalies reached 3 to 4 σ above normal. It will be shown that above normal PW values and anomalies were predicted several days in advance. The 850 hpa winds (Fig 6) and v-wind (Fig. 7) show an atypically strong low-level jet (LLJ). Wind anomalies with the surge of high PW air exceed 6 σ above normal. The surge of anomalously strong low-level winds was closely

4 coincident with both the area and timing of heavy rainfall. It should be noted 6 σ wind anomalies outside of tropical storms are extremely rare events. It will be shown that these 6 σ winds were predicted by the NCEP models and EFS. iii. Ensemble pattern Forecasters like patterns and use pattern recognition often to forecast the weather. In this section several forecasts of key heavy rainfall patterns are presented. The ensemble mean used here to limit the images shown. The data is limited to forecasts valid at 30/1200 UTC for the pattern as it was a period of heavy rainfall. The evolution at other times was equally impressive but prohibitive to display. Figure 8 shows 9 GEFS runs of PW anomalies from the GEFS. The earliest forecast is from 24/1200 UTC. The longer range forecasts (Figs. 8a-b) implied that the plume of moisture might just miss the coast. No probabilities are shown so the western limb is not truly known. The shorter range forecasts all keyed in on a plume of high PW coming into the southeastern United States near North Carolina. The potential of a synoptic rain event type is evident in these data. The 850 hpa winds are not shown, but they indicated strong southerly jet with shorter range forecasts showing 3 to 5SD above normal 850 hpa winds from forecast initialized after 25/1800 UTC. Figure 9 shows the 850 hpa winds and wind anomalies from a second set of GEFS forecasts all valid at 30/1200 UTC. The earliest forecast here is from 25/1200 UTC. They all show a strong LLJ aimed at the southeastern United States. The 2-3SD anomalies at 25/1200 UTC (Fig 9a) may reflect uncertainty issues limiting the maximum anomalies. This effect is mitigated at shorter ranges where uncertainty is smaller and thus anomalies are larger. The 5 to 6SD anomalies in the last 3 forecasts suggests a very strong LLJ. The PW plume or AR tightened up and the axis shifted to the west as shown in Figure 10. The surge of 2 to 4SD PW air into North Carolina and the Mid-Atlantic region and the potential for PW values above 60 mm indicated a potentially significant rainfall event. Combined with 5-6SD wind anomalies, this event had some potential to be a record maker. As the forecast range decreased, the GEFS forecasts converged and produced ever larger anomalies. For brevity at the shorter ranges we can pare down to the NCEP SREF with 32km resolution. The PW forecasts from 28/0300 through 30/0300 UTC are shown in Figure 11. The overall pattern and anomalies are similar to those in the GEFS, similar to the GEFS, there is a hint of an increase on anomaly values over time as uncertainty diminishes. This effect is better illustrated in the 850 hpa winds and wind anomalies (Fig. 12). The low-level jet was forecast to be over 5SDs above normal. iv. Ensemble Probabilities of rainfall In the previous section it was shown that the ensemble forecast systems got the generalize pattern correct, to include very anomalous winds and PW values. With the correct pattern, the EFSs showed some large QPF amounts. The precipitation will be displayed using the ensemble mean and select probabilistic displays. The latter are a better mean to display the data but produce far too many images. Figure 13 shows the GEFS forecast of ensemble mean accumulated rainfall from the pattern shown in Figures 9 & 10. This event came on the heels of an earlier event which contributes to some of the QPF amounts in the earlier images. But the clear signal in these data is a north-south oriented band of heavy rainfall suggesting a band of 96 to 128 mm (4 to 5 inches) of QPF. Shorter range forecasts showed a similar pattern (Fig. 14) but retained the same general concept. It is notable that there was some shifting of the axis and location of the heavy rainfall axis. Thus why probabilities are necessary. Figure 15 shows 9 SREF forecast of mean accumulated QPF valid at 01/0000 UTC. The SREF generally showed lower QPF amounts than the GEFS. This was true of all cycles and is not a unique aspect of these data. Probabilistic displays of the GEFS QPF from 27/0000 and 28/0000 UTC are shown in Figures

5 These data provide 4 time thresholds and the probability of exceedance. Figure 16 shows lower probabilities than Figure 17 due to forecast length and uncertainty issues. The mean 4 inch contour is evident in both forecasts for longer accumulation intervals. In the shorter range forecast, the convergence of forecasts produces a larger area covered by the 4 inch (100 mm) contour and increase the probabilities of exceedance A similar SREF image from 29/0300 UTC is shown in Figure 18. These data show slightly lower values and probabilities than produced by the GEFS. Figure 19 shows a more traditional QPF display for a 48 hour period ending at 01/1200 UTC. In this context the SREF had bot a larger area and higher probability of 4 inches in 48 hours relative to the GEFS. In the lower panels the individual 4 inch contours by member in the SREF show slower and faster members relative to the GEFS and a smaller region covered by each closed 4 inch contour 2. In this case the coarse resolution of the GEFS may have produced more QPF over a larger area. 4. CONCLUSIONS A meteorologically and climatologically rainfall event impacted the eastern United States on 30 Setember This event will likely be an historic rainfall event too. The rainfall in some locations began on 29 September and lingered in the northeastern United States into 1 October But the bulk of the rainfall in the Mid- Atlantic region fell on 30 September. For climatological purposes, the event will appear to have occurred on 1 October due to the method of recording 24 hour rainfall for the period ending at 1200 UTC the following day. the NCEP ensemble forecast systems. No explanations as to why this event was relatively well predicted are tendered. It is also acknowledged that as successful as these forecasts were, the details as to where the heaviest rain would fall and when were not as well predicted and may be beyond the state of NWP to accurately do so. But overall the ensembles got the pattern and general threat areas correct. These data suggest that the ensembles correctly predicted the pattern conducive for a synoptic heavy rainfall event. And the forecasts showed a surge of tropical energy into the favorable system. A good indication of heavy rainfall. Combined with the knowledge that most of the top 20 heaviest rainfall events in the Mid Atlantic region are Synoptic-hybrid events, this event had record rainfall written all over it. Based on the pattern, with above normal PW values and extremely anomalous 850 hpa winds, it is not surprising that both the NCEP GEFS and SREF showed the potential for extremely heavy rainfall. Get the pattern and the potential is clear that the probabilities will likely be a reliable first guess. The axis of heavy rainfall in the GEFS and SREF was aligned with the plume of high PW and in close proximity to the strong low-level jet. The axis and exact area affected by the heavy rainfall showed some run-to-run variations, attributed to model and initial condition uncertainty. This event, in terms of impacts and the region threatened by heavy rainfall was relatively well predicted by the NCEP models (not shown) and 2 Viewed on smaller projections like State scales this is even more apparent. The SREF pays for resolution and details.

6 Figure 2. GFS 00-hour forcecasts of 500 hpa heights (m) and height anomalies valid at a) 1200 UTC 29 September, b) 1800 UTC 29 September, c) 0000 UTC 30 September, d) 0600 UTC 30 September, e) 1200 UTC 30 September, f) 1800 UTC 30 September, g) 0000 UTC 01 October, h) 0600 UTC 01 October and i) 1200 UTC 01 October Return to text. Clearly, the accumulated mean QPF in the GEFS routinely produced higher QPF values than the SREF. This despite that fact the SREF typically produced higher overall amounts at discrete locations. Part of the reason for this involves model resolution and ensemble forecast system diversity. The higher resolution SREF pays a price of where it places the higher precipitation amounts. Additionally, with model diversity there can be significant differences between members as to where the convection or larger scale forcing produces rainfall. This can lead to a larger envelope of solutions but the averaging process can obfuscate the higher end values. The single model core GEFS lacks diversity which can contribute to higher values due to a lack of spread due more convergent forecasts. The model diversity is strength of the SREF which can cause the mean to show lower overall rainfall amounts relative to the GEFS. Figure 20 is shown to indicated how the EFS data can be leverage to define the potential of a significant event. In this image, the probability of exceedance for anomalies often associated with heavy rain are presented. These values

7 were obtained from an analysis of heavy rainfall in the Mid-Atlantic region from which the data in Table 1 were taken. This case was dominated by strong southerly winds and above normal PW values. These data show nearly all members predicted the PW anomalies to exceed 2SDs above normal and v-wind anomalies to exceed 2.5SDs above normal. A high probability of a strong AR and Maddox Synoptic heavy rainfall event. Data like this can be used to add value and confidence to ensemble QPFs. How does a forecaster use these data to produce a forecast when faced with a potential record event based on the anomalies of the pattern and the high probabilities of large QPF values? First, the patterns should provide positive feedback as to the potential of a significant event. The ensembles should then provide high probability outcomes and define the general area of the threat. Due to uncertainty issues, the specifics and details should be avoided at longer ranges. Focus should be on the threat, the significance and the generalized area. The area can be slowly trimmed down as the forecast length decreases and confidence increases. It is unlikely that the exact areas can be defined. However, the region of the high threat, the probabilities and the range of expected outcomes can and should be conveyed. The less confident (high uncertainty) the forecast is the shorter time one will have to get specific. The more confident (lower uncertainty) the forecast is the more likely longer lead-times can be provided. Though the edges on either side of a high probability axis can be problematic. In forecast extreme events with varying predictability horizons, we should know the threat 1-4 days out and the general area. This is the key phase to engage decision makers and raise awareness. The details on what actions to take where will likely be in the 1-3 day time frame. But often, taking actions to mitigate losses with mesoscale intense band of precipitation may limit specific actions to the order of hours. Being better to be prepared to act on short notice is the achievable goal. 5. Acknowledgements We would like to thank the NWS SCEP program. The precipitation typing project was part of the local SCEP training program. 6. References Graham, Randall A., Richard H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16, Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp Maddox, R. A., C. F. Chappell, L. R. Hoxit, 1979: Synoptic and Meso-α Scale Aspects of Flash Flood Events1. Bull. Amer. Meteor. Soc., 60, Ralph, F. M., G. A. Wick, S. I. Gutman, M. Dettinger, C. R. Cayan, and A. B. White, 2006: Flooding on California s Russian River: The role of atmospheric rivers. Geophys.Res. Lett., 33, L13801, doi: /2006gl

8 Figure 3. As in Figure 2 except 250 hpa winds and wind anomalies. Return to text.

9 Figure 4. As in Figure 2 except mean sea level pressure (hpa) and pressure anomalies. Return to text.

10 Figure 5. As in Figure 2 except for precipitable water (mm) and anomalies focused over the Mid-Atlantic region. Return to text.

11 Figure 6. As in Figure 3 except NCEP GFS 850 hpa winds and v-wind anomalies. Return to text.

12 Figure 7. As in Figure 3 except NCEP GFS 850 hpa winds and total wind anomalies. Figure 7. GFS 00-hour forecasts of 850 hpa winds (kts) and total wind anomalies. Data (a-i) every 6-hours from 0000 UTC 27 September to 0000 UTC 29 September Return to text.

13 Heavy Rainfall Classification Start Date End Date Max Precip (mm) Classification Tropial Storm 12Z11AUG Z12AUG Tropical Connie 12Z08OCT Z09OCT Hybrid TS Tammy 12Z20AUG Z21AUG Frontal 12Z18SEP Z19SEP Tropical Isabel 12Z06OCT Z07OCT Hybrid TS Tammy 12Z14OCT Z15OCT Hybrid 12Z26SEP Z27SEP Hybrid Gloria 12Z17OCT Z18OCT Hybrid Irene 12Z16SEP Z17SEP Hybrid Floyd 12Z02MAY Z03MAY Synoptic 12Z27JUL Z28JUL Mesoscale 12Z01SEP Z02SEP Hybrid Ernesto 12Z17SEP Z18SEP Hybrid Ivan 12Z15APR Z16APR Hybrid 12Z10SEP Z11SEP Frontal 12Z28SEP Z29SEP Hybrid Jeanne 12Z05OCT Z06OCT Frontal Tropical 12Z12AUG Z13AUG Tropical Connie 12Z12NOV Z13NOV Hybrid 12Z27NOV Z27NOV Synoptic 12Z07OCT Z08OCT Hybrid TS Tammy Table 1. Largest 24-hour rain events based on the Unified Precipitation data from 1948 to The period of the rainfall, the maximum rainfall amount (mm) and event classification are shown. Tropical events were pure tropical cyclones, frontal events involved an east-west frontal boundary, Synoptic events involved a northsouth boundary, mesoscale events were unique locally heavy rainfall events. Hybrid events represent the interaction of a tropical system with a north-south oriented frontal boundary. Known tropical storms are listed. Return to text.

14 Figure 8. NCEP GEFS forecasts valid at 1200 UTC showing ensemble mean precipitable water and precipitable water anomalies from forecasts initialized at a) 1200 UTC 24 September 2010, b) 0000 UTC 25 September 2010, c) 1200 UTC 25 September 2010, d) 1800 UTC 25 September 2010, e) 0000 UTC 26 September 2010, f) 0600 UTC 26 September 2010, g) 1200 UTC 26 September 2010, h) 1800 UTC 26 September 2010, and i) 0000 UTC 27 September Return to text.

15 Figure 9. As in Figure 8 except for GEFS 850 hpa winds and wind anomalies a) 1200 UTC 25 September 2010, b) 0000 UTC 26 September 2010, c) 1200 UTC 26 September 2010, d) 1800 UTC 26 September 2010, e) 0000 UTC 27 September 2010, f) 0600 UTC 27 September 2010, g) 1200 UTC 27 September 2010, h) 1800 UTC 27 September 2010, and i) 0000 UTC 28 September Return to text.

16 Figure 10. As in Figure 9 except for GEFS precipitable water and precipitablew anomalies. Return to text.

17 Figure 11. NCEP SREF forecasts valid at 1200 UTC showing ensemble mean precipitable water and precipitable water anomalies from forecasts initialized at a) 0300 UTC 28 September 2010, b) 0900 UTC 28 September 2010, c) 1500 UTC 28 September 2010, d) 2100 UTC 28 September 2010, e) 0300 UTC 29 September 2010, f) 0900 UTC 29 September 2010, g) 1500 UTC 29 September 2010, h) 2100 UTC 29 September 2010, and i) 0300 UTC 30 September Return to text.

18 Figure 12. As in Figure 11 except for SREF 850 hpa winds and total wind anomalies. Return to text.

19 Figure 13. NCEP GEFS ensemble mean QPF (mm) valid for the period ending at 0000 UTC 01 October 2010 from forecasts initialized at a) 1200 UTC 25 September 2010, b) 0000 UTC 26 September 2010, c) 1200 UTC 26 September 2010, d) 1800 UTC 26 September 2010, e) 0000 UTC 27 September 2010, f) 0600 UTC 27 September 2010, g) 1200 UTC 27 September 2010, h) 1800 UTC 27 September 2010, and i) 0000 UTC 28 September Return to text.

20 Figure 14. NCEP GEFS ensemble mean QPF (mm) valid for the period ending at 0000 UTC 01 October 2010 from forecasts initialized at a) 1200 UTC 26 September 2010, b) 0000 UTC 27 September 2010, c) 1200 UTC 27 September 2010, d) 1800 UTC 27 September 2010, e) 0000 UTC 28 September 2010, f) 0600 UTC 28 September 2010, g) 1200 UTC 28 September 2010, h) 1800 UTC 28 September 2010, and i) 0000 UTC 29 September Return to text.

21 Figure 15. NCEP SREF ensemble mean QPF (mm) valid for the period ending at 0000 UTC 01 October 2010 from forecasts initialized at a) 1500 UTC 27 September 2010, b) 2100 UTC 27 September 2010, c) 0300 UTC 28 September 2010, d) 0900 UTC 28 September 2010, e) 1500 UTC 28 September 2010, f) 2100 UTC 28 September 2010, g) 0300 UTC 29 September 2010, h) 0900 UTC 27 September 2010, and i) 1500 UTC 29 September Return to text.

22 Figure 16. GEFS QPF probability forecasts initialized at 0000 UTC 27 September showing the probability of a) 12 mm in the 6 hours ending at 0000 UTC 1 October 2010, b) 25 mm in the 24 hours ending at at 0000 UTC 1 October 2010, c) 35 mm in the 30 hours ending at at 0000 UTC 1 October 2010, and d) 50 mm in the 36 hours ending at at 0000 UTC 1 October Probabilities in percent and the mean 4 inch contour is shown. Return to text.

23 Figure 17. GEFS QPF probability forecasts initialized at 0000 UTC 28 September showing the probability of a) 12 mm in the 6 hours ending at 0000 UTC 1 October 2010, b) 25 mm in the 24 hours ending at at 0000 UTC 1 October 2010, c) 35 mm in the 30 hours ending at at 0000 UTC 1 October 2010, and d) 50 mm in the 36 hours ending at at 0000 UTC 1 October Probabilities in percent and the mean 4 inch contour is shown. Return to text.

24 Figure 18. SREF QPF probability forecasts initialized at 0300 UTC 29 September showing the probability of a) 12 mm in the 6 hours ending at 0000 UTC 1 October 2010, b) 25 mm in the 24 hours ending at at 0000 UTC 1 October 2010, c) 35 mm in the 30 hours ending at at 0000 UTC 1 October 2010, and d) 50 mm in the 36 hours ending at at 0000 UTC 1 October Probabilities in percent and the mean is plot in powers of 2 contours beginning with the 4 mm contour. Return to text. Return to text.

25 Figure hour accumulated QPF by the SREF 0300 UTC 29 September cycle and the 0000 UTC 29 September GEFS cycle. Upper panels show the probability of exceeding 4 inches in 48 hours and each systems 4 inch contour. Lower panels show the ensemble mean QPF (shaded) and each members 4 inch contour (contours) and the ensemble mean contour (thick black). Return to text. Return to text.

26 Figure 20. NCEP GEFS forecasts of exceedance of anomaly values from the GEFS initialized at 0000 UTC 28 September 2010 showing a) 850 hpa u-wind anomalies less than -2.5SDs below normal, b) 850 hpa v-wind anomalies greate rhan 2.5SDs above normal, c) PW anomalies in excess of 2SDs above normal and d) mean sea-level pressure anomalies less than -1.5SDs below normal. Return to text.