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Some Behaviors of CPC Monthly Precipitation Forecasts In Nebraska

Some Behaviors of CPC Monthly Precipitation Forecasts In Nebraska. James McCormick University of Nebraska-Lincoln School of Natural Resources August 16, 2007. Outline. Purpose Definitions Methodology Accuracy Physical Explanations Conclusions Future Work Acknowledgements References.

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Some Behaviors of CPC Monthly Precipitation Forecasts In Nebraska

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  1. Some Behaviors of CPC Monthly Precipitation Forecasts In Nebraska James McCormick University of Nebraska-Lincoln School of Natural Resources August 16, 2007

  2. Outline • Purpose • Definitions • Methodology • Accuracy • Physical Explanations • Conclusions • Future Work • Acknowledgements • References

  3. Purpose • To examine tendencies and accuracy of monthly precipitation products in Nebraska • Discover physical reasoning for ‘missed’ forecasts

  4. Definitions • Time Period: October 1995 – November 2006 • Monthly Precipitation Outlook • Above, Below, Near Normal, Equal Chances

  5. Methodology • 9 Stations In Nebraska Chosen • Ainsworth, Broken Bow, Chadron, Hastings, Lincoln, McCook, Norfolk, North Platte, Omaha • Values Collected By Reading Maps – ‘The Average User’ • Rainfall Data Taken From High Plains Regional Climate Center

  6. Methodology (cont.) • Use 30-Year Data To Create Verification Thresholds • Find the Median Value For Each 30 Year Period • Divide Each 30 Year Period Into Thirds

  7. Methodology (cont.) • Assign forecasts values of ‘1’ for above normal, ‘0’ for near normal or equal chances, and ‘-1’ for below normal • For this binomial analysis, assign all precipitation values (with 2 exceptions) values of ‘1’ for above 30-year median, and values of ‘-1’ for below 30-year median • Subtract observation value from forecast value; take absolute value • Values of 0 are considered accurate forecasts; values of 2 considered poor forecasts

  8. Verification Issues… • How does the generic user see the product? • How does the meteorological user see the product? • What to do with the ‘Near Normal’ forecast product, which has never been used in monthly precipitation forecasts for Nebraska? • How to deal with ‘Equal Chance’ Forecasts

  9. Point Verification • Advantages • Use of rain gauge data • Precipitation measurement as opposed • Disadvantages • Individual points do not necessarily represent atmospheric behavior over larger spaces

  10. Binomal Verification • Ainsworth: 32 Forecasts – (20A/12B) • Broken Bow: 31 Forecasts – (21A/10B) • Chadron: 29 Forecasts – (21A/8B) • Hastings: 29 Forecasts – (19A/10B) • Lincoln: 32 Forecasts – (21A/11B) • McCook: 28 Forecasts – (20A/8B) • Norfolk: 32 Forecasts – (21A/11B) • North Platte: 28 Forecasts – (19A/9B) • Omaha: 33 Forecasts – (21A/12B) • Scottsbluff:

  11. Binomial Verification Results(for above/below forecasts) • Ainsworth: 18/32 Accurate (56.25%) • Broken Bow: 8/30 Accurate (26.67%) • Chadron: 20/29 Accurate (68.97%) • Hastings: 15/29 Accurate (51.72%) • Lincoln: 15/32 Accurate (46.88%) • McCook: 13/28 Accurate (53.57%) • Norfolk: 14/32 Accurate (43.75%) • North Platte: 19/28 Accurate (67.86%) • Omaha: 14/33 Accurate (42.42%) • Overall: 136/273 Accurate (49.618%)

  12. # Of Errors Ainsworth: (9/5) Broken Bow: (13/10) Chadron: (7/2) Hastings: (8/6) Lincoln: (9/8) McCook: (10/5) Norfolk: (11/7) North Platte: (6/3) Omaha: (8/11) Scottsbluff: # Of Forecasts Ainsworth: (18/15) Broken Bow: (17/13) Chadron: (19/10) Hastings: (18/11) Lincoln: (18/14) McCook: (18/10) Norfolk: (17/11) North Platte: (16/12) Omaha: (18/15) Scottsbluff: Binomial Results(pre-June 2001 vs post-June 2001)

  13. Regional Verification • National Weather Service Precipitation Analysis • Examined in Cases of Missed Precipitation Forecasts

  14. Higher Confidences… • Current standards of confidence implemented in early 2003 • 1st Level: 33-40% confidence of above/below normal conditions • 2nd Level: 40-49% confidence of above/below normal conditions • 3rd Level: 50-59% confidence of above/below normal conditions • And so on…

  15. Higher Confidences (Cont.) • Since the confidence levels were implemented, none of the selected Nebraska stations have been placed in a confidence level of 50% or greater, for above or below normal forecasts

  16. Higher Confidence Results • Ainsworth: (0/2) • Broken Bow: (0/2) • Chadron: (0/1) • Hastings: (0/2) • Lincoln: (0/1) • McCook: (1/1) • Norfolk: (0/2) • North Platte: (0/2) • Omaha: (1/2) • OVERALL: (2/15) – 13.333% Accuracy!!

  17. Forecast Normal Values (in) Ainsworth Lower Third: 2.495 Median: 3.19 Upper Third: 3.64 May 2006

  18. May 2006 • Results

  19. What Happened? • 28 Days Produced .74 inches of rain… while 2 days produced 4.42 inches of rain

  20. December 2006 • Forecast

  21. Results

  22. Pre-December 15th Ainsworth: 0.00 Broken Bow: 0.00 Chadron: 0.00 Hastings: 0.00 Lincoln: 0.00 McCook: 0.00 Norfolk: 0.07 North Platte: 0.00 Omaha: 0.11 Total (in) Ainsworth: 1.81 Broken Bow: 2.16 Chadron: .37 Hastings: 2.48 Lincoln: 3.05 McCook: 4.08 Norfolk: 2.62 North Platte: 2.56 Omaha: 2.25 Precipitation Totals(Inches)

  23. Total (in) Ainsworth: 1.81 Broken Bow: 2.16 Chadron: .37 Hastings: 2.48 Lincoln: 3.05 McCook: 4.08 Norfolk: 2.62 North Platte: 2.56 Omaha: 2.25 30-Year High (in) Ainsworth: 1.36 Broken Bow: 1.24 Chadron: 1.89 Hastings: 2.81 Lincoln: 3.41 McCook: 2.1 Norfolk: 2.25 North Platte: 1.22 Omaha: 5.42 December 2006 (continued)

  24. Physically… What happened? - Two synoptic weather systems affected the central plains within the last 10 days of the month

  25. Conclusions • No significant increase in forecast aggressiveness – equal chance forecasts remain much more common in Nebraska than forecasts of either above or below normal • Overall trends suggest that even fewer above/below normal forecasts are being issued currently compared to the late 1990s • Current monthly precipitation forecast confidence levels are fairly conservative in relation to other CPC products, especially those of equal or shorter forecast duration • Forecasts of above normal precipitation are more common during the last 10 years • This result may suggest that forecasting ‘below’ normal precipitation is inherently more difficult than forecasting ‘above’ normal precipitation • The forecasts of higher confidence do not show a significant tendency to be accurate

  26. Conclusions • Precipitation forecasts for monthly and seasonal values, at one single point, are frequently at the whim of the mesoscale behavior of thunderstorms… especially in spring and summer months • This behavior is unpredictable at short ranges… much less 15-20 days in advance • While winter weather patterns may seemingly be more predictable (synoptically driven), there is less ‘margin for error’ • The difference between the top 1/3 and bottom 1/3 of the 30 year normals is less than .25 inches of precipitation in many cases • This behavior is unpredictable at short ranges… much less 15-20 days in advance

  27. Conclusions (Cont.) • Short term weather patterns, even for the majority of the month, do not, in all cases, indicate the precipitation tendency (as evidenced by December 2006)

  28. Concurrent/Future Work • Forecast Analysis for Monthly Temperature and Seasonal Forecasts • Forecast Analysis for 5 Other Regions of the Nation • Continued, in-depth physical exploration for missed and accurate forecasts • Forecast Analysis of Short Term Forecasts

  29. Acknowledgements • Dr. Steve Hu, School of Natural Resources, University of Nebraska-Lincoln • Donald Van Dyke and Clark Evans, Department of Meteorology, Florida State University • Nich Smith, Computer Support

  30. Sources • Climate Prediction Center, http://www.cpc.noaa.gov • High Plains Regional Climate Center, http://www.hprcc.unl.edu • National Weather Service Precipitation Analysis, http://www.srh.noaa.gov/rfcshare/precip_analysis_new.php • Storm Prediction Center Image Archive

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