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Updated Thunderstorm Probability Forecast Guidance in Support of Gridded Model Output Statistics

Updated Thunderstorm Probability Forecast Guidance in Support of Gridded Model Output Statistics. NWA Annual Meeting, Cleveland, OH October 16, 2006 E-mail: Kathryn.Gilbert@noaa.gov. Current State of Ensemble MOS.

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Updated Thunderstorm Probability Forecast Guidance in Support of Gridded Model Output Statistics

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  1. Updated Thunderstorm Probability Forecast Guidance in Support of Gridded Model Output Statistics NWA Annual Meeting, Cleveland, OH October 16, 2006 E-mail: Kathryn.Gilbert@noaa.gov

  2. Current State of Ensemble MOS • MOS equations are applied to all 15 members of the GFS ensemble and the operational run to create 16 text messages • Local applications like getENS generate summary values • The current gridded MOS is an example of a lagged ensemble. • Temperature, dew point, max and min temperature grids are created from two runs, weighted 2:1 • Future ensemble MOS requirements are vague Image from PSU web site

  3. Enhanced-Resolution MOSSupport for the operational forecasters Two Approaches • “True” Gridded MOS • Observations and forecasts valid on fine grid • Uses remotely-sensed predictand data • cloud-to-ground lightning data • “MOS at any point” • Emphasis on high-density surface networks • Uses high-resolution geophysical data

  4. Remotely-Sensed Lightning Data • NLDN cloud-to-ground lightning observations define a thunderstorm event • Observations are the predictand • Monthly lightning climatologies were developed from ten years of data for 3-, 6-, 12-, and 24-h time periods, to be used as predictors • Observed lightning data are used to verify the thunderstorm guidance

  5. = thunderstorm = no thunderstorm Creating the Gridded Lightning Datasets from ObservationsStrikes are summed over the appropriate time period and assigned to the center of the grid boxes

  6. Probabilities for Different Grid Resolutions 40-km 21-24h period 10% contour interval 20-km 21-24h period 10% contour interval

  7. Probabilities for Different Time Periods As the resolution increases in time and space, the magnitude of the probabilities will decrease as the likelihood of an event at an exact time and point in space approaches zero

  8. MOS Thunderstorm Probability Guidance • Given that: • The area covered by the forecast should be at least as large as the event • Thunderstorm forecast probabilities traditionally represent an area within 25 miles of a point, and • We don’t want the probabilities to become so small they lose information and credibility… • The gridded MOS thunderstorm probability is defined as the probability of one or more cloud-to-ground lightning strikes in a 40-km square grid box during a given time period. • So: • The guidance is interpolated to the 5-km NDFD grid using bilinear interpolation • It represents the likelihood of a cloud-to-ground lightning strike 40-km around each 5-km grid box, and • Stations have the value of the 40-km grid box they are sitting in, using nearest neighbor assignments, to fill in the text messages

  9. Thunderstorm Equation Development • Equations developed from 5 years of GFS model output • All 40-km “stations” were combined into one region • increased sample size and eliminated boundaries • Separate equations were developed for 3 seasons • Spring: March 16 – June 30 • Summer: July 1 – October 15 • Cool: October 16 – March 15 • 3-h time periods through day 3; 6- and 12-h time periods through day 7. • Important Predictors • Stability Indices • Convective precipitation amount • Product of vertical velocity * relative humidity • K-index * thunderstorm relative frequency • Thunderstorm relative frequencies

  10. Gridded MOS Thunderstorm Products • NDGD web-based graphics • Distributed in GRIB2 format • 3-,6-, 12-h forecast periods • Available in AWIPS with OB7.1 release, as part of the gridded MOS system

  11. “Enhanced-Resolution” MOS • “True” gridded MOS • Uses remotely-sensed data, i.e. the probability of a thunderstorm • “MOS at any point” • Emphasis on high-density surface networks; Co-Op, buoy, mesonet, etc • Uses high-resolution geophysical data • Analyze forecasts on high-resolution grid • Traditional MOS downscales the model output to specific sites • Examples of weather elements • temperature • dew point temperature • max/min temperature • wind speed and direction • relative humidity • probability of precipitation (6- and 12-h periods)

  12. Gridded MOS ObjectiveGenerate guidance sufficient for forecast initialization at WFOs

  13. Forecaster Feedback is InvaluableGridded MOS wind speed problem • Strange dots • Bad “if test” in a wind post-processing subroutine created calm winds, when the direction was near 0. • Too many calm mountain winds • A control file setting caused excessive calm winds over the mountains 20060921 1200 UTC 21 hour forecast

  14. Forecaster Feedback is InvaluableThank you The subroutine and control files have been fixed and implemented 20060921 1200 UTC 21 hour forecast 20060921 1200 UTC 21 hour forecast

  15. Summary • Gridded MOS thunderstorm probabilities are valid over a 40-km area, for each 5-km NDFD grid box • Gridded MOS is a work in progress • more elements are coming • improvements to current algorithms and equations expected • increased resolution from 5-km to 2.5-km • User feedback is critical to define requirements and improve the products • Alaska and other OCONUS areas are in the early stages of development Link to documentation and products http://www.weather.gov/mdl/synop/gmos.html

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