1. Developing GFS-based MOS Thunderstorm Guidance for Alaska Phillip E. Shafer* and Kathryn Gilbert Meteorological Development Laboratory, NWS, NOAA http://www.nws.noaa.gov/mdl/synop/ * Contact Information: Phillip E. Shafer, Meteorological Development Lab., NWS, SSMC2, Rm. 11314, 1325 East West Highway, Silver Spring, MD 20910; email Phil.Shafer@noaa.gov Figure 1. Polar Stereographic grid used for the 48-km GFS MOS thunderstorm development. The gridpoints used in equation development are enclosed by the blue polygon. The operational forecast gridpoints are enclosed within the larger polygon. Figure 2. 12-h lightning relative frequency for July during 1800-0600 UTC. 0000, 1200 UTC GFS0600, 1800 UTC GFS Model Grid (See Fig. 1) 47.625 km, Polar Stereographic (97 x 69) 703 developmental gridpoints 2066 operational forecast gridpoints Forecast Projections 3-h forecasts out to 84 hours 6-, 12-, and 24-h forecasts out to 156 hours 3-h forecasts out to 84 hours 6-, 12-, and 24-h forecasts out to 84 hours Warm Season Sample (May 15 – Sept 15) 2001-2006 warm seasons (6-84 h) 2002-2006 warm seasons (90-156 h) 2001-2006 warm seasons (6-84 h) Cool Season Sample Cool season equations not developed Predictors Used (continuous and binary form) Height, temperature, dewpoint, theta-e, and relative humidity (RH) Layer average dewpoint, theta-e, and RH Moisture divergence, u and v, wind speed, and vertical velocity (VV) Temperature and vorticity advection Convective precip, best lifted index, CAPE, K-index (KI), Total Totals, SWEAT Lightning relative frequency (RF), sine/cosine day of the year KI x RF, RH x VV cross products Introduction Cloud-to-ground (CG) lightning is one of the leading causes of weather related fatalities in the United States (U.S.). In heavily forested areas such as Alaska, CG lightning also is a major contributor to the initiation of forest fires. Graphical, probabilistic guidance for thunderstorms over Alaska would allow the fire weather community and other users to better assess the CG lightning threat, and thereby aid in the protection of life and property. For many years, National Weather Service (NWS) forecasters have used Model Output Statistics (MOS) guidance produced by the Meteorological Development Laboratory (MDL) as an aid in generating text forecast products issued to the user community. However, forecasters now are required to produce forecasts on a high-resolution grid in support of the National Digital Forecast Database (NDFD). Recently, updated thunderstorm probability guidance based on output from the Global Forecast System (GFS) was implemented for the contiguous U.S. (CONUS) to satisfy NDFD grid requirements. This new gridded thunderstorm guidance system has now been expanded to cover the state of Alaska. Equations giving the probability of a thunderstorm at 3-, 6-, 12-, and 24-h intervals have been developed on a 48-km grid for each GFS model cycle. The equations were developed for the warm season (May 1 – September 30). Development Due to limited data availability, and the rareness of thunderstorms during the cool season, equations were developed only for the warm season—the period May 1 through September 30. Separate 3-, 6-, 12- and 24-h forecast equations were developed for each projection, for all four GFS model cycles. Additional information on forecast projections and the developmental data sample is shown in the table to the left. An archive of CG lightning observations from the Bureau of Land Management (BLM) was used to define thunderstorm events for the MOS system. Each lightning observation was assigned to a grid cell on a 48-km Polar Stereographic grid (Fig. 1). Flash counts were tabulated for each 3-h period (e.g., 0000-0259, …, 2100-2359 UTC), and binary indicators were assigned to each grid cell: a “1” if one or more CG flashes occurred during the 3-h period, or a “0” if no activity occurred. Similar binary indicators were assigned to 6-, 12-, and 24-h periods. The binary indicators served as the predictands in the MOS system. Lightning relative frequencies were developed to study the climatological characteristics of lightning in Alaska, and are also made available as potential predictors in the MOS system. An example of a 12-h lightning relative frequency during the month of July, valid for the period 1800-0600 UTC, is shown in Fig. 2. A major axis of activity is evident over the Alaska interior, bounded by the Brooks Range to the north and the Alaska Range to the south. This region is characterized by a continental climate, where convection is driven by intense solar heating. We used a generalized operator approach to develop the equations. A subset of points was selected from the 97 x 69 grid (Fig. 1) to use in equation development, and these points were combined into one region to increase the sample size and the stability of the equations. Linear screening regression was used to relate the occurrence of thunderstorms to forecast predictors from the GFS. Potential predictors offered to the screening regression are listed in the table to the left. Convective precipitation and the cross product of K-index and lightning relative frequency were found to be the most often selected predictors in the equations. Verification The 2006 warm season was withheld as an independent data sample for testing, while the 2001-2005 warm seasons were used to develop the test equations. The Brier Score and the percent improvement relative to an equation containing only climatological predictors was calculated for each projection to assess the skill of the guidance. Scores for the 3-h and 24-h guidance generated from the 0000 UTC runs are shown in Figs. 3 and 4, respectively, for the period May 15 – September 15, 2006. In general, the 3-h guidance for all cycles was found to have skill out to 84 hours, while the 6-, 12- and 24-h guidance was found to have skill out to around 156 hours. Forecasts for projections beyond 156 hours will not be included in the operational products. Case Study On July 6-7, 2006, a low pressure system and associated frontal boundary (Fig. 5a) produced widespread thunderstorms across much of southern Alaska. This event produced nearly 18,000 CG lightning strikes in a 24-h period (Fig. 5b), and was responsible for igniting two wildfires that burned 6700 acres near the Kuskokwim Delta in SW Alaska. GFS MOS forecasts generated from the 1200 UTC run on 6 July (Figs. 5c and 5d) captured the activity very well, with 24-h probabilities approaching 70% over SW Alaska. In contrast, MOS forecast probabilities were very low or near zero to the north of the front where no lightning was observed. Users of this guidance would have been alerted many hours in advance that a significant lightning event was likely during this period across southern Alaska. Products The new GFS MOS thunderstorm guidance will be available to users beginning in May 2008, in alphanumeric text format for 130 sites in Alaska, as well as on high resolution grids to support the NDFD. The short range GFS MOS station guidance (MAV) will contain the 6- and 12-h thunderstorm probabilities out to 84 hours, while the extended range message (MEX) will contain 12- and 24-h probabilities out to 156 hours. The 3-, 6-, and 12-h probabilities also will be available in graphical form at 3-km resolution as part of the National Digital Guidance Database (NDGD). Acknowledgements: Lightning data for the MOS thunderstorm development was provided by John Palmer from the Bureau of Land Management. The authors also wish to thank Becky Cosgrove and Joe Maloney from MDL for their helpful suggestions and for their assistance with the coming May 2008 implementation. Figure 5. Widespread lightning event occurring on 6-7 July 2006. The surface analysis valid at 0600 UTC 7 July is shown in panel (a), and the observed CG lightning is shown in panel (b) for the 24-h period ending at 1800 UTC 7 July. GFS MOS probability forecasts are shown for the 3-h period ending at 0300 UTC 7 July (panel c), and the 24- h period ending at 1800 UTC 7 July (panel d). The forecasts were generated from the 1200 UTC run on 6 July (12 hours prior to the peak of the event). a) Surface analysis valid at 0600 UTC 7 July 2006.b) 24-H CG lightning (ending 1800 UTC 7 July) c) GFS MOS 3-H Probability of a Thunderstorm Ending 0300 UTC 7 July 2006 d) GFS MOS 24-H Probability of a Thunderstorm Ending 1800 UTC 7 July 2006 Figure 3. Brier Skill Scores for the GFS MOS 3-h thunderstorm guidance. The zero skill line is indicated in red. Forecasts were generated out to 84 hours from the 0000 UTC runs for the independent test period of 15 May – 15 September 2006. Figure 4. Brier Skill Scores for the GFS MOS 24-h thunderstorm guidance. The zero skill line is indicated in red. Forecasts were generated out to 192 hours from the 0000 UTC runs for the independent test period of 15 May – 15 September 2006.