SPC Dry Thunderstorm Guidance Page

The SPC has started a Perfect Prog Forecast (PPF) web page focusing on dry thunderstorms for the contiguous United States (CONUS; on a 40 km grid) and Alaska (on a 10 km grid). The page utilizes JavaScript and is interactive with looping controls (and will even run on a smart phone). It also allows for toggling overlays off and on and dProg/dT (a look back at previous model runs).


Twelve years of developmental data from 2000-2011 were used to derive the predictive equations using North American Regional Reanalysis (NARR) and lightning data. The PPF system uses the GFS gridded input data and provides probabilistic forecasts from 0 to 180 hours (7.5 days) in 3 hourly increments. The methodology used to derive the new PPF equations was similar to the NAM-based perfect prog forecasts that have been available since 2006.

Web Page

The web page (www.spc.noaa.gov/exper/dryt/) will update after each of the (4) GFS cycles have been processed. The CONUS forecasts are available about 6 hours after each cycle time, while the Alaska forecasts are available about 7 hours after each cycle time.

The initial version of the web page includes the following graphics:

  1. Probability of 1 or more CG flashes with less than 0.10 inch precipitation (a "true" dry thunderstorm — NWS definition).
  2. Probability of 1 or more CG flashes with less than 0.25 inch precipitation (same as 2, but with a slightly higher QPF cutoff of a quarter inch).
  3. Probability of 0.10 inch or more of precipitation.
  4. Probability of 0.25 inch or more of precipitation.
  5. The average relative humidity over approximately the lowest 8000 ft AGL (terrain following).
  6. The 700 mb wind vectors (quick, simple look at approximate storm motion).
  7. Probability of 1 or more Cloud-to-Ground (CG) flashes.

Performance and Interpretation

To avoid over-forecasting dry thunderstorms over the CONUS, the lower tropospheric humidity must be no higher than 50% for dry thunderstorms to be forecast. However, no humidity criterion is used for the dry thunderstorm equations in Alaska.

Experience during the final testing of these fields, using cases from the summer of 2013, have generally confirmed that dry thunderstorms are more likely to occur where:

  1. dry thunderstorms are forecast (lower probabilities included);
  2. the lower tropospheric humidity (lowest 8000 ft) is below 50% (CONUS only); and
  3. where the probability of 0.10 and/or 0.25 inch remains low (higher probability of precipitation values will be more likely to have "wetting rains").

For more details about the PPF methodology, see the Power Point or PDF Slides.

Comments and questions about the web page can be sent to: Patrick Marsh. Comments and questions about the display fields can be sent to: Phillip Bothwell.