Case summary

This case examined the use of the NCEP medium-range ensemble forecast (MREF) to assess the likelihood of synoptic scale patterns conducive to winter weather in the southern and eastern U.S. in the seven to three day time frame (a so-called "heads-up" for the possibility of weather with a significant public impact). At the 7-day time range, MREFs can be used to determine, among other things:

• The large-scale flow regime affecting the area of interest, and thus the relative risk of significant weather
• Broad probabilities of exceeding critical values of variables at the large scale, such as lower tropospheric temperatures and 24-hour preciptation thresholds, and
• Features of interest which may affect the forecast area, such as large-scale jet maxima, air mass boundaries, and large-scale troughs and ridges.
  • In this case, the features of interest were:
    • Split northern and southern jet streams
    • Western U.S. ridge and its placement
    • Position of disturbances in the northern and southern streams
    • Eastern U.S. jet streak

At the shorter medium-range (3-4 days), the synoptic-scale forecast can usually be refined to the extent that the coarse resolution of the MREFs can allow. In particular, the forecaster can:

• Determine a "broad-brush" of the most-likely forecast scenario.
  • In this case, the ensemble forecasts seemed to increase the chances for a winter storm in the eastern U.S. from day 7 to day 3 prior to the event.
• Determine if there are multiple scenarios with significant probabilities of occurrence.
  • Such a situation is indicated when groups of ensemble member forecasts cluster around different solutions.
  • This was not observed in this particular case.