Improving forecasts of instrument flight rule conditions over the Upper Mississippi River Valley and beyond

Instrument Flight Rules (IFR) is defined as surface visibility less than 3 statute miles and/or a cloud ceiling less than 1000 feet above ground level. Impacts on aviation during IFR conditions can include delays in airport departure and arrivals, additional fuel consumption and resulting cost and, most importantly, pilot and passenger safety. Since all types of operations from general aviation to scheduled air carriers can be significantly impacted, the purpose of this study was to focus on utilizing climatology, as well as new forecast data sets specifically tailored to aviation forecasting, to improve National Weather Service (NWS) forecasts of IFR conditions.

Various data sets were reviewed to compile a climatology of IFR conditions at La Crosse, WI (LSE) and Rochester, MN (RST). These data suggest IFR conditions are most prevalent from November through March, with a lower frequency of occurrence from April through October. Peak daily time of occurrence is primarily between 1000 UTC and 1600 UTC throughout the year, although IFR duration is longer during the cold season.

IFR conditions are most common during the winter months, and a large percentage of these occurrences are associated with snow. From 1961-1990, approximately 70 percent of measurable snow events over 12 mm (0.5 inches) were associated with IFR conditions at both LSE and RST. This suggests the Terminal Aerodrome Forecast (TAF) can be improved by forecasting IFR as the prevailing condition during most events where measurable snow is expected. This strategy can generally be applied to measurable snow events beyond the two sites investigated in this study.

In addition to utilizing climatology, new forecast data sets such as the Localized Aviation MOS Program (LAMP), have the potential to provide NWS forecasters with increased predictability of IFR conditions in the TAF. In fact, use of LAMP data in forecasting snow events during the winter of 2008-2009 provided improved accuracy in timing the onset of snow and its attendant reductions in visibility and cloud ceilings.

Utilizing climatology and new forecast data sets should provide NWS forecasters with an opportunity to improve skill in forecasting IFR conditions. A forecast improvement in IFR conditions through the use of these techniques would provide significant benefits to the aviation community, especially related to enhancing safety and promoting more efficient use of the national airspace system.