The Use of Observations and High-Resolution Modeling in Specialized Forecasts for Western Wyoming

The National Weather Service (NWS) Weather Forecast Office (WFO) in Riverton, Wyoming (RIW) has been providing forecast guidance to the Bridger-Teton Avalanche Center in Jackson, Wyoming for well over a decade. Currently, RIW uses the Snow Avalanche Guidance (SAG) product to disseminate the forecast information. The product contains a discussion as well as seven-day point forecast matrices for a variety of locations in the Bridger-Teton National Forecast, ranging in elevation from 6451 ft. at the Jackson Hole airport to 10318 ft. at Rendezvous Mountain. During the winter season, the SAG product is sent twice daily to assist the center in snowfall and avalanche operations, primarily to protect those who use the backcountry of western Wyoming for recreational activities. Most of the forecast sites are chosen based on both manual and automated observation platforms the avalanche center has established over the years. The observations are critical in helping forecasters at RIW better understand and forecast the weather in the complex terrain of western Wyoming.

Beginning in the spring and summer of 2011, RIW began issuing a Recreational Forecast (REC) for Grand Teton National Park (GTNP). This product builds on REC for Mount Rainier National Park provided by the Seattle, Washington WFO. The REC product contains a brief discussion, a lightning threat forecast, a 24-hr cloud height forecast taken from the Terminal Aerodrome Forecast at the Jackson Hole airport, and three-day point forecast. The sites in this product range in elevation from the park headquarters in Moose, WY at 6460 ft. to the summit of Grand Teton at 13770 ft. The table of forecast values is also derived from the 2.5 km forecast database at RIW, although subjective and objective comparison indicates that the resolution of the forecast database is not fine enough for some weather elements to capture the true variation in weather around GTNP. Most of these sites do not have automated observation platforms with which to compare, so further collaboration will be needed to provide better forecasts for GTNP employees and visitors.

The Weather Research and Forecasting (WRF) Model has been utilized for several years to provide additional model guidance to RIW forecasters, primarily focusing on the complex terrain of Wyoming. The NMM core has been run at 4-km horizontal resolution twice per day using either the Global Forecast System or North American Mesoscale models for initial and boundary conditions. Starting in February 2012, the ARW core has been run at 4-km horizontal resolution twice per day to provide another option for high-resolution guidance. Forecasters at RIW have found the output particularly useful in adjusting the wind and temperature forecasts in western Wyoming that directly impact the guidance provided in the aforementioned forecasts. Subjective evaluation of the model output suggests that higher resolution nesting, perhaps down to 1- or 2-km resolution, might be required to better capture the critical weather variables around the steep and narrow terrain of western Wyoming.