I'll Melt With You: How a Regional Snow-Liquid Ratio Workshop Improved Collaborative Winter Forecasts in the Western United States

Snow-liquid ratio (SLR) is one of the most challenging, yet crucial, aspects of winter weather forecasting. Not only is an accurate SLR critical to forecasting snow amounts, but it is also, and perhaps more importantly, vital to predicting potential impacts to travel and infrastructure. Even modest changes in SLR can be the difference between fluffy snow that can be easily cleared from roadways and blow off power lines, to heavy wet snow that can snarl traffic, down trees, and cause widespread power outages. Unfortunately, SLR forecasting remains exceptionally challenging, and, in many ways, very subjective.

The National Weather Service (NWS) has multiple methods by which to forecast SLR. Although an SLR of 10:1 is commonly assigned as an “average” SLR across much of the Continental United States (CONUS), Baxter in 2005 showed that an average SLR of around 13:1 is more appropriate for most of the country. While a climatology-based SLR is helpful on longer time scales, it is rare that an individual event will fall exactly at this climatological mean. A better approach, and one used by the NWS in their Graphical Forecast Editor (GFE), is to blend climatology with other forecasting methods including the Roebber method, the Cobb method, and the University of Utah method. None of these on their own will likely produce a perfect SLR, but a meteorologist can produce a reasonable SLR forecast by blending these techniques based on the environment. However, unique SLR blended forecasts across the 116 CONUS Weather Forecast Offices (WFOs) can, and oftentimes do, result in inconsistent and low confidence messaging to core NWS partners.

To attempt to remedy this problem, in 2019 the Weather Prediction Center (WPC) began producing gridded SLR guidance as a starting point for all WFOs to use in GFE. WPC does this by initializing with the National Blend of Models (NBM) and then coordinating adjustments with the WFOs to produce the WPC SLR. This can lead to a more coherent SLR forecast, but consistency and accuracy can still be limited due to differences in how these processed SLR grids are used among WFOs.

After a series of challenging snowfall forecasts in the winter of 2023-2024, WPC and several Western Region WFOs realized the need to increase the consistency and accuracy of the WPC SLR guidance. To do this, they hosted a regional workshop to collaborate, share ideas, understand challenges, and develop best practices for forecasting SLR. This workshop included 10 WFOs, WPC, regional/national NWS headquarters, academic researchers, and proposed the following goals:

1. Improve consistency of SLR forecasting for the Pacific and Interior Northwest
2. Increase collaboration of SLR between WPC and Western Region WFOs
3. Improve efficiency of WPC/WFO Collaborative Winter Storm Watch Calls
4. Submit proposed SLR adjustments to MDL for NBMv5.0+

This presentation will outline the structure of this workshop, explain how it improved relationships to align with the NWS probIDSS roadmap, and summarize the most important findings and recommendations for future SLR forecasts. We will also explore how SLR coordination and forecasting has changed for the 2024-2025 winter season based on these recommendations. Finally, we hope to also demonstrate how this can serve as a framework for future regionally-based knowledge sharing workshops.