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Snow Squalls - A WINDEX Event

Winter forecasts in New England sometimes refer to the potential for 'WINDEX events'. WINDEX itself  stands for 'winter instability index'. It is used to assign a value to the probability of (non-Lake Effect) snow squalls.

You calculate the difference between the T1 and the T5 on the NGM, if it is over 10, and especially over 15, you then look at the lifted index.. if it rises significantly after the proposed event, and the RH at R1 is over 50%, you will have instability squalls.

T1: Temperature in the 35 mb closest to the surface
T5: Temperature at 800 mb
R1: Relative Humidity in the 35 mb closest to the surface
Lifted Index: Instability index calculated by lifting parcels from the
surface to  500 mb and subtracting the temperature of its environment
from the parcel temperature.

Further reading about WINDEX can be found at:

The following are excerpts from a 2004 Southern New England Weather Conference Abstract 

WINDEX V2.0 – An improved forecast technique to assess the threat of snow squall potential over a given area (Weir Lundstedt)

During the early 90’s a wintertime instability index (WINDEX) was developed to assess the potential for snow squalls along frontal boundaries across New England. It was that study which attempted to quantify moisture, lift and instability in the low levels of the atmosphere needed to produce snow squalls in an environment unstable in respect to shallow upright convection. WINDEX V2.0 is the second version, or next generation of WINDEX which makes use of improved model resolution, better understanding of dendritic growth and the utilization of the latest software of BUFKIT to produce what is believed to be an even more robust technique to forecast snow squalls. A review of the new parameters will be presented along with a recent case study from the winter season of 2003.

Weir Lundstedt currently works for the National Weather Service (NWS) as an aviation weather forecaster in Nashua, New Hampshire. He has been working in the Center Weather Service Unit (CWSU) within the Boston Center Air Traffic Control Facility since the fall of 1992. His duties include providing Air Traffic Controllers timely weather forecasts for such things as icing, turbulence, thunderstorms and IFR conditions for the safe and efficient movement of aircraft over the northeast corridor of the United States. Before working at the CWSU, Weir entered the NWS in the fall of 1986 and worked as meteorologist intern for over 5 years at the Weather Service Office (WSO) in Concord New Hampshire. While there he published WINDEX – A wintertime instability index to aid in forecasting snow squalls. Weir’s interest in snow squalls stem back to his days at Lyndon State College where he watched forecast “flurries” occasionally turn into several inches of snow. He graduated with a BS degree in meteorology in 1986.

Snow Squalls in Current Mesoscale Forecasting Models  (Dr. Frank Colby)

In this part of the talk, we will concentrate on the ability of currently available forecasting models to forecast the occurrence of snow squalls in advance. We will compare the operational National Centers for Environmental Prediction Eta model output with output from an NCAR/PSU Mesoscale Model (MM5) run made in the UMass Lowell Weather Lab. The Eta model ran on a 12 km grid, but the output is typically viewed on a 40 km grid, due to Internet bandwidth limitations. The MM5 model ran with 3 nested grids: 36 km , 12 km, and 4 km. The 4 km grid output will be highlighted here, illustrating the importance of the fine-scale grid in simulating snow squalls.

Frank Colby is a Professor of Meteorology at the University of Massachusetts Lowell, in the Department of Environmental, Earth, and Atmospheric Sciences. He has been a Professor since 1995, and has been at the University since 1983.

Professor Colby received his B.S. from the University of Michigan in 1976, his M.S. from the Massachusetts Institute of Technology (M.I.T.) in 1979, and his Ph.D. in Meteorology from M.I.T. in 1983, under thesis advisor Professor Fred Sanders. Dr. Colby’s research interests include a blend of analysis, forecasting, and modeling, especially relating to the atmospheric boundary layer.

PowerPoint Presentation:

SNOW SQUALLS: The Anatomy, Impact, and Forecasting of Snow Squalls in New England
PART 3:  Snow Squalls in Current Mesoscale Forecasting Models

by  Frank P. Colby, Jr. and Beth Krajewski

January 6 and 13 2004 case studies


From the Northeastern Storm Conference Oral Abstract

Application of the Wintertime Instability Index (WINDEX)
During Two Significant Snow Squall Events in Central New York and Northeast Pennsylvania

Jim Brewster
    NOAA/ National Weather Service
    Binghamton, NY


In winter, intense snow showers, often termed "snow squalls" are associated with the passage of surface frontal boundaries or troughs of low pressure, and are common to the Northeastern United States. Snow squalls pose a significant, short-term forecast problem for meteorologists concerned with public and aviation safety. Although snow squalls are not known for heavy snow accumulation, they do typically produce heavy snowfall rates, very poor visibility, and enough of a snow accumulation to wreak havoc on travelers as they pass through an area.


The Wintertime Instability Index (WINDEX) method for assessing the synoptic environments favorable for snow squalls was developed in 1993. This legacy technique was developed during an era when only a limited subset of parameters from the Nested Grid Model (NGM) was routinely available to operational forecasters. These parameters were related to moisture, low level instability, surface and upper level forcing. The method was modified in 2005 to take advantage of the much larger suite of forecast data currently available on a routine basis to National Weather Service (NWS) forecasters. The updated technique now incorporates higher resolution, point specific, numerical model data via the NWS Buffalo's Forecasting Toolkit (BUFKIT) thermal profile analysis software.


Two events, February 17, 2005 and November 24, 2005 (Thanksgiving Day), were examined using the WINDEX method. These events were chosen due to their significant, negative impact on late afternoon and evening travel in the Binghamton, NY and Scranton, PA areas on the dates noted. Upper level and surface synoptic charts, radar and satellite imagery, and plan views of the North American Mesoscale Model (NAM) forecast output were examined for each case, and will be used to lay the synoptic groundwork for each event. The specific WINDEX criteria were analyzed from hourly forecast soundings via the BUFKIT software, and tabular model output from the NAM. Forecast lapse rate data between 950 and 800 hPa for the two cases was examined, as these levels correlate best to the T1 and T5 sigma levels from the NAM model output parameter used in the current WINDEX method.

This presentation will outline the WINDEX method, and then demonstrate it's effectiveness on short term forecast and warning operations at NWS Binghamton, NY on February 17, 2005, and Thanksgiving Day 2005. The hourly forecast lapse rate data prior to the development of the snow squalls will be examined, and recommendations for using the lapse rate forecast data in lieu of the legacy T1 and T5 differential will be presented.

Norlun "NORLUN Instability Trough" AN Explanation of Two Coastal New England Unexpected Snowstorms Which Produced Over a Foot of Snow  by Weir Lundstedt, NWS, CWSU, Nashua, NH

A Method to Forecast Wintertime Instability and Non-Lake Efffect Snow Squalls ACross Northern New England by by Weir Lundstedt, NWS, CWSU, Nashua, NH