Experimental Outlook (WSO) Product Description Document (PDD) Winter 2019/2020

Part I - Mission Connection a. Product Description – The Experimental Winter Storm Outlook (WSO) is a product of the ​ ​ Weather Prediction Center (WPC) and will depict the probability of exceeding winter criteria over finite time periods out to at least 72-hours. The WSO probabilities are calculated using a combination of WPC's Probabilistic Winter Precipitation Forecasts (PWPF) and local National Weather Service (NWS) warning thresholds for forecast and accumulations. Similar to other national center outlooks, the WSO serves as a forecast aid and a probabilistic approach for NWS watch/warning decision making. An example of a WSO map for a snow storm is shown below.

The WSO will indicate the chance and location where criteria (for snow or ice accumulations) may be exceeded over 12- and 24-hour time periods over the next three days. Probabilistic precipitation forecasts are already utilized across NWS and form the foundation of the WSO while WFOs retain Winter Storm Watch and Warning issuance responsibility.

WPC will issue 24-hour outlooks out three days (72-hours) for both snow and ice on the night shift (one each for Day 1, 2, and 3, with 12 to 12 UTC valid times). These outlooks will be subsequently updated on the day shift for the same valid times and a 12-hour valid period forecast will be issued for the beginning of Day 4 (a Day 3.5 WSO). The WPC forecaster will coordinate with the NWS field offices to edit and send these outlook forecasts to this URL: https://www.wpc.ncep.noaa.gov/wwd/wso. ​

Deadline for night shift WSO issuance: 0900 UTC. Deadline for day shift WSO issuance: 2100 UTC.

This experimental version of the WSO constrains forecast probabilities to 12-, 24-, and 72-hour valid times. It should be noted the WSO will, on occasion, indicate lower than expected winter storm probabilities when a forecast storm event crosses valid time thresholds (usually 12 UTC). For example, winter storm criteria may be achieved over a 12-hour time span from the final 6-hour period of Day 2 into the first 6-hour period of Day 3. These scenarios are not currently accounted for in WSO probabilities. (Refer to Part II - Technical Description, for additional details.)

WPC will team with the NWS Winter Program, Regions, and WFOs to conduct evaluation and verification of both winter storm outlooks, watches, warnings and the WSSI products. In addition to developing technical tools to enhance the collaborative forecast process, other changes to the process may occur, including adjusting timelines to better align winter storm outlook issuance with key WFO decision deadlines. Work will expand to integrate probabilistic hazards information with winter weather impacts information.

Winter storm watch and warning decisions are complex and are not usually based entirely on accumulation thresholds. Winter storm impacts, however, can be even more challenging to quantify. As a parallel experiment to the WSO, the Winter Storm Severity Index (WSSI) is being tested to determine its utility in assessing winter storm impacts based on a multivariate approach including but not limited to snow and ice accumulation forecasts. Maps of NWS WSSI are available at this URL: https://www.wpc.ncep.noaa.gov/wwd/wssi/wssi.php. ​ ​ b. Purpose – The product will provide a likelihood of warning level wintry precipitation and serve ​ ​ as a step toward improving NWS Winter Storm Watch consistency and accuracy across the lower 48 U.S. states. c. Audience – The product is intended as an enhancement to decision support services being ​ ​ provided to NWS core partners in emergency management, broadcast/electronic media, and transportation categories as well as the general public. d. Presentation Format – The outlooks for Days 1 to 3 (night) and Days 1 to 3.5 (day) will be ​ ​ displayed as web-based maps at https://www.wpc.ncep.noaa.gov/wwd/wso. ​ ​ e. Feedback Method – Feedback from users will be collected via a NWS web-based survey at: ​ ​ https://www.surveymonkey.com/r/ExpWinterStormOutlook

Comments or questions regarding the prototype WSO can be also addressed to:

Michelle Hawkins, Chief, Severe, Fire, Public, and Winter Weather Services Branch NWS - Analyze, Forecast, and Support Office, Forecast Services Division Silver Spring, MD 20910 [email protected] 301-427-9374

Gregory Carbin, Chief, Forecast Operations Branch NWS – NCEP/Weather Prediction Center College Park, MD 20740 [email protected] 301-683-1494

Part II – Technical Description a. Format and Science Basis – The WSO will depict the probability meeting winter storm ​ ​ warning criteria for both snow and ice for the next three days, from 12 to 12 UTC, in four probability ranges (<10%, 10-30%, 30-50%, > 50%). Ice warning criteria is on a “per event” basis nationwide, while most local offices have separate 12-hour and 24-hour snow warning criteria. The WSO snow for a particular day is the highest probability for meeting the 12-hour (from 12 to 00 UTC or 00 to 12 UTC) or 24-hour (12 to 12 UTC) snow warning criteria. For offices that have a “per event” criteria for snow, the same criteria is used for both 12-hour and 24-hour probabilities. Occasional discontinuities may appear on the WSO snow map when established NWS winter storm warning criteria change across NWS office boundaries.

WPC will issue WSOs twice daily: Day 1, 2, 3 (12 to 12 UTC) for meeting WFO Winter Storm Warning criteria for snowfall and Day 1, 2, 3 (12 to 12 UTC) for meeting WFO Winter Storm Warning criteria for ice. A Day 3.5 (valid for a 12-hour period, 12 to 00 UTC) for snow (only the 12-hour or “per event” criteria used) and ice will be issued on the day shift. b. Availability – The WSO will be provided externally to users through a WPC webpage. WPC ​ ​ will develop verification statistics and a WSO product archive.