Overview of the SPC Day 1-3 Outlook Change

Service Change Notice 14-42 has been issued. These changes were implemented effective Wednesday, October 22, 2014 at 1500 UTC.

Product Description Document (PDD): https://products.weather.gov/PDD/SPC_Day_1to3_Cat_Conv_Outlook.pdf.

The public comment period ended on June 17, 2014.

Q: How are the outlooks changing for Day 1, Day 2 and Day 3?

A: The SPC has revised Day 1 through Day 3 categorical severe weather outlooks to better communicate risk and describe the likelihood of severe weather. Format changes also improve the use of SPC severe weather forecasts for customers who incorporate SPC outlooks into GIS systems.

The SPC has expanded the risk categories from four to five and clarified the risk previously labeled as "See Text". That descriptor is replaced by a categorical line and the term "Marginal" to denote areas with a 5 percent probability of severe weather. The upper end of the "Slight Risk" category is renamed "Enhanced" (short for "Enhanced Slight") to denote a threshold 30 percent probability of severe wind or hail and/or a 10 percent chance of a tornado during the Day 1 period. For Days 2 and 3, the "Enhanced" risk category denotes a 30 percent total severe probability. The Moderate and High risk thresholds will remain essentially unchanged.

Current:

1. See Text 2. Slight (SLGT) 3. Moderate (MDT) 4. High (HIGH)

Proposed:

1. Marginal (MRGL) - replaces the previous SEE TEXT and now is described with Categorical line on the SPC Outlook. 2. Slight (SLGT) 3. Enhanced (ENH) - replaces upper-end SLGT risk probabilities, but is not a MDT risk 4. Moderate (MDT) 5. High (HIGH)

The examples below juxtapose the current (left) and the previous (right) outlook graphics for the marginal (MRGL) and enhanced (ENH) categories as opposed to the previous slight (SLGT) category and SEE TEXT labels.

The examples below juxtapose the current (left) and the previous (right) outlook graphics for the marginal (MRGL) and enhanced (ENH) categories as opposed to the previous slight (SLGT) category.

The examples below juxtapose the current (left) and the previous (right) outlook graphics for the marginal (MRGL) category as opposed to the previous SEE TEXT labels which does not clearly define the geographical areas of concern.

The examples below juxtapose the current (left) and the previous (right) outlook graphics for the all categorie as opposed to the previous SLGT/MDT/HIGH.

The examples below juxtapose the current (left) and the previous (right) outlook graphics for the marginal (MRGL)/slight (SLGT)/enhanced (ENH) categories as opposed to the previous SLGT which does not clearly define the enhanced slight (ENH) area to better highlight/communicate the risks.

Below is an example of a current Public Severe Weather Graphic that includes the new categories. This graphic is enlarged on the area of greatest risk and designed for use by media partners and social media.



Current 2011/04/27 1630Z Public Severe Weather Outlook Graphic (Top)

Q: Why is the SPC doing this?

A: A primary goal of these changes is to bring better consistency to the risks communicated in SPC outlooks, from the short-range Day 1 outlooks through the extended range Day 4-8 outlooks. The changes have been made based on customer feedback and to better meet their needs.

Example: Previously, a 10 percent tornado probability including a risk of a significant tornado (>=EF2) was categorized as a Slight Risk. This was the same category used for a "low end" 15 percent risk of severe thunderstorm wind and hail events. In the new scheme, a 10 percent tornado probability that includes the chance of significant tornadoes is categorized as an Enhanced Risk.

In addition, "See Text" did not convey a threat area, due to the lack of a contour in any "See Text" categorical forecast. And the previous "Slight Risk" category covered too broad a range of severe weather probability values.

Q: Are there cases where the current categories will change based on the underlying severe weather probabilities?

A: The thresholds for traditional risk categories are essentially unchanged but there is some refinement in the underlying definitions to remain consistent with evolving trends in severe weather reporting. These refinements would only impact a couple of Day 1 tornado and severe wind outlooks during any year. A 15 percent tornado probability without a threat of an EF-2 or greater tornado at Day 1 now qualifies as an Enhanced Day 1 tornado risk as opposed to the previous scheme where it was a Moderate Day 1 Tornado Risk. Likewise, a 45 percent severe thunderstorm wind probability without a significant threat at Day 1 now qualifies as an Enhanced Day 1 wind risk as opposed to the previous scheme where it was a Moderate Day 1 wind risk.

Q: Why not a more comprehensive overhaul of all categorical outlook words (i.e. SLGT, MDT, HIGH)?

A: The categorical words Slight, Moderate and High have been used by SPC for nearly 35 years and are generally understood by the weather risk communication community. Making measured changes, we believe, is more effective than a wholesale change. These measured changes include: 1) moving to de-emphasize the specific words; and 2) working to communicate the level of risk to the public in multiple ways. This includes numerical risk categorization, appropriate colors to indicate severity, and strategic use of icons and symbols. Social scientists have encouraged us to communicate on multiple levels and not just with a single word, label or category.

Q: What role did social science play in making this change?

A: The NWS has a strong commitment to engaging the social sciences in evolving our services, and this community has helped inform our decision making for this change.

Technical Details of the Current SPC Day 1-3 Outlook Change

On October 22, 2014 the NWS Storm Prediction Center (SPC) Day 1, Day 2, and Day 3 Convective Outlooks for the CONUS include two new risk categories.

The addition of the new risk categories is based on customer feedback and the need to provide better consistency with other NWS products. Examples of these Outlooks (using historical data), are depicted below.

"MARGINAL" replaces the previous SEE TEXT used in these products.

"ENHANCED" is an additional category to delineate areas of risk in the high end of the current SLIGHT risk, but below MODERATE risk.

Click on the links below to view the section:

Below is an example of the Convective Outlook text product containing "MARGINAL" (MRGL) and "ENHANCED" (ENH) risk area delineations and Summary section. (Top)

DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0110 AM CDT WED APR 27 2011 VALID 271200Z - 281200Z ...THERE IS A HIGH RISK OF SVR TSTMS OVER PORTIONS OF NRN MS...AL...FAR NWRN GA AND SRN MIDDLE TN... ...THERE IS A MDT RISK OF SVR TSTMS OVER MUCH OF CNTRL AND NRN MS AND AL...NWRN GA...MUCH OF TN AND KY...WRN CAROLINAS... ...THERE IS AN ENH RISK OF SVR TSTMS FROM PARTS OF THE LOWER MS VALLEY TO THE UPPER OH VALLEY/CNTRL APPLACHIANS... ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE CNTRL GULF COAST TO THE NRN APPALACHIANS... ...THERE IS A MRGL RISK OF SVR TSTMS FROM THE LOWER MS VALLEY TO THE EAST COAST... ...SUMMARY... AN OUTBREAK OF TORNADOES AND DAMAGING WINDS IS EXPECTED TODAY THROUGH THIS EVENING OVER PORTIONS OF NORTHERN MS/AL...TN AND KY. FAST-MOVING SUPERCELLS WILL BE CAPABLE OF LONG-TRACKED STRONG TO VIOLENT TORNADOES. ...SYNOPSIS... (The rest of the discussion remains unchanged.)

With the addition of "MARGINAL" and "ENHANCED" categories, the new categorical Day 1-3 Outlooks include contours for up to six (6) categories as follows: (Top)

Day 1: a. General Thunderstorms - 10% or greater probability of non-severe or near severe thunderstorms. b. Severe Category 1 - Marginal - 2% tornado probability, or - 5% severe hail or severe wind probability. c. Severe Category 2 - Slight - 5% tornado probability, or - 15% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater. d. Severe Category 3 - Enhanced - 10% tornado probability WITH OR WITHOUT 10% or greater probability of an EF2+ tornado, or - 15% tornado probability, or - 30% severe hail or severe wind probability WITH OR WITHOUT 10% or greater probability of hail 2 inches or greater in diameter, or wind gusts 75 mph or greater, or - 45% probability of severe hail or wind. e. Severe Category 4 - Moderate - 15% tornado probability AND 10% or greater probability of an EF2+ tornado, or - 30% tornado probability, or - 45% severe wind probability AND 10% or greater probability of a wind gusts 75 mph or greater, or - 45% severe hail probability AND 10% or greater probability of hail 2 inches or greater in diameter, or - 60% severe wind probability, or - 60% severe hail probability WITH OR WITHOUT 10% or greater probability of hail 2 inches or greater in diameter. f. Severe Category 5 - High - 30% tornado probability AND 10% or greater probability of an EF2+ tornado, or - 45% or greater tornado probability WITH OR WITHOUT 10% or greater probability of an EF2+ tornado, or - 60% severe wind probability AND a 10% or greater probability of a wind gust 75 mph or greater.



Day 1 Outlook Probability to Category Conversion Table

Days 2 and 3: (Top) a. General Thunderstorms - 10% or greater probability of non-severe or near severe thunderstorms. b. Severe Category 1 - Marginal - 5% total severe probability. c. Severe Category 2 - Slight - 15% total severe probability WITH OR WITHOUT 10% or greater probability of significant severe. d. Severe Category 3 - Enhanced - 30% total severe probability WITH OR WITHOUT 10% or greater probability of significant severe, or - 45% total severe probability. e. Severe Category 4 - Moderate - 45% total severe probability AND 10% or greater probability significant severe, or - 60% total severe probability (Day 2 only). f. Severe Category 5 - High (Day 2 only) - 60% total severe probability AND 10% or greater probability of an EF2+ tornado or a wind gust 75 mph or greater.





Day 2 Outlook Probability to Category Conversion Table



Day 3 Outlook Probability to Category Conversion Table

The following products reflect the changes: (Top)

WMO Header AWIPS ID Description ACUS01 KWNS SWODY1 Day 1 Convective Outlook Discussion WUUS01 KWNS PTSDY1 Day 1 Convective Outlook Areal Outline PGWE46 KWNS RBG94O Day 1 Red Book Graphic Categorical Outlook LDIZ17 KWNS Day 1 NDFD Categorical Outlook ACUS02 KWNS SWODY2 Day 2 Convective Outlook Discussion WUUS02 KWNS PTSDY2 Day 2 Convective Outlook Areal Outline PGWK48 KWNS RBG99O Day 2 Red Book Graphic Categorical Outlook LDIZ27 KWNS Day 2 NDFD Categorical Outlook ACUS03 KWNS SWODY3 Day 3 Convective Outlook Discussion WUUS03 KWNS PTSDY3 Day 3 Convective Outlook Areal Outline PGWI47 KWNS RBG98O Day 3 Red Book Graphic Categorical Outlook LDIZ37 KWNS Day 3 NDFD Categorical Outlook The "points" products (PTSDY1, PTSDY2, and PTSDY3) will include new labels "MRGL" (Marginal) and "ENH" (Enhanced).