Public Severe Weather Outlooks for nighttime tornado potential will be issued December 1 through March 31.
Cool-season nighttime tornadoes pose a particular challenge to forecasters, and are also a substantial threat to the public (see: Ashley, Weather and Forecasting Volume 22, Issue 6 (December 2007) pp. 1214-1228). In the wake of the Super Tuesday Tornado Outbreak of February 2008, it was found that many people did not expect tornadoes, especially in February, and especially at night. Late night tornado events also pose a significant challenge to public safety officials when alerting the public to the impending threat can be extremely difficult after people have gone to bed. While it is recognized that the bulk of these events will remain difficult to forecast and highly uncertain, the SPC is in the best position to offer guidance and meteorological support ahead of these "high impact, low probability" events. To that end, this is an initial attempt to improve our commitment to saving lives and protecting property.
Below is an outline of what SPC and a few Southern
Region WFOs have collectively decided to do in an attempt to increase
public awareness of the potential for significant cool-season
nighttime tornadoes. This practice will commence on December
1st, 2009 and end on March 31, 2010. This is only a minor
modification of the current Public Severe Weather Outlook (PWO)
issuance criteria that requires a PWO for slightly greater forecast
probabilities of tornado/wind events. The climatology of nighttime 10
percent significant tornado forecasts over the past several years
suggests that this type of PWO may be issued about 3 to 5 times
during the cool season (December-March).
The Storm Prediction Center Station Duty Manual was updated on November 30, 2009 to include the following instructions to SPC forecasters:
A Public Severe Weather Outlook (PWO) will be composed and transmitted by the Outlook forecaster following the issuance of a 2000 UTC and/or 0100 UTC Day 1 Outlook (SWODY1) when a 10 percent probability of significant tornadoes (hatched area) is forecast after dark anywhere in the CONUS. These PWOs will be specifically worded to address the threat of nighttime tornadoes and issued even if an earlier PWO was issued for significant severe weather during daylight hours. A preformat has been added to SPC product generation software for the purpose of overnight PWOs. The forecaster will need to add the states included in the threat area to this preformat. These types of PWOs will be issued for the December 1st through March 31st period. We will assess the impacts of this change and determine if this is a practice that should be continued for every cool-season at the close of the cool season in early 2010.
Here is an example of this type of PWO:
PUBLIC SEVERE WEATHER
NWS STORM PREDICTION CENTER NORMAN OK
0600 PM CST WED JAN 06 2010
...SEVERE THUNDERSTORMS EXPECTED OVER PARTS OF XXXX OVERNIGHT...
THE NWS STORM PREDICTION CENTER IN NORMAN OK IS FORECASTING THE
POSSIBLE DEVELOPMENT OF TORNADOES OVER PARTS OF XXXX OVERNIGHT.
CONDITIONS ARE EXPECTED TO SUPPORT THE POTENTIAL FOR A FEW STRONG AND
POSSIBLY LONG-LIVED TORNADOES.
WHILE OVERALL SEVERE THUNDERSTORM COVERAGE IS NOT EXPECTED TO BE
WIDESPREAD...THE POTENTIAL FOR A SIGNIFICANT TORNADO AFTER DARK WARRANTS
HEIGHTENED SAFETY PRECAUTIONS.
TORNADOES DURING THE OVERNIGHT HOURS AT THIS TIME OF YEAR CAN BE
PARTICULARLY DANGEROUS BECAUSE THEY ARE USUALLY FAST-MOVING
AND OBSCURED BY RAIN AND DARKNESS.
THE AREAS MOST LIKELY TO EXPERIENCE THIS ACTIVITY INCLUDE
XXXX AND XXXX
STATE AND LOCAL EMERGENCY MANAGERS ARE MONITORING THIS DEVELOPING
SITUATION. THOSE IN THE THREATENED AREA ARE URGED TO REVIEW SEVERE
WEATHER SAFETY RULES AND TO LISTEN TO RADIO...TELEVISION...AND NOAA
WEATHER RADIO FOR POSSIBLE WATCHES...WARNINGS...AND STATEMENTS