Southern Illinois University
Roger Edwards, David Imy and Stephen Goss
Storm Prediction Center Page last modified: March 25, 2010
The Storm Prediction Center (SPC) at Norman, Oklahoma is a part of the National Weather Service (NWS) charged with monitoring and forecasting severe weather over the 48 continental United States. It is a division of the National Centers for Environmental Prediction (NCEP). Within SPC, the Operational Branch prepares several products that can help you prepare for hazardous and severe weather.
If you were to ask ten different people what "severe weather" means you would probably get ten different answers. The NWS definition states that a "severe" thunderstorm is any storm that produces one or more of the following elements:
Other forms of dangerous weather include heavy rain (flash flooding hazard), excessive heat and cold, tropical cyclones, and winter storms. Although forecasting these other types of dangerous weather is mainly the responsibility of other branches of NCEP, and of local NWS offices, the SPC also issues 1-6 hour short-term forecasts, or mesoscale discussions of certain heavy rain, heavy snow, freezing rain, and blizzard events in portions of states.
The convective outlooks serve as guidance to the local NWS forecast offices and are used by emergency managers, private sector meteorologists, media, and other weather customers concerned with public safety. Three separate risk areas (slight, moderate, and high) are used to describe the expected coverage and intensity for the categorical severe weather threat on days 1-3 along with severe weather probabilities for the potential threat.
The Day 4-8 Severe Weather Outlook graphic depicts those days where a 30% or higher probability for severe storms is expected.
The Day 1 Convective Outlook consists of a narrative and a graphics depicting thunderstorm and severe thunderstorm threats across the continental United States. The narratives are written in technical language, intended for sophisticated weather users, and provide the meteorological reasoning for the risk areas. The text product also provides explicit information regarding the timing, the most likely severe weather hazard and the severity of the event, when possible. The graphics include a categorical forecast of the severe risk (slight, moderate, or high) along with a 10% or greater forecast for thunderstorms. Separate probabilities for severe hail, wind and tornadoes are also issued.
The Day 1 Convective Outlooks are issued 5 times daily: at 0600 UTC (initial issuance valid 1200 UTC that day until 1200 UTC the following day), 1300 UTC and 1630 UTC (the "morning updates," valid until 1200 UTC the next day), 2000 UTC (the "afternoon update," valid until 1200 UTC the next day), and the 0100 UTC (the "evening update," valid until 1200 UTC the following day).
Please click here to see the probability to categorical outlook conversion tables.
The Day 2 Convective Outlook is similar to the Day 1 Outlook in terms of a text and graphics. The biggest difference is instead of forecasting separate probabilities for wind, hail and tornadoes, a single combined severe weather probability is issued on the Day 2 probability Outlook. Also, the Day 2 Outlook is issued only twice a day, at 100 am CST/CDT and 1730 UTC. This outlook covers the period from 1200 UTC the following day to 1200 UTC the day after that. For example, if today is Monday then the Day 2 Outlook will cover the period 1200 UTC Tuesday to 1200 UTC Wednesday.
The Day 3 Convective Outlook text and probabilities are similar to the Day 2 Outlooks. However, the categorical outlook (slight, moderate and high graphic) does not include a thunder forecast and the outlook is only issued once a day, at 400 am CST/CDT.
The Day 4-8 graphic consists of one map depicting the severe weather threat during the forecast period. A single non-probability red line will be drawn on the graphic for each separate area where there is at least a 30% probability for severe thunderstorms. This is equivalent to a higher end slight risk threat. A text box is included with the severe weather area listing the day(s) of the potential severe weather threat.
THERE IS A SLGT RISK OF SVR TSTMS TO THE RIGHT OF A LINE FROM 45 ESE YUM TRM NID P38 CDC BCE U28 EVW LND 4DG LBF OLU MKT 55 NW CMX ...CONT...40 E TOL FDY LUK 5I3 PSK 25 NE ECG.
Standard aviation identifier location codes are used to delineate the risk areas on the Day 1-3 Convective Outlooks. [A list of many of these identifiers can be found online by clicking here]. When plotted with a line drawn between each point, the outlined area forms a polygon. The points may either fall exactly on top of the location identifiers (i.e. DAL...SPS...GAG) or may be referenced from those points (ie. 20 NW FMY...10 E MIA) in which case the point would be xx number of nautical miles in the given direction from that point. The previous example would read 20 nautical miles northwest of Ft. Myers FL to 10 nautical miles east of Miami. The designator "...CONT..." is used to indicate that the risk area goes to the U.S. border, then starts again at another location on the border. For example, part of a risk area might say "MSP INL ...CONT... SSM". This means the risk area goes from Minneapolis to International Falls then runs along the Canadian border to Sault Ste. Marie. The points forecast for the each outlook can be found at a link at the bottom of the Day 1-3 Convective Outlooks.
The SEE TEXT label appears only on the graphic map. Although there is no cateogorical line drawn for the labeled area, you should read the text of the outlook discussion to be aware of the potential for a threat to develop, if environmental conditions come together. As a rule, the "SEE TEXT" is used on Days 1-3 for areas where severe weather may be possible, but enough forecast uncertainty exists (variability in model guidance, capping, moisture return, or other such factors) to not issue a risk area. Note that the SPC severe thunderstorms outlooks are not meant to cover every single possibility of a severe thunderstorm -- otherwise, severe and general thunder outlooks would often be the same.
A SLGT risk implies that well-organized severe thunderstorms are expected but in relatively small numbers/coverage, or a small chance of a more significant severe event. Not all severe storm events will be covered with a SLGT risk, especially during the summer when short-lived, "pulse-type" severe storms are relatively common during the afternoon.
A MDT risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a SLGT risk. A MDT risk is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak. Typical MDT risk days include multiple tornadic supercells with very large hail, or intense squall lines with widespread damaging winds.
The HIGH risk implies that a major severe weather outbreak is expected, with large coverage of severe weather and the likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events). The HIGH risk category is reserved for the most extreme events with the least forecast uncertainty, and is only used a few times each year.
The outlook categories are related to the specific tornado, damaging wind, and large hail probability forecast graphics on Day 1, and the total severe storm probabilities for the Day 2 and Day 3 outlooks.
In addition to the severe risk areas, general thunderstorms (non-severe) are outlined, but with no label on the graphic map. Within this area, a 10% or greater probability of thunderstorm occurrence is forecast.
...SEVERE THUNDERSTORM FORECAST DISCUSSION... ...SYNOPSIS... UPPER FLOW IS LOSING AMPLITUDE OVER U.S. AS STRONGER WLYS SHIFT NWD INTO SRN CANADA. FORMER HURRICANE JIM...NOW A MARGINAL TROPICAL STORM...IS WEAKENING AS IT MOVES NNEWD ACROSS FL KEYS/STRAITS. COLD FRONT CONTINUES TO PUSH E AND S ACROSS N-CENTRAL U.S. WITH MOIST/UNSTABLE AIR MASS AVAILABLE IN WARM SECTOR. SIGNIFICANT FRONTOLYSIS IS EXPECTED TO BE UNDERWAY OVER OH VALLEY BY END OF PERIOD. ...WRN GREAT LAKES TO CNTRL PLAINS.... VERY MOIST AND POTENTIALLY UNSTABLE LOW LEVEL AIR MASS CONTINUES AHEAD OF COLD FRONT NOW LOCATED FROM CENTRAL UPPER MI SWWD INTO SERN NEB. AS AMBIENT FLOW WEAKENS GRADUALLY THROUGH PERIOD...VERTICAL SHEAR PROFILES OVER MOST OF THE REGION WILL ONLY MARGINALLY SUPPORT A SEVERE THREAT...HOWEVER CONVECTIVE INSTABILITY WILL REMAIN FAVORABLE. FORECAST MLCAPE IN 2500-3500 J/KG RANGE WILL BE COMMON BY MID/LATE AFTERNOON PERIOD OF PEAK DIABATIC HEATING...WHILE SURFACE DEW POINTS WILL STAY IN 70-75 DEG F RANGE. THERE SHOULD BE SUFFICIENT CONVERGENCE ALONG OR JUST AHEAD OF SURFACE FRONT TO INITIATE SEVERE MULTICELL STORMS...WHICH WILL BE OUTFLOW-DOMINANT GIVEN WEAK STORM-RELATIVE INFLOW AND LARGE DEW POINT DEPRESSIONS. DAMAGING DOWNDRAFTS WILL BE THE MAIN THREAT...AND A FEW LARGE HAIL EVENTS ARE POSSIBLE AS WELL. SEVERE THREAT SHOULD DIMINISH SHORTLY AFTER DUSK...WITH LITTLE ADDITIONAL DEVELOPMENT EXPECTED ALONG RESIDUAL OUTFLOW BOUNDARIES. MODIFIED FORECAST SOUNDINGS INDICATE INSTABILITY WILL DISAPPEAR WITH A FEW DEGREES OF NOCTURNAL DIABATIC COOLING IN THE BOUNDARY LAYER.
Each risk area has its own detailed discussion describing the factors expected to produce severe weather and the type and timing of severe weather expected.
SPC outlooks are issued daily, in UTC time (subtract 6 hours for CST, 5 for CDT):
SPC outlooks are designed for more sophisticated weather customers -- although they are widely available on the Internet -- and are considered "guidance" products. The discussions are technical and useful in judging one's chances of being included in a watch later in the day. Spotters can be notified that "today is a day to keep in touch" when there is a risk over your local area. This tends to increase spotter turnout when a watch is issued. Like all guidance products, the outlooks are not a guarantee for severe weather. The Outlook must be used in conjunction with other products to get the full picture. It is a forecast product and is subject to change as additional data is evaluated. For example, what appeared to be a MDT risk situation at 0600 UTC may be downgraded to a SLGT risk at 1630 UTC as the 1200 UTC upper air soundings might show the atmosphere had stabilized more than previously forecast. The opposite can happen also.
It is important not to rigidly associate the type of risk area (SLGT, MDT, HIGH) with the severe potential for any given thunderstorm in the risk area. That is, just because a SLGT risk is forecast does not necessarily mean that the thunderstorms within the risk area will be slightly severe. Sometimes, violent tornadoes occur in SLGT or MDT risk areas as opposed to HIGH. The reason for this is the synoptic situation producing the violent tornadoes may be confined to a relatively small area. Another SLGT risk area may cover several states in which only one or two tornadoes may develop. Some SLGT situations won't involve a threat of tornadoes or supercells, but sustained multicell storms with a threat for severe hail and wind damage. HIGH risk situations, which are rarely forecast, signifies that either an outbreak of tornadoes or extreme and widespread severe wind event is likely.
Remember that almost any thunderstorm can, at some point in its lifetime, produce severe or nearly severe weather. Any thunderstorm can kill. SPC severe weather outlooks, though, forecast the development of well-organized severe weather events, most capable of damage and injury from tornadoes, damaging winds or large hail. They are not meant to cover every isolated, brief or marginally severe thunderstorm; otherwise the general thunder and SLGT risk lines would nearly always be the same.
Pulse-type thunderstorms, consisting primarily of solitary brief severe updrafts (often found in environments with weak vertical wind shear) are not considered to be organized. Convection of this type, and isolated severe storms with marginal intensities or short durations, will likely not be included in a risk area. When an unusually dense or large area of marginally severe reports is anticipated, though, the area of concern will probably be included in a SLGT risk. Examples of "organized" convection include supercells, squall lines, and multicell thunderstorm complexes.
General thunderstorm outlooks are guidance for local forecasters concerning the possibility of more than very isolated or brief thunderstorms in or near their areas. General thunderstorm outlooks forecast thunderstorm coverage of 10% or more of the broad region drawn since almost any thunderstorm may produce a brief severe weather event, it doesn't necessarily mean there is a conflict when a severe thunderstorm warning is issued by a local NWS office in an SPC general thunderstorm outlook.
In short, no two situations are alike, even within the same risk category. This is why a narrative discussion accompanies the outlook - to specifically describe and provide rationale for what kind of severe weather is expected and where/when it is most likely within the risk area.
The Public Severe Weather Outlooks (PWO) are issued for all high risks issuances for potential tornado outbreaks or widespread significant wind damage. This plain-language forecast is typically issued 12-24 hours prior to the event and is used to alert NWS field offices and other weather customers concerned with public safety of a potentially dangerous situation. A PWO is also issued for a moderate risk outlook which contains at least a 15% probability of tornadoes or a 45% probability of damaging wind gusts. The PWO is issued only for Day 1 Outlooks. If the probabilities support a PWO issuance on the 0600 UTC Day 1 Outlook, a PWO would be issued around 1000 UTC and updated around 1700 UTC. If the probabilities first support a PWO issuance on the 1300 UTC Day 1 Outlook, the PWO would be issued around 1300 UTC and updated around 1700 UTC. Below is an example of a PWO.
Here is an example of a PWO:
ZCZC MKCPWOMKC ALL WOUS40 KWNS 050850 KSZ000-NEZ000-051645- PUBLIC SEVERE WEATHER OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 350 AM CDT SAT MAY 5 2007 ...SEVERE THUNDERSTORMS EXPECTED OVER PARTS OF THE CENTRAL PLAINS THIS AFTERNOON AND TONIGHT... THE NWS STORM PREDICTION CENTER IN NORMAN OK IS FORECASTING THE DEVELOPMENT OF A FEW STRONG TORNADOES OVER PARTS OF THE CENTRAL PLAINS THIS AFTERNOON AND TONIGHT. THE AREAS MOST LIKELY TO EXPERIENCE THIS ACTIVITY INCLUDE: CENTRAL KANSAS CENTRAL NEBRASKA SURROUNDING THE MODERATE RISK AREA¿THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM WEST TEXAS NORTHWARD ACROSS MUCH OF THE CENTRAL PLAINS STATES. A STRONG MID/UPPER JET MAX IS FORECAST TO ROTATE ACROSS THE CENTRAL ROCKIES AND INTO THE PLAINS STATES THIS AFTERNOON AND EVENING. MEANWHILE STRONG SOUTHERLY LOW LEVEL WINDS WILL REMAIN IN PLACE THROUGHOUT THE CENTRAL UNITED STATES FEEDING VERY MOIST/UNSTABLE AIR NORTHWARD ACROSS PARTS OF OKLAHOMA KANSAS AND NEBRASKA. NUMEROUS THUNDERSTORMS ARE ONGOING THIS MORNING ACROSS THE REGION. THESE STORMS SHOULD MOVE EASTWARD BY LATE MORNING ALLOWING STRONG HEATING TO OCCUR ALONG THE DRYLINE FROM WEST CENTRAL NEBRASKA INTO WEST CENTRAL KANSAS. SUPERCELL THUNDERSTORMS ARE EXPECTED TO DEVELOP ONCE AGAIN THIS AFTERNOON ALONG THIS AXIS TRACKING NORTH-NORTHEASTWARD DURING THE EVENING. EXTREMELY UNSTABLE CONDITIONS ARE POSSIBLE ALONG WITH STRONG AND VEERING WINDS WITH HEIGHT. THESE PARAMETERS INDICATE A RISK OF VERY LARGE AND DAMAGING HAIL AS WELL AS A FEW STRONG TORNADOES. 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 LATER TODAY. ..HART.. 05/05/2007
The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs provide extra lead time on the severe weather development and allow you to begin gearing up operations before a watch is issued.
ZCZC SPCSWOMCD ALL ACUS11 KWNS 152317 ^^SPC MCD 152317 SCZ000-NCZ000-160215- MESOSCALE DISCUSSION 2065 NWS STORM PREDICTION CENTER NORMAN OK 0617 PM CDT TUE OCT 15 2002 AREAS AFFECTED...PAMLICO SOUND/OUTER BANKS OF NORTH CAROLINA/CAROLINA COASTAL WATERS CONCERNING...SEVERE THUNDERSTORM POTENTIAL VALID 152317Z - 160215Z WATERSPOUTS/STRONG WIND GUSTS WILL REMAIN POSSIBLE THROUGH 0200Z IN THE COASTAL WATERS OF SOUTH CAROLINA AND NORTH CAROLINA AS SURFACE LOW NOW ABOUT 80 ESE ILM TRACKS NORTHWARD TOWARD THE PAMLICO SOUND AREA THIS EVENING. SOME INCREASE IN THREAT OF TORNADOES WILL RESULT...ESPECIALLY LATER THIS EVENING. INITIAL MCS BRUSHING THE OUTER BANDS IS JUST NORTHEAST OF MAIN SURFACE LOW. ENTIRE MCS SHOWS BROAD ROTATION IN RADAR IMAGERY...AND NUMEROUS EMBEDDED ROTATING STORMS ARE EVIDENT ABOUT 15-50 MILES OFFSHORE. DUE TO LARGE SHIELD OF MODERATE/HEAVY RAIN ON NORTHERN EDGE OF MCS OVER EASTERN NORTH CAROLINA... DESTABILIZATION PROCESS OVER LAND WILL BE SLOW DURING THE EVENING. STILL...A COUPLE OF THE ROTATING CELLS WITH AN OUTSIDE CHANCE OF A TORNADO WILL BRUSH NEAR THE OUTER BANKS IN THE NEXT FEW HOURS. NORTH-SOUTH BAND OF THUNDERSTORMS THAT HAS MOVED OFFSHORE OF THE FLORIDA ATLANTIC COAST LIKELY MARKS THE LEADING EDGE OF THE REGION OF UPWARD MOTION THAT WILL CAUSE SIGNIFICANT DEEPENING OF THE SURFACE LOW AS IT SPREADS TOWARD THE NORTH CAROLINA COASTAL WATERS LATER TONIGHT. CELL 75 S CHS HAS SHOWN SOME SUPERCELL CHARACTERISTICS AS IT TRACKS ALONG SURFACE FRONT. EXPECT INCREASING THREAT OF WATERSPOUTS ABOUT 50 MILES OFFSHORE WITH THIS CONVECTION IN THE NEXT FEW HOURS AS WELL. ..CRAVEN.. 10/15/2002 ...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT... 31848001 32757916 33707776 34627680 35427636 35987545 35197453 34077533 32887698 31667909 NNNN
SPC heavy rain MCDs are typically issued for:
Heavy Rain MCDs will contain the expected location, rainfall rates, durations and, most importantly, the reasoning for the forecast heavy rain episode. They're written for somewhat narrower space and time frames than severe weather MCDs: 0-3 hours, and specific sets of counties where the greatest rains are expected. The heavy rain MD is intended as forecast guidance to local NWS offices about the most significant heavy rainfall area; and is not intended to focus on large areas of heavy rain or to be a QPF product.
If heavy rains have already occurred, a heavy rain MCD usually will not be written, unless:
Winter weather MCD's focus on the meteorological processes expected to cause hazardous winter weather: the where, when, what, and (most importantly) why. The meaning of "hazardous weather" varies; but discussions on heavy snow are issued for lake effect snowstorms, climatologically anomalous events or unexpected events. Winter weather MCDs are also issued for forecast snowfall rates of at least 1" per hour in the lowlands and plains, and 2" per hour for areas higher than 4000 feet. Winter MCDs are also issued for freezing rain events especially when amounts are expected to greater than 0.05" per three hours, or for blizzard conditions lasting over three hours.
If hazardous winter weather has already occurred, a MCD usually will not be written, except for:
When conditions become favorable for organized severe thunderstorms and tornadoes to develop, the SPC issues a severe thunderstorm or tornado watch. A tornado can occur in either type of watch, but tornado watches are issued when conditions are especially favorable for either multiple and or strong tornadoes. Watches encourage the general public to stay alert for changing weather conditions and possible warnings. For emergency managers, storm spotters, and the broadcast media, watches provide valuable lead time to gear up operations and increase staffing. Although the general watch area is approximated through the issuance of a parallelogram, the actual watch is issued by counties and collaborated with local NWS offices. Therefore, some counties in the watch may be outside the parallelogram, while come counties in the parallelogram may not be in the actual watch issuance. The watch issued by counties provides a more precise area for the severe threat than the parallelogram.
A typical watch ranges in size from 20,000 to 40,000 square miles, though some are smaller and others larger, depending on the meteorological situation. In most years, a total of 800 to 1000 severe thunderstorm and tornado watches will be issued. Watches are numbered sequentially, with the count reset at the beginning of each year. In the watch header below, the last digit of the number of this watch is 3 (e.g., SEL3), and the whole watch number is given several lines later. A typical watch duration is 6 to 7 hours, but it may be canceled, replaced, or reissued as required. A watch is not a warning, and should not be interpreted as a guarantee that there will be severe weather! When the SPC feels confident about the possibility of severe weather in a specific area, the watch is usually issued at least 1 hour prior the onset of severe weather.
SEL3 URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 843 NWS STORM PREDICTION CENTER NORMAN OK 340 PM EDT TUE AUG 19 2008 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF EAST-CENTRAL AND NORTHEAST FLORIDA PENINSULA COASTAL WATERS EFFECTIVE THIS TUESDAY AFTERNOON AND WEDNESDAY MORNING FROM 340 PM UNTIL 100 AM EDT. TORNADOES...THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS. THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 40 STATUTE MILES EAST AND WEST OF A LINE FROM 45 MILES NORTH NORTHWEST OF DAYTONA BEACH FLORIDA TO 45 MILES SOUTH SOUTHEAST OF VERO BEACH FLORIDA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU3). REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS. OTHER WATCH INFORMATION...CONTINUE...WW 841...WW 842... DISCUSSION...A FAVORABLE COLLOCATION OF STRONG LOW-LEVEL SHEAR AND MODEST INSTABILITY WILL PERSIST THIS EVENING INTO TONIGHT IN RIGHT-FORWARD QUADRANT OF TROPICAL STORM FAY. AS SUCH...THE THREAT FOR SUPERCELLS CAPABLE OF A FEW TORNADOES WILL CONTINUE WITHIN PRIMARY SPIRAL BAND WHICH IS BEING MAINTAINED FROM N OF THE CENTER SSEWD THROUGH THE ERN SEMI-CIRCLE OF THE TROPICAL SYSTEM. AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM MOTION VECTOR 16035. ...MEAD/HART
The watch describes in plain language the state(s) affected, valid times, severe weather potential, meaning of the watch, replacements (if any), a short weather discussion, and aviation information for pilots.
In the AVIATION section, the storm top numbers are in hundreds of feet; so "500" is 50,000 feet. MEAN STORM MOTION VECTOR is the average expected motion of all the storms in the watch: The first 3 digits are direction the storms will move from on a 360-degree compass; and the last two digits are the storm's expected forward (ground) speed in knots. So in the watch example above, storms are forecast to move from the southeast (from 160 degrees) at 35 knots.
When weather conditions are favorable for a potential tornado outbreak with extreme tornadoes, the SPC will often highlight a tornado watch with the following "enhanced" wording:
...THIS IS A PARTICULARLY DANGEROUS SITUATION...
Also, similar wording will be used in severe thunderstorm watches when extremely destructive and widespread thunderstorm wind events are anticipated.
Unfortunately, not all severe weather situations are clear cut. For example, severe weather may be expected IF thunderstorms form, but there may be doubt about whether storms will develop. In such cases, SPC may wait until storms actually develop before they issue a watch. Sometimes warnings may precede a watch, especially when weaker severe storms develop before the greater severe threat has developed. If severe weather develops unexpectedly, but is expected to be short lived (last less than a couple of hours) or is only very isolated, a watch probably will not be issued. Instead, the storms would be handled with warnings issued by your local NWS office.
When a watch is issued, the SPC transmits a product called a Watch Outline Update (WOU). This product contains a listing of all counties in the watch. The local NWS offices will also issue a Watch County Notification (WCN) message that lists the counties in the watch within their area of responsibility. Once the watch is issued, the local offices will issue WCN messages to take counties out of the watch, add counties as needed and even at times, extend the watch expiration time. The WOU will be updated at least hourly to incorporate the changes made in the WCNs.
Not all watches are created equal, so to provide a better perspective of what is expected severe weather-wise, a set of watch probabilities is included with each watch issuance. Two probabilities for each severe hazard (tornadoes, damaging winds and hail) are included along with the probability of 6 or more severe events. A table illustrating these probabilities is included below.
|Probability of 2 or more tornadoes
|Probability of 1 or more strong (F2-F5) tornadoes
|Probability of 10 or more severe wind events
|Probability of 1 or more wind events > 65 knots
|Probability of 10 or more severe hail events
of 1 or
more hailstones > 2 inches
|Probability of 6 or more combined severe hail/wind events
Watch Status Messages will be issued at the bottom of each hour (between 20 and 40 minutes after), during the lifetime of each severe thunderstorm and/or tornado watch. The first status message usually will not be issued until the watch has been in effect at least 30 minutes. Each watch status message uses the distance in statue miles relative to anchor points and to the right of a line for delineating where the severe weather threat continues. The watch status lines are drawn from one edge of the watch parallelogram to the other edge. The watch status process also includes deselecting those counties where the severe weather threat, to the left of the status line, appears to be over.
The watch status messages rarely include a discussion of the meteorology affecting the watch area, as this information will be contained in a mesoscale discussion (MD). The exception may be with the final watch status message, where a brief sentence sometimes will be included explaining why the watch will be re-issued or allowed to expire.
Watch Status example:
WOUS20 KWNS 252240 WWASPC SPC WW-A 252240 NYZ000-OHZ000-PAZ000-LEZ000-LOZ000-092300- STATUS REPORT ON WW 737 THE SEVERE WEATHER THREAT CONTINUES TO THE RIGHT OF A LINE FROM 50 SW HUM TO 30 NW BE. ..KERR..11/25/07 ATTN...WFO...LCH...LIX... && SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS LAC075-252300- LA . LOUISIANA PARISHES INCLUDED ARE PLAQUEMINES GMZ550-252300 CW . ADJACENT COASTAL WATERS INCLUDED ARE COASTAL WATERS FROM THE SOUTHWEST PASS OF THE MISSISIPPI RIVER TO LOWER ATCHAFALAYA RIVER LA OUT 20 NM $$ THE WATCH STATUS MESSAGE IS FOR GUIDANCE PURPOSES ONLY. PLEASE REFER TO WATCH COUNTY NOTIFICATION STATEMENTS FOR OFFICIAL INFORMATION ON COUNTIES... INDEPENDENT CITIES AND MARINE ZONES CLEARED FROM SEVERE THUNDERSTORM AND TORNADO WATCHES. $$
The purpose of the SPC Fire Weather program is to provide a national fire weather guidance product for use by the National Weather Service, as well as other federal, state, and local government agencies. The product is intended to delineate areas of the contiguous U. S. where the pre-existing fuel conditions, combined with forecast weather conditions during the next 8 days will result in a significant threat for wildfires.
There are three types of Fire Weather Outlook areas - a Critical Fire Weather Area for Wind and Relative Humidity, an Extremely Critical Fire Weather Area for Extreme Conditions of Wind and Relative Humidity, and a Critical Fire Weather Area for Dry Thunderstorms.
The SPC Fire Weather Outlook is comprised of a Day 1 and a Day 2 forecast, in addition to a Day 3-8 forecast. Each forecast period will contain text products along with corresponding graphic products. The Day 1 Fire Weather Outlook is scheduled for issuance at 4:00 AM CST/CDT and is updated at 17Z. The Day 2 Fire Weather Outlook is scheduled for issuance at 10Z and is updated at 20Z. The Day 3-8 Fire Weather Outlook is scheduled for issuance at 22Z.
The Day 1 Outlook covers the 24-hour period from 12Z on the morning of product issuance to 12Z the following morning, with the update covering the 19-hour period from 17Z at issuance to 12Z the following morning. The Day 2 Outlook covers the following 24-hour period out to 48 hours. The Day 3-8 Outlook covers the period of 48 to 192 hours from 12Z on the morning of product issuance.
The outlook type depends upon the severity of the forecast weather, antecedent conditions, and climatology relative to the given geographic region. Critical Fire Weather Areas for Wind and Relative Humidity are typically issued when strong winds (>20 mph) and low RH are expected to occur where dried fuels exist. Critical Fire Weather for Dry Thunderstorms are typically issued when widespread or numerous thunderstorms producing little wetting rain (<0.10 in) are expected to occur where dried fuels exist. Extremely Critical Fire Weather Areas for Wind and Relative Humidity are issued when very strong winds and very low RH are expected to occur with very dry fuels. Extremely Critical areas will be rarely issued, similar to the very low frequency of High Risk Convective Outlooks.
Reports are lumped under three basic categories: tornado reports, large hail/strong wind reports, and other severe reports. The "tornado reports" section is self explanatory. The "large hail/strong winds reports" section contains information on very large hail and major wind damage. The "other severe reports" section contains miscellaneous reports of marginally severe weather such as hail one inch in diameter or smaller.
The log is a raw listing of all reports received. The final list of reports is found in the monthly publication Storm Data, which is compiled by the National Climatic Data Center (NCDC) several months later from report lists submitted by local NWS offices. For details about Storm Data, contact the National Climatic Data Center thru email@example.com.
Again, there is no guarantee as to the accuracy of SPC rough log reports and should be regarded as strictly preliminary. Again, the rough log is automated and depends on properly formatted local storm reports (LSRs) sent by local NWS offices. Reports may be reclassified as time goes on, too. What's initially reported as a tornado today might be called straight-line wind damage a few days later after a survey. While SPC maintains a detailed database in Norman, they do not issue corrections to the daily report log. Further, if the report doesn't make it into the log during the 24 hour log period in which the event occurred, the event will not be listed. That is, wind damage discovered today from a storm that happened two days ago will not be listed in today's report log.
Here's what a typical report log report might look like:
ZCZC MKCSTADTS WWUS60 KMKC 131200 SPC TORNADO AND SEVERE THUNDERSTORM REPORTS UNOFFICIAL - FOR OFFICIAL REPORTS, SEE PUBLICATION 'STORM DATA' FOR 06CST TUE OCT 12 1993 THRU 06CST WED OCT 13 1993 EVENT LOCATION REMARKS (CST)TIME TORNADO REPORTS..........TORNADO REPORTS..........TORNADO REPORTS..... 80 *TORN 2 SW DUSTER TX (28 WSW SEP) 12/2145 PSBL TORNADO; HOMES DMGD; SVRL PERSONS FTW/LSR 32139865 HOSPITALIZED .......LRG HAIL/STRONG WIND RPTS.......LRG HAIL/STRONG WIND RPTS...... 55 A450 PROFFITT TX (55 WNW MWL) 12/1905 FTW/LSR 33199888 12 WNDG BRADY TX (49 NNE JCT) 12/1642 SIGNS DOWN.STEEPLE OFF CHURCH; TREES & POWER SJT/LSR 31139933 POLES DOWN. 2 G 56 DRYDEN TX (17 E P07) 12/1420 60-70 MPH WNDS; SPOTTER RPRT MAF/SVS 300510211 .........OTHER SEVERE REPORTS..........OTHER SEVERE REPORTS......... 91 A 75 ADDICKS TX (24 WNW HOU) 12/1215 DIME SIZED HAIL NR LAMAR HIGH SCHOOL HOU/LSR 29789565
This is an hourly updated version of the daily report log. It is in the same format, except for some coding in the header:
ZCZC MKCSTAHRY WWUS60 KMKC 131200 SPC TORNADO AND SEVERE THUNDERSTORM REPORTS UNOFFICIAL - FOR OFFICIAL REPORTS, SEE PUBLICATION 'STORM DATA' FOR 06CST TUE OCT 12 1993 THRU 22CST WED OCT 13 1993 EVENT LOCATION REMARKS (CST)TIME TORNADO REPORTS..........TORNADO REPORTS..........TORNADO REPORTS..... 80 *TORN 2 SW DUSTER TX (28 WSW SEP) 12/2145 PSBL TORNADO; HOMES DMGD; SVRL PERSONS FTW/LSR 32139865 HOSPITALIZED .......LRG HAIL/STRONG WIND RPTS.......LRG HAIL/STRONG WIND RPTS...... 55 A450 PROFFITT TX (55 WNW MWL) 12/1905 FTW/LSR 33199888 12 WNDG BRADY TX (49 NNE JCT) 12/1642 SIGNS DOWN.STEEPLE OFF CHURCH; TREES & POWER SJT/LSR 31139933 POLES DOWN. 2 G 56 DRYDEN TX (17 E P07) 12/1420 60-70 MPH WNDS; SPOTTER RPRT MAF/SVS 300510211 .........OTHER SEVERE REPORTS..........OTHER SEVERE REPORTS......... 91 A 75 ADDICKS TX (24 WNW HOU) 12/1215 DIME SIZED HAIL NR LAMAR HIGH SCHOOL HOU/LSR 29789565 15 B200 O45 (ORL)ORLANDO EXEC ARPT FL 12/1655 OV ORL 045008/TM 2255/FL010/TP PARO/TB ORL/UUA 28558133 2 IN DIA HAIL
The hourly log will contain all reports received by SPC from 0600 CST (12Z) through the latest full hour.
ZCZC MKCSTAMTS TTAA00 KNAW DDHHMM STORM PREDICTION CENTER (NORMAN OK) ...THROUGH 6 AM CDT 02/15/99 STATISTICS FOR TORNADO TOTALS AND TORNADO RELATED DEATHS ............NUMBER OF TORNADOES............. NUMBER OF KILLER TORNADO DEATHS TORNADOES ....1999.... ....1998.... 1997 1996 3YR 3YR PRELIM FINAL PRELIM FINAL FINAL FINAL AVG 99 98 97 96 AVG 99 98 97 JAN 169 - 20 49 50 35 45 19 - 2 1 2 9 - 2 FEB 9 - 56 78 23 14 38 - 41 1 1 14 - 4 1 MAR - - 66 80 102 71 84 - 16 28 6 17 - 4 9 APR - - 196 208 114 177 166 - 55 1 12 23 - 14 1 MAY - - 309 326 225 235 262 - 10 29 1 13 - 5 3 JUN - - 372 400 193 128 240 - 3 - - 1 - 2 - JUL - - 59 82 188 202 157 - - 4 1 2 - - 4 AUG - - 32 64 84 72 73 - - 1 - 1 - - 1 SEP - - 61 109 32 101 81 - 2 1 - 1 - 2 1 OCT - - 64 66 100 68 78 - 2 - - 1 - 2 - NOV - - 18 19 25 55 33 - - - 2 1 - - - DEC - - 1 - 12 15 15 - - - 1 1 - - - ---- ---- ---- ---- ---- ---- --- -- --- -- -- -- -- -- -- SUM 178 - 1254 1481 1148 1173 1272 19 129 67 25 77 9 33 22 MCCARTHY NNNN
The statistics are broken down by month and contain data for the last four years. An "-" in a column means the data is missing or not yet available. There is also a column called "3 YR AVG" that gives the average number of tornadoes per month (based on the 3 years' data). In the 1998 columns under "NUMBER OF TORNADOES" there were 20 tornadoes initially reported in January under the PRELIM (preliminary) category. SPC does not include reports of "unconfirmed" or "possible" tornadoes in the PRELIM numbers.
When the digital Storm Data database arrives from the NWS Office of Meteorology, FINAL numbers go in that column. Those include removal of any erroneous/duplicate reports or added reports which were initially missed or misclassified. The FINAL numbers are not whole tornadoes, but instead county-segments of tornado tracks, which accounts for much of the increase between PRELIM and FINAL. The NWS uses county-segments because warnings are verified by county. For example: Tornado A stays in one county. Its whole path stays counted as 1. Tornado B is a long-tracker which crossed parts of 8 counties. Its FINAL breakdown is 8. Therefore, in most cases, the FINAL totals will be more than the PRELIM values, because of the county-segment breakdown.
The DEATHS columns are simply the number of people killed by month for the years 96 through 98 and the average killed (3 years) per month. The KILLERS columns represent killer tornado events for the current year and the 3-year average. A tornado is counted as a killer if one or more persons were killed. If 100 people were killed by a single tornado it would be counted as one killer event. Multiple killer tornadoes on the same day are counted as separate events.
Along the bottom of the report are totals for the columns and a simplified re-cap. For example, there were 1254 preliminary (PRELIM) reports of tornadoes in 1998, versus 1481 tornado segments FINAL through Dec 1, 1998.
ZCZC MKCSTATIJ TTAA00 KNAW DDHHMM STORM PREDICTION CENTER (NORMAN OK) CST # DATE TIME LOCATION DEATHS A B C D WATCH F CIRCUMSTANCES = ====== ==== ===================== ====== = = = = ====== == ============= 1 JAN 02 0040 BUNA TX 1 1 0 0 0 WT0003 F2 1M 2 JAN 17 1825 JACKSON TN 7 7 0 0 0 WT0012 F4 07? 3 JAN 17 1900 SAULSBURY TN 1 1 0 0 0 WT0012 F1 01? 4 JAN 17 1905 ATWOOD TN 1 1 0 0 0 WT0013 F? 01? 5 JAN 21 1720 CENTER HILL AR 2 2 0 0 0 WT0018 F2 01M 01V 6 JAN 21 1720 PLEASANT PLAINS AR 1 1 0 0 0 WT0018 F2 01M 7 JAN 21 1847 LITTLE ROCK AR 3 3 0 0 0 WT0018 F3 01M 01V 01P 8 JAN 21 1935 BEEBE AR 2 2 0 0 0 WT0018 F3 02H 9 JAN 22 0330 7N CAMDEN TN 1 1 0 0 0 WT0027 F3 01O -- -- - - - TOTALS: 19 19 0 0 0 BY STATE: TN 10 AR 08 TX 01 BY CIRCUMSTANCE: 04M 02H 02V 01O 01P 09? PRELIMINARY THRU 30 JAN 99. ... EDWARDS ...
The killer tornadoes are listed in the chronological order they happened, by DATE and CST TIME. LOCATION is self-explanatory. DEATHS is number of deaths in the whole tornado path -- not juct the given location. The ABCD column letters represent the number of deaths:
If the tornado was in a watch, the watch type and number is given. For example, WT0012 is Tornado Watch number 12. If known, the F-scale damage rating of the tornado is listed; if not, a "?" mark is entered. The deaths are broken down by the following circmstances of the victims, if known:
Information for the killer tornadoes list comes from Local Storm Reports (LSRs) and Public Information Statements (PNS) issued by local NWS offices, supplemented by news from internal NWS event memos and Internet media accounts. Since killer tornado information -- especially death counts, circumstances and F scale, is often not complete until many days later, these numbers are very preliminary and subject to change as more information arrives.