SPC and its Products

Chris Hayes Novy

Sinclair Broadcast Group

Roger Edwards, David Imy (ret.) and Stephen Goss

Storm Prediction Center

Page last modified: 3 Nov 2014

The Storm Prediction Center (SPC) at Norman, Oklahoma, is a part of the National Weather Service (NWS) charged with monitoring and forecasting severe weather and fire 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.

  1. What is Severe Weather?
  2. Convective Outlooks
    1. Day 1 Convective Outlook
    2. Day 2 Convective Outlook
    3. Day 3 Convective Outlook
    4. Day 4-8 Severe Weather Outlook
    5. Probabilistic Outlook
    6. Plotting the Points
    7. Levels of Risk
    8. Severe Thunderstorm Discussion
    9. Using the Technical Outlooks
    10. The Public Severe Weather Outlook (PWO)
  3. Mesoscale Discussions (severe thunderstorms)
    1. Winter Weather MCD
  4. Severe Weather Watches
  5. Watch Description
  6. Watch Outline Update (WOU) and Watch County Notification (WCN)
  7. Watch Probabilities
  8. Watch Status Messages
  9. Fire Weather Outlooks
  10. Statistics

What is 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:

  1. A tornado.
  2. Damaging winds, or winds measured 50 knots (approx. 58 MPH) or more.
  3. Hail 1 inch in diameter or larger.

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 heavy snow, freezing rain, and blizzard events in portions of states. SPC also forecasts tornadoes in mainland U.S. tropical cyclones, in coordination with the National Hurricane Center.


Convective Outlooks

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. Five separate risk areas (marginal, slight, enhanced 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.

Day 1 Convective Outlook

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 (marginal, slight, enhanced 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 for all outlooks.


Day 2 Convective Outlook

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.


Day 3 Convective Outlook

The Day 3 Convective Outlook text and probabilities are similar to the Day 2 Outlooks. However, this outlook is only issued once a day, by 400 am CST/CDT.


Day 4-8 Severe Weather Outlook

The Day 4-8 graphic consists of one map depicting the severe weather threat during the forecast period. A single 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.


Plotting the Points
   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 (i.e. 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 comes back in from the Canadian border at 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.


Levels of Risk
Risk areas come in five varieties and are based on the expected number and intensity of severe thunderstorm reports over an area:
  1. GEN TSTMS (not labelled on the graphic outlook but listed in the discussion) - General (non-severe) thunderstorms
  2. MRGL - Marginal risk, both graphic and text
  3. SLGT - Slight risk, both graphic and text
  4. ENH - Enhanced Slight risk, both graphic and text
  5. MDT - Moderate risk, both graphic and text
  6. HIGH - High risk, both graphic and text

The former SEE TEXT label was replaced by the MRGL outlook in October, 2014. As a rule, MRGL is used on Days 1-3 for areas where severe weather may occur if some low-probability ingredients come together, or if severe weather is forecast but should be isolated and mrginal in nature. Note that the SPC convective outlooks are not meant to cover every single remote possibility of a severe thunderstorm -- otherwise, severe and general thunder outlooks often would be the same.

A SLGT risk implies that well-organized severe thunderstorms are expected but either in relatively low numbers or coverage, or a small chance of a more significant severe event.

An ENH risk implies a greater concentration of organized severe thunderstorms with varying levels of intensity.

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 strong confidence. A large coverage of severe weather is likely with some instances of extreme severe (i.e., violent tornadoes or very damaging convective wind events) embedded in that. The HIGH risk category is reserved for the most intense and dangerous events with the most forecast certainty, and is only used a few times each year at most.

The outlook categories are driven by the specific tornado, damaging wind, and large hail probabilities 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. An example from a real outlook (24 May 2011 HIGH risk) follows.


Severe Thunderstorm Discussion
   
...CENTRAL/SOUTHERN PLAINS...
MORNING SATELLITE LOOPS SHOW A VIGOROUS AND PROGRESSIVE MID/UPPER
LEVEL SHORTWAVE TROUGH OVER THE SOUTHERN ROCKIES.  THIS FEATURE 
IS FORECAST TO ROTATE RAPIDLY EASTWARD INTO THE PLAINS AND BECOME
NEGATIVELY TILTED THIS EVENING.  LATEST TCC PROFILER DATA SHOWS 
90+ KNOT FLOW AT 6KM...INDICATIVE OF THE MID/UPPER LEVEL WIND MAX 
THAT WILL NOSE INTO THE PLAINS THIS EVENING.  MEANWHILE...
SOUTHERLY LOW LEVEL WINDS WILL MAINTAIN UPPER 60S AND LOWER 70S
DEWPOINTS ACROSS MUCH OF NORTH TX...OK...AND SOUTHERN/CENTRAL KS.
BY MID AFTERNOON THE DRYLINE WILL EXTEND FROM WEST-CENTRAL KS
INTO WESTERN OK AND WESTERN NORTH TX.  STRONG HEATING AND RICH
MOISTURE...COUPLED WITH NEAR DRY-ADIABATIC MIDLEVEL LAPSE 
RATES...WILL YIELD AN EXTREMELY UNSTABLE AIR MASS WITH MLCAPE 
VALUES OF 4000-4500 J/KG.
   
PRESENT INDICATIONS AND LATEST OPERATIONS/MESOSCALE MODEL
GUIDANCE INDICATE THAT DISCRETE SUPERCELL THUNDERSTORMS WILL FORM 
ALONG THE KS/OK DRYLINE BY MID AFTERNOON AND MOVE RAPIDLY
NORTHEASTWARD WITH A RISK OF A FEW TORNADOES AND VERY LARGE HAIL.
EARLY IN THE EVENT...THE GREATEST TORNADO THREAT WILL BE OVER KS 
AS STORMS INTERACT WITH PRE-EXISTING OUTFLOW BOUNDARY AND BACKED 
LOW LEVEL WINDS.

AS THE AFTERNOON PROGRESSES...CONTINUED HEATING AND LARGE-SCALE 
LIFT DUE TO APPROACHING UPPER TROUGH MAY LEAD TO MULTIPLE ROUNDS 
OF CONVECTIVE DEVELOPMENT ALONG THE DRYLINE.  THESE STORMS WILL 
ALSO TRACK ACROSS THE HIGH RISK AREA DURING THE EVENING.  LOW- 
LEVEL WINDS WILL BE STRENGTHENING...WITH ALL CONDITIONS FAVORING 
THE POTENTIAL FOR LONG-TRACKED STRONG/VIOLENT TORNADOES AND VERY 
LARGE HAIL OVER PORTIONS OF NORTH TX...CENTRAL OK...AND CENTRAL 
KS.
   
DURING THE LATE EVENING...STORMS ARE EXPECTED TO CONGEAL AND 
SPREAD EASTWARD INTO PORTIONS OF MO/AR WITH AN ENHANCED RISK OF 
WIDESPREAD DAMAGING WINDS AND HAIL.

   

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.


Using the Technical Outlooks

SPC outlooks are issued daily, in UTC time (subtract 6 hours for CST, 5 for CDT):

  1. The Day 1 Outlooks will be issued at 0600 UTC, 1300 UTC, 1630 UTC, 2000 UTC and 0100 UTC year-round.
  2. The Day 2 Outlooks will be issued at 0700 (0600 UTC daylight time) and 1730 UTC.

SPC outlook discussions are designed for weather customers with a strong meteorology background -- 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 (MRGL, SLGT, ENH, 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, ENH 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 or a very conditional, uncertain situation. 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, may not be included in a risk area, unless confidence is good enough to draw MRGL risk. When an unusually dense or large area of low-end 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.


Public Severe Weather Outlooks

The Public Severe Weather Outlooks (PWO) are issued for all HIGH risks and for some MDT areas, just on Day 1. This plain-language forecast is typically issued the morning of the event and is used to alert non-technical 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. 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. PWOs now are in mixed-case text.

Here is an example of a PWO:

   
PUBLIC SEVERE WEATHER OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK
0350 AM CDT MON OCT 13 2014

   ...Severe thunderstorms expected over parts of the Mid South and Lower Mississippi Valley today...

   * LOCATIONS...
     Central and eastern Arkansas
     Western and northern Mississippi
     Western Tennessee
     Northern Louisiana
     Southeastern Missouri
     Western Kentucky
     Southern Illinois

   * HAZARDS...
     Widespread damaging winds, some hurricane force
     A few intense tornadoes
     Isolated large hail

   * SUMMARY...
     Strong to severe thunderstorms are expected today and
     tonight from East Texas through the lower and middle
     Mississippi Valley into parts of the Tennessee Valley, Ohio
     Valley and Gulf Coast states. Widespread damaging winds and
     a few tornadoes are likely, especially over the Mid South 
     and Lower Mississippi Valley regions today.

   Preparedness actions...

   Review your severe weather safety procedures for the
   possibility of dangerous weather today. Stay tuned to NOAA
   Weather Radio, weather.gov, or other media for watches and 
   warnings. A watch means that conditions are favorable for
   severe thunderstorms over the next several hours. If a severe
   thunderstorm warning is issued for your area, move to a place
   of safety, ideally in an interior room on the lowest floor of 
   a sturdy building.

   &&

   ..Edwards.. 10/13/2014
   


Mesoscale Discussions

When conditions appear favorable for severe storms development, SPC issues a Mesoscale Discussion (MCD), normally 1 to 3 hours before issuing a weather watch. SPC also puts out MCDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards and freezing rain (see below).

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. For severe-storm situations, they also offer a statement of watch potential and the probability that SPC will issue a watch within the valid time of the MCD.

 
   MESOSCALE DISCUSSION 1897
   NWS STORM PREDICTION CENTER NORMAN OK
   1148 PM CDT TUE OCT 14 2014

   AREAS AFFECTED...ERN NC

   CONCERNING...SEVERE POTENTIAL...WATCH UNLIKELY 

   VALID 150448Z - 150545Z

   PROBABILITY OF WATCH ISSUANCE...20 PERCENT

   SUMMARY...THE POTENTIAL FOR A BRIEF TORNADO WILL EXIST
   OVERNIGHT WITH PERSISTENT TSTM ACTIVITY OCCURRING ALONG A 
   CONFLUENCE ZONE. A WATCH IS NOT CURRENTLY ANTICIPATED.

   DISCUSSION...A LONG-LIVED CONFLUENCE ZONE HAS BEEN A GENESIS 
   REGION FOR TSTM DEVELOPMENT THIS EVENING INTO TONIGHT FROM 
   NEAR AND NORTH OF EWN TO OFF THE COAST WELL SSE OF ILM. RECENT 
   TRENDS IN VOLUMETRIC RADAR DATA INDICATE A GRADUAL DEEPENING 
   OF TSTMS FROM BEAUFORT TO CARTERET COUNTIES WITHIN A VERY 
   MOIST AND AT LEAST WEAKLY UNSTABLE ENVIRONMENT BASED ON THE 
   00Z MHX SOUNDING. SINCE THAT OBSERVATION WAS TAKEN...CURRENT 
   VAD DATA FROM MHX SHOW PERHAPS A SLIGHT INCREASE IN DEEP-LAYER 
   SLY FLOW WITH 0-1 KM SHEAR OF AROUND 25 KT. 

   FURTHER STRENGTHENING OF THE WIND FIELD IS ANTICIPATED
   OVERNIGHT IN RESPONSE TO THE MOVEMENT OF A POTENT VORTICITY 
   MAXIMUM INTO THE SERN STATES. AS SUCH...THE ENVIRONMENT WILL
   BECOME PROGRESSIVELY MORE SUPPORTIVE OF UPDRAFT ROTATION WITH 
   AT LEAST SOME RISK FOR A BRIEF TORNADO. A MORE ROBUST SEVERE 
   WEATHER THREAT IS EXPECTED TO BE PRECLUDED BY WEAK 
   TROPOSPHERIC LAPSE RATES AND RESULTING MARGINAL INSTABILITY. 
   THUS...A WATCH IS NOT CURRENTLY ANTICIPATED.

   ..MEAD.. 10/15/2014
   


Winter Weather MCD

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:

  1. 1) Changing meteorological parameters that indicate an end to the event
  2. 2) An episode that may have been overlooked,
  3. 3) Further hazardous winter weather across the same area where recent heavy snow, freezing rain or blizzard conditions have occurred, or
  4. 4) When an event is expected to continue and shift into a different area.


Severe-weather Watches

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 8 hours, but it may be canceled, replaced, or reissued as required. Watches are canceled only at the local (WFO) level. 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.


   SEL9

   URGENT - IMMEDIATE BROADCAST REQUESTED
   SEVERE THUNDERSTORM WATCH NUMBER 549
   NWS STORM PREDICTION CENTER NORMAN OK
   310 PM EDT TUE OCT 14 2014

   THE NWS STORM PREDICTION CENTER HAS ISSUED A

   * SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 
     SOUTHEASTERN GEORGIA
     THE SOUTH CAROLINA COASTAL PLAIN
     COASTAL WATERS

   * EFFECTIVE THIS TUESDAY AFTERNOON FROM 310 PM UNTIL MIDNIGHT
     EDT.

   * PRIMARY THREATS INCLUDE...
     SCATTERED DAMAGING WIND GUSTS TO 70 MPH POSSIBLE

   THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND
   55 STATUTE MILES EITHER SIDE OF A LINE FROM 5 MILES SOUTH- 
   SOUTHWEST OF VIDALIA GEORGIA TO 30 MILES NORTH NORTHEAST OF 
   CHARLESTON SOUTH CAROLINA.  FOR A COMPLETE DEPICTION OF THE 
   WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS 
   WOU9).

   PRECAUTIONARY/PREPAREDNESS ACTIONS...

   REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
   FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN AND
   OCCASIONALLY DO PRODUCE TORNADOES.

   &&

   OTHER WATCH INFORMATION...CONTINUE...WW 547...WW 548...

   DISCUSSION...PRE-FRONTAL TSTM CLUSTERS NOW OVER THE SE GA CSTL
   PLN EXPECTED TO MOVE/DEVELOP NNE INTO THE LWR SAVANNAH RVR VLY
   AND...LATER...THE SC CSTL PLN AS HEATING CONTINUES IN VERY 
   MOIST SLY FLOW E OF THE STORMS. GRADUALLY INCREASING SSWLY 
   DEEP SHEAR...AND STRENGTHENING DCVA/UPR DIFLUENCE EXPECTED BY 
   THIS EVE...SUGGEST POSSIBLE UPSCALE DEVELOPMENT INTO A MORE 
   STRONGLY ORGANIZED SQLN/MCS...WITH A ASSOCIATED RISK FOR DMGG 
   WIND AND POSSIBLY A BRIEF TORNADO.

   AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND
   ALOFT TO 0.5 INCH. EXTREME TURBULENCE AND SURFACE WIND GUSTS 
   TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN 
   STORM MOTION VECTOR 22030.

   ...CORFIDI
   


Watch Description

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.


Watch Outline Update (WOU) and Watch County Notification (WCN)

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.


Watch Probabilities

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.

Tornadoes
Probability of 2 or more tornadoes

Mod (40%)

Probability of 1 or more strong (F2-F5) tornadoes

Low (20%)

Wind
Probability of 10 or more severe wind events

Low (20%)

Probability of 1 or more wind events > 65 knots

Low (10%)

Hail
Probability of 10 or more severe hail events

Low (<5%)

Probability of 1 or more hailstones > 2 inches

Low (<5%)

Combined Severe Hail/Wind
Probability of 6 or more combined severe hail/wind events

Mod (50%)


Watch Status Messages

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:

   
STATUS REPORT #2 ON WW 544

VALID 141125Z - 141240Z

SEVERE WEATHER THREAT CONTINUES RIGHT OF A LINE FROM 35 NNE ABY
TO 10 NE MCN TO 45 NNE MCN TO 20 S AHN TO 35 NW AHN TO 60 NNW AHN.

FOR ADDITIONAL INFORMATION SEE MESOSCALE DISCUSSION 1889

..DIAL..10/14/14

ATTN...WFO...FFC...

&&

STATUS REPORT FOR WT 544 

SEVERE WEATHER THREAT CONTINUES FOR THE FOLLOWING AREAS 

GAC009-011-023-059-081-091-093-133-157-219-221-235-237-271-311-
315-319-141240-

GA 
.    GEORGIA COUNTIES INCLUDED ARE

BALDWIN 	     	BANKS		 BLECKLEY	     
CLARKE		     CRISP		 DODGE		     
DOOLY		     	GREENE	 JACKSON	     
OCONEE		     OGLETHORPE PULASKI	     
PUTNAM		     TELFAIR	 WHITE		     
WILCOX		     WILKINSON		 
$$

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.
$$


Fire Weather Outlooks

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 five types of Fire Weather Outlook areas: Elevated, Critical or Extremely Critical for wind and relative humidity, and Isolated and Scattered areas 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. Extremely Critical areas will be rarely issued, similar to the very low frequency of High Risk Convective Outlooks. Scattered for Dry Thunderstorms is typically issued when widespread or numerous thunderstorms producing little wetting rain (<0.10 in) are expected to occur where dried fuels exist. [Isolated dry-thunder fire risk is covered in Isolated Dry Thunderstorm risk areas.] 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.

An example of SPC Fire Weather Outlook text follows for a day with all risk areas up to and including Extremely Critcal, as well as Isolated Dry Thunderstorms.

   ZCZC SPCFWDDY1 ALL
   FNUS21 KWNS 140846

   DAY 1 FIRE WEATHER OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0346 AM CDT SAT JUN 14 2014

   VALID 141200Z - 151200Z

   ...EXTREMELY CRITICAL FIRE WEATHER AREA FOR FAR NORTHEAST 
   ARIZONA AND NORTHWEST NEW MEXICO...
   ...CRITICAL FIRE WEATHER AREA FOR EASTERN ARIZONA...EXTREME
   SOUTHEASTERN UTAH...EXTREME SOUTHWEST COLORADO...AND NORTHWEST 
   NEW MEXICO...

   ...SYNOPSIS...
   LARGE-SCALE TROUGH /CURRENTLY OVER THE WEST COAST/ WILL
   CONTINUE TO MOVE EAST THROUGH THE FORECAST PERIOD. A STRONG 
   SHORT-WAVE TROUGH /EMBEDDED WITHIN THIS LARGER-SCALE TROUGH/ 
   WILL RAPIDLY EJECT EAST/NORTHEASTWARD FROM THE SOUTHWEST 
   UNITED STATES INTO THE NORTHERN PLAINS. AS IT DOES...SURFACE 
   CYCLOGENESIS WILL BE INDUCED ACROSS EASTERN COLORADO EARLY IN 
   THE FORECAST PERIOD BEFORE EJECTING NORTHEAST AHEAD OF THE 
   AFOREMENTIONED TROUGH.

   ...EASTERN ARIZONA...EXTREME SOUTHEASTERN UTAH...EXTREME 
   SOUTHWEST COLORADO...AND NORTHWEST NEW MEXICO...
   AS CYCLOGENESIS OCCURS ACROSS EASTERN COLORADO...THE SURFACE
   PRESSURE GRADIENT WILL STRENGTHEN AREA WIDE...RESULTING IN
   STRONG SURFACE WINDS. ADDITIONALLY...THE PRESENCE OF A DEEP 
   BOUNDARY LAYER WILL MIX DOWN STRONGER FLOW ALOFT THAT WILL 
   OVERSPREAD THE AREA AS THE SHORT-WAVE TROUGH RACES BY. THE 
   COMBINATION OF THE STRONG/GUSTY SURFACE WINDS WITH RELATIVE-
   HUMIDITY VALUES BETWEEN 5 AND 15 PERCENT AND DRY FUELS WILL 
   RESULT IN WIDESPREAD CRITICAL FIRE-WEATHER CONDITIONS.

   AT THIS TIME...IT APPEARS THE STRONGEST SURFACE WINDS AND 
   LOWEST RELATIVE HUMIDITY WILL BE JUXTAPOSED OVER PORTIONS OF 
   NORTHEAST ARIZONA AND NORTHWEST NEW MEXICO. HERE...SUSTAINED 
   SURFACE WINDS GREATER THAN 30 MPH /WITH GUSTS NEARING 50 MPH/ 
   AND RELATIVE-HUMIDITY VALUES AROUND 7 PERCENT WILL SUPPORT
   EXTREMELY-CRITICAL FIRE-WEATHER CONDITIONS.

   SURROUNDING THIS AREA...SUSTAINED SURFACE WINDS ARE EXPECTED 
   TO BE SOMEWHAT LIGHTER /BETWEEN 20-30 MPH/ AND/OR RELATIVE-
   HUMIDITY VALUES SOMEWHAT HIGHER /10-20 PERCENT/. THIS WILL 
   RESULT IN CRITICAL FIRE-WEATHER CONDITIONS.

   SURROUNDING THE CRITICAL FIRE-WEATHER AREA...QUESTIONS ABOUT 
   THE DURATION OF CRITICALLY LOW RELATIVE-HUMIDITY AND 
   CRITICALLY HIGH WIND SPEEDS...AND/OR FUEL DRYNESS WILL RESULT 
   IN AN ELEVATED TO LOCALLY CRITICAL FIRE-WEATHER THREAT.

   ...CENTRAL NEW MEXICO MOUNTAINS...
   BY EARLY AFTERNOON...OROGRAPHIC CIRCULATIONS COUPLED WITH WELL 
   ABOVE SEASONAL PRECIPITABLE-WATER VALUES /AROUND 0.6 INCHES/ 
   WILL SUPPORT ISOLATED THUNDERSTORM DEVELOPMENT. DESPITE THE 
   HIGH PRECIPITABLE-WATER VALUES...A DEEP...WELL-MIXED BOUNDARY 
   LAYER /AND THE RESULTING VERY DRY SUB-CLOUD LAYER/ AND FAST 
   STORM MOTIONS SHOULD ACT TO LIMIT THE AMOUNT OF PRECIPITATION 
   ANY ONE LOCATION RECEIVES...RESULTING IN THE POTENTIAL FOR DRY 
   THUNDERSTORMS.

   FARTHER EAST /ACROSS THE HIGH PLAINS OF EASTERN NEW MEXICO/...
   A WELL-MIXED BOUNDARY LAYER BUT HIGHER PRECIPITABLE-WATER 
   VALUES WILL ACT TO SUPPORT A MIXED-MODE OF WET/DRY 
   THUNDERSTORMS. RECENT RAINS AND ASSOCIATED GREENUP SHOULD ACT 
   TO LIMIT THE POTENTIAL FOR LIGHTNING-STARTED FIRES.

   ..MARSH.. 06/14/2014

   ...PLEASE SEE WWW.SPC.NOAA.GOV/FIRE FOR GRAPHIC PRODUCT...



Statistics from the Storm Prediction Center

The SPC rough log is compiled by an automatic PC logging program running at the Storm Prediction Center at Norman, OK, and is issued daily. On the Web, you can find it here. The log is a raw listing of all continental U.S. severe weather reports that SPC received during the 24 hour period from 6:00 AM CST the previous day up until 6:00 AM CST on the day of issuance. This is only a preliminary list. Because the logging process is automated, missing or improperly formatted reports from NWS field offices will not get into the data base. Also, reports could arrive after the daily log is compiled, and be missed in the list (which is updated periodically). All occurrance times are referenced to Central Standard Time -- even if the event occurred in a different time zone or during Daylight Savings Time.

Reports are lumped under three basic categories: tornado, hail and thunderstorm-wind reports.

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 their website.

Again, there is no guarantee as to the accuracy of SPC rough log reports and should be regarded as strictly preliminary. 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 thunderstorm wind damage a few days later after a survey is done to verify the cause of the damage. While SPC maintains a detailed database in Norman, they do not issue corrections to the daily report log. The log often contains duplicate reports, especially of tornadoes when multiple sightings of the saem tornado were sent in the local storm reports.


Monthly Tornado Statistics

The monthly tornado statistics are issued by SPC at irregular intervals. Let's look at the product:

TORNADO TOTALS AND RELATED DEATHS...THROUGH SUN OCT 26 2014
NWS STORM PREDICTION CENTER NORMAN OK
0203 PM CDT MON OCT 27 2014

       ...NUMBER OF TORNADOES...  NUMBER OF          KILLER
                                  TORNADO DEATHS     TORNADOES
    ..2014.. 2013 2012 2011 3YR                 3YR             3YR
    PREL ACT  ACT  ACT  ACT  AV   14  13  12 11  AV  14 13 12 11 AV
---  --   -- ---  ---- ---- ----  --  -- --- -- ---  -- -- -- -- --
JAN   4    4   75   79  16   57    0   1   2  0   1   0  1  2  0  1
FEB  41   42   39   57  63   53    0   1  15  1   6   0  1  7  1  3
MAR  25   20   18  154  75   82    0   0  43  1  15   0  0 10  1  4
APR 220  128   86  206 758  350   35   1  6 363 123   8  1  1 43 15
MAY 150  130  268  121 326  238    0  41  0 178  73   0  5  0  9  5
JUN 331  280  125  111 160  132    2   1   4  3   3   2  1  2  1  1
JUL  85   83   72   37 103   71    7   0   0  0   0   2  0  0  0  0
AUG  30   --   46   38  57   47    0   0   0  2   1   0  0  0  2  1
SEP  45   --   21   39  51   37    0   0   0  0   0   0  0  0  0  0
OCT  69   --   61   37  23   40    1   0   0  0   0   1  0  0  0  0
NOV  --   --   79    7  44   43   --   8   0  5   4  --  3  0  2  2
DEC  --   --   18   53  15   29   --   2   0  0   1  --  2  0  0  1
---  --   --  ---  ---- ---- ---- --  -- --- -- ---  -- -- -- -- --
SUM 1000 687  908 939 1691 1179   45  55  70 553 227 13 14 22 59 33

PREL = 2014 PRELIMINARY COUNT FROM ALL NWS LOCAL STORM REPORTS.
ACT  = ACTUAL TORNADO COUNT BASED ON NWS STORM DATA SUBMISSIONS.

COMPARISONS BETWEEN PRELIMINARY AND ACTUAL COUNTS SHOULD BE AVOIDED.

..CARBIN..10/27/2014

$$

The statistics are broken down by month and contain data for the last four years. PREL stands for preliminary, which should match the rough-log totals. ACT is actual Storm Data number if available; AV is average. ACT numbers include removal of any erroneous/duplicate reports or added reports which were initially missed or misclassified. An "-" in a column means the data is missing or not yet available. There is also a column called "3 YR AV" that gives the average numberof tornadoes per month (based on the 3 years' data).

The NUMBER OF TORNADO DEATHS columns are simply the number of people killed by month for the years listed and the average killed (3 years) per month. The KILLER TORNADOES 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.


Killer Tornado Statistics

The killer tornado statistics are issued by SPC at irregular intervals, as new information rolls in for this year's killer tornado events. Let's look at the product:

2014 PRELIMINARY KILLER TORNADOES
NWS STORM PREDICTION CENTER NORMAN OK
0205 PM CDT MON OCT 27 2014
                         
# DATE TIME-CST COUNTIES   STATE DEATHS A B C D  WATCH EF LOCATION
------ ------- ---------  ------ ------ -------  ----- -- -------
01 APR 25 1830 CHOWAN CO       NC   1   1 - - -  WT091  2  01H
02 APR 27 1430 WAPELLO-JOHNSON IA   2   - 2 - -  WS098  1  02M
03 APR 27 1630 OTTAWA CO       OK   1   1 - - -  WT097  2  01V
04 APR 27 1825 PULASKI-WHITE   AR  16  16 - - -  WT100  4  13H 02M
                                                           01V
05 APR 28 1500 WINSTON CO      MS  10  10 - - -  WT108  4  09H 01P
06 APR 28 1600 LIMESTONE CO    AL   2   2 - - -  WT108  3  02M
07 APR 28 1730 RANKIN CO       MS   1   1 - - -  WT108  3  01V
08 APR 28 1915 LINCOLN CO      TN   2   3 - - -  WT108  3  02M
09 JUN 16 1500 STANTON CO      NE   1   1 - - -  WT303  4  01M
10 JUN 16 1515 STANTON CO      NE   1   1 - - -  WT303  4  01V
11 JUL 08 1702 MADISON CO      NY   4   - 4 - -  WS403  2  03M 01H
12 JUL 24 0630 NORTHHAMPTON CO VA   3   - - - 3  NONE   1  03O
13 OCT 13 0440 LITTLE RIVER CO AR   1   1 - - -  WT533  2  01M
                                   ___ __ _ _ _
TOTALS:                            45  36 6 - 3

FATALITIES BY STATE: AL02 AR17 IA02 MS11 NC01 NE02 NY04 OK01 TN02 VA03

FATALITIES BY CIRCUMSTANCE/LOCATION: 24H 13M 03O 01P 04V

A = IN TORNADO WATCH
B = IN SEVERE THUNDERSTORM WATCH
C = CLOSE TO THE WATCH /15 MINUTES OR 25 MILES/
D = NO WATCH IN EFFECT
H = HOUSE
M = MOBILE HOME
O = OUTDOORS
P = PERMANENT BUILDING/STRUCTURE
V = VEHICLE
U = UNKNOWN
WS = SEVERE THUNDERSTORM WATCH /NUMBER/
WT = TORNADO WATCH /NUMBER/
EF = ENHANCED FUJITA SCALE RATING

MAP OF ANNUAL U.S. KILLER TORNADOES (LOWER CASE):
HTTP://WWW.SPC.NOAA.GOV/CLIMO/TORN/FATALMAP.PHP

..CARBIN..10/27/2014

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:

  1. A = In tornado watch
  2. B = In severe thunderstorm watch
  3. C = "Close" to the watch (15 minutes or 25 miles)
  4. D = No watch in effect

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:

  1. H = House (permanent foundation)
  2. M = Mobile home (a.k.a. "manufactured home")
  3. O = Outdoors (not inside any vehicle, mobile home or permanent building)
  4. P = Permanent structure (school, garage, factory, store, warehouse, truck stop, etc.)
  5. V = Vehicle (includes parked RVs)

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. The SPC Warning Coordination Meteorologist or designee prepares this listing as necessary. 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.

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