Severe Weather FAQ

The Day-1 Convective Outlook, sometimes called the "AC", and 2nd Day Severe Weather Outlook are guidance products issued by the Operational Guidance Branch (OGB) unit of the Storm Prediction Center (SPC) in Norman, Oklahoma. The Day 1 outlook outlines areas in the continental United States where severe thunderstorms may develop during the next 6 to 30 hours. The Convective Outlook is issued 5 times daily: at 06Z (the initial day-1 outlook, valid 12Z that day until 12Z the following day), 11Z (the "two-part outlook"), 15Z (the "morning update," valid until 12Z the next day), 19Z (the "afternoon update, " valid until 12Z the next day), and the 02Z (the "evening update," valid until 12Z the following day),

The Day 2 Outlook is very similar to the Day 1 Outlook. It is issued only twice a day, at 08Z and 18Z, and covers the period from 12Z the following day to 12Z the day after that. For example, if today is Monday then the Day 2 Outlook will cover the period 12Z Tuesday to 12Z Wednesday. The outlook issued at 08Z now qualifies the degree of risk like the Day 1 has (i.e. SLGT, MDT, and HIGH risk areas). The Day 2 Outlook has also begun to include a general thunderstorm outline.

Weather watches that are in effect at the time the outlook is issued will be referenced in the beginning of the product.

Standard aviation identifier location codes are used to delineate the risk areas. [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. A single product may describe several separate risk areas. For example, there might be one risk area for the Central Plains and an another one for southern Florida.

• GEN TSTMS - General (non-severe) thunderstorms
• SEE TEXT - A label on the graphic only
• SLGT - Slight risk, both graphic and text
• MDT - Moderate risk, both graphic and text
• HIGH - High risk, both graphic and text

  The SEE TEXT label appears only on the graphic map. "SEE TEXT" will be used for those situations where a SLGT was considered, but at the time of the forecast, is not warranted. Although there is no severe outlook for the labeled area, you should read the text of the outlook discussion to learn more about the potential for a threat to develop if some particular conditions do come together. As a rule, the "SEE TEXT" is used on the Day 2 Outlooks for areas where severe weather is possible, but there is too much forecast uncertainty (questionable model data, capping, moisture return, or other such factors) to draw a risk area. The "SEE TEXT" will be used in the Day 1 Outlooks to discuss areas where a few severe storms are possible or storms may approach severe levels, but the coverage or intensity is expected to be too small for a marginal for a risk area. Again, note that SPC severe thunderstorms outlooks are not meant to cover every single possibility of a severe thunderstorm -- otherwise, severe and general thunder outlooks would be the same.

  A SLGT risk implies well-organized severe thunderstorms are expected but in small numbers and/or low coverage. Here are the guidelines: a high probability of 5-29 reports of 1 inch of larger hail, and/or 3-5 tornadoes, and/or 5-29 wind events...OR...low/moderate probability of moderate to high risk being issued later if some conditions come together.

  MDT risks imply a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather. Many NWS offices will "beef-up" their forecasts to include the phrase "some thunderstorms may be severe" when a SLGT or MDT risk is issued. Here are the guidelines: a high probability of at least 30 reports of hail 1 inch or larger, or 6-19 tornadoes, or numerous wind events (30 that might be associated with a squall line, bow echo or derecho).

  The HIGH risk area almost always means a major severe weather outbreak is expected, with great coverage of severe weather and enhanced likelihood of extreme severe (i.e., violent tornadoes or extreme convective wind events over a large area). Here are the guidelines: a high probability of at least 20 tornadoes with at least 2 of them rated F3+, or an extreme derecho causing 50+ widespread wind events (50+) with numerous higher end wind (80+ mph) and structural damage reports.

  The report criteria for each of those risks is valid for an area the size of Oklahoma without the panhandle, or about 50,000 square miles. As the size of the risk area increases (decreases) from 50,000 square miles, those expected severe weather numbers would increase (decrease) proportionally.

  In addition to the severe risk areas, general thunderstorms (non-severe) are outlined, but with no label on the graphic map.

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.

A separate discussion, sometimes composed by a different forecaster, describes areas where mostly non-severe thunderstorms are possible and the factors expected to produce them. This part will often be more generalized than the severe thunderstorm discussion. General thunderstorm discussions are not included in many cases when severe thunderstorm risks are in the outlook, due to time constraints and the greater priority placed on severe weather.

Here is when you can expect SPC outlooks every day, in UTC or "Z" time (subtract 6 hours for CST, 7 for CDT):

• The Day 1 Outlooks will be issued at 0600Z, 1300Z, 1630Z, 2000Z and 0100Z year-round.
• The Day 2 Outlooks will be issued at 0830Z and 1730Z during standard time, and 0730z and 1730z during daylight time.

  As the name implies these products are outlooks. They are designed for forecasters and not the general public -- although they are widely available on the Internet -- and are considered "guidance" products because they are technical forecast discussions. They are, however, very 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 sure bet. The AC must be used in conjunction with other products to get the full picture. It must be kept in mind that it is a forecast product and is subject to change as additional data is evaluated. What appeared to be a HIGH risk situation at 06Z may be downgraded to a SLGT risk at 15Z because the 12Z upper air soundings indicate the atmosphere had significantly stabilized. The opposite can happen too.

  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. This is because 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 don't involve a threat of torndoes or supercells at all, but only large hail and/or downbursts. Some HIGH risk situations are expected to result in a large number of severe reports, but not necessarily tornadic or extreme in magnitude.

  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, are keyed to the well-organized severe weather events most capable of damage and injury. 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 weakly sheared environments) 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 meant to be guidance to local forecasters to alert them to the possibility of more than very isolated or brief thunderstorms in or near their areas. General thunderstorm outlooks are meant to forecast thunderstorm coverage of 10% or more of the broad region drawn. For that reason, it is acceptable and normal to have very isolated, brief or sparse thunderstorms "outside the lines." Also, the lack of a severe risk within a general thunder outlook does not imply there will be zero severe thunderstorms. Again, almost any thunderstorm may produce a severe weather event. It doesn't necessarily mean there is a conflict or bad forecast 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 a risk area.

Acronyms Used in the SPC Discussions:

Note: for a lengthy list of technical meteorological terms you may wish to visit the Weather Glossary on AgriWeather (click on Help/FAQs then pick "Weather Glossary" under "The Weather.")

Mesoscale Discussion: This product is issued by the Storm Prediction Center. When conditions actually begin to shape up for severe weather, SPC often issues a Mesoscale Discussion (MCD) statement anywhere from roughly half an hour to several hours before issuing a weather watch. SPC also puts out MCDs for hazardous winter weather events on the mesoscale, such as locally heavy snow, blizzards and freezing rain (see below). MCDs are also issued on occasion for heavy rainfall, convective trends, and other phenomena, when the forecaster feels he/she can provide useful information that is not readily available or apparent to field forecasters.

MCDs are based on mesoscale analysis and interpretation of observations and of short term, high resolution numerical model output. 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 can help you get a little extra lead time on the weather and allow you to begin gearing up operations before a watch is issued.

The MCD begins with a numerical string that gives the LAT/LON coordinates of a polygon that loosely describes the area being discussed. The string "424,0812 433,0784 413,0784 404,0812" would be read as "the discussion area is bounded by a line that runs from 42 degrees 40 minutes North/81 degrees 20 minutes West to 43 degrees 30 minutes North/78 degrees 40 minutes West to 41 degrees 30 minutes North/78 degrees 40 minutes West to 40 degrees 40 minutes North/81 degrees 20 minutes West."

SPC heavy rain MCDs are issued for:

• 1) Rainfall rates up to 3 inches per hour are expected with slow moving convection (e.g., storms moving at 10 knots or less),
• 2) rainfall amounts of at least 2 inches expected at any one location within one hour,
• 3) rainfall rates of at least 1 « inches/hour are expected to last at least 3 hours with a total rainfall of at least 4.5 inches, or
• 4) the forecast of an end to a heavy rain event.

  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:

• 1) Changing meteorological parameters that indicate an end to heavy rains,
• 2) A heavy rain episode threatens within an area of severe thunderstorms,
• 3) Additional storms with heavy rains are expected across the same area where recent heavy rains have occurred, or
• 4) Heavy rains are expected to continue while moving into a different area.

Winter weather MCD's focus on the meteorological processes expected to cause hazardous winter weather: the where, whe, 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 sent 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 canalso be done for freezing rain events 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) Changing meteorological parameters that indicate an end to the event
• 2) An episode that may have been overlooked,
• 3) Further hazardous winter weather across the same area where recent heavy snow, freezing rain or blizzard conditions have occurred, or
• 4) When an event is expected to continue and shift into a different area.

ACCAS- Altocumulus Castellanus
AOA -At or Above
AOB -At or Below
ASOS -AutomatedSurface Observing System
ATTM -At this time
AWIPS - Advanced Weather Interactive Processing System
AWOS - Automated Weather Observation System
BRN -Bulk Richardson Number
BWE -Bounded Weak Echo Region
CAA - Cold Air Advection
CAPE -Convective Available Potential Energy
SBCAPE- CAPE calculated using a Surface based parcel
MUCAPE - CAPE calculated using a parcel from a pressure level that results in the Most Unstable CAPE possible
MLCAPE- CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level
*ETA* SBCAPE - SBCAPE values obtained using ETA model soundings
*ETA* MUCAPE - MUCAPE values obtained using ETA model soundings
*ETA* MLCAPE - - MLCAPE values obtained using ETA model soundings
CB - Cumulonimbus
CCL - Convective Condensation Level
CIN - Convective Inhibition
CISK - Conditional Instability of the Second Kind
CONUS - Continental United States
CSI - Conditional Symmetric Instability
CU - Cumulus
DELMARVA - Delaware/Maryland/Virginia
ECMWF - European Centre for Medium-Range Weather Forecasting model
EHI - Energy Helicity Index
EL - Equilibium Level
ETA - ETA Model
FFG - Flash Flood Guidance
GOES - Geostationary Operational Environmental Satellite
HPC - Hydrometeorological Prediction Center
INVOF - In Vicinity of
IR - Infrared
J/KG - Joules per Kilogram
LI - Lifted Index
SBLI - LI calculated using surfaced based parcel
MULI - LI calculated using a parcel from the pressure level that results in the Most Unstable value (lowest value) of LI possible
MLLI - LI calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level ETA SBLI - SBLI values obtained from ETA model soundings
ETA MULI - MULI values obtained from the ETA model soundings
ETA MLLI - MLLI values obtained from ETA model soundings
ETA LI - LI gridpoint values calculated by ETA model postprocessing
NGM LI - LI gridpoint values calculated by NGM model postprocessing
RUC LI - LI gridpoint values calculated in-house using values from the RUC model
LCL - Lifted Condensation Level
LEWP - Line Echo Wave Pattern
LFC - Level of Free Convection
LLJ - Low Level Jet
M2/S2 - m²/s² (meters squared per second squared, equivalent to J/kg)
MAX - Maximum
MB - Millibars
MCC - Mesoscale Convective Complex
MCD - Mesoscale Discussion
McIDAS - Man-computer Interactive Data Access System
MCS - Mesoscale Convective System
MOS - Model Output Statistics
NCEP - National Centers for Environmental Prediction
NGM - Nested Grid Model
NVA - - Negative Vorticity Advection
PVA - Positive Vorticity Advection
QG - Quasigeostrophic
QPFERD - NCEP Excessive Rainfall Discussion
QPFHSD - NCEP Heavy Snow Discussion
QPFPFD - NCEP Precipitation Forecast Discussion
RAOB - Radiosonde Observation
RAFS - Regional Analysis and Forecasting System
RUC - Rapid Update Cycle (model)
SPC - Storm Prediction Center
SPENES - NESDIS Satellite Precipitation Estimates
SRH - Storm-relative Helicity
SWODY1 - Severe Weather Outlook-Day 1
SWODY2 - Severe Weather Outlook-Day 2
TCU - Towering Cumulus
T.D. - Tropical Depression
TPC - Tropical Prediction Center (National Hurricane Center)
T.S. - Tropical Storm
ULJ - Upper Level Jet
UVV - Upward Vertical Velocity
VWP - VAD (Velocity-azimuth Display) Wind Profile
VIL - Vertically Integrated Liquid
VSB - Visible (satellite imagery)
WAA - Warm Air Advection
WW - Severe Thunderstorm Watch or Tornado Watch

Source: National Severe Storms Laboratory - Storm Prediction Center