STORM PREDICTION CENTER

The 'Storm Prediction Center' (SPC), located in Norman, Oklahoma, is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), which in turn is part of the National Oceanic and Atmospheric Administration (NOAA) of the United States Department of Commerce (DoC). Until October 1995, the SPC was known as the National Severe Storms Forecast Center (NSSFC) and was located in Kansas City, Missouri.

Contents

Overview

Convective outlooks

Categories

The outlooks

Mesoscale discussions

Weather watches

See also

External links

Overview

The Storm Prediction Center is responsible for forecasting (identifying, describing, and quantifying) the risk of severe weather caused by severe convective storms - specifically, those producing tornadoes, hail 3/4" (2 cm) or larger, and winds 58 mph (93 km/h) or greater - as well as winter and fire weather. It does so primarily by issuing convective outlooks (AC), severe thunderstorm watches (WS) and tornado watches (WT) ("weather watches" (WW)), and mesoscale discussions (MCD or MD).
There is a three-stage process in which the area, time period, and details of weather parameters forecasted goes from more general to more specific, also with heightening alertness.

Convective outlooks

The Storm Prediction Center issues categorical and probability forecasts describing the general threat of severe convective storms over the contiguous United States for the next 6-73 hours. They are labeled and issued by day and are issued up to five times a day (current convective outlooks).
The categorical risks are general thunderstorms (GEN TSTMS) (brief textual description only), "SEE TEXT" (area on map for focusing attention only for isolated severe or near-severe weather), "SLGT" (slight risk of severe weather), "MDT" (moderate risk of severe weather), and "HIGH" (high risk of severe weather). Critical areas (referred to as "hatched areas" because of their representation on outlook maps) refer to a threat of increased magnitude; that is of "significant severe" (F2 or stronger tornado, 2" (5 cm) or larger hail, 75 mph (120 km/h) winds or greater) either generally or specific for the phenomenon. Public severe weather outlooks (PWO) are issued when a significant or widespread outbreak is expected, especially for tornadoes.

Categories

A 'slight risk' day typically will mean the threat exists for scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes. During the severe weather season, most days will have a slight risk somewhere in the US.
A 'moderate risk' day indicates that more widespread and/or more dangerous severe weather is possible, with some significant severe weather. Numerous tornadoes (including some strong tornadoes), more widespread or severe wind damage and very large hail could occur. Some moderate risk days will bring a significant tornado outbreak or a major derecho event. These are not uncommon and are typically issued several times each month during the peak season.
A 'high risk' day indicates a considerable likelihood of a major tornado outbreak or extreme derecho event. On these days, the potential exists for extremely severe and life-threatening weather, including widespread strong or violent tornadoes and/or very destructive straight-line winds. (Hail cannot verify or produce a high risk on its own.) Many of the most prolific severe weather days were high risk days. This is quite rare; a high risk is typically issued only a few times each year.
Note that these are merely forecasts and are not always correctly hit; tornado outbreaks are known to have happened when only a slight risk was forecasted, and high risk busts are not uncommon.

The outlooks

The Day 1 Convective Outlook, issued five times per day at 0600Z (the initial day-1 outlook, valid 1200Z that day until 1200Z the following day), 1300Z and 1630Z (the "morning updates," valid until 12Z the next day), 2000Z (the "afternoon update," valid until 1200Z the next day), and the 0100Z (the "evening update," valid until 1200Z the following day), provides a textual forecast, map of categories and probabilities, and chart of probabilities. The Day 1 is currently the only outlook to issue probabilities specifically for tornadoes, hail, or wind. It is the most descriptive and highest accuracy outlook.
Day 2 outlooks, issued twice daily at 08Z and 1730Z, refer to tomorrow's weather (12Z-12Z of the next calendar day) and include only a categorical outline, textual description, and a probability graph for severe convective storms generally. Day 2 moderate risks are fairly uncommon, and a Day 2 high risk has only been issued once (for April 7, 2006).
Day 3 outlooks refer to the day after tomorrow, and include the same "break outs" as the Day 2 outlook. Higher probability forecasts are less and less likely as the forecast period increases due to lessening forecast ability farther in advance. No attempt is made to forecast general thunderstorms and a high risk is never issued. Day 3 moderate risks are also very rare - it has only been used five times; for June 10, 2005, for January 2, 2006, for April 24, 2007, for June 6, 2007, and for June 7, 2007.
Day 4-8 outlooks primarily are only text-based, and can often completely change from day to day due to low confidence levels and high volatility. They were an experimental product until March 22, 2007 when they became an official product. Maps only show areas with at least a 30% chance of severe weather in the day 4-8 period, which is fairly uncommon.
Local forecast offices of the National Weather Service, radio and television stations, and emergency planners often use the forecasts to gauge the potential severe weather threats to their areas.
The categories refer to the following risk levels for the specific severe weather event occurring within 25 miles of any point in the delineated region (H = Hatched area; 2% and 10% are only used for tornadoes):

	'Tornado'	'Wind'	'Hail'	'Day 2/3'
'2%'	SEE TEXT
'5%'	SLGT	SEE TEXT	SEE TEXT	SEE TEXT¹
'10%'	SLGT
'15%'	MDT	SLGT	SLGT	SLGT
'30%'	HIGH	SLGT	SLGT	SLGT
'45%'	HIGH	MDT	SLGT	SLGT
'45%' (H)	HIGH	MDT	MDT	MDT
'60%'	HIGH	MDT	MDT	MDT²
'60%' (H)	HIGH	HIGH	MDT	HIGH²

¹Can be a SLGT if the risk is only for tornadoes (usually occurs as a result of a tropical system).

²Not issued for Day 3.

Mesoscale discussions

Mesoscale discussions (current mesoscale discussions) generally precede a tornado watch or severe thunderstorm watch, by 1-3 hours when possible. Mesoscale discussions are designed to give local forecasters an update on a region that is becoming a severe weather threat and an indication of whether a watch is likely and details thereof, as well as situations of isolated severe weather when watches are not necessary. MCDs contain meteorological information on what is happening and what is expected to happen in the next few hours, and forecast reasoning in regard to weather watches. Mesoscale discussions are often issued to update information on watches already issued, and sometimes when one is to be canceled. Mesoscale discussions are also issued for winter weather and heavy rainfall events.
Here is an example of a typical MD:

MESOSCALE DISCUSSION 0685

   NWS STORM PREDICTION CENTER NORMAN OK

   0253 PM CDT FRI MAY 04 2007

   AREAS AFFECTED...WRN KS...PARTS OF WRN OK/ERN TX PNHDL

   CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY 

   VALID 041953Z - 042200Z

   TRENDS ARE BEING CLOSELY MONITORED FOR SIGNS OF CONVECTIVE

   INITIATION.  ALTHOUGH TIMING IS STILL A BIT UNCERTAIN...ONE OR MORE

   TORNADO WATCHES WILL PROBABLY BE REQUIRED LATE THIS AFTERNOON.

   MOISTENING SOUTHERLY LOW-LEVEL FLOW AND STRONG DAYTIME HEATING ALONG

   SOUTHERN HIGH PLAINS DRY LINE...INTO THE VICINITY OF WEAK SURFACE

   LOW OVER SOUTHWEST KANSAS...IS CONTRIBUTING TO STRONG

   DESTABILIZATION.  RUC GUIDANCE INDICATES MIXED LAYER CAPE IS

   INCREASING INTO THE 3000-4000 J/KG RANGE...THOUGH MID-LEVEL

   SUBSIDENCE/SHORT WAVE RIDGING ALOFT IS CURRENTLY INHIBITING

   CONVECTIVE DEVELOPMENT.

   HOWEVER...MID/UPPER FORCING ASSOCIATED WITH AN IMPULSE LIFTING OUT

   OF AMPLIFIED WESTERN TROUGH IS BEGINNING TO SHIFT EAST OF THE

   CENTRAL/SOUTHERN ROCKIES.  AND...LATEST RUC GUIDANCE SUGGESTS WEAK

   LOWER/MID TROPOSPHERIC COOLING HAS OCCURRED ACROSS WESTERN KANSAS

   INTO THE TEXAS PANHANDLE ASSOCIATED WITH IMPULSE ALREADY LIFTING

   NORTHWARD THROUGH THE NORTH CENTRAL HIGH PLAINS.  ALTHOUGH

   UNCERTAINTY DOES EXISTS CONCERNING TIMING OF CONVECTIVE

   INITIATION...MUCH OF MODEL GUIDANCE SUGGESTS THAT THIS COULD OCCUR

   AS EARLY AS 22-23Z.  THIS SEEMS MOST PROBABLE WHERE FORCING WILL BE

   STRONGEST NEAR SURFACE LOW...BUT INITIATION OF WIDELY SCATTERED

   STORMS MAY QUICKLY FOLLOW SUITE.

   STORM DEVELOPMENT/INTENSIFICATION WILL LIKELY BE VERY RAPID ONCE CAP

   BREAKS.  AND...LARGE CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS BENEATH

   40-50 KT CYCLONIC WEST SOUTHWESTERLY 500 MB FLOW WILL BE FAVORABLE

   FOR TORNADOES...IN ADDITION TO THE RISK OF VERY LARGE HAIL. 

   ISOLATED STRONG TORNADOES ARE POSSIBLE...PARTICULARLY AS LOW-LEVEL

   JET STRENGTHENS NEAR/SHORTLY AFTER 04/00-01Z.

   ..KERR.. 05/04/2007

   

   ATTN...WFO...ICT...OUN...GID...DDC...GLD...LUB...AMA...

Weather watches

Main articles: Tornado watch, Severe thunderstorm watch

Watches (current severe weather watches) issued by the SPC are generally less than 20,000-50,000 square miles in area and are normally preceded by a mesoscale discussion. Watches are intended to be issued preceding arrival of severe weather by 1-6 hours. They indicate that conditions are favorable for severe thunderstorms or tornadoes. In the case of severe thunderstorm watches ("blue box"), organized severe thunderstorms are expected but conditions aren't thought to be especially favorable for tornadoes, whereas for tornado watches ("red box") conditions are thought favorable for severe thunderstorms producing tornadoes. In situations where a forecaster expects a significant threat of extremely severe and life-threatening weather, a watch with special wording of "particularly dangerous situation" (PDS) is subjectively issued. It is occasionally issued with tornado watches, normally for the potential of major tornado outbreaks. A PDS severe thunderstorm watch is very rare and usually issued for the potential of major derecho events.
"Watches are not 'Warnings', where there is an immediate severe weather threat to life and property. Although Severe Thunderstorm and Tornado Warnings are ideally the next step after watches, watches cover a threat of organized severe thunderstorms over a larger area and may not always precede a warning. Warnings are issued by local National Weather Service offices, not the Storm Prediction Center, which is a national guidance center."
A watch will usually be something like the following:

URGENT - IMMEDIATE BROADCAST REQUESTED

   TORNADO WATCH NUMBER 232

   NWS STORM PREDICTION CENTER NORMAN OK

   955 AM CDT SAT MAY 5 2007

   THE NWS STORM PREDICTION CENTER HAS ISSUED A

   TORNADO WATCH FOR PORTIONS OF 

          PARTS OF WESTERN AND CENTRAL KANSAS

          PARTS OF SOUTHWEST AND CENTRAL NEBRASKA

   EFFECTIVE THIS SATURDAY MORNING AND EVENING FROM 955 AM UNTIL

   1000 PM CDT.

   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...

   DESTRUCTIVE TORNADOES...LARGE HAIL TO 4 INCHES IN DIAMETER...

   THUNDERSTORM WIND GUSTS TO 90 MPH...AND DANGEROUS LIGHTNING ARE

   POSSIBLE IN THESE AREAS.

   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE

   MILES EAST AND WEST OF A LINE FROM 45 MILES NORTH NORTHWEST OF

   BROKEN BOW NEBRASKA TO 55 MILES SOUTHWEST OF RUSSELL KANSAS.  FOR

   A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH

   OUTLINE UPDATE (WOUS64 KWNS WOU2).

   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.

   DISCUSSION...VERY POTENT TORNADIC SUPERCELL PATTERN IN PLACE ACROSS

   WATCH AREA AS AIR MASS IS EXTREMELY UNSTABLE WITH VERY FAVORABLE

   SHEAR PROFILES.  WITH LITTLE INHIBITION REMAINING ALONG E OF DRY

   LINE...STORMS WILL RAPIDLY BECOME SEVERE BY EARLY THIS AFTERNOON WRN 

   KS INTO SWRN NEB.  TORNADIC SUPERCELLS WILL DEVELOP WITH POTENTIAL

   FOR LONG TRACK/VIOLENT TORNADOS.  AS DRY LINE REMAINS WRN KS THRU

   THE AFTERNOON...ADDITIONAL DEVELOPMENT OF TORNADIC SUPERCELLS ARE

   LIKELY  OFF THE DRY LINE THRU THE EVENING HOURS.

   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL

   SURFACE AND ALOFT TO 4 INCHES. EXTREME TURBULENCE AND SURFACE

   WIND GUSTS TO 80 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO

   600. MEAN STORM MOTION VECTOR 22040.

   

   ...HALES

See also

★ Severe weather terminology

★ List of SPC High Risk days

External links

★ Storm Prediction Center (SPC homepage)

★ SPC products descriptions