SPC Severe Weather
Forecasting
Overview
David Imy
david.imy@noaa.gov
SPC Organization
Director
Admin Officer
WCM
Admin Coord.
Operations Branch Chief
Science Branch Chief
5 Lead Forecasters
Science Operations Officer
10 Mesoscale/Outlook Forecasters
3 Technologists
5 Assistant Mesoscale Forecasters
2 System Analysts
Software Analyst
Contract Computer Tech
Contract Programmer
Storm Prediction Center Evolution
• Congress recognized the need for a severe weather
center in late 1940s (after two tornadoes at Tinker
AFB)
• 1950 in Washington, D.C.
• 1953 in Kansas City, MO as National Severe Storms
Forecast Center
• 1996 in Norman, OK as the NOAA
Storm Prediction Center
National Weather Center
National Weather Center Occupants
Storm Prediction Center
National Severe Storms Laboratory
WFO Norman
Warning Decision Training Branch
University of Oklahoma
Meteorological Dept.
 Oklahoma Climate Survey
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SPC Mission Statement
SPC exists to protect life and property
of the American people through the
issuance of timely, accurate watch and
forecast products dealing with
tornadoes and other mesoscale
hazardous weather.
Severe Criteria
** Tornadoes
** Hail ¾” diameter or larger
** Thunderstorm winds > 57 mph
How Do We Accomplish Mission?
• Issue Severe Thunderstorm and Tornado Watches
for the CONUS
• Issue Convective Outlooks for Days 1-8 and Fire
Outlooks Days 1-8
• Issue needed Mesoscale Discussions for short term
hazardous weather (severe, winter and heavy rain)
SPC Products
 Convective
Outlooks
 Day
1 (today)
 Day 2 (tomorrow)
 Day 3 (day after tomorrow)
 Day 4-8
 Severe
Weather Watches
 Tornado
 Severe
Thunderstorm
Other SPC Products
 Mesoscale
Discussions
 Watch Status Messages
 Severe Weather Stats
 Fire Weather Outlooks
 Day
1 (today)
 Day 2 (tomorrow)
 Experimental Day 3-8
I. Pattern Recognition/Forecaster Experience
Classical synoptic-dynamic setting for some severe weather
outbreaks (but other patterns are regionally / event dependent)
• Southwest monsoon, northeast U.S., pulse severe, hurricane-tornado, etc.
II. Climatology
Knowledge of climatology is a good first step to
provide information about when/where events
more likely to occur
Probability of F2 or greater tornado within
25 miles of a point
EXAMPLES:
Week of Feb 19th
Week of May 6th
III. Parameter Evaluation
This changes more rapidly than pattern recognition
and climatology
OLD:
Lifted Index, Showalters, Total-Totals, etc.
NEW:
CAPE, SRH, LCL Height, 0-1km shear, etc.
• Also, technology and research allows more meaningful
parameters that link observable scales to storm-scale (cloud-scale
models).
Analysis of Surface and Upper Air Parameters
Deep Layer Shear (0-6 km)
Instability (MLCAPE)
Low Level Shear (0-1km)
Supercell Composite Parameter
Significant Tornado Parameter
Severe Weather Outlooks
Categorical
•Slight
•Moderate
•High
Probabilistic
•Tornadoes
•Hail
•Convective Winds
Categorical Outlooks

Slight (SLGT)




Moderate (MDT)




5-20 Severe Hail Events
5-20 Severe Wind Events
2-5 Tornadoes
20-50 Severe Hail Events
20-50 Severe Wind Events
6-19 Tornadoes
High


> 19 Tornadoes with 2+
potentially producing F3-F5
damage
Derecho - producing
extreme wind damage (> 50
reports)
Probability
Outlooks
 Provides the threat of severe
weather occurring within 25
miles of any point within the
area
 Tornadoes
 Large hail
 Severe convective winds
 Also threat for extreme
severe
Feb 16, 2006 Categorical Outlook
Feb 16, 2006 Hail Probabilities
Feb 16, 2006 Wind Probabilities
Feb 16, 2006 Tornado Probabilities
Probability Outlook Intervals

Tornadoes


Hail


5%, 15%, 25%, 35%, 45%, 60%
Convective Wind


2%, 5%, 10%,15%, 30%, 45%, 60%
5%, 15%, 25%, 35%, 45%,60%
Extreme (> 10%)



Tornadoes F2+ damage
Hail 2.0+ inches diameter
Winds 65+ kt
Severe Weather Watches
Tornado Watches
Issued when:
 strong/violent
tornado
(F2 – F5) damage is
possible
2
or more tornadoes
are expected
Not all tornadoes
will occur in a watch!
“Particularly Dangerous
Situation”
(PDS) Watches
• Tornado watches
– Multiple strong or
violent (F2 – F5
damage) events
• Severe
Thunderstorm
Watches
– Long lived wind
events (derechoes)
ZCZC MKCSEL9 ALL 250500;365,0980 321,0985 321,1013 365,1004;
WWUS9 KMKC 242144
MKC WW 242144
OKZ000-TXZ000-250500URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 49
STORM PREDICTION CENTER NORMAN OK
344 PM CST THU FEB 24 2000
THE STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF
WESTERN OKLAHOMA
NORTHWESTERN TEXAS
EFFECTIVE THIS THURSDAY AFTERNOON AND EVENING FROM 430 PM UNTIL
1100 PM CST.
...THIS IS A PARTICULARLY DANGEROUS SITUATION...
DESTRUCTIVE TORNADOES...LARGE HAIL TO 2 INCHES IN DIAMETER...
THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE
POSSIBLE IN THESE AREAS.
Severe Thunderstorm Watch
Organized Severe Storms
Supercells
Squall lines
Multicell complexes
Extreme severe storms
Wind gusts > 64 kt (73 mph)
Damage to permanent
structures
Hail > 2.0 inches diameter
Tornado Watch 33
Watch #33 Severe Weather Probabilities
Watch-By-County Tornado Watch
Coming Soon- New Watch Web Presentation
Watch Outline Update (WOU)
**
All counties in watch included in initial
issuance
**
As local NWS offices clear counties
from watch (WCN), the WOU will be
updated with those counties removed
**
The SPC does not remove counties from
a watch; the local offices control the
watch after issuance
Mesoscale Convective
Discussions (MD)

Goal is to issue pre-watch MDs 1 to 3 hours prior to a Severe
Thunderstorm or Tornado watch issuance.
- Define area(s) of concern
- State expected watch type
- Provide meteorological reasoning – most important
Also issued to address following hazards:
- Outlook Upgrade
- Heavy Rainfall
- Winter Weather
Mesoscale Discussions
ZCZC MKCSWOMCD ALL;334,0996 373,0979 353,0979 314,0996;
ACUS3 KMKC 032023
>MKC MCD 032023
TXZ000_OKZ000_032300_
SPC MESOSCALE DISCUSSION #0345 FOR...SW OK/NW TX...
CONCERNING...SEVERE THUNDERSTORM POTENTIAL...
WATER VAPOR IMAGERY SHOWS A LEAD MID LEVEL SHORT
WAVE TROUGH MOVING ENEWD OVER E/NE NM THIS
AFTERNOON...AND THIS IS CONFIRMED BY PROFILER TIME SERIES
FROM AZC/GDA/TCC/JTN. MID/UPPER 60 DEWPOINTS AND
TEMPERATURES NEAR 80 ARE CONTRIBUTING TO SURFACE_BASED
CAPE VALUES OF 3500_5000 J/KG OVER WRN OK AND NW TX TO
THE E OF THE DRYLINE. CONVERGENCE ON THE DRYLINE IS NOT
STRONG AND A CIRRUS SHIELD OVER THE TX PANHANDLE/ NW TX/
WRN OK SHOULD LIMIT ADDITIONAL SURFACE HEATING... BUT
VISIBLE /RADAR IMAGERY HAS SHOWN THE FIRST ATTEMPTS AT
TCU OVER FAR NW TX AS OF 20Z WITHIN A BREAK IN THE CIRRUS.
MID LEVEL FLOW AND VERTICAL SHEAR WILL INCREASE OVER NW
TX AND WRN OK THROUGH LATE AFTERNOON... WITH AN
INCREASING THREAT OF SUPERCELLS NEAR THE DRYLINE FROM
00 to 03Z. THIS AREA IS BEING MONITORED FOR A POSSIBLE
TORNADO WATCH.
..THOMPSON.. 05/03/99
...PLEASE SEE WWW.SPC.NOAA.GOV/ FOR GRAPHIC PRODUCT...
NNNN
Fire Weather Outlook

Three Types of
Areas



Critical
Extremely Critical
Dry Thunderstorm
Day One Fire Weather
Outlook
Day Two Fire Weather
Outlook
SPC Web Page
www.spc.noaa.gov