Using Ensemble Probability Forecasts And High Resolution Models
To Identify Severe Weather Threats
Josh KorotkyNOAA/NWS, Pittsburgh, PA
andRichard H. Grumm
NOAA/NWS, State College, PA
Overview
This study illustrates the operational value of combining diagnostic and probabilistic information from higher resolution models and SREF forecast products to better understand the nature of severe weather potential
SREF probability forecasts are examined for a vigorous severe weather event that occurred across much of the central Mississippi and lower Ohio Valleys on 2 April 2006
Probabilities of exceedance for Convective Available Potential Energy (CAPE), Storm-Relative Helicity (SRH), mean shear, and the Energy Helicity Index (EHI) are examined
Single and combined probabilities of MUCAPE, effective shear, and 3 hr. convective precipitation are also considered
Overview
A forecast strategy is proposed which utilizes:
Ensemble data for assessing the likelihood of a severe weather event and the confidence level of NWP forecasts
Climatological anomalies for evaluating the historical context of a model forecast
High resolution model data for determining the timing, evolution, mode, and intensity of forecast convection, including important mesoscale structures and relevant forcing mechanisms
SREF Configuration
EMC runs a 21 member multi-model, multi-analysis mesoscale SREF system with enhanced physics. The SREF is run four times daily at 03, 09, 15, and 21 UTC, with forecasts to 87 hours
The current SREF configuration contains 10 NAM-Eta members, 5 Regional Spectral Model (RSM) members, and 6 WRF members
Introduction
A strong cold front brought severe weather to much of the central Mississippi and lower Ohio Valleys on 2-3 April 2006
There were 871 severe weather reports and 85 tornadoes. 29 people lost their lives in this deadly early-spring tornado outbreak
SPC SREF Forecasts 0402/09 UTC Valid 04(02-03)/(21 – 00) UTC
Single and combined SREF probabilities - David Bright
http://www.spc.noaa.gov/exper/sref/
Prob Conv Precip ≥ .01 in x Prob MUCAPE ≥ 1000 x Prob Eff Shr > 40 kts
04/02/09Z valid 0402/21 UTC
04/02/09Z valid 0403/00 UTC
Prob Conv Precip ≥ .01 in x Prob MUCAPE ≥ 1000 x Prob Eff Shr > 40 kts
Prob Effective Shear ≥ 40 kts, Mean Effective Shear ≥ 40 kts (yellow) Prob Effective Shear ≥ 40 kts, Mean Effective Shear ≥ 40 kts (yellow)
Prob Supercell Composite 3, Mean Supercell Composite = 3 (yellow) Prob Supercell Composite 3, Mean Supercell Composite = 3 (yellow)
04/02/09Z valid 0402/21 UTC
Prob Sig Tor 3, Mean Sig Tor = 3 (yellow) Prob Sig Tor 3, Mean Sig Tor = 3 (yellow)
04/02/09Z valid 0403/00 UTC
SPC SREF Summary
The SREF single and combined probabilities illustrate an environment favoring supercells and tornadoes across the lower and central Mississippi Valley
SREF and Consensus Forecast Departure from Climatology
21Z01APR2006 valid 00Z03APR2006
SREF Probabilities, composite probabilities, climate anomalies; Rich Grumm
http://nws.met.psu.edu/ensembles/index.html
http://eyewall.met.psu.edu/ensembles/
SREF init: 21Z01APR2006 valid 00Z03APR2006 Mean MSLP and Anomaly (shaded)
SREF init: 21Z01APR2006 valid 00Z03APR2006 Mean PWAT and Anomaly (shaded
a.
b.
SREF MSLP and PWAT Anomalies21Z01APR2006 valid
00Z03APR2006
SREF forecasts indicate a strong surface cyclone, with a central pressure > 2 SD below normal over the upper Mississippi Valley
In the warm sector, moist air is surging poleward; PWAT anomalies are forecast to be 2 to 3 SDs above normal
SREF init: 21Z01APR2006 valid 00Z03APR2006 Mean CAPE (shaded) and EHI
SREF init: 21Z01APR2006 valid 00Z03APR2006 SR-Helicity (shaded);1.5 km Shear (103) & vectors
a.
b.
SREF CAPE, SRH, EHI, Shear21Z01APR2006 valid
00Z03APR2006
Mean CAPE (shaded) and EHI
Mean SRH (shaded) and 1.5 km Shear ≥ .006 s-1
Mean CAPE forecast 1200-2500 Jkg-1 and EHI values 1-3 from Illinois southward across much of the lower Mississippi Valley
Mean SRH in the warm sector 300-400+ m2s2 from Indiana to Wisconsin, generally along and north of a strong warm front
SRH 200-300 m2s2 extends southward into the lower Mississippi valley along and east of the cold front
1.5 km mean (normalized) shear is .009-.010+ s-1 (~30 kt) across the Mississippi Valley
SREF NARR 21Z01APR2006 valid 00Z03APR2006 Prob CAPE > 2000 Jkg-1; Mean CAPE ≥ 1200 Jkg-1
SREF 09Z01APR2006 valid 00Z03APR2006 Probability CAPE > 1000 Jkg-1
a.
b.
Prob CAPE ≥ 2000 m2s2 (shaded) and Mean CAPE ≥ 1200 J/kg
Prob CAPE ≥ 1000 J/kg
SREF CAPE Probabilities21Z01APR2006 valid
00Z03APR2006
Exceedance probabilities indicate CAPE will exceed 2000 J/kg from TX to southern IL
…CAPE will likely exceed 1000 J/kg across much of the Mississippi Valley
SREF NARR 21Z01APR2006 valid 00Z02APR2006 Probability 1.5km shear > .006 s-1
SREF 09Z01APR2006 valid 00Z03APR2006 Probability SRH > 200 m2s2
a.
b.
Prob 1.5 km shear ≥ .006 s-1 (shaded) and mean shear ≥ .006 s-1
Prob SRH ≥ 200 m2s2
SREF Shear and SRH Probabilities
21Z01APR2006 valid 00Z03APR2006
Mean shear will likely exceed .006 s-
1 across entire outlook region …with mean values .012 - .018 s-1
SRH will likely exceed 200 m2s2 across entire warm sector
SREF Summary
SREF forecasts established a high likelihood of severe weather across much of the lower and central Mississippi Valley on 2 April 2006
Probability forecasts indicated an environment favoring supercells, and SREF probabilities indicated a high degree of agreement among the ensemble members
NCEP Operational NAM-WRF Graphics
00 UTC and 1200 UTC: http://www.emc.ncep.noaa.gov/mmb/mmbpll/nampll12_fullcyc_2mbtop/index.html
0600 UTC and 1800 UTC: http://www.emc.ncep.noaa.gov/mmb/mmbpll/opsnam_offtime/index.html
NAM-WRF and SREF
SREF: Greater than 90% probability of the surface dew point > 600 F across central and southern Mississippi Valley
NAM-WRF illustrates details and magnitude of warm sector moisture content
SREF Probability of the 2m Dew Point > 60o F valid 0403/00 UTC
NAM-WRF Dew point valid 0403/00 UTC
NAM-WRF Best LI
Best Li reveals addition structure of the unstable warm sector
NAM-WRF highlighted important forcing mechanisms in a forecast of significant low-level frontogenesis and moisture flux convergence along the frontal features (not shown)
Instant Pcp Rate - 36 H Forecast valid 0403/00 UTC
Conv Pcp Rate - 36 H Forecast valid 0403/00 UTC
NAM-WRF Instant and Convective Precipitation
NAM-WRF instant and convective precipitation shows convective potential along banded frontal structures and grid scale precipitation northwest of the surface cyclone
a.
b.
Although it is not generally possible to make direct comparisons between actual and simulated radar, the simulated radar can reveal the nature of model-derived mesoscale forcing
The simulated radar indicates banded frontal and pre-frontal structures which correspond rather well with the actual radar, even though the actual radar shows much greater reflectivities in the convection
This information can be very useful to an analyst trying to understand how moisture, instability, and vertical wind shear are forecast to interact in a severe storm environment.
NAM-WRF Simulated Radar Reflectivity
Summary
Ensemble data indicated the likelihood of a severe weather event with a high potential for supercells and tornadoes across the mid Mississippi Valley on 2-3 April 2006. Probability forecasts indicated a high degree of agreement between the 21 SREF members… increasing confidence in the forecast
Climate anomalies indicated the event would be associated with a an anomalous surface cyclone containing an anomalously moist warm sector
High resolution model data helped fill in the details of the mode, evolution, and intensity of forecast convection, and highlighted important mesoscale structures, including relevant forcing mechanisms