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 Day one Outlook04/02/06 UTC valid 0402/12 UTC –

0403/12 UTC

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 CAPE

CAPE forecast 2000-3000J/kg along and ahead of the cold front

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