Matthew Parker - Convective Storms

NCSU home page for HSLC and SHERB information

HSLC="high-shear low-CAPE", a convective environment broadly defined by surface-based CAPE < 500 J/kg and 0-6 km bulk wind difference > 18 m/s

SHERB="severe hazards in environments with reduced buoyancy", a family of diagnostic indices designed for HSLC environments

Why does this page exist?

Severe convective storms in environments with large vertical wind shear but limited instability ("HSLC" environments) have received only modest research attention compared to their higher CAPE counterparts. We find that they occur at all times of day, across all seasons, and throughout the entire U.S. HSLC environments represent a unique challenge because they occur frequently, but produce severe weather a relatively low percentage of the time.
We find that traditional composite parameters based upon CAPE often have limited skill in HSLC environments, and also tend to have values that are well below their conventional thresholds. This prompted the statistical development of a diagnostic parameter (the "SHERB") that appears to have greater skill specifically in HSLC environments.
The SHERB indices are still considered experimental, and they are subject to several important caveats. For this reason, we felt it was important to provide a central set of resources for forecasters and also to solicit feedback from forecasters who are using the SHERB.

Origins of the SHERB

The SHERB parameters were introduced and compared to other conventional indices in the following journal article.

Sherburn, K. D., and M. D. Parker, 2014: Climatology and ingredients of significant severe convection in high shear, low CAPE environments. Wea. Forecasting, 29, 854-877.

If you can't wade through the full article at the moment, here we briefly summarize the definition of the two most skillful SHERB indices, as well as some considerations for their use.

Definitions

Important usage notes:

The SHERB parameters are most skillful at separating significant* HSLC severe reports from non-severe HSLC storms using a threshold of 1 (with significant severe weather more likely for SHERB above 1).
Like many convective indices, the SHERB parameters do not forecast whether convection will occur. Forecasters need to inspect other products to establish the probability of convection.
The SHERB parameters were designed to include all significant severe weather (winds and hail, in addition to tornadoes). Even so, they do still have skill for HSLC tornadoes alone.
The SHERB parameters were specifically developed using an HSLC dataset. Although our tests reveal some skill across all CAPE environments, we cannot recommend use of SHERB in higher CAPE situations.
SHERBS3 can appear to have large false alarm areas because its inputs can be large where there is no CAPE (and thus storms are impossible). We recommend ignoring SHERBS3 where the most unstable parcel LI is > +6 K.
SHERBE largely avoids this false alarm problem because the effective shear magnitude becomes 0 where CAPE is absent. However, for this reason, SHERBE may miss events where CAPE is under-forecast/diagnosed.
The SHERB indices may be artifically inflated if steep boundary layer lapse rates extend upward into the 700-500 mb layer. For this reason, elevated SHERB values over high terrain in the afternoon are dubious.

* Significant severe weather includes tornadoes rated EF2 or higher, wind reports of 65 knots or higher, and hail reports of 2" diameter or larger

New developments: the Modified SHERB

To address some of the deficiencies in the SHERB, the MOSH/E ("Modified SHERB/SHERBE) parameters were introduced and compared to other conventional indices in the following journal article.

Sherburn, K. D., M. D. Parker, J. R. King, and G. M. Lackman, 2016: Composite environments of severe and non-severe high-shear, low-CAPE convective events. Wea. Forecasting, 31, 1899-1927.

If you can't wade through the full article at the moment, here we briefly summarize the definition of the two most skillful MOSH indices, as well as some considerations for their use.

Definitions

in which LLLR is the 0-3 km lapse rate, S15MG is the 0-1.5 km shear vector magnitude, ESHR is the effective bulk shear vector magnitude, and MAXTEVV is the lower troposheric maximum of the product between omega (pressure vertical velocity) and the lapse rate of equivalent potential temperature. This added MAXTEVV term helps to reduce false alarm area by localizing and maximizing the MOSH/E parameters where there is strong synoptic ascent in the presence of potential instability. It is also useful in identifying areas where synoptic forcing may lead to rapid local increases in CAPE in short order (see King et al. 2017 for more on this)

Important usage notes:

The MOSH parameters are most skillful at separating significant* HSLC severe reports from non-severe HSLC storms using a threshold of 1 (with significant severe weather more likely for MOSH above 1).
Like many convective indices, the MOSH parameters do not forecast whether convection will occur. Forecasters need to inspect other products to establish the probability of convection. But, the inclusion of large scale forcing in the MOSH parameters makes them somewhat more robust than the original SHERB parameters (which could become large in cmopletely non-convective scenarios).
Because of the weighting toward synoptic forcing, MOSH may miss events in weakly forced environments. Strongly forced scenarios are the most common sort in the eastern U.S., but there are HSLC events in the western U.S. that look somewhat different.
The MOSH parameters were designed to include all significant severe weather (winds and hail, in addition to tornadoes). Even so, they do still have skill for HSLC tornadoes alone.
The MOSH parameters were specifically developed using an HSLC dataset. Although our tests reveal some skill across all CAPE environments, we cannot recommend use of MOSH in higher CAPE situations.

* Significant severe weather includes tornadoes rated EF2 or higher, wind reports of 65 knots or higher, and hail reports of 2" diameter or larger

Real-time data

SPC mesoanalyses of SHERB:

CONUS: SHERBS3 SHERBE MOSHE

Southeast/Gulf Coast: SHERBS3 SHERBE MOSHE

Mid-Atlantic/Ohio Valley: SHERBS3 SHERBE MOSHE

Northeast/Great Lakes SHERBS3 SHERBE MOSHE

Southern Plains/Lower Mississippi Valley: SHERBS3 SHERBE MOSHE

Central Plains/Upper Mississippi Valley: SHERBS3 SHERBE MOSHE

Northern Plains/Upper Midwest: SHERBS3 SHERBE MOSHE

Northwest/Northern Rockies: SHERBS3 SHERBE MOSHE

NC State Real-time HSLC SHERB Forecast Plots:

RAP: CONUS SHERB/SHERBE/MOSH/MOSHE all plots

NAM: CONUS SHERB all plots

GFS: CONUS SHERB/SHERBE/MOSH/MOSHE all plots

SREF: E. US SHERB

Feedback

As a part of our ongoing efforts to assess and improve the SHERB indices, we ask operational forecasters to submit feedback on the performance of the SHERB, for both HSLC events and null cases. Thanks!

Submit SHERB feedback here

Code and instructions for adding SHERBS3 to NWS tools

NWS AWIPS-1 SHERBS3 Volume Browser (requires NWS login)

NWS AWIPS-1 and AWIPS-2 SHERBS3 GFE tool (requires NWS login)