Welcome to the Storm Prediction Center's "Hourly Mesoscale
Analysis" page. This page features numerous diagnostic fields that are
commonly used at the SPC to determine the short term potential for
severe thunderstorms and tornadoes. Many of the parameters have been
documented in AMS, NWA, and NWS publications. Others are currently
being evaluated and tested for their utility.
The fields shown are calculated using a combination "real-time OA" and "model forecast" method. The SPC runs a 2-pass Barnes surface objective analysis around :05 after each hour, using the latest RAP forecast as a first guess. Next, the surface data is merged with the latest RAP forecast upper-air data to represent the best 3-dimensional atmospheric analysis available. Finally, each gridpoint is inputed into a sounding analysis rountine called "NSHARP" to calculate about 100 new fields.
The area depicted on this page will change from day to day, and will generally show a region where the SPC is closely monitoring for severe thunderstorm development. These fields are offered by the SPC to share the latest severe weather diagnostic techniques with local forecasters.
The diagnostic variables and parameters presented herein represent a best guess current state of the atmosphere. Any attempts to make short term forecasts based on the mesoanalysis data must consider short term trends, which can be deduced from the 5 hour animations available with each plot. The majority of the parameters displayed have not been tested as prognostic tools (in the strict sense of relating current values to the probability of a particular event at some time in the future).
These products are
usually updated by :15 after each hour.
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Lifted air parcels are constrained by the curves on the diagram. Unsaturated parcels follow dry adiabats upward until they become saturated, then they follow "moist" adiabats. A saturated parcel, if moving downward, follows a moist adiabat until it is no longer saturated, then if follows a dry adiabat (i.e., the parcel warms at a faster rate while sinking and unsaturated). In using "parcel theory" and this diagram to estimate various parameters, it is assumed that: 1) the condensed water is not carried with the parcel and all falls out, 2) the pressure of the lifted parcel adjusts immediately to the environment, 3) there are no sources or sinks of heat and moisture external to the lifted parcel, 4) and ice processes are ignored.
The SPC uses two different methods to calculate instability parameters - "virtual" parcels, and "non-virtual" parcels. The "virtual" parcel is used to calculate CAPE and LI on our web page, since it includes the effects of moisture on density via the virtual temperature. Operational experience suggests the ml (virtual) parcel values are more useful than the sb parcel values in diagnosing the potential for convective initiation rooted in the boundary layer.
For additional information, please see:
There is some confusion over which of these various parcel choices is most relevant to forecasting thunderstorms. Unfortunately, the science of meteorology is still inexact, and we just don't know! However, recent observational evidence suggests that late afternoon cumulus cloud base heights are best estimated using the ml parcel.
Craven, J. P., R. E. Jewell, and H. E. Brooks, 2002: Comparison between observed convective cloud base heights and lifting condensation level for two different lifted parcels. Wea. Forecasting, 17, 885-890.
LCL = lifting condensation level. This is the level at which a lifted parcel becomes saturated, and is a reasonable estimate of cloud base height when air parcels experience forced ascent. The LCL is this example is for the lifted surface parcel (virtual).
LFC = level of free convection. The LFC is the level at which a lifted parcel begins a free acceleration upward to the equilibrium level. Preliminary research suggests that tornadoes become more likely with supercells when LFC heights are less than 2,000 m above ground level, and thunderstorms are more easily initiated and maintained when LFC heights are lower than about 3,000 m. The LFC is this example is for the lifted surface parcel (virtual).
LFC-LCL = the height difference between the LFC and the LCL. The smaller the difference between the LCL and LFC, the more likely deep convection becomes.
EL= equilibrium level. The EL is the level at which a lifted
parcel becomes cooler than the environmental temperature and is no
longer
buoyant (i.e., "unstable"). The EL is used primarily to estimate the
height
of a thunderstorm anvil. You may notice that the "virtual" and
"non-virtual"
lifted parcels both end up with the same EL. This happens because the
virtual
temperature converges to the actual temperature when temperatures are
very
cold (less than -20 C) and moisture effects become negligble.
CAPE = Convective Available Potential Energy. CAPE is a measure of instability through the depth of the atmosphere, and is related to updraft strength in thunderstorms. SPC forecasters often refer to "weak instability" (CAPE less than 1000 Jkg-1), "moderate instability" (CAPE from 1000-2500 Jkg-1), "strong instability" (CAPE from 2500-4000 Jkg-1), and "extreme instability" (CAPE greater than 4000 Jkg-1). The CAPE in the sample sounding above is about 3200 Jkg-1 lifting the "virtual" surface parcel. In the real world, CAPE is usually an overestimate of updraft strength due to water loading and entrainment of unsaturated environmental air.
CIN = convective inhibition. Convective inhibition represents the "negative" area on a sounding that must be overcome for storm initiation. The CIN in the sample sounding above is about 20 Jkg-1, lifting the "virtual" surface parcel.LI = Lifted Index. The lifted index is the temperature difference between the 500 mb temperature and the temperature of a parcel lifted to 500 mb. Negative values denote unstable conditions. LI is more of a measure of actual "instability" than CAPE because it represents the potential buoyancy of a parcel at a level, whereas CAPE is integrated through the depth of the troposphere. The LI is the sample sounding above is about -10 C, lifting the "virtual" surface parcel.
Normalized CAPE = CAPE divided by the depth of the layer where CAPE is present (units of m/s2). Normalized CAPE can be interpreted in much the same way as the LI (e.g., a "tall, skinny" CAPE gives a low normalized CAPE value and a small negative LI, while a "short, wide" CAPE gives a large normalized CAPE and larger negative LI.
Blanchard, D. O., 1998: Assessing the vertical distribution of convective available potential energy. Wea. Forecasting, 13, 870-877.
DCAPE = Downdraft CAPE. DCAPE can be used to estimate the potential strength of rain-cooled downdrafts with thunderstorms convection, and is similar to CAPE. Larger DCAPE values are associated with stronger downdrafts.
Gilmore, M.S., and L.J. Wicker, 1998: The influence of midtropospheric dryness on supercell morphology and evolution. Wea. Forecasting, 126, 943-958.
Effective Inflow Layer = the first layer in a sounding where all contiguous lifted parcels have CAPE >= 100 J kg-1 and CIN >= -250 J kg-1. The effective inflow layer is meant to represent the inflow layer for a thunderstorm, where lifted parcels have sufficient CAPE and CIN that is not excessive. The effective inflow layer technique is applicable to both surface-based and elevated thunderstorms, and this layer is used in calculations of effective storm-relative helicity. The cyan vertical bar in the sounding above (left of the temperature and dewpoint traces) marks the bottom and top of the effective inflow layer in meters above ground level. such that
Weisman, M.L., 1996: On the use of vertical wind shear versus helicity in interpreting supercell dynamics. Preprints, 18th Conf. on Severe Local Storms, San Francisco, CA, Amer. Meteor. Soc., 200-204.
Rasmussen, E.N., and D.O. Blanchard, 1998: A baseline climatology of sounding-derived supercell and tornado forecast parameters. Wea. Forecasting, 13, 1148-1164.
Weisman, M.L., and J.B. Klemp, 1982: The dependence of numerically simulated convective storms on vertical wind shear and buoyancy. Mon. Wea. Rev., 110, 504-520.
Stensrud, D.J., J.V. Cortinas Jr., and H.E. Brooks, 1997: Discriminating between tornadic and nontornadic thunderstorms using mesoscale model output. Wea. Forecasting, 12, 613-632.
SRH = Storm-Relative Helicity. SRH is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1 and 3 km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. However, larger values of 0-3 km SRH (greater than 250 m2s-2) and 0-1 km SRH (greater than 100 m2s-2) do suggest an increased threat of tornadoes with supercells. For SRH, larger is generally better, but there are no clear "boundaries" between nontornadic and significant tornadic supercells.
Davies-Jones, R.P., 1984: Streamwise vorticity: The origin of updraft rotation in supercell storms. J. Atmos. Sci., 41, 2991-3006.
Davies-Jones, R.P., D.W. Burgess, and M. Foster, 1990: Test of helicity as a forecast parameter. Preprints, 16th Conf. on Severe Local Storms, Kananaskis Park, AB, Canada, Amer. Meteor. Soc. 588-592.
Rasmussen, E.N., and D.O. Blanchard, 1998: A baseline climatology of sounding-derived supercell and tornado forecast parameters. Wea. Forecasing, 13, 1148-1164.
The 0-2 km SR winds are meant to represent low-level storm inflow. The majority of sustained supercells have 0-2 km storm inflow values of 15-20 kt or greater. The red vertical bar in the upper right inset shows the 0-2 km mean SR speed (see sample hodograph above).
Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.
Rasmussen, E. N., and J.M. Straka, 1998: Variations in supercell morphology, Part I: Observations of the role of upper-level storm-relative flow. Mon. Wea. Rev., 126, 2406-2421.
Hart, J.A., and W. Korotky, 1991: The SHARP workstation v1.50 users guide. National Weather Service, NOAA, US. Dept. of Commerce, 30 pp. [Available from NWS Eastern Region Headquarters, 630 Johnson Ave., Bohemia, NY 11716.]
Davies, J.M., 1993: Hourly helicity, instability, and EHI in forecasting supercell tornadoes. Preprints, 17th Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 107-111.
Rasmussen, E.N., and D.O. Blanchard, 1998: A baseline climatology of sounding-derived supercell and tornado forecast parameters. Wea. Forecasting, 13, 1148-1164.
This index is formulated as follows:
SCP = (muCAPE / 1000 J kg-1) * (ESRH / 50 m2
s-2) * (EBWD /
20 m s-1)
EBWD is divided by 20 m s-1
in the range of 10-20 m s-1. EBWD less
than 10 m s-1
is set to zero, and EBWD greater than 20 m s-1 is set to
one.
STP = (mlCAPE / 1500 J kg-1) * ((2000 - mlLCL) / 1000 m) * (ESRH / 150 m2 s-2) * (EBWD / 20 m s-1) * ((mlCIN + 200) / 150 J kg-1)
where the mlLCL term is set to 1.0 for mlLCL heights < 1000 m AGL; the EBWD term is capped at 1.5 for EBWD > 30 m s-1, and set to 0.0 for EBWD < 12.5 m s-1; the mlCIN term is set to 1.0 for mlCIN > -50 J kg-1; the entire index is set to 0.0 when the effective inflow base is above the ground.
A majority of significant tornadoes (F2 or greater damage) have
been
associated with STP values greater than 1, while most nontornadic
supercells
have been associated with vales less than 1 in a large sample of RAP
analysis
proximity soundings. Inclusion of the mlCIN term tends to reduce the
size of
contoured areas, thus reducing false alarms.
Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.
Thompson, R. L., R. Edwards, C. M. Mead, 2004: An update to the Supercell Composite and Significant Tornado Parameters. Preprints, 22nd Conf. Severe Local Storms, Hyannis, MA (134K PDF)
STP = (sbCAPE / 1500 J kg-1) * ((2000 - sbLCL) / 1000 m) * (0-1 km SRH / 150 m2 s-2) * (6BWD / 20 m s-1)
where the sbLCL term is set to 1.0 for sbLCL heights < 1000 m AGL; the 6BWD term is capped at 1.5 for 6BWD > 30 m s-1, and set to 0.0 for 6BWD < 12.5 m s-1.
A majority of significant tornadoes (F2 or greater damage) have
been
associated with STP values greater than 1, while most nontornadic
supercells
have been associated with vales less than 1 in a large sample of RAP
analysis
proximity soundings.