I saw the below discussion in today's weather outlook for Gwinnett County, GA. Can anyone help explain terms like vort max, capes, indicies, and K index? Thanks!
.DISCUSSION...
MAIN LOW CENTER ASSOCIATED WITH THE REMNANTS OF HURRICANE DENNIS
HAS STALLED OVER THE OHIO/MISSISSIPPI VALLEY TODAY. THIS WILL
INCREASE MOIST SOUTHWESTERLY FLOW OVER THE AREA TODAY. THE FORECAST
MODELS ARE SHOWING A VORT MAX ROTATING AROUND THE LOW CENTER
AND MOVING ACROSS THE REGION THIS AFTERNOON. THIS WILL HELP IN THE
DEVELOPMENT OF SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS ACROSS
THE COUNTY WARNING AREA. INSTABILITY WILL ALSO INCREASE WITH CAPES
EXPECTED AROUND 1500 AND LIFTED INDICES RANGING FROM MINUS 2 TO
MINUS 4. COMBINE THIS WITH K INDEX VALUES BETWEEN 30 AND 35 AND
GEORGIA COULD SEE SOME STORMS PRODUCING HEAVY RAIN AND GUSTY WINDS.
Need help with NOAA discussion
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Need help with NOAA discussion
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Had to look up K-index. I knew it had to do with temperatures and dewpoint temps, but exactly what I didn't know. 
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The K-index was composed for forecasting air mass thunderstorms, or thunderstorms with no dynamic triggering mechanism. To compute this index, first take the 850mb temperature minus the 500mb temperature. Secondly, add the 850mb dew point temperature to this difference. Larger values of this dew point indicate low level moisture present and increase the chance of convection. Finally, we subtract the 700mb dew point depression for moisture input at the mid levels. A small dew point depression at 700mb indicates a possibility for deep convection. If there is no significant moisture at 700mb then there is a greater chance that entrainment of dry air would occur, given a parcel were lifted from beneath the 700mb level. If entrainment of dry air occurs, the parcel will become less buoyant (Blustein, 1993).
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Jan
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The K-index was composed for forecasting air mass thunderstorms, or thunderstorms with no dynamic triggering mechanism. To compute this index, first take the 850mb temperature minus the 500mb temperature. Secondly, add the 850mb dew point temperature to this difference. Larger values of this dew point indicate low level moisture present and increase the chance of convection. Finally, we subtract the 700mb dew point depression for moisture input at the mid levels. A small dew point depression at 700mb indicates a possibility for deep convection. If there is no significant moisture at 700mb then there is a greater chance that entrainment of dry air would occur, given a parcel were lifted from beneath the 700mb level. If entrainment of dry air occurs, the parcel will become less buoyant (Blustein, 1993).
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Jan
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Severe weather, this is my better realm of territory, Jan handled a good bit of it though.
Vorticity maximum is center of circulation of energy that creates instability and turbulence that can spark off thunderstorms. CAPE is the convective available *potential* energy, which means how much potential energy is present necessary for thunderstorm. The higher the number the better the chance. Note again that its potential energy. . You need a kicker, the vort max is the kicker here. The value is 1500 which is marginally unstable. Liftted index is another measure of instability. You want *negative* numbers here, the lower the value is the better the instability. Here those -2 to -4 are marginally unstable. And the K-index is another value that measures the potential for thunderstorms. It does well for predicting heavy rain and deeper convection. Those values of 30 to 35 bode well for stronger thunderstorms with heavier rains.
So, you have marginal instability with a vort max nearby, and a typical summer time pattern, which leads to an increased chance for a few thunderstorms in the afternoon. Given the instability, the threat of severe weather is quite marginal, although other factors can compensate the above given values to create a better chance of wet microbursts (localized areas of damaging winds)
Vorticity maximum is center of circulation of energy that creates instability and turbulence that can spark off thunderstorms. CAPE is the convective available *potential* energy, which means how much potential energy is present necessary for thunderstorm. The higher the number the better the chance. Note again that its potential energy. . You need a kicker, the vort max is the kicker here. The value is 1500 which is marginally unstable. Liftted index is another measure of instability. You want *negative* numbers here, the lower the value is the better the instability. Here those -2 to -4 are marginally unstable. And the K-index is another value that measures the potential for thunderstorms. It does well for predicting heavy rain and deeper convection. Those values of 30 to 35 bode well for stronger thunderstorms with heavier rains.
So, you have marginal instability with a vort max nearby, and a typical summer time pattern, which leads to an increased chance for a few thunderstorms in the afternoon. Given the instability, the threat of severe weather is quite marginal, although other factors can compensate the above given values to create a better chance of wet microbursts (localized areas of damaging winds)
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Good answers from everyone.
A good resource to help explain all that technical talk in the AFDs (Area Forecast Discussion) is here: http://theweatherprediction.com/
Click on SKEW-T, which explains each of the indices on upper-air soundings and what values signal increased instability and risk for severe wx.
A good resource to help explain all that technical talk in the AFDs (Area Forecast Discussion) is here: http://theweatherprediction.com/
Click on SKEW-T, which explains each of the indices on upper-air soundings and what values signal increased instability and risk for severe wx.
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BTW, a nice line of thunderstorms has formed off in AL and is prompting Flash Flood Warnings. There is also an SPC MCD (mesoscale discussion) outlining AL and GA in a threat for isolated strong winds and heavy rains today.
Given the rainfall from Dennis, I would imagine you'll see far more flash flood warnings than severe thunderstorm warnings today.
isobar posted a great link, one of my favorite sites. It has about anything you would need or want to know.
Given the rainfall from Dennis, I would imagine you'll see far more flash flood warnings than severe thunderstorm warnings today.
isobar posted a great link, one of my favorite sites. It has about anything you would need or want to know.
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