GRR NEXRAD Loop

Michigan NEXRAD Radar

Grand Rapids Webcam

SPC Day 1 Convective Outlook

National StormWatch

North Central Region Surface Map

NE Region Lightning Strikes

Conditions

Temperature

Lifted Index
This is an index used to determine the stability of the lower half of the troposphere. The LI field shows instability in the atmosphere by lifting a parcel of air from the surface to 500 mb and comparing its temperature to that of the environment. Where LIs are less than 0, thunderstorms are possible. The lower the number, the more unstable the atmosphere is and as a result, the stronger the thunderstorms could become. Values of -4 or lower indicate areas where severe thunderstorms are possible. Values > 10 indicate areas of stable weather where skies are generally clear. The risk of severe weather activity is defined as follows:
LI > 2 No significant activity
0 < LI < 2 Showers probable, isolated thunderstorms possible
-2 < LI < 0 Thunderstorms probable
-4 < LI < -2 Severe thunderstorms possible
LI < -4 Severe thunderstorms probable, tornadoes possible

K-Index
The K Index field shows instability in the atmosphere as it relates to the development of air mass thunderstorms. The KI is similar to Total Totals index except that it takes into account moist air at 700 mb contributing to air mass thunderstorm development. It is based on the 850 to 500 mb lapse rate plus 850 mb dewpoint minus the 700 mb dewpoint depression. It is defined as follows: K = T850 - T500 + Td850 - (T700 - Td700). Strong Springtime thunderstorms often require dry air at mid levels to cap the convection. On the other hand, summer air mass thunderstorms need a very moist atmosphere at mid levels to prevent evaporation through entrainment. Where KIs are greater than 35, air mass thunderstorms are likely. the higher the number, the higher the probability. Values < 10 indicate areas of stable weather where skies are generally clear. The risk of air mass thunderstorms is defined as follows:
K < 15 0% Air mass t-storm probability
15-20 <20% Air mass t-storm probability
21-25 20-40% Air mass t-storm prob.
26-30 40-60% Air mass t-storm prob.
31-35 60-80% Air mass t-storm prob.
36-40 80-90% Air mass t-storm prob.
K > 40 >90% Air mass t-storm probability

Showalter Index
This is an index used to determine the stability of the lower half of the troposphere. An air parcel is lifted from an initial position at 850 mb where localized low level influences are greatly reduced. It is lifted dry adiabatically to its LCL and then pseudo-adiabatically to 500 mb. The environmental temperature is then subtracted from the parcel temperature to obtain the value of the Showalter index. The risk of severe weather activity is defined as follows:
SI > 3 No significant activity
1 < SI < 3 Showers probable, isolated thunderstorms possible
-2 < SI < 1 Showers probable, isolated thunderstorms possible
-2 < SI < 1 Thunderstorms probable
-6 < SI < -2 Severe t-storms possible
SI < -6 Tornadoes possible

Vertical Total Totals Index
A measure of the thunderstorm potential, according to the following scale and color fill scheme:
< 25 Thunderstorms unlikely
25-26 Scattered thunderstorms
27-30 Scattered t-storms/ few severe/ isolated tornadoes
31-32 Scattered-numerous t-storms/ few tornadoes
33-34 Numerous thunderstorms/ few tornadoes
> 34 Numerous t-storms/ scattered severe/scattered tornadoes

Total Totals Index
The TT field shows instability in the atmosphere based on the lapse rate from 850 to 500 mb plus dewpoint at 850 mb: TT = T850 + Td850 - 2T500. Where TTs are greater than 45, thunderstorms are possible. The higher the number, the more unstable the atmosphere is and as a result, the stronger the thunderstorms could become. Values of 52 or higher indicate areas where severe thunderstorms are possible. Values < 40 indicate areas of stable weather where skies are generally clear. The risk of severe weather activity is defined as follows:
44-45 Isolated moderate thunderstorms
46-47 Scattered moderate/ few heavy thunderstorms
48-49 Scattered moderate/ few heavy / isolated severe thunderstorms
50-51 Scattered heavy/ few severe thunderstorms and isolated tornadoes
52-55 Scattered to numerous heavy/ few to scattered severe thunderstorm/ few tornadoes
> 55 Numerous heavy/ scattered severe thunderstorms and scattered tornadoes

SWEAT Index
Severe Weather thrEAT - This is an United States Air Force index that takes into account thermal instability, low level moisture content, vertical wind shear and horizontal wind speeds. The risk of severe weather activity is defined as follows:
SW < 300 No activity expected
300 < SW < 400 Isolated moderate to heavy thunderstorms
400 < SW < 500 Severe thunderstorms and tornadoes probable
SW > 500 SW < 500 Severe thunderstorms and tornadoes probable
SW > 500 Severe thunderstorms and tornadoes likely

CAPE Index
Convective Available Potential Energy - 
Another instability tool. CAPE represents the amount of energy a parcel might have if it were lifted. Often this reflects the strength of updrafts within a thunderstorm. CAPE values of greater than 2000 represent enough energy to produce thunderstorms. A value greater than 3000 represents enough energy to produce strong thunderstorms. Values < 1000 denote a relatively stable atmosphere.
< 300  Very weak convection
300-1000  Weak
1000-2500  Moderate
2500-3000  Strong
3000+  Wizard of Oz

Helicity Contour
This is a contour plot of the amount of storm relative rotation/shear in the atmosphere. Helicity is used to indicate where rotation/shear is high enough to allow thunderstorms to organize into severe or supercell storms. In the lack of helicity, storms develop vertically and the precipitation will snuff out the updraft killing the thunderstorm. Severe storms need helicity to maintain an organized structure allowing the storm to develop to severe limits. A value of 400-500 is often needed to produce severe storms. Often this is used in conjunction with CAPE to determine severe storm location.

Cross Totals Index
A measure of the frequency and intensity of thunderstorms:
<= 17 Thunderstorms unlikely
18-19 Isolated thunderstorms
20-21 Scattered thunderstorms
22-23 Scattered t-storm/ isolated severe
24-25 Scattered thunderstorms/ few severe/ isolated tornadoes
26-29 Scattered to numerous t-storms/ few severe/ few tornadoes
>= 30 Numerous t-storms/scattered severe/scattered tornadoes

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