NEXRAD GR Level 3 help me understand elements


First off..

I realize this is a tall order..
But i feel it will benefit many of us including myself.
I rely on weather radio, internet and others to forecast for me so that i can set up on my storms correctly.. this site has been very helpful and i wouldnt have been able to attain the success i have without learning and nowcasting from this site.

I have used the interwarn software last year and i think I may consider using GRlevel3 this year.. I mostly use the reflectivity to determine storms postitioning etc.. i realize that the height of storm etc plays roles in the determined severity. as the interwarn would give alarms.. while listening to weather radio my uneducated mind would attempt to put the info together with the data given in interwarn stormlab with what the NWS radio is disseminating.. Make sense?

I know to some of you this is probably easy stuff but Im still learning..

Can someone or a few of you please take the time to break down the different elements in this software and explain the levels I need to look for? Again I relaize this is a tall order that might take some thought but I am sure more than just I would benefit from it.

Im not sure really what Im asking but if someone could break down the elements that the software outlines and elaborate on each one in laymans terms to use as a guide..

Storm attributes..
Base reflectivity
Base velocity
Storm relative velocity
Composite reflectivity
echo tops

I understand some of this.. But if someone could maybe give a crash course for a Noob .. im not afraid to admit or ask...

Maybe it would be easier for some examples of storms.. I know you guys save the info on particular storms in these software programs.. If someone could take the time to show each measurement in the thick of a storm and break it down into laymans terms?? I also realize barometirc pressure and dew point is imporant in a given area.. Can anyone elaborate on these things a little further?

I realize Im basically asking for a quick meteorology degree here.. lol
But hopefully it could be simpler than that.

This would save people like myself who cant go to college for a meteorology degree. But still love chasing and fitting in learning weather in our lives. it would also make a wonderful addition to Stormtrack for a person to be able to go to a thread here or a link and understand in laymans terms how this software works..

Thanks in advance. If this is too much and out of the realm I understand..
As a quick commercial self-plug my Weather Map Handbook goes into substantial detail on all the WSR-88D products. I'm working on something even more thorough right now.

Algorithm info is very tough to find online.... I had a lot of trouble finding references on the Internet and had to rely on my tech library, NEXRAD ICD's, and my contacts at NSSL and ROC when developing the above named book. This also hints at the fact that people probably don't understand these algorithms very well and may be getting theirselves lost in arcane things like storm attribute tables when analysis of the reflectivity & velocity products would be much more appropriate (and faster).. I've seen a lot of Snellman's "meteorological cancer" developing even in NEXRAD (though thankfully not at the WFOs).

I assume you were asking general NEXRAD questions; I didn't get a sense you were looking for exact instructions on using grlevel3.

That is correct.. Reading the radar is easy Its some of the other attributes I have an idea of but am unsure about....
Base reflectivity

I don't know the exact definitions of any of these. I imagine base reflectivity is just the lowest scan of the radar. The further away from the radar the higher this lowest scan will be. Like the central NE storm of July 12 this year had a base scan that was around 14,000 feet becuase it was so far away from the North Platte radar.

Base velocity

Same lowest scan but just measuring wind direction and speed in a toward and away means.

Storm relative velocity

I suppose it just subtracts the storm movement speed from the velocity the radar is detecting. So if a storm is moving toward a radar at 30mph and has an RFD blowing toward the radar at 100mph then the actuall storm relative winds in the rfd would be 70mph. Same but opposite would go for the outbound winds blowing away from the radar as a storm moves towards it(you would add the storm speed to the wind speed). Someone that actually knows what they are talking about can add to that if they decide to post.

Composite reflectivity

Not exactly sure on this one....would it be an image of all scan heights combined together?

echo tops

Highest level the radar detects an echo giving you an estimated storm top?

The echotops are defined as the highest altitude at which a Radar reflectivity strength equivalent to a rainfall rate of 0.1 mm/h is still found.


Another quantity that can be calculated from the large 14-elevation radar scan is the so-called Vertically-Integrated-Liquid (VIL). The VIL-value at a certain location is the sum of all observed radar reflectivities (converted to liquid watercontent) in a vertical column above this location. The unit of VIL is kg/m2 or mm, and it can be regarded as a measure for the potential rainfall. The rate at which precipitable waterdroplets are formed is proportional to the updraft-speed, and therefore the VIL-value is both a function of updraft-speed and cloud thickness. It has been noted already by others that observations of high VIL-values and the occurrences of severe thunderstorms correlate quite well.


dBZ levels correspond to levels of precipitation intensity in terms of inches of rainfall per hour. The higher the dBZ level, the more intense the precipitation ; they are directly proportional. \"dBZ\" stands for decibels of Z, Z being the reflectivity factor. The higher the reflectivity factor, the heavier the rainfall. This scale is logarithmic, so there is no factor one can use to multiply the dBZ level and arrive at the corresponding rainfall rate.


The NSSL HDA consists of the Probability of Severe Hail (POSH) and the Probability of Hail (POH).The NEXRAD and the POH HDAs are designed to detect hail of any size (>0 mm) while the POSH HDA is designed to detect severe (>19 mm) hail.
Wow really great stuff Mike. Thank you very much for taking the time to post.

Now if someone had screen shots from storm lab or on GRlevel3 to show where each one was at when the storm was at its peak I can applicate some of Mikes info.

Any pointers on which of these that are used most when forecasting storms.. Other than the radar of course..
Obviously severe hail in an area would give us a clue to the wall cloud possibly behind it.

VIL would show heavy rain. When taking that into account with a nice radar image some predictability can be given.

DBZ as i always thought was the height of the storm.. My understanding.. Someone correct me if wrong.. But a DBZ of 45,000 feet is a tell tale sign of severe activity

TVS Tornado Vortex signature?? How is this applicable and what will I see on the software??

I notice in the field with other experienced chasers they like the Goes satellite images as well. Thats basically what was used to determine initiation if I remmebr correctly.. Of course dewpoint etc.. was used...

There sure is alot to learn . Im coming along right nicely. : ) Thanks to all the helpful people this year..

Mike Deason?? Where are you? I notice you post alot on GR forum... :)

Thanks for the info on the book Tim.. ill look into that and some of your newer stuff coming.. I hope to maybe meet you in Colo. this Feb and get a forecasting class i hear you put on. Dave D gave me a heads up on..

Weather spotting classes are so rudimentary and I have already taken one.. Maybe Ill take another for fun and network with local chasers..

Anyone using GR or Stormlab i would like to know your process flow and what measurements you most lean on. Again with maybe some screen shots and a short online course of workflow etc...

Thanks again Mike..

WOW Tim.. i think ill buy that book..It looked awesome.. What else you got coming out?? Try to keep it at Fred level... lol :D

Sometimes when Im with experienced chasers and they talk about cape or other complicated subjects etc.. I just smile and shake my head yes.. i have not a clue.. And of course they dont have an hour to explain their 5 second conversation to me.. lol
Originally posted by fplowman+--><div class='quotetop'>QUOTE(fplowman)</div>
VIL would show heavy rain.[/b]

It is actually your lowest base reflectivity scan that shows heavy rain (or heavy precip) near the surface. VIL is a weighted sum of all the reflectivity above a given point, and is more appropriately used to determine precipitable water. It's expressed in kg/m^3 (density, if you will) and can be loosely likened to how much water is suspended above a point. In many cases VIL and reflectivity may be skewed by hail.


DBZ as i always thought was the height of the storm.. My understanding.. Someone correct me if wrong.. But a DBZ of 45,000 feet is a tell tale sign of severe activity

dBZ is the effective power (energy) received by the radar, thus it is "intensity" of a radar echo. Maybe you're thinking of 45 dBZ, which sounds like a threshold for severe weather. Generally at that reflectivity level, the rain is quite heavy or hailstones are getting added into the mix. Storm height is not a good indicator... tornadic storms of 30,000 ft happen during the cold season, while the subtropics occasionally gets weaker cells up to 70,000 ft. The depth of the troposphere is not always related to CAPE (i.e. energy).

Originally posted by fplowman+--><div class='quotetop'>QUOTE(fplowman)</div>
TVS Tornado Vortex signature?? How is this applicable and what will I see on the software??[/b]

You probably will as it's part of the Level III products. This uses a tornado detection algorithm developed by NSSL and some other agencies. It's subject to basic radar limitations like resolution, range, and beam height, so it's always best used with manual analysis of the velocity and reflectivity products (and spotter reports!)

Originally posted by fplowman@
I notice in the field with other experienced chasers they like the Goes satellite images as well.

Yes, 1 km visible imagery is excellent for this. Sample.


they talk about cape
CAPE = potential energy of an updraft. The greater the CAPE, the greater the potential speed of an updraft, and this brings all kinds of severe weather possibilities out of the woodwork.