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EuroWX Map Definitions

Posted: Sun Sep 08, 2013 11:36 am
by Andrew Revering
Note: This list may not be a complete list of all the maps available as we try to add more maps from time to time.

APRWX Severe Index

- A propriety severe weather index developed by Andrew Revering. This index attempts to do the job of a severe storm forecaster by combining the various parameters of model output to decide if there will be severe weather, and if there will, its intensity. More information can be found in this thread:



CAPE stands for 'Convective Available Potential Energy'. This is the area of a sounding bounded by the environmental temperature and the parcel temperature lines. When the parcel temperature is warmer than the environmental temperature, you have conditionally unstable atmosphere and the area where the parcel is warmer then the environmental temperature is the area of CAPE or instability.

CAPE represents the amount of buoyant energy available to accelerate a parcel vertically, or the amount of work a parcel does on the environment. CAPE is the positive area on a sounding between the parcel's assumed ascent along a moist adiabat and the environmental temperature curve from the level of free convection (LFC) to the equilibrium level (EL). The greater the temperature difference between the warmer parcel and the cooler environment, the greater the CAPE and updraft acceleration to produce strong convection.

CAPE below 0: Stable.
CAPE = 0 to 1000: Marginally unstable.
CAPE = 1000 to 2500: Moderately unstable.
CAPE = 2500 to 3500: Very unstable.
CAPE above 3500-4000: Extremely unstable.



CINH is the Convective Inhibition value. This is the most common parameter used to determine the strength of the 'cap'. CIN is calculated by looking at a sounding and integrating the area below the LFC (Level of Free Convection) downward through the area where the parcel temperature is cooler than the environmental temperature. When the parcel temperature is cooler than the environmental temperature the parcel naturally wants to sink... this is the cap; negative buoyancy.


Dew Point

The dew point is the point at which condensation will occur if the environmental temperature is cooled to this point. When this occurs the humidity becomes 100%.

Dew point is extremly useful for just about every aspect of Meteorology. It is useful in the winter for determining moisture content of a snowstorm, how low the temperature might get at night, if we have enough moisture for severe storms, if there is enough dry air in the mid levels for strong downdrafts and large hail, etc.


Dew Point Depression

The dew point depression is just another way to look at atmospheric humidity. Rather than a humidity, we are looking at the span between the environmental temperature and the dew point. The greater this value is, the drier the atmosphere is. This is typically most useful for looking at dry mid levels for thunderstorm development, or perhaps for smaller values where humdities would be high indicating cloudiness.


Directional Shear

Directional shear by itself is nothing new, but the way we display this parameter may be a little different than what you're used to. We are simply looking at the differences between the directions between two levels. Directional shear can be useful when looking for storm inflow or rotation with storms, although I don't place nearly as much importance on this as I would speed shear.


Freezing Level

The Freezing Level is pretty self explanatory. Generally speaking the temperature cools with height. During the spring, summer and fall, the surface is usually not freezing, but it's important to know where the freezing level is. This can be very important to pilots who might have to fly through freezing rain, snow or hail. It might also be important to a meteorologist forecasting hail.


Heat Ind / Wind Chill / Temp

This is a proprietary composite map from the developers of F5Data. We simply are making a "feels like" map where we are plotting either the heat index, wind chill or temperature depending on the situation. For example, if it's in Florida and the humidity is high and there is a heat index, we will plot the heat index rather than the temperature. Meanwhile farther north we might be plotting a wind chill if there is one. In the central U.S. there may not be a heat index or a wind chill, so we plot the surface temperature.


Heights (Pressure)

The atmosphere is made up of different pressure levels. At surface level the pressure is typically around 1013mb. The pressure is what we are feeling from atmosphere pushing down on us from the forces of atmosphere and gravity. As you go up in the atmosphere the pressure decreases because there is essensially less atmosphere above that point to push down on you. So half way up through our atmosphere we have about 500mb of pressure.

These various pressure levels are used by meteorologists as a reference for what is going on at those heights. Rather than saying x meters or x thousand feet, we typically use millibars of pressure. We do this because heights can change with pressure systems that force atmosphere and evacuate atmosphere out of a column of air (high and low pressure centers).

We use the Heights maps to give us an indication of what height a specific pressure level is located. This is similar to a surface pressure map with isobars, but rather than using lines of equal pressure we are assuming the entire map is at a single pressure (say 700 mb) and the lines on the map are lines of equal height.

Kinks in the height contours and closed centers can give us a feel for boundaries aloft such as shortwaves and where our pressure centers are above us. Their orientation with respect to the surface pressure centers can help us to determine if a system is going to weaken or strengthen. For example, if you have a pressure center at the surface which is to the east of the pressure center aloft, it is likely that the storm will strengthen. However if both pressure centers are vertically stacked, it is likely that the storm will occlude and begin to die.


LCL Height

LCL is the Lifted Condensation Level. This is the level at which a parcel will condense into a cloud if it is forced upward. This can be considered the height of the cloud base.


Mixing Ratio

The Mixing Ratio is another way of looking at moisture content in the atmosphere, much like one would use the Dew Point.


Moisture Convergence

Moisture Convergence is a calculated parameter which moisture convergence. Duh! It may be indicative of locations of a boundary. Areas of strong moisture convergence may be good to pinpoint areas of developing rain/snow/thunderstorms, however I have found that the model guidance moisture convergence maps seem to be quite useless in predicting such a fine mesoscale feature.



The precipitation maps are model generated areas and amounts of precipitation created with large quantities of physics packages. This is ultimately the final summary of all the model output. However, meteorologists like to look at all of the other specific parameters to do their own forecasting because you can't always trust a model. You have to first determine if the model is intializing correctly to be at all useful, then you have to look at other specific output parameters to see if you think it might be missing something.


Relative Humidity

Relative Humidity at various levels is extremly important in forecasting. Through looking at the humidity levels you can determine cloud layers which can have an impact of high and low temperature. You can determine if there is enough humidity through several layers to indicate precipitation. You can also determine if there is enough dry air in the mid levels along with moist air at the surface levels to support thunderstorms.



The shear maps are giving you some very simple things generally... speeds between 850 and 500 for example are simply looking at the speed differences between those layers. Speed shear is very important in storm development because the tilt of the updraft can mean keeping the updraft base clean enough for good continuous rising motion without being rained on and having a downdraft stop the updraft. Generally values of 10 knots + would be required for a tornadic producing environment.

The 6km Shear parameter is a vector shear parameter which considers both speed and direction. This is crucial for determining storm mode as well as support for tornadoes. Values above 30 would support both of these things. The higher the better. 50+ would be outstanding. Some food for thought: The Palm Sunday tornado outbreak had a pretty basic atmosphere. There wasn't anything terribly unusual about it except one thing... it had outstanding 500mb winds. This created a 6km shear of something on the order of 120 knots! I've never seen numbers that high in my lifetime, but I'd say that was probably the deciding factor in such a huge tornado outbreak.


MSLP (Pressure)

SLP is the Sea Level Pressure value at the surface. This is the typical surface pressure map you see on TV.


MSLP Trend

The SLP Trend map is a pressure change map over the last 2 hours. This can be fun to watch for mesoscale changes in the RUC computer model. Often times behind a cold pool from a MCS or Derecho you will find rapid pressure changes in response to the storms. Sometimes you will find rapid pressure falls just prior to storm development, or in response to a rapidly developing cyclone.


Spot Index

The SPOT Index is a index developed by the Navy to be used in conjunction with the SWEAT Index. The SPOT Index uses only surface based parameters which can be measured hourly. Through hundreds of climatological cases they were able to identify the values which typically lead to severe weather events. The SPOT Index highlights these areas, and when used together with the SWEAT Index as an overlay, you are able to identify the locations of severe weather prior to its development.


Summer Simmer Index

The Summer Simmer Index is similar to the Heat Index, and is meant to give us a rating of how its expected to feel outside.



The SWEAT Index was developed by the Navy to work in conjunction with the SPOT Index for determining areas of severe weather. SWEAT values higher than 273 are areas to be concerned about, especially where SPOT index values are enhanced over the same areas.



We display maps with temperatures at various heights from the surface all the way up to jet stream level. These temperatures can be used in any number of ways to determine things from forecast temperatures by mixing the 850mb temperature down to lapse rates to freezing levels to be concerned with hail or flight dangers to precipitation type in the winter, fall and spring.



Theta-E is a mixture of temperature, pressure and moisture giving you an idea of the stability of a layer.



The Total Totals index is to be used only as a thunderstorm coverage predictor.
It has a strong bias toward steep lapse rates that can yield high numbers under cold cores with very little moisture. Values generally greater than 50 are areas to be concerned with.

The Total Totals index is comprised of the Vertical Totals and Cross Totals indexes.


TQ Index

The TQ Index is an index intended to forecast the potential for low-topped supercells. I have found this index to be somewhat useless.


Wet Bulb

The wet bulb temperature is the temperature at which the air can cool to if it is cooled by means of precipitation. This differs from radiational cooling down to the dew point temperature because the air is saturating itself. This process prevents the air to cool that far because the dew point will actually meet it somewhere in the middle at the air moistens.


Wind Barbs

The wind barb overlays are indicating the wind speed and direction at a given layer. Half barbs indicate 5 knots, a full barb is 10 knots, and a pennent is 50 knots. You add up the barbs and lines to determine the speed. For example a pennent, long barb and a half barb would mean a 65 knot wind. The direction is determined by the orientation of the barb. If the barb extends from its nose to the west with the 'feathers' on the west side of the barb, then the winds are from the west. If the 'feathers' are on the north side of the barb, then the wind is from the north.


Wind Direction

This wind direction map is a proprietary map created by Nick Benson. This indicates the wind directions as colors. It's a unique map that may take some getting used to, but it can be quite useful in finding areas of circulation. You will find these areas show up best when the black contour lines all cross each other. Overlaying a wind barb map on top of this may be useful in seeing the air flow more clearly.


Wind Speed

Pretty self explanatory... I usually overlay wind barbs on top of this map.


WMAX (Max Updraft Speed)

WMAX is directly proportional to the amount of instability (CAPE) in the atmosphere. The calculations done are intended to give you an idea of how strong an updraft might be in a convective storm.