GB Weather – Forecast Model Variables Tutorial and Snow Forecast Criteria.
Overview: Find below summary information on some of the variables that are present on our forecast weather models. Sometimes it can of be interest and of use to understand what these variables mean and given the time of year also find below some more specific information as to what to look out for, on the models, to gauge whether wintry precipitation is possible.
DEW POINT: The dew point is the temperature at which the water vapour within the atmosphere condenses into liquid water. The nearer the dew point value is to the temperature the more moist the atmosphere is, the greater the difference between the temperature and the dew point value the drier the air. Dew point is also an important indicator of air masses. Sometimes, as an example, in a cold air mass the temperature may well be 5C, but the dew point could be -1C. So whilst the dew point provides us with information on the amount of moisture in the atmosphere, it can show us different types of air masses too.
WINDCHILL: Wind-chill or the wind-chill factor is a variable that takes into account the air temperature and the wind speed. For example, if the air temperature was 0C with no wind then there would be little or no wind-chill present. However, the greater the wind speed when coupled the colder it ‘feels’ to the human body. So a temperature of 0C, with a wind speed of 20mph would ‘feel’ more like -7C.
500hPa and 850hPa WINDS: To start with let’s take a look at the hPa means first and it stands for Hectopascal and is essentially the same as mb (millibars) as it is the variable for pressure. Now 500hPa also means 500mb, which is approximately 18,000ft up in the atmosphere, whilst 850mb is 5000ft. Wind speeds at this altitude can provide information on air masses and also position of troughs and ridges, especially so at the 500hPa level.
THICKNESSES: Thicknesses and especially 1000-500mb thickness, is essentially a measure of the temperature between 1000mb and 500mb, so that’s essentially between the surface and 18,000ft up in the atmosphere. What thicknesses are very important for are gauging the potential for wintry precipitation and precipitation type, as varying temperatures between the surface and 18,000ft can mean the difference between rain, sleet and snow.
0°C ISOTHERM: The 0°C isotherm is essentially showing the height at where the temperature reaches 0°C. Clearly this is a very important variable when it comes to forecasting wintry precipitation as if the air temperatures is near to freezing then precipitation, most often than not, will fall as sleet or snow. So knowing the altitude of where the temperature reaches 0°C is very important.
CAPE (Surface Based, ML Cape and LI): CAPE stands for Convective Available Potential Energy and whilst they may sound rather complicated and confusing, essentially CAPE is a measure of the instability or available instability within the atmosphere. Surface based CAPE (SBCAPE) is essentially a measure of instability from the surface to certain height and Mean Layer CAPE (MLCAPE) is a measure of instability but, as the name would imply, a mean of a certain section of the atmosphere.
LI stands for Lifted Index and is another variable that aids to gauge the instability within the atmosphere. Values often range between -6 and +6C with values of +6 or more indicating very stable atmosphere conditions with little or no risk of thunderstorms or convective weather. Clearly the further you progress towards values of -5 or -6 then these values indicate extreme and potentially severe convective weather and thunderstorms.
LOW LEVEL and DEEP LEVEL SHEAR: Low level and deep level shear within the atmosphere is essentially a measure of the wind speed within a certain column or section of the atmosphere. In severe weather forecast especially during summer, the greater the amount of shear within the atmosphere the greater the risk that where thunderstorms develop they could be organised and potentially severe. Low Level Shear is often measured between the surface and 1KM up into the atmosphere, whilst Deep Level Shear is often measured between the surface and 6KM.
850mb TEMPERATURES (upper 850 temperatures): 850mb temperatures are a measure of the temperature, within the atmosphere, at the 850mb level, or approximately at 5000ft in the air. 850mb temperatures can be a focus, in terms of model data, to gauge the risk of wintry precipitation and also highlight changing air masses. As a general rule of thumb an 850mb temperature below -5C during the winter months will lead to a risk of sleet and snow. In the UK 850mb temperatures below -10C are often associated with quite significant winter weather and those values would allow snow to reach very low altitudes. In summer 850mb temperatures can reach up to +15C and even near +20C which signals hot and often humid conditions.
JET STREAM: The jet stream is a fast flowing region of winds high up within the atmosphere, often at an altitude of between 30,000ft and 40,000ft. The jet stream is persistently present because it is there due to temperature differences between the Polar Regions and the tropical regions across the northern hemisphere. When there is a significant temperature gradient (sharp change in temperature across a distance) then the jet stream is often powerful and can aid in the development of deep low pressure systems. Equally and importantly during the winter months when the jet stream is to the south of the UK, the UK can be at risk of cold weather. Conversely, if the jet stream moves over or to the north of the UK then the weather is often milder.
PRECIPITATION TYPE: The precipitation type charts are there to provide an idea of whether precipitation will fall as rain, sleet and snow. Through computer and model calculations the precipitation type can be gauge. However, we do urge some caution as especially in the UK the margin between whether precipitation falls as rain, sleet or snow is particularly delicate and the model can be incorrect. However, if you combine the precipitation type images with the other forecast charts you will then have a better understanding as to what the precipitation type will be.
SNOWFALL CRITERIA – What’s Required?
Firstly we would like to highlight that while the information below will provide a good guide as to whether snow is likely to fall or not, local and regional variations across the UK can make a significant difference at times.
500mb Temps: -25C (below -30C preferable)
850mb Temps: -5C (below -7C preferable)
850mb theta-w (wet bulb potential temp): 2C (below 0C preferable)
850mb theta-e (equivalent potential temp): 15C (below 10C preferable)
Surface Temperature: 1.0C (below 0.0C preferable)
Surface Dew Point: -0.5C (below -1.0C preferable) *Highly Variable*
1000-500mb thickness: 528-525dam (below 522dam preferable)
Wet Bulb Zero (WB0): Height figures will vary, but gives a general/good idea as to what altitudes (200m, 300m etc) snow will fall down to. Calculated in conjunction with evaporational cooling:
Below 1000ft (approx 300m) – (below 500ft preferable (approx 150m preferable))
3000ft or over: Always rain, snow highly unlikely.
2000ft-3000ft: Mostly rain, snow unlikely, low risk of sleet.
1000ft-2000ft: Snow more likely than rain/sleet.
below 1000ft: Mostly snow, light precipitation and low lying ground may still see sleet/rain.
Freezing Level (Height of 0C isotherm): 300m (below 250m preferable)