Question on EURO Model

[WEATHERGURU]

On the EURO's 500 mb map what number is the rain/snow line(i.e. 5400)? Thanks

[Stormsfury]

On the EURO's 500 mb map what number is the rain/snow line(i.e. 5400)? Thanks

That's the 500mb heights ... anyway, the EURO isn't a good model to use in go time (IOW, in the short range) ...

To read rain/snow, you need a combination of things to look at ... but heights themselves aren't necessarily one of those ..

1) 850mb temperatures - generally for the best snow growth, the 850mb temperatures (C) need to be below -10ºC for best dendrite growth. But this alone cannot determine rain/snow in all or most cases ... Just because 850mb temperatures are below zero doesn't guarantee a location will see snowfall b/c of it ... on the other hand, just like the current cold air damming scenario, 850mb temperatures are progged to be well above that zero mark, however, it doesn't reflect the actual SFC temperature ... that warm nose (or Warm Air Advection - WAA) punching over a cold dome below is what is responsible for the sleet/freezing rain situations that so commonly occur in CAD/wedging).

2) Thickness schemes (ETA/GFS/RUC) have those ... the ECMWF does not run or release their thickness schemes to us hobbiests and there are many different scheme values with rain/snow ... or determining sleet/frz rain.

Let's start with the basic scheme values ...

1000mb-500mb Thickness -- commonly that's where the 5400 line comes in ...

ETA 24 hour forecast ... as an example ...

And another example, and we'll use the Christmas Snowstorm of 1989 for the South Carolina Coast which brought Snows all the way into Northern Florida ... the light blue is where the 1000mb-500mb thickness schemes were 5400m and BELOW ... and that's where your rain/snow line was in this storm ... however, this isn't totally failsafe, either.

December 24th, 1989 at 00z Critical Thickness Schemes 1000mb-500mb...

December 24th, 1989 at 00z 850mb Temperatures... perfect snow growth even along the coast with 850mb temperatures at -10ºC)

Now 1000-500mb thickness schemes would totally fail you with last December's icestorm that hit North Carolina so severly ... the 1000mb-500mb thickness scheme was above 5520m ... in fact, the icestorm itself looked so unimpressive on thickness schemes or even general upper air maps, it would generally be missed by the amateur who didn't know specifically what they were looking for ...

The 1000mb-850mb Thickness scheme is another value for rain/snow ... and the general threshold here is 1320m ... and again, we'll use the Christmas Snowstorm as an example ...

December 24th, 1989 --- 1000mb-850mb Thickness Schemes and the 1320m line is deep into Florida ...

But as a whole --- The distance (thickness) between 1000 mb and 500mb pressure levels. As a general rule of thumb, 5400m (540dm) is used as a typical rain versus snow line, where thicknesses below 5400m indicate snow and thicknesses above indicate rain. Since the thickness is based on a mean temperature profile in the layer, relatively warm air in the upper levels of this layer can still be below freezing, but still result in thicknesses above 5400m, supporting snowfall. Likewise, a shallow, warm boundary layer can result in rain with thicknesses below 5400m. However, significant changes can occur with subtle temperature adjustments and major moisture adjustments, as thickness calculations are based on virtual temperature, which incorporates moisture. Thickness adjustments are typically on the order of five to ten meters, but changes as high as about fifty meters are possible.

850mb - 700mb thickness. At very high elevations (low surface pressure), a thickness from 1000mb-500mb cannot be calculated. In these cases, an 850mb-700mb thickness is more appropriate (though it is still calculated for the lower elevation stations). The same assumptions apply for this thickness as for the 1000mb-500mb thickness; a general rule of thumb regarding snow/rain in 1555m.

Hope this helps..

SF