Question about GFS

[brandybugg4180]

Ok I have been told that the 540 blue line is the freezing line and anything within that is considered wintry precipitation....so now i need to know how do you tell if its snow or freezing rain. IM asking because for the past three days now the gfs has been showing the 540 line in sc but the precip is green not blue and i know on most maps that blue means snow and green means rain..Can anyone help me out?

[Donwx]

Well on the GFS the green would still mean frozen precip just light to moderate precip the blue & beyond on the GFS means heavy precip.

[Air Force Met]

Ok I have been told that the 540 blue line is the freezing line and anything within that is considered wintry precipitation....so now i need to know how do you tell if its snow or freezing rain. IM asking because for the past three days now the gfs has been showing the 540 line in sc but the precip is green not blue and i know on most maps that blue means snow and green means rain..Can anyone help me out?

OK...by 540 line I am assuming you mean on the thickness chart.

540 Thicknesses being used for for a freezing line/frozen precip line is a FIRST GUESS only and is NOT a hard and fast rule by any stretch of the imagination. There are many other variables.

What the 540 line represents is the thickness of the atmosphere b/w the 1000mb height and the 500 mb height. In theory, the thicker the atmosphere...the warmer it is. The thinner it is...the colder it is since cold air is more dense and takes up less room. So more molecules of air can fit in less space if the air is cold.

Now...it should only be used as a first guess. In the mountains, thickness values for snow can be higher. Down in the south...higher or lower. I've seen snow at 546...and rain at 536.

The only way to tell the difference if something is going to be snow or freezing rain is to look at a temp profile of the atmosphere...not the thicknesses. Again, thicknesses is a first guess only. For instance....5 days out if I see the GFS forecasting rain and thicknesses in the 540-546 range...I will start looking at the temp profiles closer. I won't pay any more attention to the thicknesses because that is not what makes snow...the actual temp profile of the atmosphere is what makes snow or freezing rain.

So you have to look at forecast skew-t's or meteograms. You have to see if you have any layers above freezing..and if there are...how thick are they. If they are more than 1200' thick and you have freezing temps at the sfc...the snow will melt and freezing rain or sleet will fall. If there is a warm layer but it is about 700-1000' thick...snow grains will fall. If the warm layer is above 3C, it will still melt all the way. If it is above freezing at the sfc but the freezing level is below 1200' agl...you will get snow...above 1200' the snow will melt and you will get all rain at the sfc. If you are above freezing at the sfc and the freezing level is b/w 1000-1200 '...you may get a mix of rain and snow...depending on how warm it is at the sfc.

So thickness is a general guide...one that is WAY overused by most people. The snow thicknesses also vary from one part of the country to another. Mountains are higher....northern parts of the country are lower....generally. However, you can have a thickness of 528...and if you have a 2000' layer of 2C air at 850 MB...you will get sleet or freezing rain.

Lot's of stuff to look at.

[Extremeweatherguy]

Well on the GFS the green would still mean frozen precip just light to moderate precip the blue & beyond on the GFS means heavy precip.

the blue means snow on the GFS and the green means rain. The colors do not reflect the heaviness. A purple/red color would mean ice.

[Aslkahuna]

Depends upon which source you get your GFS from since from the NCEP the colors on the QPF are related to amount only and not type.

Steve

[sphinx]

The 540 line!

Good explanation "Air Force Met". Could this be saved in a FAQ section? Can anyone explain some of the other rules of thumbs used. Critical lines.

Another useful concept is the triple point. I've heard cyclogenesis may occur at the point where a cold front catches up with a warm front ..... Can anyone show me a map with the triple point on it? and a following map showing cyclogenesis?

For this particular event, I believe boundary layer temperatures are forecast to be too warm to support snow in the southeast.

What a boring Wx pattern for much of the east! I can't even find any fantasy storms in the GFS runs.

[Air Force Met]

The 540 line!

Another useful concept is the triple point. I've heard cyclogenesis may occur at the point where a cold front catches up with a warm front ..... Can anyone show me a map with the triple point on it? and a following map showing cyclogenesis?

Yes...the triple-point is a favored place for cyclogenesis. The triple-point is the spot where the cold front, warm front and occluded front meet. The jet lies above it. Because of this...and because major short wave trofs (mid-level energy) travel along the jet, and because the triple point is an area of natural convergence, it is a good place to get an unstable wave spun up. Basically the cycle is this:

1) Unstable wave forms on the front
2) becomes a mature wave...low moves north of the jet and occludes.
3) Piece of upper level energy moves along the jet stream and intersects the triple point causing triple-point cyclogenesis...and it forms into an unstable wave...then a mature wave.
4) Meanwhile the older mature wave has lost it's baroclinicity and become a cold barotropic low...and is now a decaying low...
5) The new maturing wave moves north of the jet forming it's own occlusion...and the old occlusion shears out around the circulations b/w the two lows.

That's the cycle. Meanwhile, all along the front you can get unstable waves forming as pieces of energy move along it...and as they pass the unstable wave by...they become stable waves and fill.

The life cycle of the low.

[WindRunner]

Well on the GFS the green would still mean frozen precip just light to moderate precip the blue & beyond on the GFS means heavy precip.

the blue means snow on the GFS and the green means rain. The colors do not reflect the heaviness. A purple/red color would mean ice.

No, he has it right. Look at the chart on the side when you're looking at a map with the precip. That's the QPF values - how much rain an area would get if all precip fell in the liquid form. The GFS does not predict precip type.

[Extremeweatherguy]

Well on the GFS the green would still mean frozen precip just light to moderate precip the blue & beyond on the GFS means heavy precip.

the blue means snow on the GFS and the green means rain. The colors do not reflect the heaviness. A purple/red color would mean ice.

No, he has it right. Look at the chart on the side when you're looking at a map with the precip. That's the QPF values - how much rain an area would get if all precip fell in the liquid form. The GFS does not predict precip type.

I guess we are looking at different types of GFS maps. The maps I look at do show the precipitation type, because I look at more than just the basic info. I usually look at the 850mb temp/wind/height map, then I will look at the precip. type and amount map, and in the case of a wind storm or hurricane, I will look at the surface winds map. I also will look at the surface temp. map if that is going to be a big player in a forecast.

[Air Force Met]

Well on the GFS the green would still mean frozen precip just light to moderate precip the blue & beyond on the GFS means heavy precip.

the blue means snow on the GFS and the green means rain. The colors do not reflect the heaviness. A purple/red color would mean ice.

No, he has it right. Look at the chart on the side when you're looking at a map with the precip. That's the QPF values - how much rain an area would get if all precip fell in the liquid form. The GFS does not predict precip type.

I guess we are looking at different types of GFS maps. The maps I look at do show the precipitation type, because I look at more than just the basic info. I usually look at the 850mb temp/wind/height map, then I will look at the precip. type and amount map, and in the case of a wind storm or hurricane, I will look at the surface winds map. I also will look at the surface temp. map if that is going to be a big player in a forecast.

What link are you going to that shows purple/red color is ice and blue is snow? Post the link and that will clear it up what type of GFS map it is that shows the precip type.

[P.K.]

So thickness is a general guide...one that is WAY overused by most people. The snow thicknesses also vary from one part of the country to another. Mountains are higher....northern parts of the country are lower....generally. However, you can have a thickness of 528...and if you have a 2000' layer of 2C air at 850 MB...you will get sleet or freezing rain.

Lot's of stuff to look at.

I tend to look at the 1000-850hPa thickness (1290m) rather than the 500hP one (5280m), as well as the 850hPa temperature when looking for snow.

Currently there are a few snow grains falling from the sky here. The 850hPa temp is around -9C, the the two thicknesses are around 5295m, and 1285m.

[Air Force Met]

So thickness is a general guide...one that is WAY overused by most people. The snow thicknesses also vary from one part of the country to another. Mountains are higher....northern parts of the country are lower....generally. However, you can have a thickness of 528...and if you have a 2000' layer of 2C air at 850 MB...you will get sleet or freezing rain.

Lot's of stuff to look at.

I tend to look at the 1000-850hPa thickness (1290m) rather than the 500hP one (5280m), as well as the 850hPa temperature when looking for snow.

Currently there are a few snow grains falling from the sky here. The 850hPa temp is around -9C, the the two thicknesses are around 5295m, and 1285m.

Different areas have different rules and some areas work better under different thickness rules.

For those in the US...thee is a link for critical thicknesses:
http://weather.cod.edu/forecast/fcst.thknsside.html

Pretty good tool for looking at every thickness value and gettnig a good sense of where the transition lines will be. My military unit forecasts for a large area...and it's a great tool to forecast for the transition from rain to snow...etc.

[P.K.]

Makes sense, just thought that 540DM was rather high for snow going by here. The 1000-850hPa thickness on your link is pretty close to what I would look for here though.

[Extremeweatherguy]

Well on the GFS the green would still mean frozen precip just light to moderate precip the blue & beyond on the GFS means heavy precip.

the blue means snow on the GFS and the green means rain. The colors do not reflect the heaviness. A purple/red color would mean ice.

No, he has it right. Look at the chart on the side when you're looking at a map with the precip. That's the QPF values - how much rain an area would get if all precip fell in the liquid form. The GFS does not predict precip type.

I guess we are looking at different types of GFS maps. The maps I look at do show the precipitation type, because I look at more than just the basic info. I usually look at the 850mb temp/wind/height map, then I will look at the precip. type and amount map, and in the case of a wind storm or hurricane, I will look at the surface winds map. I also will look at the surface temp. map if that is going to be a big player in a forecast.

What link are you going to that shows purple/red color is ice and blue is snow? Post the link and that will clear it up what type of GFS map it is that shows the precip type.

I usually get the precip. type values on the accuweather professional model maps...but I did find a public link that offers the same thing. Here it is:

http://www.rap.ucar.edu/weather/model/i ... ?model=gfs

^^After clicking on link; click on the forecast hour and then go to the map option of "precip. type".^^

On this links maps...ice is yellow and red instead of purple and red, but blue is still snow and green is still rain.

[WindRunner]

Sorry for bringing this back up, but I wanted to clarify one more thing.

Unfortunately, I can't get any of the rap.ucar.edu pages to load, not sure why, so I can't see this miracle map or what it's based on, so I'll ask.
Is that actual GFS output (which I doubt), or an analysis of the GFS run based on a certain factor (or factors)?