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Possible snow->rain->snow? event, Tuesday-Thursday 3/3-5/2015


famartin

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Trying to learn.

 

I can't remember if it's been every storm lately, but it seems like the HRRR comes in WAY drier than the forecast models.  Using 1 model example, the RGEM at 6z tomorrow has fairly decent precip levels over all of NJ and up to NE PA, yet the HRRR has a dry slot in NE PA and much lighter precip through NJ.  Other hours are similar with HRRR way down in many areas for expect precip amounts.

 

That drier prediction has also been off from what has actually fallen recently.

Any reason, or is that just a bias the HRRR has?

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Surely.  Same caveat applies:  These are based on my interpretation of the QPF and thermal profiles based on 6-hour time intervals...so there's a little guesswork.

 

MMU ~0.50" as snow

EWR ~0.51" as snow

BLM ~0.67" as snow

 

It will, at least if you want someone to do you a favor :P

Thanks, Ray!  

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Well, if it cools aloft all the way through the column, then you will go from rain to snow.  Sleet would mean that a layer of warm air remains aloft, as was expected with this system.  However, the most recent guidance has mostly done away with a significant cold layer beneath warmth during the changeover, opting more for an isothermal look as the entire column cools.  A small warm layer near 800 mb during changeover may be enough to get some sleet into the mix, but it probably won't last too long.  Surface temps only influence whether it makes it to the ground as sleet versus rain... if its saturated and in the 40s, it won't.  Usually during changeovers from rain to snow, you can get some sleet when temps fall to the upper 30s.  Once the column cools enough it goes to snow, and that is not directly related to the surface temp.

 

Confused yet? ;)

 

Also, in this kind of situation, dynamics can play a role.  It's always snow way up in the snow growth region and in snow to rain scenarios (i.e., largely isothermal, at least for the last 5000 feet from 850 mb down to the surface), the key is the integrated time at temperature above freezing (and how far above freezing the column is) that determines if the falling snowflakes melt into raindrops before hitting the surface or not.  If the last 5000 feet are at 33F, it'll still be snow, whereas if the last 5000 feet are 36F, it'll be rain - it's the in-between temps and distances the snowflakes fall at those temps that are the grey areas.  

 

And if the precip rate is really high, then the cold snowflakes can transport cold air with them down the column, cooling the column and allowing for snow when the surface temp is even a few to several degrees above freezing.  I wrote a small treatise, lol, on the kinetics and integral approach to showing how one might calculate whether a warm nose aloft and the temps and distances at those temps below the warm nose would lead to either sleet or freezing rain.  If I had gone into meteorology instead of chem eng'g I probably would've written a program to figure this all out - best I can do now is offer qualitative input...

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Also, in this kind of situation, dynamics can play a role.  It's always snow way up in the snow growth region and in snow to rain scenarios (i.e., largely isothermal, at least for the last 5000 feet from 850 mb down to the surface), the key is the integrated time at temperature above freezing (and how far above freezing the column is) that determines if the falling snowflakes melt into raindrops before hitting the surface or not.  If the last 5000 feet are at 33F, it'll still be snow, whereas if the last 5000 feet are 36F, it'll be rain - it's the in-between temps and distances the snowflakes fall at those temps that are the grey areas.  

 

And if the precip rate is really high, then the cold snowflakes can transport cold air with them down the column, cooling the column and allowing for snow when the surface temp is even a few to several degrees above freezing.  I wrote a small treatise, lol, on the kinetics and integral approach to showing how one might calculate whether a warm nose aloft and the temps and distances at those temps below the warm nose would lead to either sleet or freezing rain.  If I had gone into meteorology instead of chem eng'g I probably would've written a program to figure this all out - best I can do now is offer qualitative input...

 

Actually, if the last 5000 feet are at 33 and its saturated, it will be rain at the surface.  5000 feet is a long way for a snowflake and you actually have liquid water droplets at much lower temps than freezing.  If its above freezing and saturated, its not hard to melt the flakes as it runs into all those little droplets.  However, melting takes energy from the atmosphere, so gradually, assuming no advection, the continued melting of snowflakes would slowly cool the column from 5000 feet on down to the surface to 32, at which point it would snow. 

 

The whole "transporting cold air down" is actually mainly based on the heat absorbed from melting snowflakes. 

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You would at least get some mixing with sleet and frz rain.  Also it would depend on rates; lighter precip would be more mixed, heavier would be snowier.

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