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Feb 6-7 storm?


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The band from BRL to VYS is where it's at for getting the high ratio dendrites. It looks like that should slide across the southwest burbs and then probably south of downtown Chicago. Might be close to getting clipped by the band at my location.

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50 minutes ago, Chicago Storm said:


It’s wasn’t really supposed to be snowing down that way yet. The atmosphere is no drier south than it is north... Give it time.


.

It surprisingly saturated pretty quick. Flake size not great but coming down at a decent clip. A heavier band to my nw is trying to sink se very slowly. Hoping for 1-2in at this point but maybe can luck out with 2-3in if I get into some banding. 

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1 minute ago, hawkeye_wx said:

I have received only 0.7" of snow this afternoon.  I expected a bit better, but the ratio is lousy.  The 0.7" of snow melted down to 0.07" liquid, so a 10:1 ratio.

Crazy. Just goes to show you even with very cold temps, crappy ratios are possible if your best omega doesn't line up well with DGZ. 

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When I was looking at forecast soundings over the past couple days, there was a decent amount of omega in the dgz, but there was a decent amount above it as well.  I'm sure the better banding has higher ratios but unfortunately it's not that surprising to see reports of struggling ratios.  I also wonder if the dgz has been able to saturate all the way because the NAM at least was suggesting a potential problem.

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31 minutes ago, ChiTownSnow said:

Good write up/chart regarding ratios and snowflake growth by skilling

Unfortunately, as today is proving, it's far more complicated than that. 

Take DSM for example. Assuming the snowfall and QPE numbers are right, they recorded 1" of snow on 0.10" of liquid between 6AM and noon. That's a 10:1 ratio. Their surface temperature was 6-7F, which according to Skilling's chart, would end up producing snow of 4". 

It has more to do with the vertical velocities in the cloud layer than just about anything else. Those 20-40:1 ratios would only occur where the best lift coincided with a saturated cloud layer with a temperature from ~-10 to -20C (the middle of that range is best).  

There are many other variables that affect snow:liquid ratios that involve cloud microphysics, chemistry, physics and so on. It's a difficult thing to model perfectly, and we probably have a ways to go before your typical model websites are able to accurately portray what's going on.

Ricky covered this at the top of the page, so re-read his message to see a little bit of why today didn't work out for everyone.

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Had about 30 min of fat flakes and vis down to a 1/4 mile, at least thats how I measure vis to my neighbor thats a half mile away.  I couldnt see his street light for about 20 min thats usually a 1/4 mile vis.  Swept a half inch off the deck.  Fat flakes were 3/4 by themselves lol.  I'll call it a disappointing half when the offices were forecasting 1-3.  Temps rose from 11 to 16, but finally now falling towards the forecast 0 low.  We shall see

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Revisiting this, some of the early HRRR/RAP runs and to a bit lesser extent the NAMs and HRWs clearly were far too aggressive. Had thought 2-4" was attainable across a good chunk of the CWA in expectation that the ratios could perform even outside banding. This was before it started to become more clear that this would be one of those fairly common setups where only in the banding would ratios perform to their capability and the CAMs were overdoing QPF.

 

It appears that the CAMs were reacting to mesoscale banding and distributing this QPF over much too widespread an area and were in general too wet. These are tough forecasts because the globals don't necessarily do as good a job picking up on the f-gen banding, while the CAMs do but might be too aggressive in doing so. Plus the ratio question really takes being precise in ascertaining where banding will set up to forecast the localized 15-20:1 ratios while the rest of the area comes in close to 10:1.

 

 

 

 

 

 

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36 minutes ago, RCNYILWX said:

Revisiting this, some of the early HRRR/RAP runs and to a bit lesser extent the NAMs and HRWs clearly were far too aggressive. Had thought 2-4" was attainable across a good chunk of the CWA in expectation that the ratios could perform even outside banding. This was before it started to become more clear that this would be one of those fairly common setups where only in the banding would ratios perform to their capability and the CAMs were overdoing QPF.

It appears that the CAMs were reacting to mesoscale banding and distributing this QPF over much too widespread an area and were in general too wet. These are tough forecasts because the globals don't necessarily do as good a job picking up on the f-gen banding, while the CAMs do but might be too aggressive in doing so. Plus the ratio question really takes a being precise in ascertaining where banding will set up to forecast the localized 15-20:1 ratios while the rest of the area comes in close to 10:1.

Sent from my SM-G965U using Tapatalk
 

A deep-dive into the system’s innards on AWIPS and BUFKIT was revealing yesterday. They all did a nice job showing the best crosshairs signal (lift + DGZ) where the relatively heavier snow occurred, despite QPF placement being incorrect. Shame the forcing was so high up when the super deep DGZ was based at the surface. Good reminder that model QPF and weenie snow maps have a long way to go.

NE did see some stellar ratios out of this at least. Omaha saw 31:1 ratios and Lincoln had 36:1. 

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