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Major Nor'easter snow storm (possible top 20) Noon Wednesday-Noon Thursday Dec 16-17, 2020

wdrag

By wdrag,
in New York City Metro

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Rjay
Rjay
RCNYILWX
RCNYILWX

Lurker on this thread, originally from College Point Queens, been working at NWS Chicago since 2010. Jealous of this storm, as it's been exceptionally boring out here and big snowstorms like Superstor

jbenedet
jbenedet

I think you’re going to see a correction back southeast today as the arctic airmass that’s building into the region throughout the day begins to get ingested into the models.  The temp drops thro

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wdrag

wdrag

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No changes yet to TOPIC headline (MAJOR Nor'easter snowstorm because I|'d like to try isolate some numbers  but nothing is backing down thru the 12z cycle... at least not yet.

I add the late morning NWS expected qpf and D5-6 chances of 3+" of snow.  They're qpf looks up a little as do their chances. 

I may not add much more for a few hours.  Have seen the 12z GEFS ensembles for 850 LOW,  24 hr qpf and  snowfall.  As mentioned by others earlier...  it seems a KU storm is at hand...  things can still shatter (or be less extensive), but I think when you combine both Monday and Wed-Thu, 2 feet ""possible"" in some places between I84 and I80...especially if banding occurs near the southern edge of the deep RH.  i sure hope these ensembles don't let me down.  Also, the southern edge of the snow which will be wetter and especially windy LI NJ coast,  Power outage potential...  more speculative details tomorrow or Monday.  

Only one thing stands out to me that differs from my own earlier days thought process, a little slower start- still an 18-24 hour snow blitz.

 

I know several others always thought further s of I80.  I'm still not convinced and await model guidance to confirm for me, one way or the other, which may be two days in the future. 

Awaiting model guidance that says this will not happen as intensely modeled now.1232P/12

 

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Screen Shot 2020-12-12 at 12.11.45 PM.png

Screen Shot 2020-12-12 at 12.12.00 PM.png

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wdrag

wdrag

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2 hours ago, Rjay said:

@wdrag.  Here's more info on the effect of wind on snow ratio.  I hope this is useful info for you.

 

Influence of surface wind speed on snow ratio

"Surface wind speed can affect the snow ratio. Surface winds exceeding approximately 9 m s−1 can move ice crystals at the surface, fracturing the crystal during saltation and increasing the snow density (e.g., Gray and Male 1981, 345–350). Therefore, only snow events occurring at wind speeds below 9 m s−1 were considered. Classifying the snow events by snow-ratio bin and plotting the fraction of events as a function of wind speed results in Fig. 6. Although the fraction of average snow ratios is constant with wind speed, the fraction of heavy snow ratios increases with wind speed from about 10% at low wind speeds to about 20% at 9 m s−1 (correlation coefficient of 0.572). In addition, the fraction of light snow ratios decreases with wind speed from about 53% at low wind speeds to 40% at 9 m s−1 (correlation coefficient of −0.507). These relationships are statistically significant at the 98% level. If considered in isolation, the wind speed plays a small, but significant, role in determining the snow ratio by shifting the distribution of snow ratio toward lower values at higher wind speeds (Fig. 6). This minor role for wind speed may be due partly to the way Roebber et al. (2003) compiled their data, using 6-h observations to minimize effects of prolonged wind compaction on snow ratio. However, Roebber et al. (2003) also showed that wind speed can be quite important when combined with other parameters. In addition, accounting for the gauge undercatch (e.g., Goodison 1978; Yang et al. 1998) may also lead to a stronger relationship between snow density and wind speed."

 

https://journals.ametsoc.org/view/journals/wefo/21/1/waf903_1.xml?tab_body=fulltext-display

 

Just scanned the abstract and closing remarks, including the caveats. 

I myself am not sure how to account for this in a forecast, except to keep it in mind.  I use the variable density snow forecast from the HRRRX, sometimes good, and other times not so.

I'll presume surface wind above 18 knots shatters dendrites a bit, making the layer more densely packed.  Here's my concern...  minor as in the closing pgh. I cant even pinpoint snow ratio itself, or exactly where best banding (in advance)...  then I need to use model winds... for the near surface, and the winds we see are generally 8-10 meters... need face level wind, not commonly available.  Snow measuring every 6 hours will reduce this problem, especially in events where wind is primarily at the tail end. 

Later, Walt

 

The surface wind speed plays a minor role by decreasing snow ratio with increasing wind speed. Although previous research has shown simple relationships to determine the snow ratio are difficult to obtain, this note helps to clarify some situations where such relationships are possible.

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