Winter Storm Benji 12/7/17 - 12/8/17

Updated forecast graphic from this evening:

That's not a definite line though. There will likely be more moisture on the south shore than north shore so if the thicker, colder air moves in fast, you never know.

As is always the case with our winter weather events, how much dynamic cooling occurs during the precipitation will be key?

The NAM goes crazy with snow because it cools the atmosphere off so much in the heaviest precipitation.

The GFS is the exact opposite and doesn't cool much at all. Raw numbers from the GFS are below, the right two numbers are those important atmospheric levels that need to be at or below freezing for snow. No issue there. The precip is still falling according to the GFS at that time. Then look at the boundary layer temp, mid 40's? That will melt any snow. BUT look at your dew point, near freezing. If rain is falling, the temperature will have to dynamically cool closer to the dew point. "Wet bulbing" The GFS doesn't make sense because it has rain falling and temp struggling to fall much.

60 Fri 12/08 06Z 46 ° 46 ° 46 ° 36 ° 0.02 -2 ° 0 °
63 Fri 12/08 09Z 44 ° 46 ° 44 ° 34 ° 0.04 -3 ° -1 °
66 Fri 12/08 12Z 43 ° 46 ° 43 ° 34 ° 0.02 -3 ° -1 °
69 Fri 12/08 15Z 42 ° 43 ° 42 ° 33 ° 0.00 -2 ° -1 °

No, I'm not jumping on the NAM train but we have some interesting model runs coming. I want to see more precipitation than what is being shown and colder temps before I feel comfortable in saying the S word.

My weatherbug keeps lowering my temps for friday. Now they give me a high of only 40 with 80% chance of rain.

0z NAM brings back frozen precip in a big way for most of southern LA. The prior two runs had trended a bit further east. Look at the totals through Miss/Alabama!!!

TropicalTidbits added a really helpful feature to the NAM and GFS... The total positive snow depth change is likely more realistic than the total snow accumulation. The total positive snow depth change takes more things into consideration rather than just the presumed 10:1 snow to liquid ratio accumlation maps.

Until the GFS comes aboard, I’m just not going to take it too seriously