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Feb 7th discussion/obs


The Iceman
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3 minutes ago, Ralph Wiggum said:

Dafuq is going on here?

nam3km_mslp_pcpn_frzn_neus_45.png

There is intense frontogenesis (FGEN) forecast by the NAM which is resulting in a period of robust precipitation rates. This setup looks to favor a robust FGEN band. 

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7 minutes ago, MGorse said:

There is intense frontogenesis (FGEN) forecast by the NAM which is resulting in a period of robust precipitation rates. This setup looks to favor a robust FGEN band. 

Thanks Gorse :lol:

My post was tongue in cheek as I have been mentioning this for a couple days. I was more caught off guard by how robust/vigorous the 3k is depicting. 

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Insane omega values in that FGEN band on the 18z NAM (pull a forecast sounding in the 3+"/hr rate zone....).  Also happens to maximize right in the optimal snow growth zone temperature-wise. That would be quite epic. It is notable that models are still oscillating a bit closer to the coast. Monitor that trend--too close and we have the classic "oops just the poconos" setup. W.r.t. precip types, Ralph is right  I think w/95 being a dividing line. As such, I think the 6abc map, and many of the snowfall maps on the models, are a bit generous S of the city. Great dynamics only help if you've got sufficient cold air. Good positioning of the 850 mb low though. FWIW, latest 4km RPM has the dividing line smack dab across Philly and 95--almost to the mile. It does suggest that the corridor will be the locus of the fgen band for at least some period, and that some of that will be in the form of heavy snow. Towards the end of the event, heavy precip rates and less WAA as the low moves NE will likely push the snow line SE.

On basis of this being a somewhat classical setup, I'm expecting 4-6 in the city and the 95 corridor, isolated areas of 8" where banding aligns with the best thermal profiles. On basis of the RPM, I think this will be a prototypical "nail biter" for city folks like me, as small deviations right now could really change this forecast and move the axis out to the N&W burbs, or even to C. NJ though I view that as less likely. That's probably the reason for the broad brush 3-6" abc call. Regardless, this doesn't look to be a 10:1 ratio snow..it'll be heavy and wet. If you look at the ratio estimates for this on the models, it's more 1:6,1:7...so even if you buy a more "snowy" solution from these models, you won't see those 10:1 ratio estimates verify. 

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