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Possible ligh snow event 3/18-3/19 & Obs


Zelocita Weather

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40 minutes ago, bluewave said:

The narrowest Norlun I can remember was in SE PA back in Feb 2009. But they do tend to be wider that that one. They still can have a narrower axis of focused heavy snows when compared to other storms here.

 

Heavy Snow in Lancaster, PA 3-4Feb2009

A band of very heavy snow set up right over Lancaster and Lititz, PA during the evening of Tuesday, Feb 3rd, 2009, and lasted into the early morning hours of Wednesday, the 4th. Snowfall rates were estimated to be up to 3 inches per hour at times.

Hardly any snow fell to the Southwest and Northeast of the band of snow. The band was generally stationary for 5-6 hours in 10-15 mile-wide a line from Lititz to Lancaster and Strasburg. Locations in Northeast Lancaster County (Reamstown for example) hardly had a dusting. To the southwest, Millersville University had just a half of an inch.

Hints at the potential convective nature of the snow were given by the NWS CTP local WRF model run during the daytime on the 3rd. The location of the (potential) heavy snow was near to Lancaster County, but not perfectly located. It was within a few grid points for a 12km model, though.

Larger/Broader-scale models will not catch such a small-scale event, and underestimated the potential for snowfall in that tiny area.

A look at the LWX (Sterling, VA) balloon/upper-air sounding from 00Z on the 4th (7 PM EST on the 3rd) portrays a very steep lapse rate in the lowest 15kft of the atmosphere. The atmosphere appeared to be unstable all the way up to the tropospheric inversion at about 25kft. The unstable atmosphere most likely led to convective enhancement inside the snow band which began to show up early in the day over Northern and Central PA. An upper-level low pressure area moving eastward from the Great Lakes provided additional lift. The focusing mechanism for the band – that is why it developed and stayed where it was – is much more difficult to explain.

For many in Lancaster and Lititz, this will probably be the heaviest snow they receive this winter.

The really interesting part of that read? The ''focusing mechanism'' , thats the billion dollar question, whoever figures that out can become a Norlun 'expert'!

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One of the reasons for the NAM's high precip totals is a small band of high VVs that pivots over the area. If this occurs, then high snowfall can happen, but this type of banding is very difficult to predict. You can only be sure of its occurrence if the banding is developing right over your head. One of the better things about this setup is the influence from a developing coastal instead of a pure Norlun trough. Be wary (the majority of you already are). 

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Do you care to explain why it's a red flag and what that would mean for the area?

 

NAVGEM is normally the most progressive. When the navgem is west or equal to the placement of globals, a good rule of thumb is the global is too far east. For example, for the 3/14 interior blizzard, the GFS was in the same spot or a bit East of the navgem and this was a red flag for the gfs (granted I believed an eastern track.. And ate crow).

 

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1 hour ago, USCG RS said:

NAVGEM is normally the most progressive. When the navgem is west or equal to the placement of globals, a good rule of thumb is the global is too far east. For example, for the 3/14 interior blizzard, the GFS was in the same spot or a bit East of the navgem and this was a red flag for the gfs (granted I believed an eastern track.. And ate crow).

 

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Thank you for the explanation.

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7 minutes ago, allgame830 said:

So then you think that the globals could in fact come further West or this is strictly a nowcast situation at this point?

I think that the banding will be a nowcast situation, as they are usually really fickle. The only way to know if you'll get hit is to see it moving over you. :lol: But seeing models have it gives the banding a higher probability.

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BTW the euro was much closer to something on the 12Z run than was indicated here.  It had a bullseye with an IVT SW of NYC near 00Z and was very close to something from 00-12Z across NYC and LI, the QPF was not exactly fully there but the setup was for sure


Yes, the euro looked to go right over bm. I don't have paid maps, so I can't dissect it, however, it looked like a hit.

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So then you think that the globals could in fact come further West or this is strictly a nowcast situation at this point?


Yes, there is the possibility this could actually be a coastal storm for the area. Meso s are showing this just inside bm and I believe they're underestimating the backside.

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