Feb 18-19 long duration manageable snow and ice event

[HVSnowLover]

Models still sort of all over the place with this, NAM focused much more on overrunning tomorrow and Friday with coastal, Euro/CMC weaker with overrunning but have snow on Friday.

[HVSnowLover]

8 minutes ago, jm1220 said:

You can't discount the NAM getting warmer aloft. It's usually superior to other models sniffing out these warm layers that can mean changing over. Last two runs have gotten a little warmer in mid levels, no doubting that. It's north of other models generally but there's often a last minute north trend anyway. RGEM at 6z was also a good bit north it seemed. 

12Z RGEM also seems to agree more with NAM about overrunning being the main show but also has the sleet line reaching us pretty fast so agree this is a trend to monitor.  

[jm1220]

2 minutes ago, Brett said:

I just realized the snow maps count sleet as inches? So how many inches do you have to take off that NAM map to compensate?

I don't think they all count sleet as snow. I don't use Tropical Tidbits because I know they do. It's a much better idea to use soundings instead of snow maps and look at the 700/850mb maps. Lots of SW flow at those levels which will produce the heavy snow but also warm those layers up and everything in between. It's usually a good idea to use the warmer mid level temp models in these type of SWFE events since those layers often warm up faster than modeled. 

[HVSnowLover]

Just now, jm1220 said:

I don't think they all count sleet as snow. I don't use Tropical Tidbits because I know they do. It's a much better idea to use soundings instead of snow maps and look at the 700/850mb maps. Lots of SW flow at those levels which will produce the heavy snow but also warm those layers up and everything in between. It's usually a good idea to use the warmer mid level temp models in these type of SWFE events since those layers often warm up faster than modeled. 

Yea its tough because the stronger the WAA push the heavier snow we get tomorrow but also the faster change to sleet. 

[jm1220]

3 minutes ago, HVSnowLover said:

12Z RGEM also seems to agree more with NAM about overrunning being the main show but also has the sleet line reaching us pretty fast so agree this is a trend to monitor.  

Very typical scenario for our area for this-hopefully the snow comes in like a wall, accumulates fast and can hold the sleet off until it's about over. 

[NJwx85]

Ouch, if this is correct the city will start off as some light snow and then quickly change over to sleet.

[SnowGoose69]

4 minutes ago, jm1220 said:

Very typical scenario for our area for this-hopefully the snow comes in like a wall, accumulates fast and can hold the sleet off until it's about over. 

The totals may be underdone a bit on the NAM...if you stop at 20Z when it shows NYC going to sleet the 3 and 12km average out to only 3-4 inches...given the frontogenesis showing up I think the period from 17-20 see could have moderate-heavy snow and it really is not indicating that so I still think 4-6 is more likely.

[SnowGoose69]

1 minute ago, NJwx85 said:

Ouch, if this is correct the city will start off as some light snow and then quickly change over to sleet.

I am suspicious of where it shows that warm layer...given the winds I more think that the warming in the 650-700 layer is underdone and that warming below 800 is overdone...I still think NYC flips over to sleet by 20 or 21Z at the latest 

[jm1220]

12z RGEM looks pretty good actually, it's a little cooler in the mid levels than the Nam. 

[Blizzwalker]

Multiple storms have recently shown a modest NW shift late in the game.  Are there reasons making such a shift less likely with this storm tomorrow?

[SnowGoose69]

1 minute ago, jm1220 said:

12z RGEM looks pretty good actually, it's a little cooler in the mid levels than the Nam. 

the 3km NAM is correct about 8 out of times but those 2 where it is overly aggressive on the warming can really screw up a forecast because 2-3 hours too early on the changeover can be 5 inches more of snow if you're seeing heavy rates just out ahead of the changeover line which you often will

[HVSnowLover]

5 minutes ago, Blizzwalker said:

Multiple storms have recently shown a modest NW shift late in the game.  Are there reasons making such a shift less likely with this storm tomorrow?

Unfortunately I would usually go with the NAM on mid level warming scenarios (but the 12K Nam looked okay, it's like its the 3K that is warming us fast) 

[mikem81]

17 minutes ago, jm1220 said:

Very typical scenario for our area for this-hopefully the snow comes in like a wall, accumulates fast and can hold the sleet off until it's about over. 

In my mind the sleet comes in as precip goes from heavy to lighter. If we sleet it means we got a good front end thump right before.

[Blizzwalker]

1 minute ago, HVSnowLover said:

Unfortunately I would usually go with the NAM on mid level warming scenarios. 

I was wondering more about the track of the storm as opposed to the intrusion of a warm layer.  ( I know the more N track can lead to warming).

[li_wx88]

From the NWS area forecast discussion;

"Guidance continues to trend colder with the system with the low
track generally near the 40n...70W benchmark. However, global
models are in good agreement with a long period of light to
occasionally moderate snow through Friday as multiple waves 
passes to the SE. The NAM remains an outlier with strong 
frontogenetic banding to the NW of the first low on Thursday 
with the potential for heavy snow, especially along the coast. 
It produces liquid equivalent amounts of a half inch at the far
NW corner of the Lower Hudson Valley to over an inch across the
NYC metro and LI by early Thursday evening. There is also a
strong upper jet to the NW of the area. However, in assessing 
the guidance, the better jet dynamics appear to come in Friday 
as the RRQ of the upper jet approaches. Additionally, while 
there is a strong baroclinic zone to the NW of the low track, 
the low is fairly weak and the best thermal forcing may reside 
just south of the area. Overall,the NAM seems to be overdone 
with the lift, but at the same time it has been consistent over 
the last several runs. Still though, it like the global models 
has continued to shift to the south with the frontal zone and 
low tracks. This will have to be watched closely today to see if
it falls into the consensus with a longer, drawn out snowfall 
event. In fact, much of the guidance points to snowfall amounts 
of 6 to 8 inches across the area, but over a 24 to 36h period. 
Warning criteria is 6 inches or more in 12h, or 8 inches or more
in 24h. The latter is looking more likely at this time."

[HVSnowLover]

1 minute ago, Blizzwalker said:

I was wondering more about the track of the storm as opposed to the intrusion of a warm layer.  ( I know the more N track can lead to warming).

If the middle levels are warming faster it probably means the primary is further north which has been the trend on most storms this winter. Our ideal dream scenario is primary further South and coastal further NW closer to the coast but this is unlikely. 

[mikem81]

GFS basically a miss with part 1....

[justinj]

going with 3-4” for my area, south central suffolk. NW suffolk could see 6-8

[justinj]

12z nam shifted the heavier snow a bit north. Looks like more sleet will be mixing on Long Island and places south

[mikem81]

3 minutes ago, justinj said:

12z nam shifted the heavier snow a bit north. Looks like more sleet will be mixing on Long Island. 

NAM is the most north of the models.. GFS actually misses up to the south until the coastal takes over Thursday night.

[Northof78]

GFS prolonged, and starts early, just a long duration 80% to 100% snow storm for metro....all models get us to 6" - 10", just in different ways...

[MJO812]

Para has majority of the snow on Friday

[jm1220]

Comical differences on the models this close in. NAM essentially is all front end snow to sleet and most of the GFS snow is from a coastal storm. 

[White Gorilla]

21 minutes ago, jm1220 said:

Comical differences on the models this close in. NAM essentially is all front end snow to sleet and most of the GFS snow is from a coastal storm. 

GFS snow map? 

[MJO812]

3 minutes ago, jm1220 said:

Comical differences on the models this close in. NAM essentially is all front end snow to sleet and most of the GFS snow is from a coastal storm. 

Rgem is also the coastal route

[SnowGoose69]

Just now, jm1220 said:

Comical differences on the models this close in. NAM essentially is all front end snow to sleet and most of the GFS snow is from a coastal storm. 

The NAM idea wins IMO 75-25...the NAM is probably underdone on the post 22Z activity but the GFS is overdone I think Thu night and Fri.  The setup screams to me overall that nothing big is coming from the coastal outside of eastern SNE..

[mikem81]

Just now, MJO812 said:

Rgem is also the coastal route

RGEM has both....most precip from NAM and RGEM are between 12Z and an 20Z tomorrow. 

[SnoSki14]

It's a front end thump followed by some snow on Friday.

Thump is good up to NYC. I think the forecast is pretty cut & dry.

Don't model hug too much or you'll get burnt like with the last 5-8" storm on Superbowl Sunday.

6-8" looks good from combined parts. 

[SnowGoose69]

Just now, MJO812 said:

Rgem is also the coastal route

The RGEM is probably way too late getting the snow in...the one thing I learned this winter is never go against start time on the 3km NAM.  It has repeatedly won that war while the RGEM has lost.  The RGEM still gives pretty good snows 17-21z like the NAM does, it just has nothing really from 10-16z

[jm1220]

3 minutes ago, SnowGoose69 said:

The RGEM is probably way too late getting the snow in...the one thing I learned this winter is never go against start time on the 3km NAM.  It has repeatedly won that war while the RGEM has lost.  The RGEM still gives pretty good snows 17-21z like the NAM does, it just has nothing really from 10-16z

Snow always seems to start earlier than expected in these SWFE type storms.