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February 19-20th Threat - STORM MODE

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It has lower totals than almost every other model, which is odd for the NAM, but given the CAD setup, it's hard not to lean on it. And if it can't be used for this type of storm, what can it be used for?

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12Z NAM3 continues to be underwhelming with the QPF prior to the changeover (~17Z). Generally .15-.20" liquid equivalent. I trust these Ferrier-based SLRs implicitly when riming snow and eventual ptype issues arise, but I still think the continued 1-2" general area-wide average for the cities and I95 corridor through 17Z is lacking with the QPF.  I hope so, anyway. 

nam3km_asnow_ferrier_neus_30.png

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1 minute ago, osfan24 said:

It has lower totals than almost every other model, which is odd for the NAM, but given the CAD setup, it's hard not to lean on it. And if it can't be used for this type of storm, what can it be used for?

A euro/3k NAM blend is usually a good forecast in a CAD situation like this. That’s where I’d lean.

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Per the NAM, the northern folks don't flip to pure rain until 00z Thursday, over 1 inch QPF by then. 

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whats frustrating is the 850s are good through 2pm. Is there any chance of a slight error at 700mb? Is that what the canadien high res is seeing?

 

nam3km_T850_neus_32.png

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It still very much feels like a 2-4" thump for most before a layer of sleet and a coating of ice for us along the urban corridor. Hoping for closer to 4" with the snow IMBY, but I have a feeling it'll be closer to 2-3" before the flip. 

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22 minutes ago, WxUSAF said:

Outside of yesterday’s 12z run, the 3k NAM has been pretty consistent with the front end snow. 2-4”/3-5” depending on latitude and elevation. 

Where’s your bar set on this one? I’m sure the inquiring minds of the Columbia / Elkridge snow geek crew would appreciate your thoughts. :)

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Just now, Ji said:

the NAM being dry...what happened to this  model? 

did someone try to unplug and restart it?

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I know it isn't worth using as a forecasting tool, but as an example of how we at the DC latitude could potentially maximize this storm is the 12z extended HRRR.  The sleet line approaches at 15z after 2-3" like the NAM, but then the precipitation ramps up in intensity and effectively "beats back" the mix line so that we end up with 6-7" by 19z.

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13 minutes ago, Ji said:

whats frustrating is the 850s are good through 2pm. Is there any chance of a slight error at 700mb? Is that what the canadien high res is seeing?

 

nam3km_T850_neus_32.png

The issue with the NAM's are where they set up the best banding.  They push the best fronto banding north of our area.  Even north of my area.  After an initial short thump around 7am it focuses the best banding up in PA.  You can see the cooling from the heavy band in the morning at 700 mp and then once that lifts north with only lighter rates the WAA takes over and without heavy rates to mix the column we torch at the mid levels.  That is how this works.  If we stay under heavy banding like the GFS/FV3/GEM/RGEM/HRDRPS/ICON suggest the flip will be delayed a couple hours plus combined with more qpf and we get those 4-8" numbers NWS is pimping.  But if the NAM's are right that the best lift and banding ends up to our north in PA, this will be very disappointing to most in here IMO.  I am NOT saying I think the NAM is right here...but the NAM progression of this would be a quick 1-3" followed by ice across most of our area.  That would probably be considered a fail by most in here considering what the official NWS forecast is right now!

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12 minutes ago, midatlanticweather said:

Jet streak is north - Precip is going north!

 

         That's extremely apparent if one looks at the NAM QPF for southern MD.

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8 minutes ago, WxMan1 said:

It still very much feels like a 2-4" thump for most before a layer of sleet and a coating of ice for us along the urban corridor. Hoping for closer to 4" with the snow IMBY, but I have a feeling it'll be closer to 2-3" before the flip. 

You literally took the words right out of my mouth. I'd say in Baltimore it snows approximately from 7 AM- 12 PM. That's 5 hours. I'm not buying the NWS 4-6 inches. That would mean averaging inch per hour rates and during the day I just don't see that happening. I think 2-3 inches is a much safer call and if I'm being honest, I could even see areas near the cities busting low on that, NAM supports that. Typical colder areas could get in the 4-6 inch zone. I expect the sleet to last from 12 PM until about 2 PM at the latest, then freezing rain/rain rest of the day. Honestly when all is done, I think it's going to look a lot like the outcome of last week's storm. 

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2 minutes ago, psuhoffman said:

The issue with the NAM's are where they set up the best banding.  They push the best fronto banding north of our area.  Even north of my area.  After an initial short thump around 7am it focuses the best banding up in PA.  You can see the cooling from the heavy band in the morning at 700 mp and then once that lifts north with only lighter rates the WAA takes over and without heavy rates to mix the column we torch at the mid levels.  That is how this works.  If we stay under heavy banding like the GFS/FV3/GEM/RGEM/HRDRPS/ICON suggest the flip will be delayed a couple hours plus combined with more qpf and we get those 4-8" numbers NWS is pimping.  But if the NAM's are right that the best lift and banding ends up to our north in PA, this will be very disappointing to most in here IMO.  I am NOT saying I think the NAM is right here...but the NAM progression of this would be a quick 1-3" followed by ice across most of our area.  That would probably be considered a fail by most in here considering what the official NWS forecast is right now!

Usually safe to bet against the NAM. If one of the globals jumps on with it, that's more concerning.

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1 minute ago, psuhoffman said:

The issue with the NAM's are where they set up the best banding.  They push the best fronto banding north of our area.  Even north of my area.  After an initial short thump around 7am it focuses the best banding up in PA.  You can see the cooling from the heavy band in the morning at 700 mp and then once that lifts north with only lighter rates the WAA takes over and without heavy rates to mix the column we torch at the mid levels.  That is how this works.  If we stay under heavy banding like the GFS/FV3/GEM/RGEM/HRDRPS/ICON suggest the flip will be delayed a couple hours plus combined with more qpf and we get those 4-8" numbers NWS is pimping.  But if the NAM's are right that the best lift and banding ends up to our north in PA, this will be very disappointing to most in here IMO.  I am NOT saying I think the NAM is right here...but the NAM progression of this would be a quick 1-3" followed by ice across most of our area.  That would probably be considered a fail by most in here considering what the official NWS forecast is right now!

It's certainly possible the 3k NAM is right for several reasons:

1.) the jet streak was well sampled by the the RAOBs out of New York this morning.

2.) we've seen the last minute north trend all winter, this could be it.

3.) ian is coming back from vacation in time for the "storm" so this thing is falling apart just in time for him to get back in town

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5 minutes ago, Ji said:

the NAM being dry...what happened to this  model? 

It's not dry...its just shifting the best forcing NORTHWEST of us...its aiming that stj moisture plume across PA instead of our area.  Lets see what the other guidance does today before deciding were in trouble.  

IF...if if if...everything else moves towards the NAM then I agree we are in trouble if people expect the 4"+ snowfall totals around here.  But as of last night the NAM was pretty much on its own with that look.  Sometimes the NAM is onto something...and often the NAM is just off on a tangent smoking some bad mojo.  

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Just now, psuhoffman said:

It's not dry...its just shifting the best forcing NORTHWEST of us...its aiming that stj moisture plume across PA instead of our area.  Lets see what the other guidance does today before deciding were in trouble.  

IF...if if if...everything else moves towards the NAM then I agree we are in trouble if people expect the 4"+ snowfall totals around here.  But as of last night the NAM was pretty much on its own with that look.  Sometimes the NAM is onto something...and often the NAM is just off on a tangent smoking some bad mojo.  

the NAM has been shifting north since yesterday. 

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8 minutes ago, Scraff said:

Where’s your bar set on this one? I’m sure the inquiring minds of the Columbia / Elkridge snow geek crew would appreciate your thoughts. :)

Eh...I feel pretty good with 2”. Think maybe 3” bar snow/sleet?

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5 minutes ago, MN Transplant said:

I know it isn't worth using as a forecasting tool, but as an example of how we at the DC latitude could potentially maximize this storm is the 12z extended HRRR.  The sleet line approaches at 15z after 2-3" like the NAM, but then the precipitation ramps up in intensity and effectively "beats back" the mix line so that we end up with 6-7" by 19z.

THIS THIS THIS...  people need to focus on where the best banding sets up.  It's that simple.  We have seen this type setup over and over.  Wherever those intense bands set up during the mid morning to mid-day will delay the flip to sleet as the intense rates and vv's will help to mix out the warm layers for a time.  That extra 2 hours (along with a LOT of qpf in those 2 hours) makes all the difference.  The guidance can't agree on where that banding sets up.  Euro kind of splits into two areas...one south of DC early and one north of DC later.  GFS and RGEM like to crush DC.  ICON and HRDRPS likes MD.  NAM is way up over PA.  But that banding feature is the key to who gets good snow and where is left kind of disappointed by all this imo.   

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2 minutes ago, mappy said:

the NAM has been shifting north since yesterday. 

It has actually been pretty consistently north for 2 days...it had that one great 0z run that went south and NAMd DC and MD and got everyone excited...but it had been north before that and went right back at 6z and has only been shifting further north every run since.  Everything else has kind of been shifting north (except the euro which has been rock solid steady) also...but is still way way south of the NAM.  Interesting model war.  Unfortunately this is kind of the exact situation the NAM was designed for.  Still doesn't make it right.  I suppose the high res Canadian SHOULD be able to handle this kind of thing also.  

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25 minutes ago, WxUSAF said:

A euro/3k NAM blend is usually a good forecast in a CAD situation like this. That’s where I’d lean.

Me too!  I also like the idea of the heavier precip focusing a little to our north because as the low lifts to north west of the mountains temps will warm across Kentucky and western WV setting up a frontal boundary to focus the precipitation.  That's what the NAM and Euro seem to be doing.  Heck no one likes to look at the SREF.  It has a mean of 3 inches over DC because it has members that fringe us to the north but also have a four 5 inch plus members to balance it out.  its mean is pretty much in the Euro, NAM camp.  The other caveat is for the snow changeover go with the fastest model but at the surface relay on the slowest one to warm the temps. 

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Just now, psuhoffman said:

It has actually been pretty consistently north for 2 days...it had that one great 0z run that went south and NAMd DC and MD and got everyone excited...but it had been north before that and went right back at 6z and has only been shifting further north every run since.  Everything else has kind of been shifting north (except the euro which has been rock solid steady) also...but is still way way south of the NAM.  Interesting model war.  Unfortunately this is kind of the exact situation the NAM was designed for.  Still doesn't make it right.  I suppose the high res Canadian SHOULD be able to handle this kind of thing also.  

I sorta hope the 12z NAM is not correct, 2" of snow followed by a bunch of sleet before the flip, sounds awful. 

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Just now, mappy said:

I sorta hope the 12z NAM is not correct, 2" of snow followed by a bunch of sleet before the flip, sounds awful. 

Of course down here, we're hoping the GFS is correct with the snow part, but it's pretty bad with CAD vs NAM.  NAM is probably more reliable in this situation.   I never expected more than 2-4 anyway, so I'm unmoved by the last few NAM runs.  

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If we're being honest with ourselves we should count ourselves lucky that we even have a shot at a few inches with no major cold air before or after this storm and a low track like this. That high pressure is saving us from a 40 degree rain storm. You gotta take what you can get in a winter like this. It's part of the mess of living in this region. Gotta get used to it if you're going to stay here lol. 

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